List of Contributors ........................................... xv
Preface ...................................................... xvii
1 The Development of Spin-Crossover Research ................... 1
Keith S. Murray
1.1 Introduction ............................................ 1
1.2 Discrete Clusters of SCO Compounds ...................... 4
1.2.1 Dinuclear FeII - Fen SCO Clusters ................ 6
1.2.2 Tri-, Terra-, Penta- and Hexa-nuclear FeII SCO
Clusters ........................................ 18
1.3 ID Chains of FeII SCO Materials ........................ 22
1.4 ID Chains of FeIII SCO Materials ....................... 28
1.5 2D Sheets of FeII SCO Materials ........................ 29
1.6 3D Porous SCO Materials ................................ 30
1.7 Some Recent Developments in Mononuclear SCO FeII,
Fe111 and CoII Compounds ............................... 33
1.7.1 Iron(II) and Iron(III) .......................... 33
1.7.2 Cobalt(II) ...................................... 35
1.8 Multifunctional/Hybrid SCO Materials ................... 37
1.8.1 SCO and Porosity ................................ 38
1.8.2 SCO and Electrical Conductivity ................. 38
1.8.3 SCO and (i) Short-Range Exchange Coupling or
(ii) Long-Range Magnetic Order .................. 38
1.8.4 SCO and Liquid Crystals ......................... 39
1.8.5 SCO and Gels .................................... 39
1.8.6 SCO and NLO ..................................... 39
1.9 Developments in Instrumental Methods in Spin-
Crossover Measurements ................................. 40
1.10 Applications of Molecular Spin-Crossover Compounds ..... 41
1.11 Summary ................................................ 42
Acknowledgements ............................................ 42
References .................................................. 43
2 Novel Mononuclear Spin-Crossover Complexes .................. 55
Birgit Weber
2.1 Introduction and General Considerations ................ 55
2.2 Novel Coordination Numbers (CN), Coordination
Geometries and Metal Centres ........................... 57
2.2.1 Coordination Number 7 ........................... 57
2.2.2 Coordination Number 6 ........................... 58
2.2.3 Coordination Number 5 ........................... 60
2.2.4 Coordination Number 4+1 ......................... 62
2.2.5 Coordination Number 4 ........................... 63
2.3 Iron Complexes with Novel Ligand Donor Atoms and New
Ligand Systems ......................................... 65
2.3.1 N6 Coordination Sphere .......................... 65
2.3.2 N4O2 Coordination Sphere ........................ 66
2.4 Other Examples ......................................... 70
2.5 Conclusion and Outlook ................................. 72
References .................................................. 72
3 Spin-Crossover in Discrete Polynuclear Complexes ............ 77
Juan Olguín and Sally Brooker
3.1 Introduction ........................................... 77
3.2 Dinuclear Iron(II) Complexes ........................... 79
3.2.1 Supramolecular Approach ......................... 79
3.2.2 'Controlled/Designer-Ligand' Approach ........... 84
3.2.3 Ligands with Two Isolated Binding Pockets ....... 84
3.2.4 Ligands with Potential for Communication
between Binding Pockets ......................... 91
3.3 Higher Nuclearity Iron(II) Compounds ................... 98
3.3.1 Trinuclear Iron(II) Complexes ................... 98
3.3.2 Tetranuclear Iron(II) Complexes ................ 100
3.3.3 Higher Nuclearity Mixed Metal/Valent Iron(II)
Complexes ...................................... 103
3.4 Iron(III) ............................................. 104
3.4.1 Dinuclear Iron(III) Complexes .................. 104
3.4.2 Mixed Metal Iron(III) Complexes ................ 108
3.4.3 Mixed Valence Iron(II)/(III) Complexes ......... 108
3.5 Cobalt(II) ............................................ 109
3.5.1 Dinuclear Cobalt(II) Complexes ................. 109
3.5.2 Trinuclear Cobalt(II) Complexes ................ 110
3.6 Dinuclear Chromium(II) Complex ........................ 111
3.7 Concluding Remarks .................................... 112
References ................................................. 113
4 Polymeric Spin-Crossover Materials ......................... 121
