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Coordination Polymer Structures

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Title:
Coordination Polymer Structures Design and Photomagnetic Properties
Creator:
Risset, Olivia N
Place of Publication:
[Gainesville, Fla.]
Florida
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University of Florida
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english
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Degree:
Doctorate ( Ph.D.)
Degree Grantor:
University of Florida
Degree Disciplines:
Chemistry
Committee Chair:
TALHAM,DANIEL R
Committee Co-Chair:
MURRAY,LESLIE JUSTIN
Committee Members:
CHRISTOU,GEORGE
TOTH,ANNA F
MEISEL,MARK W
Graduation Date:
12/19/2014

Subjects

Subjects / Keywords:
Chemicals ( jstor )
Conceptual lattices ( jstor )
Coordination polymers ( jstor )
Eggshells ( jstor )
Irradiation ( jstor )
Magnetic fields ( jstor )
Magnetism ( jstor )
Magnetization ( jstor )
Magnets ( jstor )
Nanoparticles ( jstor )
Chemistry -- Dissertations, Academic -- UF
coordinationpolymerheterostructures -- magneto-optical -- morphology -- photomagnetism -- prussianblueanalogues
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bibliography ( marcgt )
theses ( marcgt )
government publication (state, provincial, terriorial, dependent) ( marcgt )
born-digital ( sobekcm )
Electronic Thesis or Dissertation
Chemistry thesis, Ph.D.

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Abstract:
Coordination polymers have been the subject of extensive research over the past 20 years owing to their potential applications in various fields, including gas separations and storage, catalysis, ion transport and storage, nonlinear optics, luminescence, magnetism, electronics, biomedical imaging and drug delivery. This thesis focuses on designing heterostructures of cyanide-bridged coordination polymers at the mesoscale (50 - 500 nm). The high level of control achieved over the heterostructures morphology is illustrated by the synthesis of size-controlled monodisperse particles, core@shell and core@islands heterostructures or even more intricate architectures such as hollow shell@shells. The design of heterostructures has proven a successful strategy toward new and enhanced properties. One example is in the area of photomagnetism. The coupling of a photo-responsive core and a photo-inert ferromagnetic shell within a core@shell structure gives rise to synergistic effects beyond the added contributions of both components. Additionally, these effects, shown to be magnetomechanical in nature, extend well beyond the few unit cells across the interface. The light-induced response, which persists after the light is turned off, is thermally reversible, thus defining an on and off state. Two illustrative examples of photo-responsive coordination polymer heterostructures are presented herein. Remarkably, a new system was designed, that shows persistent light-induced effects upon irradiation above liquid nitrogen temperature. Such materials provide new opportunities for the engineering of magneto-optical recording technologies. ( en )
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In the series University of Florida Digital Collections.
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Includes vita.
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Includes bibliographical references.
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Description based on online resource; title from PDF title page.
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This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Thesis:
Thesis (Ph.D.)--University of Florida, 2014.
Local:
Adviser: TALHAM,DANIEL R.
Local:
Co-adviser: MURRAY,LESLIE JUSTIN.
Electronic Access:
RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2015-12-31
Statement of Responsibility:
by Olivia N Risset.

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UFRGP
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Applicable rights reserved.
Embargo Date:
12/31/2015
Resource Identifier:
974372478 ( OCLC )
Classification:
LD1780 2014 ( lcc )

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COORDINATION POLYMER STRUCTURES: DESIGN AND PHOTOMAGNETIC PROPERTIES By OLIVIA NATHALIE RISSET A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2014

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© 2014 Olivia Risset

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To

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4 ACKNOWLEDGMENTS It has been quite a journey from growing up in the middle of nowhere in France to obtaining my doctorate at the University of Florida. I would probably still be where I started without the love and support of countless individuals. I will f orever be grateful to my family for their unconditional love and for providing me with the opportunity to follow my own path. I want to particularly thank my mother and grandmother for fostering my interest in learning and encouraging me to study hard and get an education, as they never had the chance to. I am indebted to my friends who became like family over the years. Although without them. I would like to give a speci al merci to Yousoon Lee, who has been My last personal acknowledgement goes to my husband, Matt, and his family who welcomed me like one of their own. On a more professio nal note, I am very grateful to my advisor, Prof. Daniel Talham, for being such a great boss and mentor and for providing me with countless opportunities to learn, make mistakes and develop my problem solving skills. I thank Prof. Mark Meisel, who has been my unofficial co advisor, for his patience and support. I am very thankful to Dr. Matthieu Dumont who mentored me throughout my first year in the Talham group. I would also like to acknowledge Dr. Rodrigue Lescouezec, my former French advisor who got me s tarted on molecular magnetism, as well as Dr. Laurent Lisnard, who taught me everything I know about bench work. I want to thank Prof. Leslie Murray for his patience and for always keeping his door opened. Finally, I would like to thank the faculty members who dedicated their time for serving on my

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5 committee, Prof. Daniel Talham, Prof. Mark Meisel, Prof. Leslie Murray, Prof. Anna Brajter Toth and Prof. George Christou. A wide variety of techniques involving state of the art equipment has been used in the co urse of this work and access to all the fun toys was possible thanks to our funding sources. Last but not least, I am indebted to the numerous people who have shared their expertise including fellow chemists, physicists and materials scientists. So I would like to thank all our collaborators, including my former and current coworkers from the Talham and Meisel groups, who have had the patience to bear with me over the be Phi nally Done, I will miss you all greatly.

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6 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 8 LIST OF FIGURES ................................ ................................ ................................ .......... 9 LIST OF ABBREVIATIONS ................................ ................................ ........................... 12 ABSTRACT ................................ ................................ ................................ ................... 15 CHAPTER 1 OVERVIEW ................................ ................................ ................................ ............ 17 2 Rb j M k [Fe (CN) 6 ] l · n H 2 O (M = Co, Ni) PRUSSIAN BLUE ANALOGUE HOLLOW NANOCUBES: A NEW EXAMPLE OF A MULTILEVEL PORE SYSTEM ............... 23 Introduction ................................ ................................ ................................ ............. 23 Experimental sec tion ................................ ................................ ............................... 25 Material preparation ................................ ................................ ......................... 25 Characterization ................................ ................................ ............................... 28 Synthesis an d morphology of hollow RbCoFe and RbNiFe nanoparticles .............. 29 Analysis of the templating process ................................ ................................ ......... 31 Properties of the hollow particles ................................ ................................ ............ 32 Porosity ................................ ................................ ................................ ............ 32 Surface reactivity ................................ ................................ .............................. 33 Conclusion ................................ ................................ ................................ .............. 34 3 EFFECTS OF LATTICE MISFIT ON THE GROWTH OF COORDINATION POLYMER HETEROSTRUCTURES ................................ ................................ ...... 44 Introduction ................................ ................................ ................................ ............. 44 Experimental section ................................ ................................ ............................... 45 Material preparation ................................ ................................ ......................... 45 Characterization ................................ ................................ ............................... 48 Synthesis and morphology ................................ ................................ ...................... 50 Structural study ................................ ................................ ................................ ....... 51 Conclusion ................................ ................................ ................................ .............. 54 4 LIGHT INDUCED CHANGES IN MAGNETISM IN A COORDINATION POLYMER HETEROSTRUCTURE Rb 0.24 Co[Fe(CN) 6 ] 0.74 @K 0.10 Co[Cr(CN) 6 ] 0.70 · n H 2 O AND THE ROLE OF THE SHELL THICKNESS ON THE PROPERTIES OF BOTH CORE AND SHELL ....... 66

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7 Introduction ................................ ................................ ................................ ............. 66 Experimental sec tion ................................ ................................ ............................... 68 Material preparation ................................ ................................ ......................... 68 Characterization ................................ ................................ ............................... 71 Results ................................ ................................ ................................ .................... 73 Synthesis and morphology ................................ ................................ ............... 73 Magnetization measurements ................................ ................................ .......... 75 Temperature dependent powder X ray diffraction ................................ ............ 76 Discussion ................................ ................................ ................................ .............. 78 Controlling the magnetism of the shell material ................................ ................ 78 Influence of the shell on the core ................................ ................................ ...... 84 Conclusion ................................ ................................ ................................ .............. 86 5 SWITCHING MAGNETISM WITH LIGHT ABOVE 77 K IN A BISTABLE COORDINATION POLYMER HETEROSTRUCTURE ................................ .......... 106 Introduction ................................ ................................ ................................ ........... 106 Experimental s ection ................................ ................................ ............................. 107 Material preparation ................................ ................................ ....................... 107 Characterization ................................ ................................ ............................. 108 Synthesis and morphology ................................ ................................ .................... 109 Magnetization measurements ................................ ................................ ............... 111 Conclusion ................................ ................................ ................................ ............ 112 6 CONCLUDING REMARKS ................................ ................................ ................... 118 APPENDIX A CTIST ANALYSIS IN THE CORE@SHELL SAMPLES ................................ ........ 120 B SIZE TRENDS OF VARIOUS PRUSSIAN BLUE ANALOGUES PARTICLES ..... 123 LIST OF REFERENCES ................................ ................................ ............................. 128 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 144

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8 LIS T OF TABLES Table page 3 1 Predicted size vs. measured size for core@shell heterostructures 1 and 2 ....... 61 3 2 Unit cell constants for pure KNiCo, 1 , 2 and 3 determined from X ray diffraction patterns collected at room temperature ................................ .............. 62 3 3 Strain parameter from the Williamson Hall analysis for heterostructures 1 , 2 and 3 ................................ ................................ ................................ .................. 64 4 1 Predicted size vs. measured size for RbCoFe@KCoCr heterostructures 1 , 2 and 3 ................................ ................................ ................................ .................. 90 4 2 Refined lattice constants at 300 K and 160 K for RbCoFe cores and RbCoFe@KCoCr 1 ( t = 11 nm), 2 ( t = 23 nm) and 3 ( t = 37 nm) ........................ 93 B 1 Size trends of various PBAs prepared by co precipitation method 1, 2 or 3. .... 124

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9 LIST OF FIGURES Figure page 1 1 Scheme of the PB A face centered cubic structure ................................ ............. 22 2 1 S ynthetic strategy to prepare PBA hollow shells ................................ ................ 36 2 2 TEM images of RbMnFe cores, core@shell particles of RbMnFe@RbCoFe and RbMnFe@RbNiFe, RbCoFe and RbNiFe hollow shells .............................. 36 2 3 EDS line scans on an individual RbMnFe@RbCoFe and RbMnFe@RbNiFe core@shell particle ................................ ................................ ............................. 37 2 4 EDS line scans on an individual RbCoFe hollow particle ................................ ... 38 2 5 Room temperature PXRD patterns for RbCoFe an d RbNiFe hollow shells ....... 38 2 6 FT IR spectra for RbMnFe cores , RbMnFe@RbCoFe and RbMnFe@RbNiFe core@shells , RbCoFe and RbNiFe hollow shells ................................ ............... 39 2 7 N 2 sorption isotherms of RbCoFe and RbNiFe hollows shells ............................ 40 2 8 TEM image of hollow nanocubes with a shell thickness of 23 ± 6 nm ................ 41 2 9 EDS line scans on an individual RbCoFe@RbNiCr shell@shell particle ............ 42 2 10 FT IR spectra for RbCoFe hollow shells and RbCoFe@NiCr hollow heterostructures ................................ ................................ ................................ .. 43 2 11 Magnetization vs. temperature plot for RbCoFe@NiCr shell@shell in an applied magnetic field H o = 100 G ................................ ................................ ...... 43 3 1 FT IR spectra for KNiCo, 1 , 2 , 3 , 4 as synthesized and 4 after 10 days ............. 55 3 2 TEM image of KNiCo ................................ ................................ .......................... 56 3 3 TEM images showing the core@shell morphology of 1 and 2 in contrast with the core@islands morphology of 3 and 4 ................................ ........................... 57 3 4 EDS line scans displaying the core@shell morphology of KNiCo@KNiCr 2 and the core@islands morphology of KNiCo@KCoCr 3 ................................ .... 58 3 5 EDS line scan displaying the core@shell morphology of 1 ................................ . 59 3 6 EDS line scan displaying the core@islands morphology of 4 ............................. 60 3 7 Size dispersion for KNiCo, 1 and 2 core@shell structures. ................................ 61

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10 3 8 PXRD patterns collected at room temperature for KNiCo@KNiFe 1 , KNiCo@KNiCr 2 and KNiCo@KCoCr 3 ................................ ............................. 62 3 9 Williamson Hall analysis for KNiCo@KNiFe 1 , KNiCo@KNiCr 2 and KNiCo@KCoCr 3 ................................ ................................ ................................ 63 3 10 Investigation of the heterogeneous growth of a series of Prussian blue analogues on nickel hexacyanocobaltate seeds ................................ ................. 64 3 11 Effect of lattice misfit on the morphology of two series of PBA heterostructures, using RbCoFe (blue) or KNiCo (red) as substrate .................. 65 4 1 TEM images of RbCoFe@KCoCr core@shell nanoparticles .............................. 87 4 2 EDS line scans on an individual RbCoFe@KCoCr core@shell particles with a shell thickness of 35 nm ................................ ................................ .................. 88 4 3 TEM image of RbCoFe cores ................................ ................................ ............. 89 4 4 Size dispersion for RbCoFe cores, 1 , 2 and 3 ................................ .................... 89 4 5 FT IR spectra for RbCoFe cores and RbCoFe@KCoCr core@shell particles 1 , 2 and 3 ................................ ................................ ................................ ........... 91 4 6 PXRD patterns at 300 K for RbCoFe cores and RbCoFe@KCoCr 1 ( t = 11 nm), 2 ( t = 23 nm) and 3 ( t = 37 nm) ................................ ................................ ... 92 4 7 and RbCoFe@KCoCr samples 1 , 2 and 3 . Right: Measured and predicted low field magnetic responses in the range 150 K to 300 K. ................................ 94 4 8 Left: Field cooled magnetization vs. temperature for RbCoFe@KCoCr 1 , 2 and 3 in the dark state and in the light state after irradiation at 5 K. Right: Magnetization (light dark) vs. temperature for RbCoFe@KCoCr 1 , 2 and 3 ..... 95 4 9 Magnetization vs. Field at T = 5 K for 1 , 2 and 3 in the dark state and light state, after irradiation at 5 K ................................ ................................ ............... 96 4 10 PXRD pa tterns at 300 K and 160 K for uncoated R bCoFe and RbCoFe@KCoCr samples 1 , 2 and 3 ................................ ................................ . 97 4 11 PXRD patterns at 160 K for RbCoFe cores and RbCoFe@KCoCr 1 ( t = 11 nm), 2 ( t = 23 nm) and 3 ( t = 37 nm) ................................ ................................ ... 98 4 12 Left: PXRD patterns stacked as a function of temperature, from 300 K to 160K, for the uncoated RbCoFe . Right: PXRD patterns stacked as a function of t emperature (300 160 K) for the RbCoFe@KCoCr sample 2 ...................... 99

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11 4 13 PXRD patterns stacked as a function of temperature, from 300 K to 160K f or the RbCoFe@KCoCr samples 1 and 3 . ................................ ............................ 100 4 14 PXRD patterns for 1, showing the (200) reflection, collected at 300 K, 160 K and at 100 K after irradiation with white light ................................ .................... 101 4 15 Williamson Hall plots for core@shell sample 1 at 300 K and 160 K in the dark state and at 100 K after irradiation with white light ................................ ........... 102 4 16 Schematic of the three component model for the core@shell heterostructure, dividing the shell into two distinct reg ions, a bulk like a nd strained region ....... 103 4 17 A plot of shell vs. temperature as derived from Equation 4 2 fo r the three core@shell samples ................................ ................................ ......................... 103 4 18 Plot of from Equation 4 3 vs. volume, V shell , for the three core@shell samples and the fitting of the model from Equation 4 5 ................................ .... 104 4 19 Simulated T vs. T data ................................ ................................ .................... 105 5 1 FT IR spectra for CoFe PBA cores and CoFe PBA@CrCr PBA heterostructures ................................ ................................ ................................ 113 5 2 TEM images of CoFe PBA@ CrCr PBA heterostructures ................................ . 113 5 3 EDS line scan on an individual CoFe PBA@CrCr PBA particle ....................... 114 5 4 TEM image of CoFe PBA particles ................................ ................................ ... 115 5 5 Size dispersion for CoFe PBA cores and CrCr PBA nanoparticles in CoFe PBA@CrCr PBA heterostructures ................................ ................................ .... 115 5 6 Field cooled magnetization vs. temperature for CoFe PBA@CrCr PBA under an applied field of 100 G in the dark state and in the light state after irradiation at 5 K ................................ ................................ ............................... 116 5 7 Field cooled magnetization vs. temperature for CoFe PBA@CrCr PBA under an applied field of 100 G in the dark state and in the light state after irradiation at 80 K ................................ ................................ ............................. 116 5 8 Left: Time dependence of the magnetization upon irradiation at 5 K . Right: Time dependence of the magnetization upon irradiation at 80 K ...................... 117 5 9 light), normalized to the magnetization in the dark state, M dark , plotted as a function of temperature ................................ .............. 117

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12 LIST OF ABBREVIATIONS A Alkali ion a Lattice constant a x Size of the x component (core, shell or core@shell) B Field BET Brunauer Emmett Teller BJH Barrett Joyner Halenda CHN Carbon Hydrogen Nitrogen cm 1 Inverse centimeter CoFe PBA Cobalt hexacyanoferrate CP Coordination polymer CrCr PBA Chromium hexacyanochromate CTIST Charge transfer induced spin transition DFT Density functional t heory EDS Energy dispersive X ray spectroscopy emu Electromagnetic unit FeCr Iron hexacyanochromate FT IR Fourier transform infrared FWHM Full width at half maximum G Gauss H Field h hour H c Coercive field HS High spin Irradiation

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13 K Kelvin KCoCr Potassium cobalt hexacyanochromate KNiCo Potassium nickel hexacyanocobaltate KNiCr Potassium nickel hexacyanochromate KNiFe Potassium nickel hexacyanoferrate kV Kilovolt l Thickness of the strained region L c Crystallite size LS Low spin M Magnetization m 2 /g Square meter per gram mg Milligram min Minute mL Milliliter mM Millimolar mmol Millimol MW Molecular weight n x Number of moles of the x component (core, shell or core@shell) nm Nanometer P Pressure PBA Prussian blue analogue PXRD Powder X ray diffraction RbCoFe Rubidium cobalt hexacyanoferrate RbMnFe Rubidium manganese hexacyanoferrate RbNiFe Rubidium nickel hexacyanoferrate

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14 sh Shoulder SQUID S uperconducting quantum interference device SR Strained region T Temperature or Tesla t Shell thickness T C Curie temperature TEM Transmission electron microscopy TGA T hermogravimetric analysis UV vis Ultraviolet visible V Volume W H Williamson Hall Vacancy Å Angstrom °C Degree Celsius Strain parameter Change in temperature x D ifference of the magnetic responses in the light and dark states for the x component (core, shell or core@shell) Bragg angle Wavelength Micrometer Stretching frequency Magnetic susceptibility

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15 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy COORDINATION POLYMER STRUCTURES: DESIGN AND PHOTOMAGNETIC PROPERTIES By Olivia Nathalie Risset December 2014 Chair: Daniel R. Talham Major: Chemistry Coordination polymers have been the subject of extensive research over the past 20 years owing to their potential applications in various fields, including gas separa tions and storage, catalysis, ion t ransport and storage, nonlinear optics , luminescence, magnetism, electronics, biomedical imaging and drug delivery. This thesis focuses on designing heterostructures of cyanide bridged coordination polymers at the mesoscale (50 500 nm). The high level of control achieved over the heterostructures morphology is illustrated by the synthesis of size controlled mono disperse particles , core@shell and core@islan ds heterostructures or even more intricate architectures such as hollow shell@shells . T he design of heterostru ctures has proven a successful strategy toward new and enhanced properties. One example is in the area of photomagnetism. The coupling of a photo responsive core and a photo inert ferromagnetic shell within a core@shell structure gives rise to synergistic effects beyond the added c ontributions of both components. Additionally, these effects, shown to be magnetomechanical in nature, extend well beyond the few unit cells across the interface. The light induced response , which persist s af ter the light is turne d off, is thermally reversible, thu s defining an on and off state. Two illustrative examples of photo responsive coordination polymer

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16 heterostructures are presented herein. Remarkably, a new system was designed, that shows persistent light induced effects upon irradiation above liquid nitrogen temperature. Such materials provide new opportunities for the engineering of magneto optical recording technologies.