M. Carmen Munoz and Jose Antonio Real
4.1 Introduction .......................................... 121
4.2 One-Dimensional SCO-CPs ............................... 121
4.2.1 Triazole Based Bridges ......................... 121
4.2.2 Tetrazole Based Bridges ........................ 124
4.2.3 Bis-Monodentate Pyridine-Like Bridges .......... 124
4.2.4 Polydentate Chelate Bridges .................... 126
4.2.5 Anionic Bridging Ligands ....................... 127
4.3 Two Dimensional SCO-CPs ............................... 128
4.3.1 Neutral Organic Bridging Ligands ............... 128
4.3.2 Dicyanometalate [MI(CN)2]- Bridging Ligands
(MI = Cu, Ag, Au) .............................. 130
4.3.3 Tetracyanometalate [MII(CN)4]2- Bridging
Ligands (MII = Ni, Pd, Pt) ..................... 132
4.4 Three-Dimensional SCO-CPs ............................. 133
4.4.1 Neutral Organic Bridging Ligands ............... 133
4.4.2 Dicyanometalate [MI(CN)2]- Bridging Ligands .... 134
4.4.3 Tetracyanometalate [MII(CN)4]2-Bridging
Ligands ........................................ 136
4.4.4 Hexa- and Octacyano-metallate Bridging
Ligands ........................................ 137
4.5 Conclusion ............................................. 138
Acknowledgement ............................................ 138
References ................................................. 139
5 Structure:Function Relationships in Molecular Spin-
Crossover Materials ........................................ 147
Malcolm A. Halcrow
5.1 Introduction .......................................... 147
5.2 Molecular Shape ....................................... 150
5.2.1 Molecular Shape Inducing Cooperativity ......... 153
5.2.2 Molecular Shape Inhibiting Spin-Crossover ...... 154
5.3 Crystal Packing ....................................... 155
5.3.1 Short Intermolecular Contacts .................. 156
5.3.2 Inhibition of Spin-Crossover by Steric
Congestion ..................................... 157
5.4 Cooperativity Mediated by Disorder .................... 158
5.5 Compounds Showing Wide Thermal Hysteresis ............. 158
5.5.1 Compounds with Symmetric Hysteresis Loops ...... 159
5.5.2 Compounds with Structured Hysteresis Loops ..... 161
5.6 Other Noteworthy Compounds ............................ 162
5.6.1 Iron(II) Triazole Coordination Polymers ........ 162
5.6.2 Cooperative Complexes of Other Metal Ions ...... 163
5.7 Conclusions ........................................... 164
References ................................................. 164
6 Charge Transfer-Induced Spin-Transitions in
Cyanometallate Materials ................................... 171
Kim R. Dunbar, Catalina Achim and Michael Shatruk
6.1 Introduction .......................................... 171
6.2 Characterization of CTIST Compounds ................... 173
6.3 CTIST in Coordination Polymers ........................ 174
6.3.1 Co-Fe Prussian Blue Analogs .................... 174
6.3.2 Other Prussian Blue Analogs .................... 183
6.3.3 Coordination Polymers Based on
Octacyanometallates ............................ 185
6.4 CTIST in Nanoscale Materials .......................... 189
6.4.1 Thin Films ..................................... 189
6.4.2 Nanoparticles .................................. 192
6.5 CTIST in Polynuclear Transition Metal Complexes ....... 195
6.6 Summary and Outlook ................................... 198
Acknowledgement ............................................ 199
References ................................................. 199
7 Valence Tautomeric Transitions in Cobalt-dioxolene
Complexes .................................................. 203
Colette Boskovic
7.1 Introduction .......................................... 203
7.2 Induction of Valence Tautomeric Transitions ........... 205
7.2.1 Thermally Induced Valence Tautomerism .......... 205
7.2.2 Pressure Induced Valence Tautomerism ........... 205
7.2.3 Light Induced Valence Tautomerism .............. 207
7.2.4 Magnetic Field Induced Valence Tautomerism ..... 208
7.2.5 X-Ray Induced Valence Tautomerism .............. 209
7.3 Other Factors that Contribute to the Valence
Tautomeric Transition ................................. 210
7.3.1 Ancillary Ligand Effects ....................... 210