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17 CHAPTER 1 OVERVIEW The rational design of molecular materials at the nanoscale (nominally less than 100 nm) and mesoscale (nominally 100 to 900 nm) is of great interest from a fundamental and technological standpoint. 1 9 The ability to control matter at such small scales holds the key to tailor made and enhanced properties. The molecular approach possesses major advantages over the more common solid state approach. In particular, the versatility , the ability to self assemble and mild synthetic conditions allow for a fine tuning of the materials nature and function alities. 10 13 Among molecule based systems, coordination polymers (CPs) have attracted growing interest over the past 20 years due to their potential applications in various fields, including catalysis, ion transport and storage, gas separa tions and storag e, nonlinear optics , luminescence, magnetism, electronics, biomedical imaging and drug delivery . 5,9,14 25 CPs consist of metal ions bridged by ligands, forming infinite arrays that can extend in one, two, or three dimensions. The wide variety of building b locks used in the preparation of hundreds of CPs exemplifies the chemical and structural versatility of such materials. 26,27 In the past two decades, research has been mainly focused on the synthesis of new structures. Aiming for materials with tailor made properties, recent efforts have been directed toward controlling the morphology of CPs beyond the molecular scale to form higher order superstructures and heterogeneous structures, such as hybrids and composites. 28 34 Prussian blue analogue s (PBAs) , proto typical cyanide based CPs, are ideal candidates to perform exploratory work on the preparation of CP structures at the nano and mesoscale. PBAs have the general formula A x (CN) 6 ] z (1 z) · y H 2 O where M n+

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18 m+ are metal ions linked by cyanide ligands i n an octahedral fashion and x = [(6 m) z ] n ( Figure 1 1). 35 The presence of alkali metal cations A + and/or intrinsic vacancies ensures the electroneutrality of the compound. For z < 1, water molecules complete the coordination spheres of M water molecules and alkali ions occupy the interstitial sites. The most attractive feature of PBAs is their chemical versatility. A plethora of PB like compounds can be prepared with various metal ions, al kali ions and stoichioemetries. Additionally, PBAs are synthesized from widely available chemicals, in mild conditions (usually room temperature and pressure), and are for the most part air stable. This vast library of compounds provides access to a wealth of electric, redox, magnetic and porous properties. Consequently, studies of PBAs have expanded to ion transport and storage, gas separation and storage, waste recovery, MRI contrast agents, negative thermal expansion, electrochromism and magnetism. 36 45 The field of PBA based nanoparticles has significantly expanded over the last ten years, starting from the development of microemulsion techniques. 46,47 The use of surface stabilizers allowed for the preparation of monodisperse nanoparticles with cubic or worm like shape as well as more intricate architectures such as shell in shell hollow cubes. 48 51 Later on, the preparation of self stabilized colloidal suspensions enabled the use of PBA particles as cores for the design of heterogeneous core@shell stru ctures. 52,53 As no protective chemical agent is used, the surface reactivity of the particles remains intact, making them suitable seeds for the epitaxial growth of different coordination networks.

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19 Through a judicious choice of the core and shell component , the first example of self stabilized hollow shells and hollow shell@shell heterostructures was prepared, as described in Chapter 2. 54 Rb 0.4 Mn [Fe(CN) 6 ] 0.8 · 4.8 H 2 O serves as a sacrificial/removable core in the synthesis of core@shell heterostructures of fo rmula Rb 0.4 Mn[Fe(CN) 6 ] 0.8 ·1.2H 2 O@ Rb 0.1 M[Fe(CN) 6 ] 0.7 ·1.8H 2 O (M = Co, Ni) . After dissolution of the cores under very mild conditions, the crystalline hollow nanocubes feature well defined micro , meso and macropores. Epitaxy has been extensively studied in the context of traditional solid state materials but nucleation and growth of CPs thin films remains relatively unexplored. 22,28,55 60 Chapter 3 examines the epitaxial relationship in a series of PBA heterostructures, varying the lattice misfit between sub strate and overlayer from 0.6% to 5.0%. Small lattice misfits (< 3.5%) result in the growth of a strained pseudomorphic shell whereas, to accommodate larger lattice misfits, lattice relaxation occurs through the formation of islands. The structural stu dy indicates the presence of anisotropic strain and confirms the efficient mechanical coupling between the substrate and overlayer materials. The mechanical coupling between core and shell materials, arising from the epitaxial relationship, enables synergistic effects which are more than the added contributions of both components. A notable example is in the area of alkali ion storage . The analo gue K 0.1 Cu[Fe(CN) 6 ] 0.7 ·3.8H 2 O has high lithium storage capacity but poor cycling performance due to a cubic to tetragonal phase change associated with the Cu + /Cu 2+ redox couple. 61,62 The growth of a more robust analogue as a shell suppresses the structural changes and leads to enhanced performance. 63,64

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20 Similar interface mediated synergy is observed in magnetic heterostructures and allows for the design of light switchable magnets with enhanced performance. The nickel hexacyanochromate analogues are not light responsive, but when incorporated into heterogeneous structures, thin films or core@ shell s, with light switchable spin t ransition compounds such as cobalt hexacyano ferrate PBA or the Hofmann like phase Fe(azpy)[Pt(CN) 4 ]· n H 2 O , their magnetization can be altered with light. 65 70 Chapter 4 offers a deeper insight into the nature of the synergistic effects giving rise to this new photomagnetic behavior and provides an estimate of the length scales involved. 71 For this study, a new phot omagnetic core@shell system was designed with a light responsive rubidium cobalt hexacyanoferrate (RbCoFe) core and a magnetic potassium cobalt hexacyanochromate (KCoCr) shell: Rb 0.24 Co[Fe(CN) 6 ] 0.74 @K 0.10 Co[Cr(CN) 6 ] 0.70 · n H 2 O. A single batch of 135 nm RbCoF e particles are used as seeds to generate three core@shell samples, with KCoCr shell thicknesses of approximately 11 nm, 2 3 nm and 37 nm . Synchrotron PXRD reveals that structural changes in the shell accompany the charge transfer induced spin transition (C TIST) of the core, giving direct evidence that the photomagnetic response of the shell is magnetomechanical in origin. Analysis of the magnetic response in the dark and light states indicates the depth of the shell material responding to the core changes t o be approximately 24 nm. In turn, a shell as thin as 11 nm alters the phase transition of the 135 nm core. In addition to improving our fundamental understanding of the phenomenon, a new heterogeneous light switchable magnet was designed, whose magnetic properties can be optically switched above liquid nitrogen temperature. As presented in Chapter 5,

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21 b istable RbCoFe@KCrCr heterostructures combining RbCoFe as the photoactive core and chro mium hexacyanochromate (KCrCr ), a ferrimagnet with T C = 190 K 24 0 K , exhibit a persistent photo induced decrease in magnetization up to 140 K, the temperature above which the RbCoFe core relaxes to the ground state. Concluding remarks are provided in Chapter 6, followed by appendix A detailing the CTIST analysis in the co re@shell heterostructures introduced in Chapter 4 and appendix B describing the size trends for various PBA particles.

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22 Figure 1 1. Scheme of the PBA face centered cubic structure. Zeolitic water molecules are omitted for clarity.

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23 CHAPTER 2 Rb j M k [Fe(CN) 6 ] l · n H 2 O (M = Co, Ni) PRUSSIAN BLUE ANALOGUE HOLLOW NANOCUBES: A NEW EXAMPLE OF A MULTILEVEL PORE SYSTEM Introduction Since the 1990s, the prolific development of nanoporous materials has extended their application range beyond traditional uses in ion exchange, adsorptive separation and catalysis. 72,73 Increased control over the distribution of sizes, shapes and volumes of the void spaces has opened up new opportunities in fields as diverse as medical diagnosis and imagi ng, drug delivery, lithium ion batteries, optics and photonics. Among porous materials, hierarchical systems with ordered micro , meso and even macropores have attracted a growing interest owing to the potential to optimize properties arising from a compl ex multilevel architecture. 74,75 The last ten years have seen tremendous progress in the synthesis of hollow structured materials. 76 The macroscale internal cavities of hollow particles when coupled with nanoporous shells exemplify the concept of hierarchi cal pore systems and lead to materials with high specific surface area, low density and surface permeability. 77 Recently, Lou et al. reported SnO 2 hollow structures with enhanced lithium storage capacity and improved cyclability. 78,79 Hollow mesoporous nan ospheres used as carriers for controlled drug release have also showed promising results demonstrating the wealth of properties originating from the unusual hollow morphology. 80 The most popular strategy to achieve hollow structures involves the coating of a R eprinted with permission from Risset, O. N.; Knowles, E. S.; Ma, S.; Meisel, M. W.; Talham, D. R. Chem. Mater. 2013 , 25 , 42. Copyright 2013 American Chemical Society.

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24 effective in the synthesis of a wide variety of hollow particles. 5 Diverse and versatile templates are available ranging from hard silica spheres 81,82 or metallic nano particles 83,84 to soft emulsion droplets 77,85 or gas bubbles. 86 Template methods possess major advantages in control over the size and shape of the products. However, they also have some inherent drawbacks attributed to tedious synthetic procedures. For ex ample, the removal of hard templates often requires calcination or chemical etching with hazardous chemicals such as hydrofluoric acid. The harsh treatment can lead to partial collapse of the hollow structures, significantly affecting the quality and prope rties of the hollow particles. Promising materials for the design of hollow shells are porous coordination polymers (PCPs). 48,77,87 89 Owing to their large and uniform porosity, 90,91 PCPs are attractive materials in applications that require selective perm eability. Even though PCP films have demonstrated outstanding properties in optics, gas separation and sensors, 23,59 PCPs with complex morphologies such as hollow structures and more exotic multilevel interior designs are scarce. 74 Prussian blue analogues (PBAs) are a widely studied family of PCPs. PBAs adopt cubic structures of formula A x (CN) 6 ] z (1 z) · y H 2 O where M n+ m+ are metal ions linked by cyanide ligands in an octahedral fashion and x = [(6 m) z ] n. 35 The presence of alkali metal cations A + and/or intrinsic vacancies ensures the electroneutrality of the compound. For z < 1, water molecules complete the coordination spheres of M ions occupy the interstitial sites. T he field of PBA based nanoparticles has significantly expanded over the last ten years, from the development of microemulsion

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25 techniques 46,47 to the preparation of self stabilized colloidal suspensions. 52 Fine control over the size and composition of the particles has been achieved, allowing for the design of even more complex architectures such as core@shell heterostructures. 53,66,92 Recently, Wang et al. 88,93 and MacLachlan et al. 48 developed elegant methods for the synthesis of PBA hollow nanostructure s. The nanoshells are prepared by using an organometallic directing agent to control the growth and stabilize the surface of the particles. Furthermore, Yamauchi et al. reported the synthesis of PBA hollow shells by controlled chemical etching in the prese nce of polyvinylpyrrolidone as a coating agent. 50,94 Herein, the synthesis of uniform hollow PBA nanocubes of formula Rb 0. 1 M[Fe(CN) 6 ] 0.7 · 1.8 H 2 O (M = Co or Ni) is reported, using a sacrificial/removable template approach. Core@shell heterostructures Rb 0.4 Mn[Fe(CN) 6 ] 0.8 · 1.2 H 2 O @Rb 0. 1 M[Fe(CN) 6 ] 0.7 · 1.8 H 2 O are first synthesized and the cores are dissolved under mild conditions that prevent the hollow particles from collapsing. This surfactant free route yields hollow shells with a surface that remains chemicall y active, as evidenced by the subsequent synthesis of hollow shell@shell heterostructures. The crystalline nanoboxes are a new example of hierarchical PCPs featuring micro , meso and macropores. Experimental section Material preparation All chemical reage nts were purchased from Sigma Aldrich and used without further purification. The nanoparticles were filtered on rapid flow bottle top filters with size PES membrane (Nalgene).

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26 Core particles : In a typical experiment, 50 mL of an aqueous soluti on of MnCl 2 .4H 2 O (99 mg; 0.50 mmol) were added dropwise (2.5 mL/min) to an equal volume of an aqueous solution containing K 3 [Fe(CN) 6 ] (181 mg; 0.55 mmol) and RbCl (181 mg; 1.50 mmol). After maturation for 4 hours under vigorous stirring, the particles were filtered under vacuum using a 0.45 filter and redispersed in 250 mL of a water/methanol mixture (4:1) to give the suspension of core particles. Except for a 1 mL aliquot used for characterization, the particles were not isolated and the suspension was used immediately in the next step. Rb 0.4 Mn[Fe(CN) 6 ] 0.8 · 1.2 H 2 O (RbMnFe): Deep brown powder. IR (KBr) 66,95,96 Mn II NC Fe I I I III NC Fe II ) cm 1 . EDS: 0.4:1:0.8 (Rb:Mn:Fe). Core @shell particles : Core@shell particles were synthesized using a modification of the method reported by Dumont et al . MCl 2 .6H 2 O (M = Co or Ni; 100 mg; 0.42 mmol) and RbCl (100 mg; 0.83 mmol) dissolved in 100 mL of a water/methanol mixture (4:1) and an equal volume of the same solvent mixture containing K 3 [Fe(CN) 6 ] (66 mg; 0.20 mmol to form RbCoFe/ 50 mg; 0.15 mmol to form RbNiFe) were simultaneously added (8 mL/h using a peristaltic pump) to the core particle suspension under vigorous stirring for 18 hours. The product was filtered under vacuum using a 0.45 fi lter. Except for a 1 mL aliquot used for characterization, the particles were not isolated and treated immediately as described in the next step. Rb 0.4 Mn[Fe(CN) 6 ] 0.8 ·1.2 H 2 O@Rb 0. 1 Co [Fe(CN) 6 ] 0.7 ·1.8 H 2 O ( RbMnFe@RbCoFe ): Deep II NC Fe I I I , Co II NC Fe I I I high spin (HS)), 2109 I I I NC Fe II low spin (LS), Co II NC Fe I I III NC Fe II ) cm 1 . EDS: 0.4:1:0.8 (Rb:Mn:Fe), 0.1:1:0.7 (Rb:Co:Fe), 1.6:1 (Mn:Co).

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27 Rb 0.4 Mn[Fe(CN) 6 ] 0 .8 ·1.2 H 2 O@Rb 0. 1 Ni [Fe(CN) 6 ] 0.7 ·1.8 H 2 O ( RbMnFe@Rb Ni Fe ): Yellow powder . IR (KBr) 40 II NC Fe I I I , Ni II NC Fe I I I II NC Fe I I ), III NC Fe II ) cm 1 . EDS: 0.4:1:0.8 (Rb:Mn:Fe), 0.1:1:0.7 (Rb:Ni:Fe), 0.9:1 (Mn:Ni). Hollow particles: The core@shell particles were allowed to stir in 1 L of water at 45 °C for 45 min. The product was then filtered using a 0.45 filter and the procedure was repeated twice. The particles were filtered one last time, redispersed in a minimum amo unt of water and air dried. For each product, the yield was calculated from the maximum theoretical amount determined by the quantity of divalent metal salt introduced. Rb 0. 1 Co [Fe(CN) 6 ] 0.7 ·1.8 H 2 O ( RbCoFe ): Purple powder (83 mg; 80% yield). IR (KBr): 2159 ( II NC Fe I I I I I I NC Fe I I low spin (LS)), 2095 II NC Fe I I ) cm 1 . EDS: 0.1:1:0.7 (Rb:Co:Fe). Rb 0. 1 Ni [Fe(CN) 6 ] 0.7 ·1.8 H 2 O ( Rb Ni Fe ): Yellow powder (79 mg; 78% yield). IR (KBr): 2165 II NC Fe I I I Ni II NC Fe I II II NC Fe I I ) cm 1 . EDS: 0.1:1:0.7 (Rb:Ni:Fe). Hollow shell@shell: NiCl 2 .6H 2 O (0.40 mmol; 4.0 mM) dissolved in 100 mL of water and an equal volume of an aqueous solution containing K 3 [Cr(CN) 6 ] (0.42 mmol; 4.2 mM) were s imultaneously added (8 mL/h using a peristaltic pump) to a 400 mL aqueous solution of the previously synthesized hollow RbCoFe core particles, under vigorous stirring for 18 hours. The product was filtered under vacuum using a 0.45 filter and washed twice with 500 mL of water. The light purple powder was isolated by centrifugation and air dried.

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28 Rb 0. 1 Co [Fe(CN) 6 ] 0.7 ·1.8 H 2 O@Ni[Cr(CN) 6 ] 0.7 ·2.0H 2 O (RbCoFe@NiCr): Light purple powder (97% yield). IR (KBr): 2170 II NC Cr I I I ) 2157 ( II NC Fe I I I HS), I I I NC Fe I I LS, Co II NC Fe I I ) cm 1 . EDS: 0.1:1:0.7:1. 2 : 0.8 (Rb:Co:Fe:Ni:Cr). Characterization Fourier transform infrared spectroscopy (FT IR) was performed on a Nicolet 6700 Thermo Scientific spectrophotometer taking 32 scans per spectrum between 4000 and 400 cm 1 with a precision of 0.482 cm 1 . Samples were placed onto the face of a KBr pellet by dispersing 1 mg of p owder in methanol or acetone and dropping the dispersion onto the preformed pellet. The spectrum of a p ure KBr pellet is taken as a background reference. Transmission electron microscopy (TEM) was performed on a JEOL 2010F high resolution transmission electron microscope at 200 kV. The TEM samples were prepared by dropping 40 water or methanol solut ion (1 mL) containing 2 mg of product, dispersed by sonication, onto the grid (carbon film on a holey carbon support film, 400 mesh, co pper from Ted Pella, Inc.) . Energy dispersive X ray spectroscopy (EDS ) was performed with an Oxford Instruments EDS X ray Microanalysis System coupled to the high resolution TEM (HRTEM) microscope. A total of four scans were recorded on different parts of the sample and then averaged to give relative atomic percentages for the metallic elements . Chemical formulas are based o n the metal compositions from EDS, adding water as determined by the number of trivalent metal vacancies to ensure charge balance . The particle size distribution is determined from the size measurements of a minimum of 200 particles from multiple regions i n one sample. The resulting distributions, with a binning of 1.5 nm for the shell thickness measurements and 3 nm for the particle sizes, are fitted using a log normal function.

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29 Powder X ray diffraction ( P XRD) was performed on a Philips APD 3720 powder diffractometer using a Cu K source. Diffractograms were recorded on 25 mg of sample mounted with double sided tape on a glass slide. Gas sorption measurements were performed on a Micromeritics ASAP 2020 Surface Area Analyzer. RbCoFe (55 mg) was degassed at 95 °C for 20 hours and RbNiFe (55 mg) was degassed at 150 °C for 10 hours. For each sample, nitrogen sorption measurements were performed at 77 K. Magnetic measurements were performed using a commercial superconductin g quantum interference device ( SQUID ) magnetometer (Quantum Design MPMS XL7). Powder sample (~1 mg) was immobilized between KBr pellets and mounted in a homemade optic sample rod . The magnetic data were taken after field cooling in an applied field H 0 = 100 G in warming mode. Synthesis and morphology of hollow RbCoFe and RbNiFe nanoparticles Formation of hollow structures via the template method requires difference s in physicochemical properties between the shell material or its precursors and the templat e . Our strategy takes advantage of the increased solubility of RbMnFe PBA compared to other members of the Prussian blue analogue family, known for being quite insoluble. Inspired by the synthesis of PBA core@shell heterostructures previously reported, 53,6 6 core@shell particles are prepared with the easily dissolved RbMnFe as the core (Figure 2 1). The RbMnFe cores are synthesized at high concentration of precursors (10 mM 11 mM) in aqueous medium by fast addition of the reagents (2.5 mL/min). These synth etic conditions favor the nucleation process over growth, allowing for the particle size to remain in the nanometric range. The cores are then redispersed in a water/methanol solvent mixture, which limits their dissolution. During the synthesis of the shel l, a substoichiometric amount of hexacyanoferrate is added to the divalent

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30 metal, inducing an equilibrium shift toward the dissolution of the cores during the synthesis. After the shell growth, the particles are washed in very mild conditions (1 L of water at 45 °C for 45 min) preventing the empty shells from collapsing. An intense yellow color appears in the aqueous wash due to the presence of ferricyanide ions in max = 420 nm), confirming the removal and dissolution of the RbMnFe cores. The mo rphology and chemical composition of the core, core@shell and hollow shell particles were investigated by TEM (Figure 2 2) and EDS. RbMnFe cores are polydispersed cubic particles with sizes ranging from 50 nm to 300 nm. RbMnFe@RbCoFe and RbMnFe@RbNiFe cor e@shell particles show a well defined cubic shape and are nearly uniform in size, 53 ± 12 nm and 5 2 ± 10 nm, respectively. The chemical assignments and layer segregation within the particles are confirmed by EDS line scans (Figure 2 3). The difference in s ize dispersion compared to the core particles and the presence of hollow structures in the core@shell mixture suggest that partial dissolution of the RbMnFe particles occurs during the shell growth. After the final washing procedure, the hollow morphology of RbCoFe and RbNiFe nanocubes is evident from the sharp difference in contrast between the shell and void space. Moreover, EDS line scans confirm the hollow character of the nanoparticles (Figure 2 4). The nanoshells well inherit the narrow size d istribution and shape of the core@shell precursors, which supports the retention of the structure upon removal of the template. The thickness of the shells ranges from 9 to 15 nm. In both cases, bulk EDS measurements reveal less than 10% of remaining RbMnF e in the final product. Powder X ray diffraction PXRD patterns (Figure 2 5) collected at room temperature on the RbCoFe and RbNiFe hollow structures can be indexed in the space

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31 group Fm 3m (No. 225) to give lattice constants a = 10.31 Ã… for RbCoFe and a = 10.17 Ã… for RbNiFe, consistent with literature precedent. 97,98 Diffraction peak linewidths indicate that the surfactant free approach to hollow particles yields crystallinity comparable to that seen in bulk preparations. Analysis of the templating proc ess The RbMnFe sacrificial template plays a dual role in the synthesis. Careful control over the concentration of precursors and the solvent mixture is required to prevent a complete dissolution of the template. However, a change in size and size dispersi on occurs from the template as bare particles to the core s embedded in the shells (Figure 2 2). These observations suggest a dissolution reprecipitation mechanism of the RbMnFe as the shell grows, which is supported by analyzing mass balance. The yield of hollow nanocubes, calculated from the limiting reagent, K 3 Fe(CN) 6 , is 112% for the RbCoFe hollows and 145% for RbNiFe. These abnormally high values support the partial dissolution, up to 20% , of the RbMnFe template, increasing the concentration of [Fe(CN) 6 ] 3 available in solution for the synthesis of the shell. Although part of the RbMnFe actively contributes to the shell growth, the remaining template cores can be dissolved by a gentle wash to obtain the hollow structures. Following each synthetic step b y FT IR gives insight into the evolution of the chemical makeup during subsequent steps in the synthesis. The FT IR spectra , shown in the cyanide stretching region for the RbMnFe cores ( Figure 2 6 ) , display two bands at 2151 and 2080 cm 1 characteristic of , respectively, both Mn II NC Fe III and Mn III NC Fe II sites in the network. 95,96 After the shell synthesis, the RbMnFe peaks remain predominant in the FT IR spectra for the core@shell samples because of differences in the molar extinction coefficients . How ever, additional features appear confirm ing the

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32 presence of a new material. The broad and asymmetric signal at 2152 cm 1 arises from two overlapping bands corresponding to the Mn II NC Fe III stretching of the core and Co II NC Fe III or Ni II NC Fe III forms in the shell compounds. Moreover, in both cases, a shoulder emerges on the high energy side of the core band at 2080 cm 1 , which can be assigned to characteristic cyanide stretches in the shell material: at 2109 cm 1 for RbCoFe and at 2099 cm 1 for RbNiFe . Strikingly, the FT IR spectra of the hollow structures are characteristic of the pure shell materials, supporting the nearly complete dissolution of the RbMnFe template. For the RbCoFe nanoshells, the FT IR spectrum exhibits a peak at 2159 cm 1 ass igned to the Co II NC Fe I I I sites as well as a broad asymmetric signal defining peaks at 2109 and 2095 cm 1 , corresponding to Co I I I NC Fe I I and Co II NC Fe I I respectively. The FT IR spectrum of the RbNiFe hollow shells displays peaks at 2165 and 2124 cm 1 , a ttributed to the bridging and terminal cyanides of Ni II NC Fe I I I , as well as a band at 2099 cm 1 characteristic of the Ni II NC Fe I I reduced phase. 40 Properties of the hollow particles Porosity The pore sizes and surface areas of the hollow particles were analyzed via nitrogen gas sorption studie s (Figure 2 7). The N 2 adsorption desorption isotherms measured at 77 K clearly show pseudo type IV behavior, indicat ing the org anization of hierarchical pores as expected for PBAs hollow shells. 99 The steep uptake followed by a plateau before P/P 0 = 0.1 is related to the pore filling of the micropores within the PBA framework , while the observed hysteresis in the pressure range of P/P 0 = 0.5 ~ 1 is indicative of mesopores result ing from the hollow morphology. By ap plying the Brunauer Emmett Teller (BET) model, the specific surface area is calculated to be

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33 694 m 2 /g for RbCoFe and 786 m 2 /g for RbNiFe, both of which are consistent with 39,94,100 Lite rature precedents indicate a value of 277 m 2 /g for hollow CoFe synthesized by controlled chemical etching. The higher value observed for the RbCoFe hollows described here suggests improved crystallinity of the shells using this surfactant free approach. Mi cropore size distribution analysis based upon a Density Functional Theory (DFT) 101 model reveals that the pores of RbCoFe and RbNiFe are predominantly distributed around 8.5 Å, which is in good agreement with expected cyanometallate vacancies in the PBA la ttice. Barrett Joyner Halenda (BJH) mesopore distribution analysis indicates a maximum around 38 Å for both compounds and th e mesopores presumably originate from the defects in the cry stalline shell. The well defined pores and high surface areas suggest th at the integrity of the PBAs structure remains intact upon dehydration. Surface reactivity The synthetic strategy has the major advantage of being surfactant free. As a result, the reactivity of the PBA surface, with its terminal cyanides and acidic sites , is preserved. To illustrate the reactivity of the shell, hollow shell@shell heterostructures were prepared. RbCoFe hollow shells were used as precursors for the growth of a NiCr PBA layer. TEM images (Figure 2 8) show well defined cubic hollow particles with a shell thickness of 23 ± 6 nm compared to 11 ± 4 nm for the starting hollow RbCoFe particles. The chemical assignments and layer segregation within the particles are confirmed by EDS line scans (Figure 2 9). Whereas the shell significantly thickens f rom the pure RbCoFe hollow precursor to the RbCoFe@NiCr heterostructure, the void size remains constant, which indicates a growth from the outer surface of the nanocubes. Moreover, the Ni/Co atomic ratio obtained by bulk EDS correlates with the layer

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34 thic kness ratio as expected in the absence of side nucleation. Hence, these observations suggest the NiCr deposits on the surface of the hollow precursors without side nucleation. Whereas the FT IR spectrum of the hollow shells is characteristic of pure RbCoFe , the cyanide stretch region of the hollow shell@shells shows an additional signal at 2170 cm 1 attributed to Ni II NC Cr I I I (Figure 2 10). Further evidence of the presence of two segregated layers as opposed to a mixed material can be found in the magnetic behavior of the hollow shell@shell. The field cooled magnetization vs. temperature plot (Figure 2 11) exhibits two ordering temperatures, T c = 20 K and T c = 75 K, assigned, respectively, to the ferrimagnetic ordering in RbCoFe and ferromagnetic ordering o f NiCr. 66,102,103 These two distinct ordering temperatures are indicative of two non interacting materials, confirming the growth of the NiCr layer on top of the RbCoFe shell with little mixing of ions from the two components. The ability to grow a second layer of a different PBA on the hollow nanocubes proves their reactivity toward further chemical modification. Although the hollow shell@shell particles were irradiated in a manner that generates photoinduced magnetism in core@shell samples , these experime nts showed no evidence of photoinduced effects for the hollow heterostructures. The chemical formula of RbCoFe shells indicates a high number of cyanometallate vacancies in the structure that hinders the bistabily required to observe the photoinduced magne tization. 96,97 Conclusion RbCoFe and RbNiFe PBA hollow nanocubes were successfully prepared via a facile surfactant free route using RbMnFe as a removable template. The hollow structured nanoparticles are crystalline and nearly uniform in size. With their high surface area and the presence of well defined micro and mesopores in the shell

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35 enclosing the larger cavity, these materials provide illustrative examples of multilevel pore systems. Furthermore, the preparation of self stabilized colloidal suspensio ns allows for the design of exotic coordination polymer heterostructures. Such a hierarchical porous architecture added to a reactive and magnetic shell is promising for applications in adsorptive/magnetic separation. The loading of such magnetic porous na noparticles with functional molecules would be of great interest for targeted drug delivery or selective catalysis.

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36 Figure 2 1 . The synthetic strategy consists in preparing core@shell heterostructures with RbMnFe core as a template and RbMFe (M = Co, Ni) as a shell. Dissolution of the template under mild conditions results in the formation of well preserved hollow shells. Figure 2 2. TEM images of RbMnFe cores A, D) show polydispersed cubes with a size range of 50 to 300 nm. After addition of the s hell precursors, TEM images display uniform cubic core@shell particles of RbMnF e@RbCoFe B) and RbMnFe@RbNiFe E ) . The shape and size of RbCoFe C) and RbNiFe F) hollow shells are preserved upon removal of the template. Shell thickness ranges from 9 to 15 nm. Scale bars for each image are 200 nm. Note that the holey carbon support is observed in panels A, B, D, E).

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37 Figure 2 3. EDS line scans on an individual RbMnFe@RbCoFe ( left ) and RbMnFe@RbNiFe ( right ) core@shell particle. Data represent the counts for each element detected as a function of the position of the electron beam across the particle. EDS line scans confirm the chemical assignments and the segregation between core and shell materials within the particle.

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38 Figure 2 4. EDS line scans on an indiv idual RbCoFe hollow particle. Data represent the counts for each element detected as a function of the position of the electron beam across the particle. EDS line scans confirm the hollow morphology of the particle. Figure 2 5. Room temperature PXRD pat terns for RbCoFe (left) and RbNiFe (right) hollow shells indexed using the space group Fm 3m (No. 225).

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39 Figure 2 6. FT IR spectra for: RbMnFe cores A , B ) exhibiting the two characteristic cyanide stretches from the Mn II NC Fe III and Mn III NC F e II sit es; RbMnFe@RbCoFe C ) and RbMnFe@RbNiFe D ) core@shells displaying predominant RbMnFe core bands with additional features assigned to the shell material. The difference in intensity between core and shell signals results from a difference in molar extinctio n coefficients; peaks in spectra of the RbCoFe E ) and RbNiFe F ) hollow shells are characteristic of the pure compounds, supporting the efficient removal of the template.

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40 Figure 2 7. N 2 sorption isotherms of RbCoFe A ) and RbNiFe B ) hollows shells measured at 77 K (BET surface areas: 694 m 2 /g and 786 m 2 /g respectively) . The black trace represents the absorption while the red trace represents the desorption. Pore size distribution s in RbCoFe determined by DFT C ) and BJH E ) models. Pore size distribution s in RbNiFe determined by DFT D ) and BJH F ) models.

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41 Figure 2 8. The TEM image of hollow nanocubes with a shell thickness of 23 ± 6 nm. While the shell thickens from the pure hollow shell to the heterostructure, the void size remains constant, suggesting the NiCr deposits on the surface of the RbCoFe hollow precursor. The scale bar is 200 nm.

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42 Figure 2 9. EDS line scans on an individual Rb Co Fe@Rb NiCr shell @shell particle. Data represent the counts for each element detected as a function of the posi tion of the electron beam across the particle. EDS line scans confirm the chemical assignments

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43 Figure 2 10 . FT IR spectra for RbCoFe hollow shells A ) and RbCoFe@NiCr hollow heterostructures B ), with additional band at 2170 cm 1 assigned to the Ni II NC Cr III sites. Figure 2 11. Magnetization vs. temperature plot in an applied magnetic field H o = 100 G exhibits two distinct ordering temperatures characteristic of the RbCoFe (T c = 20 K) and NiCr (T c = 75 K) shells, which confirms the segregation of layers within the nanoparticles.

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44 CHAPTER 3 EFFECTS OF LATTICE MISFIT ON THE GROWTH OF COORDINATION POLYMER HETEROSTRUCTURES Introduction The engineering of highly efficient electronic, magnetic and optical devices requires careful considerations of epitaxial growth processes and control of defects in thin film heterostructures. Strain and strain relief mechanisms are known to play a key role in determining the morphology and properties of thin films, 104 11 1 with elastic strain and dislocations both contributing to the lattice energy at the material interfaces. A small mismatch between the substrate and overlayer generally results in the growth of a strained pseudomorphic layer. To accommodate larger misfi ts, the formation of dislocations becomes more energetically favorable. These concepts are well established in the context of traditional solid state materials but nucleation and growth of coordination polymer thin films and heterostructures remains relat ively unexplored. 22,28,55 60 Growing interest in the design of heterogeneous multifunctional materials integrating coordination polymers stems from recent reports of new behavior arising from either additive properties or synergistic effects between the co mponents. 31 33,112 116 Heterostructures of Prussian blue analogues (PBAs), a widely studied family of cyanide bridged coordination polymers, provide illustrative examples in various fields such as ion transport and storage for battery applications or magn etism and light switchable magnetism. 63 70 The preparation of PBA core@multishell nanoparticles by epitaxial growth was first reported in 2009 by Catala and coworkers. 53 The past five years have seen a tremendous increase in the level of control achieved over PBA heterostructure morphology as demonstrated by the synthesis of monodisperse core@shell and

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45 multilayered architectures at the mesoscale, as well as more intricate hollow shell@shell structures. 51,54,63,64,66,68 In this paper, we investigate the in fluence of lattice misfit ( f ) on the growth mechanism of a series of PBAs on nickel hexacyanocobaltate (KNiCo) particles used as the substrate. KNiCo was chosen as a seed for its ability to form stable suspensions of uniform particles, its redox inert nat ure and its small lattice parameter ( a = 10.13 Å). Four PBA heterostructure particles were prepared by growing overlayers of potassium nickel hexacyanoferrate (KNiFe), potassium nickel hexacyanochromate (KNiCr), potassium cobalt hexacyanochromate (KCoCr) and potassium iron hexacyanochromate (KCoCr), increasing the lattice mismatch from 0.6% to 5.0%: KNiCo@KNiFe ( f = 0.6%), KNiCo@KNiCr ( f = 3.4%), KNiCo@KCoCr ( f = 3.9%) and KNiCo@KFeCr ( f = 5.0%). For f < 3.5%, the growth of a strained pseudomo rphic layer, indicative of a layer by layer mechanism, yields the commonly observed core@shell architecture. At higher lattice mismatch, the strain is relieved by the formation of islands giving a core@islands morphology. A structural study confirms effi cient mechanical coupling of the substrate and overlayer materials as well as the presence of anisotropic strain. Experimental section Material preparation All chemical reagents were purchased from Sigma Aldrich and us ed without further purification. KNiCo particles : In a typical experiment, NiCl 2 ·6H 2 O (95 mg; 0.40 mmol) dissolved in 100 mL of DI water and an equal volume of an aqueous solution containing K 3 [Co(CN) 6 ] (150 mg; 0.45 mmol) were simultaneously added (43 mL/h using a peristaltic pump) to 200 mL of DI water under vigorous stirring for 2 hours and 20 min.