7.3.2 Counterion and Solvation Effects ............... 210
7.3.3 Cooperativity .................................. 212
7.3.4 Valence Tautomerism in Solution ................ 214
7.4 Polynuclear Valence Tautomeric Complexes .............. 214
7.4.1 Dinuclear Valence Tautomeric Complexes ......... 214
7.4.2 Polymeric Valence Tautomeric Complexes ......... 217
7.5 Bifunctional Valence Tautomeric Complexes ............. 218
7.6 Concluding Remarks .................................... 220
Acknowledgements ........................................... 221
References ................................................. 221
8 Reversible Spin Pairing in Crystalline Organic Radicals .... 225
Jeremy M. Rawson and John J. Hayward
8.1 Introduction .......................................... 225
8.2 Radical Pairs: Solution and Gas Phase Studies ......... 226
8.2.1 Radical Dimerisation in Solution ............... 226
8.2.2 Computational Studies on Dimerisation .......... 226
8.3 Dimerisation in the Solid State ....................... 229
8.3.1 Structural Studies ............................. 229
8.3.2 Electronic Structure and Bonding ............... 229
8.3.3 Thermally Accessible Triplet States ............ 230
8.3.4 Spin-Transition Radical Dimers ................. 230
8.3.5 Trithiatriazinyl, TTTA: A Case Study ........... 233
8.4 Summary and Future Perspectives ....................... 234
Acknowledgements ........................................... 235
References ................................................. 235
9 Breathing Crystals from Copper Nitroxyl Complexes .......... 239
Victor Ovcharenko and Elena Bagryanskaya
9.1 Introduction .......................................... 239
9.2 Structural and Magnetic Anomalies ..................... 241
9.3 Relationship between the Chemical Step and the
Physical Property ..................................... 245
9.4 Relationship between the Thermally Induced
Reorientation of Aromatic Solvate Molecules and the
Character of the Magnetic Anomaly ..................... 251
9.5 EPR Study of Breathing Crystals ....................... 255
9.5.1 General Trends of EPR of Strongly Exchange-
coupled Spin Triads ............................ 256
9.5.2 Predominant Population of the Ground
Multiplet ...................................... 257
9.5.3 Dynamic Spin Exchange Processes ................ 259
9.6 Classification of Spin-Transitions in Breathing
Crystals and Correlations with Magnetic
Susceptibility ........................................ 261
9.7 The Detailed Magnetic Structure of Breathing
Crystals .............................................. 266
9.7.1 EPR Measurements of Temperature Dependence of
Intra-cluster Exchange Interaction ............. 266
9.7.2 EPR Measurement of Dipole-Dipole Interaction
and Inter-cluster Exchange Interaction ......... 268
9.8 EPR-detected LIESST on Breathing Crystals ............. 272
9.9 Conclusion ............................................ 275
References ................................................. 276
10 Spin-State Switching in Solution ........................... 281
Matthew P. Shores, Christina M. Klug and Stephanie
R. Fiedler
10.1 Introduction and Scope ................................ 281
10.2 Spin-Crossover: Solid State Versus Solution ........... 282
10.3 Practical Considerations .............................. 283
10.3.1 NMR Characterization ........................... 283
10.3.2 SQUID Magnetometry ............................. 285
10.3.3 Electronic Absorption Spectroscopy ............. 285
10.4 Spin-Crossover in Solution ............................ 285
10.4.1 Solution Characterization ...................... 285
10.4.2 Solvent Effects ................................ 287
10.4.3 Substituent Effects ............................ 288
10.5 Ligation Changes Driving Spin-State Switching in
Solution .............................................. 288
10.5.1 Solvent Exchange/Loss .......................... 288
10.5.2 Anion Exchange/Loss ............................ 289
10.5.3 (Photo)Isomerization ........................... 290
10.5.4 Encapsulation .................................. 291
10.6 Second Coordination Sphere Triggers for Spin-State
Switching ............................................. 291
10.6.1 External Anion-Dependent Spin Switching ........ 293
10.6.2 Using Ligand Fields to Tune Anion Triggered
Spin-State Switching in Solution ............... 293
10.7 Challenges and Opportunities .......................... 294
10.7.1 New Opportunities for Anion Reporting in
Solution ....................................... 294
10.7.2 MRI Contrast ................................... 295
10.8 Conclusions/Outlook .................................. 295
Acknowledgement ............................................ 295
Abbreviations .............................................. 295
References ................................................. 296
11 Multifunctional Materials Combining Spin-Crossover with
Conductivity and Magnetic Ordering ......................... 303
Osamu Sato, Zhao-Yang Li, Zi-Shuo Yao, Soonchul Kang and
Shinji Kanegawa
11.1 Introduction .......................................... 303
11.2 Spin-Crossover and Conductivity: Spin-Crossover
Conductors ............................................ 303
11.2.1 Conclusions .................................... 308
11.3 Spin-Crossover and Magnetic Interaction:
Spin-Crossover Magnets ................................ 308
11.3.1 Hybrid Spin-Crossover Cation and Anionic
Magnetic Framework ............................. 308
11.3.2 Incorporation of Spin-Crossover Sites in
a Magnetic Framework ........................... 310
11.3.3 Conclusion ..................................... 316
References ................................................. 316
12 Amphiphilic and Liquid Crystalline Spin-Crossover
Complexes .................................................. 321
Shinya Hayami
12.1 Introduction .......................................... 321
12.2 Unique Magnetic Properties of SCO Cobalt(II)
Compounds with Long Alkyl Chains ...................... 322
12.2.1 Reverse Spin-Transition for Cobalt(II)
Compounds ...................................... 322
12.2.2 Re-Entrant Spin-Transition for Cobalt(II)
Compounds ...................................... 324
12.3 Liquid Crystalline SCO Compounds ...................... 325
12.3.1 Metallomesogens with SCO Property .............. 326
12.3.2 Synchronization of SCO and Liquid Crystal
Transition ..................................... 327
12.4 Langmuir-Blodgett Films and Amphiphilic SCO
Compounds ............................................. 331
12.4.1 SCO Langmuir-Blodgett Films .................... 332
12.4.2 Amphiphilic SCO Compounds ...................... 333
12.5 Conclusion and Outlook ................................ 339
References ................................................. 340
13 luminescent Spin-Crossover Materials ....................... 347
Helena J. Shepherd, Carlos M. Quintero, Gábor Molnár,
Lionel Salmon and Azzedine Bousseksou
13.1 General Introduction .................................. 347
13.2 Introduction to Luminescent Materials and
Luminescence Energy Transfer .......................... 348
13.2.1 Photoexcitation of Luminescent Materials ....... 349
13.2.2 Return to the Ground State ..................... 351
13.3 Electronic Transitions and Optical Properties of
Spin-Crossover Complexes .............................. 358
13.4 Materials with Combined Spin-Crossover and
Luminescent Functionalities ........................... 361
13.4.1 General Considerations ......................... 361
13.4.2 Examples of Luminescent Spin-Crossover
Compounds (Ligands, Counterions) ............... 362
13.4.3 Luminescent Doping ............................. 366
13.5 Concluding Remarks .................................... 371
Acknowledgements ........................................... 372
References ................................................. 372
14 Nanoparticles, Thin Films and Surface Patterns from Spin-
Crossover Materials and Electrical Spin State Control ...... 375
Paulo Nuno Martinho, Cyril Rajnak and Mario Ruben
14.1 Introduction .......................................... 375
14.2 Nanoparticles and Nanocrystals ........................ 376
14.2.1 Reverse Micelle (Microemulsion) Technique ...... 376
14.2.2 Sol-Gel Techniques ............................. 386
14.3 Thin Films ............................................ 387
14.3.1 Langmuir-Blodgett Deposition ................... 387
14.3.2 Surface-Assisted Molecular Self-assembly ....... 390
14.3.3 Diverse Techniques ............................. 390