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46 After maturing for 24 hours, the particles were isolated by centrifugation at 10,000 rpm for 10 min and subsequently washed with 300 mL of DI water. Except for a 5 mL aliquot used for characterization, the particles were not isolated and used immediately in the following step. K 0.31 Ni[Co(CN) 6 ] 0.77 ·5.2H 2 O (KNiCo): Light blue suspension (80% yield based on isolated batches). IR (KBr): 2182 ( CN, Ni II NC Co III ), 2138 ( C N, Co III CN terminal) cm 1 . EDS: 1.0:0.77 (Ni:Co). KNiCo @K NiFe c ore@shell particles (1) : KNiCo cores were redispersed in 400 mL of DI water. NiCl 2 ·6H 2 O (95 mg; 0.40 mmol) dissolved in 100 mL of DI water and an equal volume of an aqueous solution containing K 3 [Fe(CN) 6 ] (150 mg; 0.46 mmol) were simultaneously added (8 mL/h using a peristaltic pump) to the core particle suspension under vigorous stirring for 15 hours. The particles were isolated by centrifugation at 10,000 rpm for 10 min and subsequ ently washed with 300 mL of DI water . The product was isolated and air dried. { K 0.37 Ni[Co(CN) 6 ] 0.79 } 0.47 @{ K 0.28 Ni[Fe(CN) 6 ] 0.76 } 0.53 ·4.7H 2 O: Dark yellow powder (222 mg; 88% yield). II NC Co III , Ni II NC Fe III ) , 2138 (sh, CN, Co III CN II NC Fe II ) cm 1 . EDS: 1.00:0.37:0.40 (Ni:Co:Fe); KNiCo:KNiFe = 0.89 . KNiCo @K NiCr c ore@shell particles (2) : KNiCo cores were redispersed in 400 mL of DI water. NiCl 2 ·6H 2 O (95 mg; 0.40 mmol) dissolved in 100 mL of DI water and an equal volume of an aqueous solution containing K 3 [Cr(CN) 6 ] (150 mg; 0.46 mmol) were simultaneously added (8 mL/h using a peristaltic pump) to the core particle suspension under vigorous stirring for 15 hours. The particles were isolated by centrifugation at

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47 10,000 rpm for 10 min and subsequently washed with 300 mL of DI water . The product was isolated and air dried. { K 0.28 Ni[Co(CN) 6 ] 0.76 } 0.45 @{ K 0.25 Ni[Cr(CN) 6 ] 0.75 } 0.55 ·5.1H 2 O: Light blue powder (214 mg; 84% yield). II NC Co III , Ni II NC Cr III ), 2138 ( CN, Co III CN terminal) cm 1 . EDS: 1.00:0.35:0.41 (Ni:Co:Cr); KNiCo:KNiCr = 0.82 . KNiCo @KCoCr core@islands particles (3) : KNiCo cores were redispersed in 400 mL of DI water. CoCl 2 ·6H 2 O (95 mg; 0.40 mmol) dissolved in 100 mL of DI water and an equal volume of aqueous solution containing K 3 [Cr(CN) 6 ] (150 mg; 0.46 mmol) were simultaneously added (8 mL/h using a peristaltic pump) to the core particle suspension under vigorous stirring for 15 hours. The particles were isolated by centrifugation at 10,000 rpm for 10 min and subsequently washed with 300 mL of water . The product was isolated and air dried. { K 0.25 Ni[Co(CN) 6 ] 0.75 } 0.45 @{ K 0.10 Co[Cr(CN) 6 ] 0.70 } 0.55 ·4.5H 2 O: Light purpl e powder (208 mg; 88% yield). II NC Co III , Co II NC Cr III ), 2138 ( CN, Co III CN terminal) cm 1 . EDS: 1.00:1.96:0.84 (Ni:Co:Cr); KNiCo:KCoCr = 0.83 . KNiCo@Fe Cr core@islands particles (4) : KNiCo cores were redispersed in 375 mL of DI water/methanol mixture (4:1). FeCl 2 (51 mg; 0.40 mmol) dissolved in 100 mL of DI water/methanol (4:1) and an equal volume of the same solvent mixture containing K 3 [Cr(CN) 6 ] (150 mg; 0.46 mmol) were simultaneously added (8 mL/h using a peristaltic pump) to the core particle suspension under vigorous stirring for 15 hours. The particles were isolated by centrifugation at 12,000 rpm for 10 min and subsequently

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48 washed with 300 mL of DI water/methanol (4:1) . The product was isolate d and air dried. { K 0.25 Ni[Co(CN) 6 ] 0.75 } 0.56 @{ K 0.04 Fe[Cr(CN) 6 ] 0.68 } 0.44 ·4.5H 2 O: Orange powder (173 mg; 73% yield). II NC Co III II NC Cr III ), 2138 ( CN, Co III CN terminal), 2080 (broad, CN, Fe II CN Cr III ) cm 1 . EDS: 1.00:0.75:0.80:0.54 (Ni:Co:Fe:Cr); KNiCo:KFeCr = 1.25 . Characterization Fourier transform infrared spectroscopy (FT IR) was performed on a Nicolet 6700 Thermo Scientific spectrophotometer taking 16 scans per spectrum between 4000 and 400 cm 1 with a precision of 0.482 cm 1 . Powder samples (1 mg) were mixed with 150 mg KBr and pressed into a pellet. The spectrum of a pure KBr pellet is taken as a background reference. Transmission electron microscopy (TEM) was performed on a JEOL 2010F h igh resolution transmission electron microscope at 200 kV. The TEM product, dispersed by sonication, onto the grid (carbon film on a holey carbon support film, 400 mesh, copper from Ted Pella, Inc.). Energy dispersive X ray spectroscopy (EDS) was performed with an Oxford Instruments EDS X ray Microanalysis System coupled to the high resolution TEM (HRTEM) microscope. A total of four scans were recorded on different part s of the sample and then averaged to give relative atomic percentages for the metallic elements. To determine the water content, thermogravimetric analysis (TGA) was performed on a TGA Q5000 from TA Instruments under nitrogen purge at a flow rate of 25 mL /min with the temperature increasing at a rate of 10°C/min. TGA data are in the Supporting Information (Figure S1). Chemical formulas are based on the metal composition from EDS, the water content from TGA

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49 and the amount of alkali ion necessary to balanc e the charges. The particle size distribution is determined from the size measurements of a minimum of 200 particles from multiple regions in one sample. The particle size is reported as the mean along with the standard deviation. High resolution synchr otron powder diffraction data (PXRD) were collected using beamline 11 BM at the Advanced Photon Source (APS), Argonne National Laboratory using an average wavelength of 0.413670 Å. Each sample was loaded in a Kapton capillary tube with an inner diameter o f 0.80 mm. Discrete detectors covering an angular range from 6 to 16 º 2 are scanned over a 34º 2 range, with data points collected every 0.001º 2 and scan speed of 0.01º/s. The 11 BM instrument uses X ray optics with two platinum striped mirrors a nd a double crystal Si(111) monochromator, where the second crystal has an adjustable sagittal bend. 117 Ion chambers monitor incident flux. A vertical Huber 480 goniometer, equipped with a Heidenhain encoder, positions an analyzer system comprised of twe lve perfect Si(111) analyzers and twelve Oxford Danfysik LaCl 3 scintillators, with a spacing of 2º 2 . 118 Analyzer orientation can be adjusted individually on two axes. A three axis translation stage holds the sample mounting and allows it to be spun, ty pically at ~ 5400 RPM (90 Hz). A Mitsubishi robotic arm is used to mount and dismount samples on the diffractometer. An Oxford Cryosystems Cryostream Plus device allows sample temperatures to be controlled over the range 80 500 K when the robot is use d. The diffractometer is controlled via EPICS. 119 Data are collected while continually scanning the diffractometer 2 arm. A mixture of NIST standard reference materials, Si (SRM 640c) and Al 2 O 3 (SRM 676) is used to calibrate the instrument, where the S i lattice constant determines the wavelength for each detector. Corrections are applied for

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50 detector sensitivity, 2 offset, small differences in wavelength between detectors, and the source intensity, as noted by the ion chamber before merging the data i nto a single set of intensities evenly spaced in 2 . The data were processed using Origin 8.5 . Synthesis and morphology The preparation of KNiCo particles is adapted from published procedures to provide a self stabilized suspension in water. 52 In the abs ence of surfactant, the synthesis yields fairly uniform particles at the mesoscale (50 nm 500 nm). The major advantage of this method resides in the formation of particles that are not passivated with a stabilizer, thus allowing for their use as a subst rate in the building of multicomponent heterostructures. The synthesis of heterostructures is achieved by the slow addition of low concentration precursor solutions to a suspension of the seed particles. As previously reported, this method favors heterog eneous precipitation while preventing side nucleation. 53,54,66,68 For the present study, four heterostructures are prepared with KNiCo seeds: KNiCo@KNiFe 1 , KNiCo@KNiCr 2 , KNiCo@KCoCr 3 and KNiCo@KFeCr 4 . X ray diffraction confirmed the presence of the targeted components ( vide infra and Figure 3 8 ). FT IR spectroscopy was used to probe the chemical makeup of the products as well as the valence states of the metal centers (Figure 3 1 ). The morphology and composition of the KNiCo particles and heterostru ctures were investigated by TEM (Figure 3 2 to 3 6 ). EDS line scans confirmed the chemical assignments and material segregation within the heterostructures (Figure 3 4 to 3 6 ). The KNiCo particles are ~ 100 nm well defined cubes with a small size dispers ion of less than 15% ( Figure 3 2 and 3 7 ). Heterostructures 1 and 2 exhibit a similar cubic

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51 shape and are fairly monodisperse, 127 ± 17 nm and 122 ± 15 nm, respectively (Figure 3 3 and 3 7 ). The measured sizes are in agreement with the predicted sizes, a ssuming quantitative heterogeneous precipitation leads to the formation of the core@shell heterostructures ( Table 3 1). Moreover, the core@shell nature of 1 and 2 is elucidated by EDS line scans (Figure 3 4 and 3 5 ). The morphology of 3 is quite differen t and consists of KCoCr cubic islands growing at the corners of the KNiCo cores (Figure 3 3 ). The islands are of variable sizes but remain below 50 nm. EDS line scans performed across the heterostructures are consistent with the presence of individual is lands (Figure 3 4 ). Heterostructure 4 shows a raspberry like morphology with sub 30 nm KFeCr islands randomly distributed on the surface of the KNiCo seeds (Figure 3 3 and 3 6 ). Overall, the core@shell morphology of 1 and 2 is consistent with the growth of a pseudomorphic layer as expected from the low lattice mismatch between the substrate and overlayer. The core@shell architecture is commonly observed for PBA heterostructures and previous studies indicate an epitaxial growth of the overlayer on the (1 00) faces. 53,120 In contrast, the larger misfits in heterostructures 3 and 4 yield a core@islands morphology in agreement with an island growth mechanism. The preferential growth at the corners of the seeds most likely results from high energy unsaturate d sites along with an increased strain from the confluence of two orthogonal faces. 120,121 As the islands grow closer along the core particle edges, they appear to coalesce presumably due to a sintering mechanis m. Structural study High resolution powder X ray diffraction was performed at room temperature with a synchrotron source at the APS beamline 11 BM. Data were collected for the pure

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52 KNiCo as well as heterostructures 1 , 2 and 3 . Linkage isomerism occurring in the KFeCr islands of sample 4 , KNiCo@KFe Cr, ( Figure 3 1 ) is associated with a significant contraction of the lattice constant of KFeCr from 10.65 Ã… to 10.05 Ã… over several days. 122,123 Since the KFeCr reflections are expected to be broad and to shift overtime, sample 4 was left out of the structural study. The X ray diffraction pattern for each heterostructure shows two sets of peaks characteristic of Prussian blue like face centered cubic lattices (Figure 3 8 ). 35 The absence of peaks corresponding to a mixed phase fu rther confirms the segregation of the two components within the heterostructure. Moreover, the lattice constants determined for each set of reflections are consistent with the expected value for the pure materials ( Table 3 2), ruling out the presence of u niform strain. 35,98 However, as discussed below, analysis of the linewidths indicates the presence of non uniform strain throughout the overlayer. The linewidth analysis for each set of reflections provides insight into the mechanical coupling between the two components of the heterostructure. In particular, the evolution of the crystallite size (L c be evaluated using the Williamson Hall analysis. 124 This method deconvolutes the size induced and stra in (3 1) where the full width at half maximum (fwhm) was determined by fitting the peak shapes to a Lorentzian function and represents the wavelength. Figure 3 9 sh ows the Williamson Hall plots for heterostructures 1 , 2 and 3 as the slopes of the linear fits ( Table 3 3). The strain in the KNiCo substrate is minimal

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53 for all three samples with mean values of = 0. 1%. The presence of anisotropic strain in the overlayer (shell or islands) is evidenced by larger values evaluated from the (hkl) reflections ( ) compared to the slope associated with the (h00) reflections ( ). This observation is consistent with a preferential epitaxial growth along the (h00) faces. In core@shell sample 1 , the seed and overlayer have similar lattice constants, a = 10.131 Å and a = 10.203 Å respectively (Table 3 2). The small lattice misfit, f = 0.6%, results in a strain parameter = 0.7 ± 0.1% in the (hkl) planes. The strain could not be evaluated in the (h00) planes due to the coalescence of the core and shell signals for the (200) reflections. The larger lattice misfit in core@shell 2 , f = 3.4%, leads to a two fold increase of the strain parameter = 1.6 ± 0.3%. As stated above, the anisotropic nature of the strain is demonstrated by weaker strain in the (h00) planes, = 0.4%. In heterostru cture 3 , the increase in lattice mismatch (f = 3.9%) is accompanied by a change in growth mode. The growth of islands, as opposed to a pseudomorphic shell, relieves the strain in the overlayer as shown by the = 0.8 ± 0.1% and = 0.1%. Interestingly, similar values are found for the pseudomorphic layer in core@shell 1 and the islands in heterostructure 3 suggesting comparable strained states. The interfacial energy between the seed and overlayer determines the growth mode that defines the heterostructure morphology. While lattice relaxation generally occurs through elastic strain for heterostructures with small lattice mismatch, the formation of edge dislocations becomes e nergetically favorable beyond a critical lattice misfit. The structural study presented herein confirms the efficient mechanical coupling between the KNiCo seed and overlayer materials. For lattice misfits lower than 3.5%,

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54 the material grows as a strai ned pseudomorphic layer while for higher values, strain is relie ved by the formation of islands . Conclusion The study of a new series of PBA heterostructures provides insights into the growth of coordination polymer heterostructures. Small lattice misfits between the substrate and overlayer result in the growth of a strained pseudomorphic shell whereas, to accommodate larger lattice misfits, lattice relaxation occurs through the formation of islands. The findings presented in this Chapter were visually sum marized during the final exam in Figure 3 10 and 3 11. Although promising, the engineering of heterogeneous functional materials integrating coordination polymers is still in its infancy. A fundamental understanding of the growth processes is needed to im plement their use in technologically viable systems.

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55 Figure 3 1 . FT IR spectra for KNiCo , 1 , 2 , 3 , 4 as synthesized and 4 after 10 days. The change in the band intensities for 4 after 10 days is due to linkage isomerism taking place in the KFeCr component. In the cyanide stretching region, the FT IR spectrum of KNiCo displays bands at 2182 and 2138 cm 1 attributed to the bridging and terminal cyanides of Ni II NC Co III , respective ly. 125,126 Upon growth of the KNiFe, the FT IR spectrum of 1 shows a broad asymmetric peak at 2177 cm 1 characteristic of overlapping Ni II NC Co III and Ni II NC Fe III signals, a band at 2138 cm 1 assigned to the terminal cyanides of Ni II NC Co III , as well a s a peak at 2101 cm 1 attributed to the Ni II NC Fe II reduced

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56 sites. 40 The FT IR spectrum of 2 and 3 are very similar and feature a broad band at 2182 cm 1 resulting from overlapping signals of Ni II NC Co III and Ni II NC Cr III for 2 or Co II NC Cr III for 3 as well as the peak at 2138 cm 1 characteristic of KNiCo. 125,127 The FT IR spectrum of 4 displays the KNiCo bands and a shoulder at 2160 cm 1 characteristic of Fe II NC Cr III along with a small signal at 2080 cm 1 attributed to Fe II CN Cr III . 122 After 10 d ays, the signal at 2080 cm 1 grows at the expense of the shoulder at 2160 cm 1 , indicating that linkage isomerism in the KFeCr occurs within the KNiCo@KFeCr heterostructure. 122,123 . Figure 3 2 . TEM image of KNiCo shows well defined cubic particles tha t are nearly uniform in size, 99 ± 16 nm. Scale bar is 200 nm.

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57 Figure 3 3 . TEM images showing the core@shell morphology of 1 A , E ) and 2 B , F ) indicative of the growth of a pseudomorphic layer, in contrast with the core@islands morphology of 3 C, G) and 4 D, H) associated with a 3D islands growth . Scale bars for the top series A, B, C, D) are 200 nm and for the bottom series E, F, G, H) scale bars are 100 nm.

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58 Figure 3 4. EDS line scans displaying the core@shell morphology of KNiCo@KNiCr 2 (left) and the core@islands morphology of KNiCo@KCoCr 3 (right) . The red dash line represents the path of the electron beam while performing the EDS line scan.

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59 Figure 3 5. EDS line scan displaying the core@shell morphology of 1. The red dash line represe nts the path of the electron beam while performing the EDS line scan.

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60 Figure 3 6. EDS line scan displaying the core@islands morphology of 4. The red dash line represents the path of the electron beam while performing the EDS line scan.

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61 Figure 3 7. Size dispersion for KNiCo , 1 and 2 core@shell structures. Table 3 1. Predicted size vs. measured size for core@shell heterostructures 1 and 2 . The equation a c s = a c (1 + n shell /n core ) 1/3 provides predictions of the size of a core@shell particle (a c s ) when the size of the core (a c ), the number of moles of core material (n core ) and the number of moles of shell material (n shell ) are known. 1 2 n core (mmol) 0.40 0.40 n shell (mmol) 0.40 0.40 a c : average core size (nm) 99 92 a c s : predicted size (nm) 126 116 measured size (nm) 127 ± 17 122 ± 15

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62 Figure 3 8. PXRD patterns collected at room temperature for KNiCo@KNiFe 1 , KNiCo@KNiCr 2 and KNiCo@KCoCr 3 . The unit cell parameters determined for each heterostructure components are consistent with the expected value for the pure materials, thus indicating the absence of uniform strain. Table 3 2. Unit cell constants for pure KNiCo, 1 , 2 and 3 determined from X ray diffraction patterns collected at room temperature with a synchrotron source at the AP S beamline 11 BM. The error is estimated to be less than 0.005 Ã…. KNiCo KNiCo@KNiFe 1 KNiCo KNiFe KNiCo@KNiCr 2 KNiCo KNiCr KNiCo@KCoCr 3 KNiCo KCoCr Lattice parameter a (Ã…) 10.131 10.143 10.203 10.131 10.472 10.133 10.526

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63 Figure 3 9. Williamson Hall analysis for KNiCo@KNiFe 1 , KNiCo@KNiCr 2 and KNiCo@KCoCr 3 showing the presence of anisotropic strain in the overlayer. Due to the small lattice misfit in heterostructures 1 and 2 , the overlayer grows as a strained pseudomorphic layer. As the lattice mismatch increases in heterostructure 3 , the strain is relieved by the growth of islands. Solid lines are linear fits to the experimental data. Parameters derived from the fits are i ncluded in Table 3 3.

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64 Table 3 3 . Strain parameter from the Williamson Hall analysis for heterostructures 1 , 2 and 3 . KNiCo@KNiFe 1 KNiCo KNiFe KNiCo@KNiCr 2 KNiCo KNiCr KNiCo@KCoCr 3 KNiCo KCoCr Strain parameter (%) 0.1 ± 0.0 (hkl) 0.7 ± 0.1 0.1 ± 0.0 (hkl) 1.6 ± 0.3 (h00) 0.5 ± 0.1 0.1 ± 0.0 (hkl) 0.8 ± 0.1 (h00) 0.2 ± 0.1 Figure 3 10. Investigation of the heterogeneous growth of a series of Prussian blue analogues on nickel hexacyanocobaltate seeds : KNiCo@KNiFe ( f = 0.6%), KNiCo@KNiCr ( f = 3.4%), KNiCo@KCoCr ( f = 3.9%) and KNiCo@KFeCr ( f = 5.0%). For small lattice misfits, the observation of a core@shell type morphology indicates the growth of a pseudomorphic layer w hile larger lattice misfits result in the growth of islands.

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65 Figure 3 11. Effect of lattice misfit on the morphology of two series of PBA heterostructures, using RbCoFe (blue) or KNiCo (red) as substrate. Small lattice misfits between the substrate and overlayer result in the growth of a strained pseudomorphic shell whereas, to accommodate larger lattice misfits, lattice relaxation occurs through the formation of islands.

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66 CHAPTER 4 LIGHT INDUCED CHANGES IN MAGNETISM IN A COORDINATION POLYMER HETEROSTRU CTURE Rb 0.24 Co[Fe(CN) 6 ] 0.74 @K 0.10 Co[Cr(CN) 6 ] 0.70 · n H 2 O AND THE ROLE OF THE SHELL THICKNESS ON THE PROPERTIES OF BOTH CORE AND SHELL Introduction Whether described as network solids or as metal organic frameworks, studies of coordination polymer solids ext end to catalysis, ion transport and storage, gas separations and storage, electrochromism, negative thermal expansion, magnetism and light switchable magnetism. 5,9,15 20,24,25,41,43,128 Work on these solid state topics parallels efforts looking at biomedic al applications of coordination polymers (CPs) such as nano carriers for drug delivery, contrast agents for MRI or for X Ray computed tomography, optical biomarkers or therapeutic agents. 9,44,45,129 131 Many of these applications benefit from producing nan oscale or mesoscale structures or from understanding interactions at these length scales. The study of nanoscale or mesoscale heterostructures of CPs is in its infancy, but just as for more traditional solid state materials, coordination polymer heterostr uctures provide routes to do more than simply combine properties. Synergy between compon ents can lead to new behaviors, and a n example is in the area of lithium ion storage using cyanometall The analogue K 0.1 Cu[Fe(CN) 6 ] 0.7 ·3.8H 2 O has high storage capacity, but the capacity degrades with successive redox cycling. 61,62 However, adding a shell of a second analogue leads to enhanced performance as the presence of the shell suppresses a R eprinted with permission from Risset, O. N.; Quintero , P . A.; Brinzari, T . V.; Andrus, M. J.; Lufaso, M. W. ; Meisel, M. W.; Talham, D. R. J. Am . Chem. Soc. 2014 , 136 , 15660 15669 . Copyright 2014 American Chemical Society.

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67 cubic to tetragonal phase change associ ated with the Cu + /Cu 2+ redox couple of the pure phase. 63,64 New behavior is also seen in magnetic heterostructures of Prussian blue analogues. The nickel hexacyanochromates are not light responsive, but when incorporated into het erostructure thin films or core@ shell particles with light switchable spin transition compounds such as the Prussian blue analogue cobalt hexacyano ferrate or the Hofmann like phase Fe(azpy)[Pt(CN) 4 ]· n H 2 O , their magnetization can be altered with light as a result of magnetomechanical 132 coupling across the interface between the components. 65 70 With these examples of new behavior arising from forming an interface between two coordination polymer components, it becomes important to perform systematic explorations into the influence of the interface in these classes of materials. In this paper, we report the synthesis and characterization of a new type of PBA core@shell heter ostructure, Rb 0.24 Co[Fe(CN) 6 ] 0.74 @K 0.10 Co[Cr(CN) 6 ] 0.70 · n H 2 O (RbCoFe@KCoCr). The system was chosen to extend light switchable magnetism to new examples as the RbCoFe analogue is light responsive and the KCoCr analogue is a ferromagnet with T c ~ 30 K. 127 The family of cobalt hexacyanoferrates undergoes a well characterized charge transfer induced spin transition (CTIST) that can be either thermally or optically activated. 96,97,102,133 140 The transition, Fe 2+ CN Co 3+ (LS) 3+ CN Co 2+ (HS) , involves a spin change on the Co 2+ ion, significantly lengthening the Co N bond and leading to an expansion of the lattice. When the charge transfer reverses , the lattice contracts. This elastic process in the core is thought to induce a magnetomechanical response in the magnetic shell. 132

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68 In support of this mechanism, direct evidence for a str uctural change in the shell in response to the CTIST of the core is provided by temperature dependent X ray diffraction. In addition, the new core@ shell system leads to insights into the influence of the interface on the morphology and properties of the h eterostructure, and in particular how these features change as the shell becomes thicker. T he epitaxial relationship between core and shell in heterostructures containing two different Prussian blue analogues was previously reported . 53,120 Here, a method was developed to prepare a series of core@shell particles with different shell thicknesses, starting from a common batch of narrowly disperse core particles . For RbCoFe@KCoCr, the preference for growth on t he (100) faces is demonstrated. With this series of particles in hand, it is possible to estimate the depth to which the magnetic properties of the shell are altered by the presence of an interface with the core particles that undergo a light induced structural change. Furthermore, an influence of the shell on the behavio r of the core is also observed. The KCoCr shell changes the nature of the structural transition associated w ith the RbCoFe thermal CTIST. Normally a discontinuous, first order transition in the single phase RbCoFe, the presence of the shell leads to a continuous phase transition in the core@shell particles. Experimental section Material preparation All chemical reagents were purchased from Sigma Aldrich and used without further purification. Deionized water was used as solvent in al l the following procedures. RbCoFe core particles : In a typical experiment, 200 mL of an aqueous solution containing CoCl 2 .6H 2 O (95 mg; 0. 4 0 mmol) and RbCl (95 mg; 0.79 mmol) were added dropwise ( 3.5 mL/min) to an equal volume of an aqueous solution contai ning

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69 K 3 [Fe(CN) 6 ] (150 mg; 0.46 mmol). After maturation for 4 hours under vigorous stirring, the particles were centrifuged at 9000 rpm for 10 min and subsequently washed with 300 mL of water. The particles were redispersed in 400 mL of water to give the su spension of core particles. Except for a 5 mL aliquot used for characterization, the particles were not isolated and the suspension was used immediately in the next step. Rb 0.24 Co[Fe(CN) 6 ] 0.74 · 3H 2 O (RbCoFe): Dark purple suspension (97% yield based on isolated batches ). IR (KBr): 2162 ( CN, Co II NC Fe III high spin (HS)) , 2110 ( sh, CN, Co III NC Fe II low spin (LS)) , 2098 ( CN, Co II NC Fe II ) cm 1 . EDS: 0. 24 :1 .0 :0. 74 (Rb:Co:Fe). Anal. Calcd for: C, 18.37; H, 2.07; N, 21.43. Found: C, 18.06; H, 1.92; N, 20 .96 . RbCoFe@KCoCr c ore@shell particles ; a s (average shell thickness) = 11 nm (1) : Co Cl 2 · 6H 2 O ( 48 mg; 0. 20 mmol) dissolved in 50 mL of water and an equal volume of an aqueous solution containing K 3 [Cr(CN) 6 ] ( 75 mg; 0. 23 mmol) were simultaneously added (8 mL/h using a peristaltic pump) to the core particle suspens ion under vigorous stirring for 15 hours. The particles were isolated by centrifugation at 9000 rpm for 10 min and subsequently washed with 300 mL of water. The product was divided in two eq ual portions; one half was isolated and air dried while the other half was redispersed in 350 mL of water to be used as seeds in the following experiment. {Rb 0.24 Co[Fe(CN) 6 ] 0.74 } 0. 67 @{K 0.10 Co[Cr(CN) 6 ] 0.70 } 0. 33 ·3. 5 H 2 O : Dark purple powder ( 86 mg; 98 % yield). IR (KBr): 2162 (asymmetric broadening , II NC Cr III , Co II NC Fe III III NC Fe II II NC Fe II ) cm 1 . EDS: 0. 03 :0. 16 : 1.00 :0. 50 :0. 23 (K:Rb:Co:Fe:Cr); KCoCr:RbCoFe = 0.49. Anal. Calcd for: C, 18.04; H, 2.40; N, 21.04. Foun d: C, 17.62; H, 2. 2 6; N, 20. 67 .

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70 RbCoFe@KCoCr c ore@shell particles ; a s = 23 nm (2) : Co Cl 2 · 6H 2 O ( 33 mg; 0. 14 mmol) dissolved in 35 mL of water and an equal volume of an aqueous solution containing K 3 [Cr(CN) 6 ] ( 53 mg; 0. 16 mmol) were simultaneously added (8 mL/h using a peristaltic pump) to the particle suspension (1) under vigorous stirring for 15 hours. The particles were isolated by centrifugation at 9000 rpm for 10 min and subsequently washed with 300 mL of water. The p roduct was divided in two equal portions; one half was isolated and air dried while the other half was redispersed in 250 mL of water to be used as seeds in the following experiment. {Rb 0.24 Co[Fe(CN) 6 ] 0.74 } 0.46 @{K 0.10 Co[Cr(CN) 6 ] 0.70 } 0.54 ·3.7H 2 O: Purple powder ( 56 mg; 89 % yield). IR (KBr): 216 3 (asymmetric broadening , II NC Cr III , Co II NC Fe III ), III NC Fe II II NC Fe II ) cm 1 . EDS: 0. 05 :0. 12 : 1.00 :0. 34 : 0.37 (K:Rb:Co:Fe:Cr); KCoCr:RbCoFe = 1.15. Anal. Calcd for: C, 17. 96; H, 2.57; N, 20.95. Found: C, 17. 5 9; H, 2.4 4 ; N, 20. 5 4. RbCoFe@KCoCr c ore@shell particles ; a s = 37 nm (3) : Co Cl 2 · 6H 2 O ( 33 mg; 0. 14 mmol) dissolved in 35 mL of water and an equal volume of an aqueous solution containing K 3 [Cr(CN) 6 ] ( 53 mg; 0. 16 mmol) were simultaneously added (8 mL/h using a peristaltic pump) to the particle suspension ( 2 ) under vigorous stirring for 15 hours. The particles were isolated by centrifugation at 9000 rpm for 10 min and subsequently washed with 300 mL of water. The p roduct was isolated and air dried. { Rb 0.24 Co[Fe(CN) 6 ] 0.74 } 0.28 @ { K 0.10 Co[Cr(CN) 6 ] 0.70 } 0.72 ·4H 2 O : Light purple powder ( 83 mg; 81 % yield). IR (KBr): 216 5 (asymmetric broadening , II NC Cr III , Co II NC Fe III ), III NC Fe II II NC Fe II ) cm 1 . EDS:

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71 0. 07 :0. 06 : 1.00 :0. 20 :0. 49 (K:Rb:Co:Fe:Cr); KCoCr:RbCoFe = 2.59. Anal. Calcd for: C, 17.76; H, 2.77; N, 20.72. Found: C, 17.31; H, 2.57; N, 20.37 . Characterization Fourier transform infrared spectroscopy (FT IR) was performed on a Nico let 6700 Thermo Scientific spectrophotometer taking 16 scans per spectrum between 4000 and 400 cm 1 with a resolution of 0.482 cm 1 . Samples were placed onto the face of a KBr pellet by dispersing 1 mg of p owder in acetone and dropping the dispersion onto the preformed pellet. The spectrum of a p ure KBr pellet is taken as a background reference. Transmission electron microscopy (TEM) was performed on a JEOL 2010F high resolution transmission electron microscope a t 200 kV. The TEM samples were prepared by dropping 40 water solution (1 mL) containing 2 mg of product, dispersed by sonication, onto the grid (carbon film on a holey carbon support film, 400 mesh, co pper from Ted Pella, Inc.) . Energy dispersi ve X ray spectroscopy (EDS ) was performed with an Oxford Instruments EDS X ray Microanalysis System coupled to the high resolution TEM (HRTEM) microscope. A total of four scans were recorded on different parts of the sample and then averaged to give relati ve atomic percentages for the metallic elements . Combustion analysis to determine carbon, hydrogen, and nitrogen (CHN) contents was performed at the University of Florida Spectroscopic Services Laboratory on an EA1108 CHNS O manufactured by Fisons Instrume nts in 1995 . Chemical formulas are based on the metal compositions from EDS as well as the elemental analysis. The particle size distribution is determined from the size measurements of a minimum of 200 particles from multiple regions in one sample. The particle size is reported as the mean along with the standard deviation as determined by descriptive statistics performed in Origin 8.5. The average shell thickness is defined as

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72 half of the difference between the size of the core@shell par ticles and the size of the cores. Powder X ray diffraction ( P XRD) was performed at Argonne National Laboratory on the beamline 17. Each sample was loaded in a Kapton capillary and mounted on the beamline. The experimental configuration of 17 BM uses a flat panel amorphous Si area detector p ositioned 500 mm from the sampl e . X ray ( = 0.72808 Ã…) exposure times were no less than five seconds. During each exposure the sample was rocked a total of 5 degrees. Data collection was continuous with temperature rampi ng between 100 K and 300 K (2 K/min) with a cryostream nitrogen blower regulating the temperature. The data were processed using the FullProf Suite and fit2d which was used for the design of the stacked plots. T he patterns were fitted using Le Bail method through GSAS . The magnetic properties were investigated using a commercial superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS XL7). The low temperature measurements were performed with the powder samples spread between tw o pieces of transparent tape and mounted in a homemade quartz optic sa mple rod 141 connected to a tungsten halogen lamp (400 nm 2200 nm) , while the high temperature measurements were performed with the sample in a gel cap inside a drinking straw in a comm ercial sample rod. The field cooled temperature dependence of the magnetization was measured in an applied field of 100 G while warming, in the 5 K 40 K region. After isothermal irradiation at 5 K and 100 G, irradiation was ceased and the light state established . The magnetization of the light state was measured in the temperature range 5 K 40 K while warming. The high temperature measurements were performed for the dark state while warming and cooling in 100 G field in the 150 K 300 K region. The sweeping rate of the temperature

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73 for all the measurements was 2 K/min in the 100 K 300 K region and 5 K/min below 100 K. Results Synthesis and morphology RbCoFe nanoparticles are synthesized as a self stabilized suspension in water , a dapting the method developed by Catala and coworkers 52 to yield uniform particles with a controllable composition in the mesoscale , typically 50 500 nm, size regime . The particles are not passi vated with any stabilizer, so their surface remains chemically active and can be used to form multicomponent heterostructures. Core@shell heterostructures are prepared 53,54,66,68 by the slow addition of low concentration precursor solutions to the suspension of the uncoated particles, leading to the heterogeneous precipitation of the shell material while preventing side nucleation . Once the initial batch of core@shell particles were prepared, part of the batch was harvested for measurements and characterization while the rest was used for augmenting the KCoCr shell. The process was the n cycled a third time to result in a series of three RbCoFe@KCoCr samples with increasing shell thickness , all derived from the same batch of RbCoFe core particles. The particle morphology is nicely visualized with TEM (Figure 4 1), while t he chemical as signments and layer segregation within the heterostructures are confirmed b y EDS line scans (Figure 4 2 ). The uncoated RbCoFe particles are well defined cubes, uniform in size, 135 ± 12 nm (Figure 4 3 ). Upon growth of the thinnest shell, core@shell sample 1 , the particle size increases to 157 ± 11 nm. Strikingly , 1 exhibits an intricate cross morphology as opposed to the normally observed cubic shape , indicating that the

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74 shell initially grows on the (100) faces . 120 Parenthetically, the initial absence of layer regions possessing increased strain due to the confluence of two orthogonal faces. 120,121 Upon i ncreasing the shell thickness to give a particle size of 181 ± 12 nm, core@she ll sample 2, the morphology evolves from cross like to cubic. Further increasing the shell thickness, core@shell sample 3 exhibits the common cubic shape with a particle size of 208 ± 13 nm. For all three samples, the mean size agrees well with expectation s assuming quantitative heterogeneous precipitation ( Figure 4 4 and Table 4 1 ). Signatures of two different Prussian blue analogues are seen in infrared spectroscopy, which also conveys the valence states of the metal ions in the RbCoFe core ( Figure 4 5 ). T he FT IR spectrum of the uncoated RbCoFe sample features characteristic bands at 2162, 2110 and 2098 cm 1 i n th e cyanide stretching region, attributed to Co II NC Fe III (high spin), Co III NC Fe II (low spin) and Co II NC Fe II sites, respectively. 96,97,102 T he presence of mixed valences is consistent with the cobalt:iron ratio of the cores (Co:Fe = 1.35). 97 As the thickness of the KCoCr shell increases, the peak at 2162 cm 1 shifts to 2165 cm 1 and becomes more intense with asymmetric broadening that results from the overlap between the RbCoFe band and a KCoCr band centered at 2168 cm 1 . 127,142 Powder X ray diffraction ( PXRD ) patterns collected at 300 K can be indexed to two different Prussian blue like face centered cubic lattices in the space group Fm 3m (No. 225) (Figure 4 6 and Table 4 2). The absence of peaks corresponding to a mixed phase in the core@shell particles further confirms the segregation of the core and shell materials. The smallest lattice constant, observed at a = 10.298 10.305 Å , is