14.4 Surface Patterns ...................................... 393
14.4.1 Surface Patterns of Spin-Crossover ............ 393
14.5 Electrical Spin State Control ......................... 396
14.6 Conclusion ............................................ 399
References ................................................. 400
15 Ultrafast Studies of the Light-Induced Spin Change in
Fe(II)-Polypyridine Complexes .............................. 405
Majed Chergui
15.1 Introduction .......................................... 405
15.2 Properties of Fe(II) Complexes ........................ 406
15.2.1 Electronic Structure ........................... 406
15.2.2 Molecular Structure ............................ 407
15.2.3 Vibrational Modes .............................. 407
15.2.4 Kinetics of Ground State Recovery .............. 408
15.3 From the Singlet to the Quintet State ................. 408
15.3.1 Departing from the MCLT Manifold ............... 409
15.3.2 Arrival into the HS State ...................... 412
15.3.3 Vibrational relaxation of the HS State ......... 414
15.4 Ultrafast X-Ray Studies ............................... 415
15.5 Summary and Outlook ................................... 417
Acknowledgements ........................................... 419
References ................................................. 420
16 Real-Time Observation of Spin-Transitions by Optical
Microscopy ................................................. 425
François Varret, Christian Chong, Ahmed Slimani, Damien
Garrot, Yann Garcia and Anil D. Naik
16.1 Introduction .......................................... 425
16.2 Experimental Aspects .................................. 426
16.2.1 Single Crystals ................................ 426
16.2.2 The Sample Cell ................................ 426
16.2.3 Cryostat, Objective, Camera .................... 427
16.2.4 Setting of the Cryostat ........................ 427
16.2.5 Observation Modes .............................. 428
16.3 Selected Investigations ............................... 429
16.3.1 The Interplay between Structure and Spin
Transformations: [Fe(bbtr)3](ClO4)2 ............ 429
16.3.2 Colorimetric Investigation of [Fe(ptz)6]
(BF4)2 ......................................... 430
16.3.3 The Transformation Front in
[Fe(btr)2(NCS)2]H2O Crystals ................... 433
16.3.4 The Evolution of the Frontline in [Fe(bbtr)3]
(ClO4)2 Crystals ............................... 436
16.3.5 An Example of a Robust Crystal:
[Fe(btr)3](ClO4)2 .............................. 437
16.4 Conclusions and Prospects ............................. 439
Acknowledgements ........................................... 439
References ................................................. 440
17 Theoretical Prediction of Spin-Crossover at the Molecular
Level ...................................................... 443
Robert J. Deeth, Christopher M. Handley and Benjamin
J. Houghton
17.1 Introduction .......................................... 443
17.2 Beginnings: Valence Bond and Ligand Field Theories .... 443
17.3 Quantum Chemistry ..................................... 446
17.4 Empirical Methods ..................................... 449
17.4.1 Semi-Empirical MO Theory ....................... 449
17.4.2 Ligand Field Molecular Mechanics ............... 449
17.5 Conclusions ........................................... 452
References ................................................. 452
18 Theoretical Descriptions of Spin-Transitions in Bulk
Lattices ................................................... 455
Cristian Enachescu, Masamichi Nishino and Seiji Miyashita
18.1 Introduction .......................................... 455
18.2 Elastic Interaction Models for Spin-Crossover
Systems ............................................... 457
18.2.1 Thermal Expansion of Volume and Pressure-
Induced Transitions ............................ 459
18.2.2 Long-Range Interactions and Nucleation
Features ....................................... 461
18.3 Mechano-Elastic Model ................................. 465
18.4 Conclusions ........................................... 471
References ................................................. 471
19 Optimizing the Stability of Trapped Metastable Spin
States ..................................................... 475