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75 chara cteristic of predominantly high spin RbCoFe , in good agreement with the elemental composition and the FT IR cyanide stretches. 96,97,102,137,139 The largest lattice constant, a = 10.5 31 10.551 Ã… , is attributed to KCoCr . 53,142,143 For both core and shell com ponents, the lattice constants are in good agreement with the single phase materials. This observation suggests that, at 300 K, despite the lattice misfit between core and shell, a structural relaxation occurs at the interface preventing uniform strain. Magnetization measurements The temperature dependent magnetic susceptibility over the temperature range 150 K 300 K provides information about the RbCoFe spin state and the influence of RbCoFe sample and for the RbCoFe@KCoCr heterostructures (Figure 4 7) each show a decrease in the paramagnetic high spin (HS) state to the diamagnetic low spin (LS) state as a result of the CTIST. The transition temperature is in the range 240 K 260 K for each sample. As the larger contribution of the ferromagnetic KCoCr shell, which in th is temperature range CTIST of the RbCoFe core (Figure 4 7a). the superposition of the core and shell magnetic responses in Figure 4 7b reveals that the magnitude of the decrease is the same for each sample, confirming that the transition is complete for both the uncoated particles and the core@shell particles. Details of this analysis are in the supporting information. The low temperature behavior is shown in Figure 4 8, plotted as temperature dependent field cooled magnetization, taken both in the dark state and after white light

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76 irradiation. The isothermal magnetization curves collected at 5 K in the dark and light states are presented in Figure 4 9. Each of the RbCoFe@KCoCr heterostructure samples, 1 3 , undergoes the characteristic ferromagnetic ordering of KCoCr with the magnetization increasing below T c ~ 30 K in proportion to the shell thickness. In the light state, the thinner shell samples 1 and 2 show another inflection below 25 K, attributed to ordering of the photo generated Co II Fe III moments in the RbCoFe cores. On the other hand, the sample with the thickest shell does not undergo a light induced enhancement below 25 K, but rather shows a sm all light induced decrease in magnetization. Although smaller in magnitude, this decrease is reminiscent of the photo induced decrease previously observed for core@shell particles and thin film heterostructures combining cobalt hexacyano ferrates with nicke l hexacyanochromates (KNiCr) . 65 67,69,70 These changes with light are seen more clearly in plots of M, M light M dark , in Figure 4 8b. Each of the core@shell systems undergoes a light induced increase in magnetization at temperatures corresponding to the magnetic ordering of the KCoCr shell. For the thin shells, this increase persists down to the ordering temperature of the RbCoFe core. The light induced increase at the KCoCr ordering temperature, discussed below, can be attributed to alterations of local exchange constants in the shell that occur when the core undergoes the optically induced CTIST. 69 The photo increase near 30 K is also observed for the thickest shell sample, before crossing over to a negative M at lower temperatures, as mentioned above. Temperature dependent powder X ray diffraction Structural changes associated with the thermal CTIST were investigated with PXRD data obtained with a synchrotron source at the APS beamline 17 BM. The

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77 uncoated RbCoFe and the three core@shell samples are com pared in Figure 4 10 at 300 K and 160 K, above and below the transition, highlighting the (200) reflections of the cubic lattices. At 300 K, the lattice constant for the RbCoFe core in the core@shell heterostructures is consistent with the value measured f or the uncoated particles (Table 4 2). Upon cooling, a significant contraction of the RbCoFe lattice occurs as a result of the CTIST. However, at 160 K, the decrease in lattice parameter associated with the conversion from the HS to the LS state becomes sm aller as the shell grows thicker. The difference in cell parameters between 300 K and 160 K for the uncoated RbCoFe is a = 0.359 Ã…, whereas this value decreases to a = 0.226 Ã… for 3 (Figure 4 11, Table 4 2). As the KCoCr shell becomes thicker, the lattic e change associated with the RbCoFe CTIST becomes limited. Additional insight is garnered by monitoring the PXRD patterns during the transition and Figure 4 12 presents stack plots of the temperature evolution of the (200) reflection for the uncoated RbCoF e and core@shell sample 2 while cooling from 300 K to 160 K. The stack plots for core@shell samples 1 and 3 are presented in Figure 4 13. Centered near 250 K, the uncoated RbCoFe shows the expected discontinuous transition, meaning reflections for the HS s tate diminish as the reflections for the LS state intensify. At 160 K, only the signals arising from the LS state are observed suggesting a complete conversion. In contrast, the temperature dependence of the diffraction pattern for the RbCoFe within the co re@shell architecture evolves differently. The core of the RbCoFe@KCoCr heterostructures undergoes the CTIST mainly through a continuous phase transition. Figure 4 12b follows the (200) reflections of both the RbCoFe core and the KCoCr shell upon cooling. The PXRD pattern displays a gradual

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78 shift of the Bragg reflections attributed to the RbCoFe cores. Instead of the HS state diminishing as the LS state appears, at any one temperature through the transition, only one intermediate lattice spacing is detected . Clearly, the nature of the transition has changed in the core@shell heterostructure. The (200) reflection of the shell also displays a shift, although to a much smaller extent than the CTIST active RbCoFe core. Nevertheless, a shift in position along wit h a slight broadening of the KCoCr Bragg peaks provides evidence of an elastic influence on the shell when the RbCoFe core undergoes the CTIST (Table 4 2). The change in lattice constant associated with the thermal CTIST is reversed upon light irradiation at low temperatures. Figure 4 14 compares the 200 reflections of the core and shell of sample 1 at 300 K, 160 K, and 100 K after irradiating with white light. The core is converted back to the high spin phase, and at the same time, the changes in the shell are reversed. In Figure 4 14, the 200 peak of the KCoCr shell, which is shifted and broadened at 160 K, returns to its or iginal position and shape after irradiation. Discussion Controlling the magnetism of the shell material The design of heterostructures that combine the photoactive CoFe PBA with a compound that orders magnetically at higher temperature has proven a success ful strategy for developing magnetic materials that can be switched with light. 65 68 The previous examples involved KNiCr as the normally non photoactive magnetic component in either core@shell or thin film heteostructures. A goal of the present work is to extend the concept to other heterostructures by coupling the CoFe PBA with a

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79 different magnetic component, in this case KCoCr, which orders ferromagnetically at approximately 30 K. 127 Indeed, changes are observed in the magnetization of the KCoCr shell in response to the light induced CTIST of the RbCoFe core. The previously studied RbCoFe/KNiCr heterostructures all showed a light induced decrease of magnetization of the KNiCr component. 65 67,69 Although to a lesser extent, a similar result is observed for the core@shell sample 3 , as shown in Figure 4 8. For the thickest shell sample, there is a light induced decrease in magnetization at lowest temperatures as a result of the demagnetization of the KCoCr shell at the interface. A question arises about why t he thinner shelled heterostructures do not show this light induced decrease, only showing the photoinduced increase attributed to the RbCoFe core. As will be explained below, in these cases, changes in the thinner shells are masked by the larger volume cor e. The light induced decrease can be attributed to magnetomechanical effects resulting from the interface between the two components. 132 Light induced, as well as thermally induced, changes in the structure of the core are transmitted to the shell across t he interface, altering magnetism. 120,121 Evidence for the shell undergoing structural changes can be seen in Figure 4 12 , where the (200) reflection of the shell broadens and moves to higher 2 as the core undergo es the thermal CTIST, from high spin to low spin, at around 250 K. accompanies a significant decrease of the KCoCr unit cell, which evidences the presence of uniform strain throughout the shell ( Table 4 2 ). As the shell grows thicker, the de crease in lattice constant upon cooling is closer to the value expected for thermal

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80 contraction only. This observation is consistent with the more bulk like behavior of the thickest shell, as is further described in more detail using a 3 component model. A dditional information pertaining to the mechanism of line broadening can be obtained from a Williamson Hall analysis applied to the line width of each set of reflections associated with the core and shell. 124 This method deconvolutes the size induced broad ening related to the crystallite size ( L c ) and the broadening due to non uniform strain ( , by using the equation : (4 1) where the full width at half maximum ( FWHM ) was determined by fi tting the peak shape to a Voigt function and rep resents the wavelength. Figure 4 15 shows the Williamson Hall plots for core@shell sample 1 at 300 K, 160 K and 100 K after irradiation. At 300 K, the strain is minimal in the core and for the (h00) planes of the shell, with values of = 0.2% and = 0.3 % respectively, consistent with a preferential epitaxial growth of the shell along the (h00) faces. 120 A larger value of 0.8 % is observed from the (hkl) reflections, indicating the presence of anisotropic strain. Upon cooling, the decrease of the core unit cell constant induces significant mismatch and a remarkable increase of the strain in the shell. The three (0.6 %) is accompanied by a four fold and five fold increase in the shell (h00) and (hkl) planes, respectively. The value of the strain parameter reaching 4.2 % along the (hkl) directions of the sh ell confirms the strongly anisotropic nature of the strain. Significantly, the strain state is then released at 100 K when the process is reversed with light. The optical CTIST returns the core to the HS state with its larger lattice constant, thereby decr easing the lattice misfit and releasing the strain in the core and shell, despite some

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81 residual strain in the shell. The (hkl) reflections show a strain parameter that remains higher than its 300 K value (1.6 %), potentially due to thermal contraction at 10 0 K. The important observation is that upon cooling to low temperature, the KCoCr shell magnetically orders near 30 K while structurally strained. Therefore, the decrease in magnetization attributed to the shell upon photoirradiation results from a return to the less strained state. The magnetization changes in response to alterations in the local anisotropy of the magnetically ordered lattice. Moments which aligned with the field upon field cooling through the magnetic ordering temperature then reorient aw ay from the applied field in response to changes in local anisotropy when the lattice strain is released by the optical CTIST. Returning now to the question of why the thinner shell heterostructures show a light induced increase instead of a decrease, it m ust be remembered that the low temperature magnetization is a sum of the contributions of the core and the shell. The RbCoFe core undergoes the characteristic light induced increase, which for thin shells is a larger influence than the decrease of the shel l. Nevertheless, the extent to which changes in the shell contribute to changes in magnetization can be estimated by comparing the core@shell particles wit h different shell thicknesses ( t ). A simple 3 component model, Figure 4 16, was considered with a cor e of fixed size, a shell of thickness t , and a strained region (SR) in the shell of thickness l , over which the magnetostructural distortions take place. The model uses two main assumptions. First, the photomagnetic properties of the RbCoFe core within the core@shell structure are the same as the uncoated sample, thereby equatin g the behavior of the core with the magnetic behavior measured for the uncoated RbCoFe sample . Additionally, the shell

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82 comprises two distinct regions: the SR and the bulk region. The model can then be used to quantify the contribution of the SR to the magnetization and to obtain an estimate of its thickness l . The core contribution to the light dark magnetization , , can be s ubtracted by using the equation , ( 4 2) where , the difference of the magnetic responses in the light and dark states for the x component (core, shell or core @shell) and represents the number of moles calculated from the equation . The ratios between the masses can be calculated from the EDS data, and the molecular weights ( MW ) are known ( see experimental section ). core is measured f or the uncoated RbCoFe particles. The quantity shell is plotted in Figure 4 17 for the temperature dependent response taken at H = 100 G . According to the model , core should be independent of the shell thickness if t > l , because in this case, the non photoactive bulk will be subtracted and only the magnetization of the SR will be left . The fact that the treatment for core@shell samples 2 and 3 lie on top of each other in the 5 K 25 K interval ( Figure 4 17) suggests that the model works well and t hat for those two shell thicknesses, t > l . On the other hand, the treatment indicates that t < l for the 11 nm shell . For all three samples, however, shell is negative at low temperatures, consistent with the response observed for the CoFe/NiCr hetero structures studied previously. 65 67,69 A different treatment affords an estimate of the size of the strained region (SR) . . (4 3)

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83 Specifically, Equation 3 gives the magnetization of the shell in the light state after subtracting the core eff e cts (the dark data have not been subtracted), where the superscripts designate the magnetic respons es after irradiation and the subscripts refer to specific contributions of the core, shell, or core@shell ensem ble. The value will be a function of the thickness t . For the shells where t < l , the magnetization will be proportional to the size of the shell, while for t > l the magnetization will be the sum of the magnetization of th e SR and the bulk region . More explicitly, . (4 4) C onsider ing that the molec ular weight is the same throughout the shell and the mass is proportional to the volume , we determine the two following equations: , (4 5) for which there are two free parameters, and . Fi tting the data for the three core@shell samples at T = 5 K and H = 100 G, using = 4475 emu G/mol (estimated from magnetic measurements in a si ngle phase KCoCr), gives values of = 2850 emu G/mol and nm 3 . The model predicts a thickness of the strained region of l ~ 24 nm ( Figure 4 18) regardless of the exact microscopic mechanism of the magnet ization change. It is important to remember that the model assumes that the pho tomagnetic response of the core in all core@shell samples of the series is the same. A refi ned model will take into account the fact that the shell also induces strain o n the core and will modify its magnetic properties.

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84 A prominent feature of the field cooled magnetization in the light state of each core@shell sample is a slightly higher ordering temperature of the KCoCr. This feature is more clearly seen in the difference plots, Figures 4 8 and 4 17, where the higher ordering temperature manifests as an increase in M around 30 K. The higher ordering temperature reflects subtle changes in the local exchange interactions in the shell associated with release of strain in the core upon undergoing the HS to LS CTIST. For the two thinner shell heterostructures, the photoincrease persists at lower temperatures due to the ordering of the RbCoFe core. For the thicker shell sample, 3 , there is a crossover to the photodecrease that is dominated by the local anisotropy changes in the shell. Influence of the shell on the core Previous reports have demonstrated the influence of various types of matrices over the spin crossover properties of embedded particles. 144 149 Although the heterostructures were designed s o that elastic changes of the core actuate property changes in the shell, it is clear that the shell strongly influences the structure and phase transitions of the core. Analysis of the magnetization change associated with the thermal CTIST indicates that the high spin to low spin conversion is complete, yet the associated contraction of the lattice decreases as the shell thickness increases. The room temperature lattice constants of the individual phases are 10.551 Å for K 0.10 Co[Cr(CN) 6 ] 0.70 ·4H 2 O and 10.30 3 Å for high spin Rb 0.24 Co[Fe(CN) 6 ] 0.74 ·3H 2 O , a difference that is small enough that there is relatively little strain when the two lattices form an interface in the core@shell particles. Upon transitioning to the low spin state, the RbCoFe lattice contrac ts to ~ 9.95 Å, which then creates a significant difference with

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85 the KCoCr unit cell. However, in the core@shell particles, the low spin RbCoFe lattice parameters range from 9.90 Ã… for sample 1 , with the thinnest shell, to 10.072 Ã… for 3 , indicating that t he low spin RbCoFe is significantly strained as part of the core@shell particle. As the shell becomes thicker, it appears to become less elastic, thereby further restricting the ability of the core to contract in response to the spin change. The presence o f the shell not only limits the lattice contraction associated with the thermal CTIST, but it also deeply affects the nature of the transition. The change can be seen in the high temperature (Figure 4 7) in which the uncoated RbCoFe particles display the normal hysteresis indicative of bistability. On the other hand, the hysteretic behavior is lost for RbCoFe@KCoCr heterostructures. These observations parallel the evolution of the structural parameters upon cooling (Figure 4 12). The C TIST in the uncoated RbCoFe particles is a discontinuous, first order phase transition. In contrast, the RbCoFe@KCoCr heterostructures undergo a continuous phase transition. In materials displaying spin crossover, both continuous and discontinuous transiti ons have been observed resulting from magnetostructural phase changes. 137,150 158 A model described by Boukheddaden et al. sheds light upon the parameters influencing the nature of the thermal CTIST in cobalt hexacyanoferrate. 147,159,160 The model predicts a continuous or discontinuous evolution of the high spin state fraction depending on the interplay of two key parameters, the ligand field energy gap and the elastic interaction. 159 Here, we report for the first time a switch between abrupt and smooth tra nsition in cobalt hexacyanoferrate induced by the presence of the shell. The shell is not likely to af fect the ligand field parameter, but a change in the elastic

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86 interaction becomes evident with the observation of structural constraints on the core result ing from the presence of the shell . Furthermore, the structural study provides evidence that the synergistic effects between core and shell materials extend well beyond the few unit cells across the interface. Conclusion The RbCoFe@KCoCr system provides a new example of a coordination polymer heterostructure in which synergistic effects between the core and shell lead to new behavior, in this case light induced alteration of the magnetization of the normally light insensit ive KCoCr. The KCoCr magnetization change is shown to be a magnetomechanical effect as the CTIST of the RbCoFe core induces structural strains in the shell accompanying the magnetization change. The design and fine control of a series of core@shell particl es with varying shell thickness allows an estimate of the depth of the shell material that responds to the core changes. Analysis of the magnetic response in light and dark states indicates this depth to be approximately 24 nm. At the same time, the presen ce of the shell alters the behavior of the core, changing the nature of the CTIST. A shell as thin as 11 nm alters the phase transition of a 135 nm particle. Overall, t his study provides a better understanding of the length scales involved in the synergist ic effects between core and shell and further e vidence that the se effect s are not limited to the few unit cells across the core@shell interface.

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87 Figure 4 1. TEM images of RbCoFe@KCoCr core@shell nanoparticles. The average shell thickness ( t ) is defined as half the difference between the edge to edge distance of the core@shell particles and the corresponding distance for the cores. With t = 11 nm, 1 shows an unprecedented cross shape A, D). By increasing t to 23 nm , the morphology of 2 evolves toward the expected cubic shape B, E). Further increasing t to 37 nm, 3 displays exclusively the cubic morphology C, F). Scale bars for frames a c are 200 nm and for d f the scale bars are 100 nm.

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88 Figure 4 2. EDS line scans on an individual Rb Co Fe@ K Co Cr core@ shell particle s with a shell thickness of 35 nm . Data represent the counts for each element detected as a function of the position of the electron beam across the particle. EDS line scans confirm the chemical assignments and the segregation between core and shell materials within the particle.

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89 Figure 4 3. TEM image of RbCoFe cores shows a well defined cubic shape and are nearly uniform in size, 135 ± 12 nm. Scale bar is 20 0 nm. Figure 4 4. Size dispersion for RbCoFe cores, 1 , 2 and 3 . The methods used to determine the mean size values and the corresponding uncertainties are described in the experimental section of this chapter. The values are summarized in Table 4 1.

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90 T able 4 1. Predicted size vs. measured size for RbCoFe@KCoCr heterostructures 1 , 2 and 3 . The equation a c s = a c (1 + n shell /n core ) 1/3 provides predictions of the size of a core@shell particle (a c s ) when the size of the core (a c ), the number of moles of co re material (n core ) and the number of moles of shell material (n shell ) are known. 1 2 3 n core (mmol) 0.40 0.30 0.22 n shell (mmol) 0.20 0.14 0.14 a c : average core size (nm) 135 157 181 a c s : predicted size (nm) 155 178 213 measured size (nm) 157 ± 11 181 ± 12 208 ± 13

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91 Figure 4 5. FT IR spectra for Rb Co Fe cores and RbCoFe @ K Co Cr core@shell particle s 1 , 2 and 3 . The red line indicates the position of the peak corresponding to Co II NC Fe III (HS). As the shell thickness increases, this peak shifts and becomes asymmetric due to the emergence of a peak around 2170 cm 1 consistent with KCoCr.

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92 Figure 4 6. PXRD patterns at 300 K for RbCoFe cores and Rb Co Fe@ K Co Cr 1 ( t = 11 nm), 2 ( t = 23 nm) and 3 ( t = 37 nm) , indexed in the space group Fm 3m (No. 225) and fitted using Le Bail method and refined lattice constants. For the heterostructures, the set of peaks at higher angles correspond to the RbCoFe core while the one at lower angles are attributed to the KCoCr shell.

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93 Table 4 2. Refined lattice constants at 300 K and 160 K for RbCoFe cores and RbCoFe@KCoCr 1 ( t = 11 nm), 2 ( t = 23 nm) and 3 ( t = 37 nm), indexed in the space group Fm 3m (No. 225) and fitted using Le Bail method. a represents the difference in cell parameters between 300 K and 160 K. RbCoFe 11 nm RbCoFe KCoCr 23 nm RbCoFe KCoCr 37 nm RbCoFe KCoCr Lattice parameter a at 300 K (Ã…) 10.30348(3) 10.30188(4) 10.5309(3) 10.30492(5) 10.5508(1) 10.29832(8) 10.55055(7) Lattice parameter a at 160 K (Ã…) 9.94490(5) 9.98994(7) 10.4455(5) 10.0290(1) 10.4931(2) 10.0719(2) 10.5112(1) 0.359 0.312 0.276 0.226 0.085 0.058 0.039

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94 Figure 4 7. Left in the region of the thermal CTIST for uncoated RbCoFe , and RbCoFe@KCoCr samples 1 , 2 and 3 , with an average shell thickness of , respe ctively, 11 nm, 23 nm and 37 nm. Data were collected while cooling and warming at 2 K/min in an applied field of 100 G . A ll the samples display a CTIST in the RbCoFe cores. Right: Measured (filled dots) and predicted (empty dots) low field magnetic responses in the range 150 K to 300 K. The simulation is based on the superposition of the core and shell responses assuming both materials behave independently (see Supporting Information, Figure 4 19). At 300 K, the predicted values differ in less than 0.04 emu K/mol from the measured values, suggesting that, at this temperature, (i) the core and shell behave as two independent magnetic components; (ii) the core is in are slightly lower than the values predicted by the simulation, which indicates that the core undergoes a complete transition to the LS state in all three core@shell structures.

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95 Figure 4 8. Left: Field cooled magnetization vs. temperature for RbCoFe@KCoCr 1 , 2 and 3 under an applied field of 100 G in the dark state (filled dots) and in the light state after irradiation at 5 K (empty dots). Right: Magnetization (light dark) vs. temperature for RbCoFe@KCoCr 1 , 2 and 3 . The feature between 25 K a nd 31 K is attributed to a modification of the superexchange in the distorted interfacial KCoCr upon irradiation, hence affecting the ordering temperature. Below 24 K, ferrimagnetic ordering of the RbCoFe cores results in further increase of Magnetizati on in 1 and 2 , whereas a decrease is observed in 3 . For all samples, light induced changes are observed up to 30 K, the ordering temperature of the normally non photomagnetic layer.

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96 Figure 4 9. Magnetization vs. Field at T = 5 K for 1 , 2 and 3 in the dar k state (filled dots) and light state, after irradiation at 5 K ( open dots ). The plots on the left show the whole field sweep while the plots on the right display only the low field region.

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97 Figure 4 10. PXRD patterns at 300 K A ) and 160 K B ) for uncoate d RbCoFe and RbCoFe@KCoCr samples 1 , 2 and 3 . For clarity purposes, only the (2 00) reflections are shown . At 300 K, the lattice constant for the RbCoFe is mainly unaffected by the presence of the shell. At 160 K, the change in lattice parameter associated with the conversion from the HS to the LS state becomes smaller as the shell thickness increases, providi ng evidence of a structural constraint resulting from the growth of the shell.

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98 Figure 4 11. PXRD patterns at 160 K for RbCoFe cores and Rb Co Fe@ K Co Cr 1 ( t = 11 nm), 2 ( t = 23 nm) and 3 ( t = 37 nm) , indexed in the space group Fm 3m (No. 225) and fitted using Le Bail method. For the heterostructures, the set of peaks at higher angles correspond to the RbCoFe core while the one at lower angles are attributed to the KCoCr shell.

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99 Figure 4 12. Left: PXRD patterns collected every 4 K and stacked as a function of temperature, from 300 K to 160K, for the uncoated RbCoFe. The RbCoFe particles undergo the CTIST through a discontinuous, first order phase transition. The inset corresponds to the PXRD pattern marked by a red line in the stacked plot and show s the coexistence of individual signals corresponding to both HS and LS states, characteristic of a first order transition. Right: PXRD patterns collected every 5 K and stacked as a function of temperature, from 300 K to 160K, for the RbCoFe@KCoCr sample, 2 . The reflection at a lower angle corresponds to KCoCr and the signal at higher angle is characteristic of RbCoFe. The RbCoFe core within the core@shell architecture undergoes the CTIST through a continuous phase transition. The inset corresponds to the P XRD pattern marked by a red line in the stacked plot and shows only one reflection attributed to the RbCoFe core that gradually shifts upon cooling. For clarity , only a small region of the PXRD patte rns is shown, focusing on the (2 00) reflection .

PAGE 100

100 Figure 4 13. PXRD patterns collected every 5 K and stacked as a function of temperature, from 300 K to 160K at a cooling rate of 2 K/min , for the RbCoFe@KCoCr sample s 1 (left) and 3 (right).

PAGE 101

101 Figure 4 14. PXRD pattern s for 1 , showing the (200) reflection , collected at 300 K, 160 K and at 100 K after irradiation with white light. Upon cooling, the contraction of the RbCoFe unit cell associated with the thermal CTIST induces structural strains on the KCoCr shell as illustrated by the broadening and shift to higher angle of the shell reflections. The changes in both the core and shell material are reversed after irradiation with light at 100 K.

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102 Figure 4 15. Williamson Hall plots for core@shell sample 1 at 300 K and 160 K in the dark state and at 100 K afte r irradiation with white light. At 300 K, minimal strain is observed in the core lattice and (h00) planes of the shell. The different slopes for the (h00) and (hkl) lines in the shell lattice are indicative of anisotropic strain. Upon cooling, the contract ion of the RbCoFe unit cell associated with the thermal CTIST induces considerable strain on the KCoCr shell, particularly affecting the (hkl) planes. The strain in both the core and shell material is released after irradiation with light at 100 K.

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103 Figure 4 16. Schematic of the three component model for the core@shell heterostructure, dividing the shell into two distinct regions, a bulk like and strained region (SR). The size of the SR is given by the length l , and t is total thickness of the shell. Figure 4 17. A plot of shell vs. temperature as derived from Equation 4 2 for the three core@shell samples . All the quantities are per mol e of the combined core@shell particles . l : strained thickness t : shell thickness

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104 Figure 4 18. Plot of from Equation 4 3 vs. volume, V shell , for the three core@shell samples and the fitting of the model from Equation 4 5. The pink area corresponds to the SR and the blue is the bulk region. The treatment estimates the thickness of the SR as l ~ 24 nm.

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105 Figure 4 19. Simulated T vs. T data are based on two assumptions: (i) the RbCoFe core is in the high spin state at 300 K; (ii) the RbCoFe core fully converts to the LS state at 150 K. Simulated T vs. T data (empty dots) at 300 K match well with the experimental data (filled dots) which confirm s that the core is in the high spin state, consistent with IR and XRD data. At 150 K, the experimental T value is lower than the simulated value for all three core@shell heterostructures which indicates a complete transition of the core to the LS state.

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106 CHAPTER 5 SWITCHING MAGNETISM WITH LIGHT ABOVE 77 K IN A BISTABLE COORDINATION POLYMER HETEROSTRUCTURE Introduction Physical properties that can be switched through external stimuli such as temperature, pressure and light are the key to developing novel recording technologies. Magneto optical effects, for example, could be used in heat assisted magnetic recording 161 166 and the use of circularly polarized laser pulses has provided new advances. 167,168 A remarkable family of bistable materials whose magn etic properties can be reversibly switched is found in the cyanide bridged coordination polymers. 41,43,157,169 172 For example, cobalt octacyanotungstate and iron octacyanonibate networks 173 175 as well as Prussian blue analogues cobalt hexacyanoosmate 176 and cobalt hexacyanoferrate 102 (CoFe PBA), undergo a charge transfer induced spin transition (CTIST) that can be either optically of thermally activated. Recently, heterostructures that combine the photoswitchable CoFe PBA with ferromagnetic components that order at higher temperature were developed, 65 69,71,120,121 leading to synergistic effects between the two materials and giving rise to unprecedented behavior. Taking advantage of a structural change associated with the CoFe 13 7 the elastic process couples across the heterostructure interface to induce a magnetomechanical response 132 in the non light sensitive component resulting in magnetization changes. 71 Using nickel hexacyanochromate as the magnetic component, light induced effects have been observed up to 70 K. 65 67,69

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107 Here, this new mechanism is employed to optically switch magnetism at temperatures well above liquid nitrogen temperature. Heterostructure particles of formula {Rb 0.2 Co[Fe(CN) 6 ] 0.7 } 0.8 @{K 0.1 Cr[Cr(CN) 6 ] 0.7 } 0.2 · 4 H 2 O comprised of CoFe PBA and chromium hexacyanochromate (CrCr PBA), a ferrimagnet with T C = 190 K 240 K depending upon composition, 177 show a persistent photo induced decrease in magnetization up to 120 K, the temperature above which the CoFe PBA r elaxes to the low spin ground state . Experimental section Material preparation CoFe PBA@CrCr PBA heterostructure: A uniform batch of the CoFe PBA particles was prepared as previously described. 71 CoFe PBA cores (20 mg; 0.07 mmol) were redispersed in 50 mL deionized water. Cr Cl 2 ( 25 mg; 0. 20 mmol) dissolved in 25 mL of dionized water and an equal volume of an aqueous solution containing K 3 [Cr(CN) 6 ] ( 75 mg; 0. 23 mmol) were simultaneously added (45 mL/h using a peristaltic pump) to the core particle suspens ion under vigorous stirring . The mixture was covered and left stirring for 3 days. The particles were isolated by centrifugation at 95 00 rpm for 10 mi n and subsequently washed with 15 0 mL of wat er . The product was isolated and air dried. {Rb 0.2 Co[Fe(CN) 6 ] 0.7 } 0.8 @{K 0.1 Cr[Cr(CN) 6 ] 0.7 } 0.2 · 4H 2 O : Reddish purple powder (26 mg; 10% yield). II NC Cr II ), II NC Fe III ), 2110 III NC Fe II , Co II NC Fe II ) cm 1 . EDS: 0. 1 :0. 2:1.0 :0. 7 :0. 3 (K:Rb:Co:Fe:Cr); RbCoFe:KCrCr = 4.0 . Anal. Calcd for: C, 17.26; H, 2.73; N, 20.13. Found: C, 17.55; H, 2.50; N, 19.70.

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108 Characterization IR spectroscopy was performed on a Nicolet 6700 Thermo Scientific spectrophotometer taking 16 scans per spectrum between 4000 and 400 cm 1 with a resolution of 0.482 cm 1 . Powder samples (1 mg) were mixed with KBr (200 mg) and pressed into a pellet at 20 MPa. Transmission electron microscopy (TEM) was performed on a JEOL 2010F high res olution transmission electron microscope at 200 kV. The TEM samples were prepared by dropping 40 water suspension containing 2 mg of product, dispersed by sonication, onto the grid (carbon film on a holey carbon support film, 400 mesh, co pper from Ted Pella, Inc.) . Energy dispersive X ray spectroscopy (EDS ) was performed with an Oxford Instruments EDS X ray Microanalysis System coupled to the high resolution TEM. A total of four scans were recorded on different parts of the sample and then averag ed to give relative atomic percentages for the metallic elements . Combustion analysis to determine carbon, hydrogen, and nitrogen (CHN) contents was performed at the University of Florida Spectroscopic Services Laboratory on an EA1108 CHNS O manufactured by Fisons Instruments in 1995. The particle size distribution is determined from the size measurements of a minimum of 200 particles from multiple regions in one sample. The particle size is reported as the mean along with the standard deviation as deter mined by descriptive statistics performed in Origin 8.5. The magnetic properties were investigated using a commercial superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS XL7). The optical measurements were p erformed with the powder sample spread between two pieces of transparent tape and mounted in a homemade quartz optic sa mple rod 141 connected to a tungsten halogen lamp (400 nm 2200 nm) via a fiber optic patch cable. The field cooled temperature

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109 dependence of the mag netization in the dark state was measured in an applied field of 100 G . After isothermal irradiation at 5 K and 100 G, irradiation was ceased and the long lived light state established . The magnetization of the light state was measured on warming. The 8 0 K data were collected in the similar manner while only cooling the sample to 80 K. The power at the sample was around 4 mW. The CTIST region was identified through the measurements of the sample in a ge l cap inside a drinking straw using a commercial s ample rod . The data was collected while cooling and warming in 100 G field in the 219 300 K region. The sweeping rate of the temperature for all the measurements was 2 K/min in the 100 300 K region and 5 K/min below 100 K. Synthesis and morphology Th e CoFe PBA particles used for the core are synthesized as a self stabilized suspension in water. Adapting the procedure described by Catala and coworkers 52 for the synthesis of PBA nanoparticles, uniform particles are obtained with characteristic lengths of 50 500 nm. The absence of stabilizer on the surface of the particles allows the CoFe PBA to act as a suitable platform for the growth of CrCr PBA. The conventional method to prepare PBA core@shell structures requires the slow addition of low concent ration solutions of the shell precursors to a suspension of the core particles, 53 favoring heterogeneous precipitation while preventing side nucleation. However, the CrCr PBA does not immediately precipitate under these conditions, presumably due to a hig her solubility than other PBAs previously grown as shells. 53,54,63,66,68 Nevertheless, the CoFe PBA@CrCr PBA heterostructures are obtained after the CoFe PBA particles are left stirring for three days in an aqueous solution containing the CrCr PBA precursors. These conditions result in both

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110 homogeneous and heterogeneous prec ipitation of the CrCr PBA as revealed by TEM images. The FT IR spectroscopy shows features for two distinct PBAs while conveying the valence states of the metal ions in the CoFe PBA core ( Figure 5 1 ). Specifically, the FT IR spectrum of bare CoFe PBA in th e cyanide stretching region displays characteristic bands at 2165, 2110 and 2098 cm 1 , assigned to Co II NC Fe III (high spin), Co III NC Fe II (low spin) and Co II NC Fe II (reduced) sites, respectively, 96,97,102 and the presence of mixed valences is in agreeme nt with the cobalt:iron ratio in the cores (Co:Fe = 1.4). After depositing the shell, an additional shoulder appears at 2185 cm 1 , characteristic of CrCr PBA. 177 At the same time, the distribution of the different charge states of the CoFe PBA core chang es, as illustrated by an increase in intensity of the bands attributed to the low spin and reduced species accompanied by a diminished high spin state signal. These observations suggest a partial reduction of the CoFe PBA by the Cr II precursor, consisten t with the electrochemical potentials of the hexaaquachromium (II) ions and the ferricyanide within the PBA structure. Surprisingly, CoFe PBA treated only with CrCl 2 does not undergo a similar change in oxidation state, suggesting that the redox process o ccurs as the CrCr PBA grows at the surface of the CoFe PBA particles. The TEM images provide insights into the morphology of the PBA particles (Figure 5 2 ) while EDS results confirm the chemical assignments and component segregation within the heterostruct ures ( Figure 5 3 ). CoFe PBA are well defined cubic particles, nearly uniform in size, 170 ± 13 nm ( Figure 5 4 and 5 5 ). After being exposed to the CrCr PBA precursors, the CoFe PBA seeds become coated with nanoparticles, 9

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111 ± 2 nm in size, identified as CrC r PBA by EDS ( Figure 5 3 and 5 5 ). While the heterogeneous growth of CrCr PBA nanoparticles on the surface of the CoFe PBA cores is evident, Figure 5 2 also pictures a few small aggregates identified as the single phase CrCr PBA. Current synthetic efforts are directed toward controlling the precipitation of CrCr PBA to prevent side nucleation . Magnetization measurements Magnetic data provide direct evidence that the two materials are coupled and behave cooperatively due to a shared interface. The magnetic response of the CoFe PBA @ CrCr PBA heterostructures in the dark state and after irradiation with white light is shown in Figure 5 6 and 5 7 . In the dark state, the temperature dependent field cooled magnetization shows a first feature around T C ~ 205 K in agreement with the ferrimagnetic ordering of the CrCr PBA. 177 A second increase in magnetization around 15 K is attributed to the ordering of remaining high spin sites in CoFe PBA, with T C = 25 K, as is often observed for this system as a resu lt of an incomplete high spin to low spin conversion when cooling through the thermal CTIST . Upon irradiation at 5 K, an increase in magnetization is observed due to the photo generated high spin Co II Fe III pairs in the CoFe PBA core (Figure 5 6, Figure 5 8 ). Above the ordering temperature of the CoFe PBA, a crossover occurs and a decrease in magnetization is observed for the light state . The magnetization decrease is attributed to the CrCr PBA in response to the spin state transition in the CoFe PBA and is the first time photo induced magnetization changes have been observed in this high ordering temperature PBA. The photomagnetic response in the normally light insensitive component is attributed to magnetomechanical effects mediated by the interface bet ween the two materials. 132

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112 Upon irradiation, the structural changes associated with the CTIST in CoFe PBA are transmitted to the CrCr PBA nanoparticles coating the surface, thereby altering the local anisotropy of the magnetically ordered lattice and resu lting in the distortion/realignment of the CrCr PBA magnetic domains. The strongest light induced changes in the CrCr PBA persist up to 120 K, the temperature at which the CoFe PBA high spin state begins a thermally assisted dynamical relaxation to the low spin ground state, as shown in Figure 5 9 which plots the change in magnetization, M dark Ml ight , normalized to the magnetization in the dark state. The mechanism requires efficient mechanical coupling, so a physical mixture of the two components does n ot exhibit similar photomagnetic behavior. The heterostructure is required. Taking advantage of the high ordering temperature of CrCr PBA, the heterostructure allows light switchable magnetization above liquid nitrogen temperature. For example, Figure 5 7 shows a significant decrease in magnetization upon irradiation at 80 K ( Figure 5 7 , Figure 5 8 ). The light induced effects are persistent until thermally reversed starting at 120 K. Conclusion In summary, a new light switchable material, designed from coordination polymers, shows an unprecedented photomagnetic response upon irradiation at 80 K. Bistable CoFe PBA @ CrCr PBA heterostructures exhibit a persistent light induced decrease in magnetization that can be recovered by thermal treatment above 120 K. New strategies and materials for switching magnetism with light open up new avenues toward the design of technologically viable magneto optical sensors and switches .