Jean-François Létard, Guillaume Chastanet, Philippe
Guionneau and Cedric Desplanches
19.1 Introduction .......................................... 475
19.2 Light-Induced Excited Spin-State Trapping (LIESST)
Effect ................................................ 476
19.2.1 LIESST Effect .................................. 476
19.2.2 Variable Temperature Fourier Transform
Infrared Spectroscopy (VTFTIR) ................. 477
19.2.3 The Low-Energy Gap ............................. 478
19.3 The T(LIESST) Approach: The Case of Mononuclear
Compounds ............................................. 479
19.3.1 Principle of the T/(LIESST) Measurement ........ 479
19.3.2 The T(LIESST) Database ......................... 482
19.3.3 Parameters Affecting the T0 Factor ............. 484
19.3.4 The T(LIESST) Approach to Fe(III) Metal
Complexes ...................................... 486
19.4 The T(LIESST) Approach: An Extension to Polynuclear
Iron(II) Complexes .................................... 488
19.4.1 Binuclear Compounds ............................ 488
19.4.2 Trinuclear/Tetranuclear Complexes .............. 492
19.4.3 Hexanuclear Complexes .......................... 493
19.4.4 Polymeric Complexes ............................ 493
19.4.5 Nanoparticles .................................. 494
19.5 Simulation and Extrapolation of a T(LIESST)
Experiment ............................................ 495
19.5.1 Simulation of T(LIESST) Curve .................. 495
19.5.2 Simulation and Extrapolation ................... 497
19.6 Conclusions ........................................... 500
Acknowledgements ........................................... 500
References ................................................. 500
20 Piezo- and Photo-Crystallography Applied to Spin-
Crossover Materials ........................................ 507
Philippe Guionneau and Eric Collet
20.1 Introduction .......................................... 507
20.2 Spin-Crossover and Piezo-Crystallography .............. 507
20.2.1 Pressure-Induced SCO: Expectation Versus
Observation .................................... 508
20.2.2 Piezo-Crystallography and SCO: Investigations .. 509
20.2.3 Piezo-Crystallography and SCO: Challenges ...... 512
20.3 Crystallography of Photoexcited SCO Materials ......... 512
20.3.1 Photo-Crystallography of SCO: Probing the
Change of Molecular Structure .................. 513
20.3.2 Light-Induced Broken Symmetry: Reaching New
States by Laser Excitation ..................... 514
20.3.3 Photoswitching between Different Excited
States ......................................... 515
20.3.4 Slow Phase Nucleation Dynamical Process and
Hysteretic Behaviour ........................... 516
20.3.5 Ultrafast Time-Resolved Crystallography of
SCO Photoswitching Dynamics .................... 517
Acknowledgements ........................................... 519
List of Abbreviations ...................................... 519
References ................................................. 520
21 Spin-Transitions in Metal Oxides ........................... 527
Jean-Pascal Rueff
21.1 Introduction .......................................... 527
21.1.1 CEF Approach to Spin State Stability ........... 528
21.1.2 Stoner Criterion for Itinerant Magnetism ....... 528
21.1.3 Probes of the Spin-Transitions ................. 529
21.2 RIXS: A Probe of the 3d Electronic Properties ......... 530
21.2.1 Overview of the RIXS Process .................. 530
21.2.2 X-Ray Emission as a Probe for the Spin State .. 530
21.2.3 Direct View of the 3d: Pre-Edge Features at
the Metal K-Edge by RIXS and PFY-XAS .......... 531
21.3 Experimental Results .................................. 533
21.3.1 High Pressure Magnetic Collapse ................ 534
21.3.2 Application to Geophysics ...................... 536
21.3.3 Occurrence of Intermediate Spin State in
Cobaltates ..................................... 537
21.3.4 Photoexcited Spin-Transition in Crossover
Compounds and ps Dynamics ...................... 538
21.4 Conclusions and Perspectives .......................... 538
References ................................................. 540
Index ......................................................... 543
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