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113 Figure 5 1. FT IR spectra for CoFe PBA cores (top) and CoFe PBA @ CrCr PBA heterostruc tures (bottom). For the heterostructures, a signal appears at 2185 cm 1, characteristic of CrCr PBA. Furthermore, a change occurs in the oxidation states of the transition metals in CoFe PBA upon epitaxial growth of CrCr PBA nanoparticles. Figure 5 2 . TEM images of CoFe PBA@ CrCr PBA heterostructures. Well defined and uniform CoFe PBA cubes are coated with CrCr PBA nanoparticles. Along with heterogeneous pre cipitation, side nucleation of CrCr PBA occurs as indicated by the presence of nanosized particl es aggregates (red arrow) . Scale bars are 200 nm for the left frame and 100 nm for the right frame.

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114 Figure 5 3 . EDS line scan on an individual CoFe PBA @ CrCr PBA particle. Data represent the counts for each element detected as a function of the position of the ele ctron beam across the particle. EDS line scans confirm the growth of CrCr PBA nanoparticles on the CoFe PBA core.

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115 Figure 5 4 . TEM image of CoFe PBA particles. Scale bar is 500 nm. Figure 5 5 . Size dispersion for CoFe PBA cores and CrCr PBA nanoparticles in CoFe PBA @ CrCr PBA heterostructures. The particle size is reported as the mean along with the standard deviation as determined by descriptive statistics performed in Origin 8.5.

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116 Figure 5 6 . Field cooled magnetization vs. temperature for CoFe PBA @ CrCr PBA under an applied field of 100 G in the dark state (black) and in the light state after irradiation at 5 K (red). Data in the light state are collected after the light is turned off. M agnetic ordering of the photo generated Co II Fe III moments induces an increase in magnetization up to 25 K, above which the change in magnetization becomes negative. The photo induced decrease in magnetization is attributed to magnetomechanical coupling, mediated by the interface between CoFe PBA and CrCr PBA. The changes are thermally reversed above 120 K Figure 5 7 . Field cooled magnetization vs. temperature for CoFe PBA@CrCr PBA under an applied field of 100 G in the dark state (black) and in the lig ht state after irradiation at 80 K (red). Irradiation induces a persistent decrease in magnetization indicative of a photo response in the normally light insensitive CrCr PBA. This new behavior arises from magnetomechanical effects at the interface between CoFe PBA and CrCr PBA. The bistable heterostructures relax to the gr ound state upon warming above 12 0 K.

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117 Figure 5 8 . Left : Time dependence of the magnetization upon irradiation at 5 K. The light was turned on at 160 min and turned off at 46 0 min, sever al hours after the magnetization reaches saturation. Right : Time dependence of the magnetization upon irradiation at 80 K. The light was turned on at 7 3 min and turned off at 3 42 min, several hours after the magnetization reaches saturation. Figure 5 9 . light), normalized to the magnetization in the dark state, M dark , plotted as a function of temperature. Irradiation at 5 K induces a maximum decrease of 37% at 60 K whereas irradiation at 80 K shifts the maximum decrease of 31% to 8 8 K.

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118 CHAPTER 6 CONCLUDING REMARKS This final chapter provides a brief overview of the findings presented in this dissertation. The first recurring theme throughout the work presented herein is the ability to finely control matter at the nano and mesoscale. T he study of nano and mesostructures of coordination polymer s (CPs) is still in its infancy. The development of simple and highly reproducible methods, as described herein, allows for the preparation of size controlled mono disperse particles , c ore@shell and core@islan ds heterostructures and even more intricate architectures such as hollow shells. Control over the size, size dispersion and morphology of various coordination polymer structures below 500 nm represents a significant achievement and provide insights into fundamental aspects of the nucleation and growth processes . An extensive library of cyanide based CPs from the PBA family has been explored in the course of this work, consequently providing access to a variety of magnetic, redox, ele ctric and structural properties. This consideration brings us to our second recurring theme: the rational design of materials with tailor made properties. As illustrated in Chapter 4 and 5, t he design of heterostructures has proven a successful strategy to ward materials with novel and enhanced properties. The coupling of functional materials through an interface gives rise to synergistic effects beyond the added c ontributions of both components. Moreover, a better understanding of the length scales involved in these synergistic effects is provided. Future magnetic and structural studies will focus on a more fundamental understanding of the mechanisms involved in the photoexcitation, the metastable state relaxation and the history dependence of these photomag netic core@shell systems.

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119 Although the design of light switchable magnetic materials is strongly emphasized, other projects focused on the building of mutliferroic materials and the assembly of supramolecular photocatalytic system s integrating PBA particl es as electron reservoirs has been explored. In particular, the post synthetic functionalization of PBA particles with Ru based photosensitizers was achieved and a transient absorption spectroscopy study is ongoing to investigate the electron transfer betw een the chromophore and PBA particle. Overall, PBA particles are sturdy and versatile platforms and hopefully this work will contribute to the development of novel heterogeneous systems and composites. Current and future directions include the building of different types of coordination polymers on PBA platforms, which would provide opportunities to design heterostructures with various stimuli sensitive components. The building of these new types of heterogeneous materials will require new synthetic strateg ies and the author hopes that the methods presented here as well as the knowledge accumulated throughout the course of this work will provide a good starting point. Beyond fundamental implications in inorganic and solid state chemistry, materials science a nd physics , such materials provide new opportunities for the design of technologically relevant magneto optical recording systems. As described in Chapter 5, we designed a new photoswitchable system exhibit ing a persistent light induced decrease in magnetization upon irradiation at 80 K. The original magnetic state can be recovered by thermal treatment above 140 K . The on and off state thus defined might be offering us a sneak peek of the next generation of rec ording technologies.

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120 APPENDIX A CTIST ANALYSIS IN THE CORE@SHELL SAMPLES In this section, the low field (100 G) magnetic response is analyzed in the high temperature region of 150 K < T < 300 K. The results clearly indicate the CTIST in the CoFe PBA is complete for the bare cores and for the three core@shell samples. This analysis was performed by Pedro A. Quintero and Prof. Mark W. Meisel. Complete CTIST for the bare cores The analysis begins by realizing that the CTIST is complete for the bare cores, as suggested from the values of the magnetic data and the absence of magnetic T value at room temperature of 2.9 (Fig. 4 19 ) is in agreement with the value expected from the formula Rb Co [Fe (CN) ] 3.5 H O and the isolated values of high spin Co (S=3/2 , g 2.3) and low spin Fe (S =1/2 , g 2.7) ( Li , D. et al., J. Am. Chem. Soc. 2008 , 130 , 252 258) yield, in cgs units per spin when using , ( A 1 ) In addition, the T value at 150 K of 0.8 (Fig. S10 ) is consistent with the formula Rb Co Co [Fe (CN) ] 3.5 H O, where all the Co Fe pairs in the (HS state) have transitioned to Co Fe (LS state), namely ( A 2 ) R eprinted with permission from Ri sset, O. N.; Quintero , P . A.; Brinzari, T . V.; Andrus, M. J.; Lufaso, M. W. ; Meisel, M. W.; Talham, D. R. J. Am . Chem. Soc. 2014 , 136 , 15660 15669 . Copyright 2014 American Chemical Society.

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121 Finally, the last piece of evidence for the completeness of the transition is the paramagnetic behavior of the bare cores down to 5 K, indicating the absence of antiferromagnetic coupled Co Fe pairs, that give raise to a ferrimagnetic transition with T Complete CTIST for the three core@shell samples If there were no interactions between the core and the shell, and they both behave independently, the magnetic data of the system would be just the superposition of the core and shell responses,which can be written as ( A 3 ) where , the the number of moles of the x component (core, shell or core@shell), can be calculated from the equation . The ratios between the masses can be calculated from the EDS d ata, and the MW are known (see Chapter 4, E xperimental section). In Figure 4 19 , the data for the core, the shell, and the three core@shells are shown, along with the res ults of the simulations corresponding to equation ( A 3 ). It is noteworthy that the room temperature values of the simulated and actual data differ in less than 0.04 emu K/mol, suggesting that, at this high temperature, the cores and shells behave as two in dependent magnetic components, and the cores are in the HS state. At 150 K, the T values of the measured data are slightly lower than those predicted by equation ( A 3 ), (Fig. 4 19 ), and this observation suggests that the cores have un dergone a complete transition to the LS state. The small difference between the actual and simulated values at 150 K is most likely linked to changes in the g values coming from the strain induced during the CTIST. Finally, in the intermediate

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122 temperature region (200 275 K), the actual and simulated data differ due to the large synergy CTIST event in the system as extensively discussed in the main text. Modeling the influence of this synergy on the magneto lattice cooperativity is beyond the scope of the pr esent discussion, and the simplier cooperative effects through the CTIST transistion, which have been modeled, are also not included . 121,178,179

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123 APPENDIX B SIZE TRENDS OF VARIOUS PRUSSIAN BLUE ANALOGUES PARTICLES Herein, we describe the size trends of va rious PBAs synthesized following three co precipitation methods described below. Experimental observations are also reported. Co precipitation method s Method 1 : 0.4 mmol of MCl 2 .nH 2 O in 200 mL DI water added to 0.46 mmol K 3 6 ] in 200 mL of DI water under vigorous stirring. Addition time: ~ 1h. Method 2 : 0.4 mmol of MCl 2 .nH 2 O in 100 mL DI water and 0.46 mmol K 3 6 ] in 100 mL of DI water simultaneously added to 200 mL DI water under vigorous stirring. Addition time: ~2h30. Method 3 : 0.4 mmol of MC l 2 .nH 2 O in 100 mL DI water and 0.46 mmol K 3 100 mL of DI water simultaneously added to 200 mL DI water under vigorous stirring. Addition time: ~10h.

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124 Table B 1. Size trends of various PBAs prepared by co precipitation method 1, 2 or 3. Hexacyanoferrate based PBAs FeFe CoFe NiFe CuFe Method 1 < 20 nm 190 ± 20 nm 69 ± 6 nm 25 to 80 nm Method 2 < 20 nm 450 ± 100 nm 150 ± 25 nm 70 to 280 nm Method 3 < 20 nm 443 ± 100 nm 265 ± 60 nm 60 to 200 nm Hexacyanochromate based PBAs F eCr C oCr N iCr CuCr Method 1 200 to 900 nm 400 to 1000 nm < 50 nm Method 2 200 ± 800 nm 85 ± 13 nm 70 to 600 nm Method 3 110 ± 20 nm Hexacyanocobaltate based PBAs F eCo C oCo N iCo CuCo Method 1 245 ± 50 nm < 50 nm Method 2 93 ± 16 nm Method 3

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125 Experimental obs ervations MnFe : Partially soluble in water and acetone, insoluble in methanol. Procedure to Talham, D. R. Chem. Mater. 2013 , 25 , 42 also been prepared using a microemulsion technique: K 3 [Fe(CN) 6 ] (0.23 mmol) in 4 mL H 2 O is added to a solution of IGEPAL CO 520 (10 mL) dissolved in 80 mL of cyclohexane under vigorous stirring. A solut ion of MnCl 2 (0.2 mmol) and RbCl (0.6 mmol) in 4 mL H 2 O is added dropwise over 5 min, yielding a brown suspension. After 10 min, the emulsion is broken with 30 mL of acetone and the particles are washed with acetone to remove the IGEPAL. Particle size: 23 ± 3 nm. Decreasing the amount of IGEPAL results in oddly shaped particles (rectangular, cage like). FeFe : Aggregates of very small particles, difficult to isolate. Product color: Prussian Blue. CoFe : Dispersity is not as good for methods 2 and 3. For all m ethods, the product is Co 3 Fe 2 (HS). To prepare RbCoFe (LS) using method 1, add RbCl (1.24 mmol) to each presursors solutions. The particles are smaller than for the HS product (111 ± 10 nm) and display a spherical shape. To prepare RbCoFe (LS) using metho ds 2 and 3, add RbCl (2.5 mmol) to the solution in which both precursors are added. To prepare Rb x CoFe y (photoswitchable) with Co/Fe ~ 1.3 for all methods, add RbCl (0.79 mmol) to the solution containing the divalent metal salt. Method 1 yields NP smaller than for the HS material, 140 ± 15 nm. Methods 2 and 3 yields particles about the same size as the HS material. To prepare Na x CoFe y (photoswitchable) with Co/Fe ~ 1.3 using method 1, add NaCl (0.6 mol not mmol! which is 35 g again, not mg! lot) to

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126 each presursors solutions. Because of the high ionic strength of the medium (3 M), the product shows ill defined aggregates of particles but the width of the thermal CTIST is 3 times wider than for Rb x CoFe y (7 K for Rb x CoFe y vs. 22 K for Na x CoFe y ). Product color: Co 3 Fe 2 (HS) is violet red; ACoFe (LS) is violet blue; A x CoFe y is violet. NiFe : Well behaved, easy to vary the size by adjusting the addition speed but the standard deviation increases with the size. Product color: Yellow. CuFe : The dispersi ty is hard to control independent of the particle size. Product color: Yellowish brown. CrCr : In 200 mL DI water, add CrCl 2 (0.81 mmol) and let stir for 5 min, the solution turns light green. Add K 3 [Cr(CN) 6 ] (s) (0.61 mmol). The suspension turns brown and, after several minutes, turns light green grayish . Particle size: ~ 100 nm. Product color: Light green. MnCr : Only precipitates from solution with high concentration of precursors. Otherwise, the solution turns slightly white and cloudy, suggesting the form ation of very small nanoparticles. Product color: White. FeCr : After addition of the precursors, the solution is orange and clear but after 5 min, the solution turns opaque and bright orange which suggests the occurrence of ripening yielding big, polydispe rse particles. Product color: Bright orange. FeCr undergoes linkage isomerism to give CrFe. FeCr has also been prepared using a Knowles, E. S.; Yamamoto, T.; Pajerowsk i, D. M.; Meisel, M. W.; Talham, D. R. Inorg. Chem. 2013 , 52 , 4494 CoCr : Big, polydisperse particles. Product color: Light pink.

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127 NiCr: For all methods, the product is Ni 3 Cr 2 . Adding KCl, NaCl or RbCl to the divalent metal salt precursor solution giv es the corresponding analogue. However, CsCl inhibates the growth of the particles so that the product is very difficult to isolate and the yield is very poor. Product color: light blue green. CuCr: Polydisperse particles. Product color: Baby blue. CoCo: P roduct color: Baby pink. NiCo: Behaves similarly to NiCr. Product color: Baby blue.

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128 LIST OF REFERENCES (1) Zhang, S. Fabrication of novel biomaterials through molecular self assembly. Nat. Biotechnol. 2003 , 21 , 1171 1178. (2) Coronado, E.; Day, P. Magnetic Molecular Conductors. Chem. Rev. 2004 , 104 , 5419 5448. (3) Barth, J. V.; Costantini, G.; Kern, K. Engineering atomic and molecular nanostructures at surfaces. Nature 2005 , 437 , 671 679. (4) Browne, W. R.; Feringa, B. L. Makin g molecular machines work. Nat. Nanotechnol. 2006 , 1 , 25 35. (5) Cheetham, A. K.; Rao, C. N. R.; Feller, R. K. Structural diversity and chemical trends in hybrid inorganic organic framework materials. Chem. Commun. 2006 , 4780 4795. (6) Shirota, Y.; Kageyam a, H. Charge Carrier Transporting Molecular Materials and Their Applications in Devices. Chem. Rev. 2007 , 107 , 953 1010. (7) Zhao, Y. S.; Fu, H.; Peng, A.; Ma, Y.; Xiao, D.; Yao, J. Low dimensional nanomaterials based on small organic molecules: preparatio n and optoelectronic properties. Adv. Mater. 2008 , 20 , 2859 2876. (8) Sanvito, S. Molecular spintronics. Chem. Soc. Rev. 2011 , 40 , 3336 3355. (9) Wang, C.; Liu, D.; Lin, W. Metal Organic Frameworks as A Tunable Platform for Designing Functional Molecular M aterials. J. Am. Chem. Soc. 2013 , 135 , 13222 13234. (10) Yaghi, O. M.; O'Keeffe, M.; Ockwig, N. W.; Chae, H. K.; Eddaoudi, M.; Kim, J. Reticular synthesis and the design of new materials. Nature 2003 , 423 , 705 714. (11) Smith, D. K.; Hirst, A. R.; Love, C. S.; Hardy, J. G.; Brignell, S. V.; Huang, B. Self assembly using dendritic building blocks towards controllable nanomaterials. Prog. Polym. Sci. 2005 , 30 , 220 293. (12) Sanchez, C.; Belleville, P.; Popall, M.; Nicole, L. Applications of advanced hybrid organic inorganic nanomaterials: from laboratory to market. Chem. Soc. Rev. 2011 , 40 , 696 753. (13) Luo, Z.; Zhang, S. Designer nanomaterials using chiral self assembling peptide systems and their emerging benefit for society. Chem. Soc. Rev. 2012 , 41 , 4736 4754.

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144 BIOGRAPHICAL SKETCH Olivia Risset was born and raised in Malesherbes, France. She obtained her state chemistry in 2010 from Université Pierre et Marie Curie, Paris VI. In the summer of 2009, she participated in the Research Ex perience for Undergraduates (REU) program back to UF to pursue her Ph.D. degree under Prof. Talham supervision. She successfully defended her dissertation in the fall of 2014.