Photoinduced Spin Crossover in Single Crystal[FeII(bpy)3](PF6)2

by

Ryan Lucas Field

A thesis submitted in conformity with the requirementsfor the degree of Doctor of Philosophy

Graduate Department of PhysicsUniversity of Toronto

c© Copyright 2016 by Ryan Lucas Field

Abstract

Photoinduced Spin Crossover in Single Crystal [FeII(bpy)3](PF6)2

Ryan Lucas Field

Doctor of Philosophy

Graduate Department of Physics

University of Toronto

2016

Spin crossover (SCO) is a phenomenon in which the spin state of a molecular system

is changed by photoexcitation or a change in temperature/pressure. Research in SCO

has been driven by potential applications in data storage and solar energy generation.

SCO also has biological relevance, as it is involved in the function of certain metallopro-

teins. [FeII(bpy)3]2+ is an SCO system that undergoes a low-spin (LS) to high-spin (HS)

transition following photoexcitation. In aqueous solutions, this process is extremely fast,

with the HS state being populated within ∼100 fs of the initial excitation. While the

dynamics of this system have been extensively studied in solution, comparatively little

research has been done in the solid phase, which would be a requirement for potential

applications.

Herein, the ultrafast SCO dynamics in single crystal [FeII(bpy)3](PF6)2 are character-

ized using transient absorption (TA) spectroscopy to elucidate the effects of the crystal

environment on the SCO process. This environment is of particular interest for probes

sensitive to the nuclear motions involved in the SCO transition such as time-resolved

electron diffraction. To this end, a TA spectrometer capable of collecting high-quality

data over a broad spectral range in both liquid and solid samples was developed.

The results show that the ultrafast dynamics following excitation are very similar

those in aqueous [FeII(bpy)3]2+, while dynamics on longer time scales are perturbed in

the crystal environment. The spectral signatures associated with subpicosecond SCO in

ii

aqueous [FeII(bpy)3]2+ are also present in the crystal case. However, oscillations observed

on this time scale suggest more complicated nuclear motions than those observed in aque-

ous samples. At longer times after excitation, the HS state decays back to the LS ground

state almost an order of magnitude faster than in the aqueous case because of chemical

pressure exerted by the lattice. The effects of a long-range acoustic phonon also manifest

themselves as periodic modulations of the absorption peaks. It is concluded that SCO in

single crystal [FeII(bpy)3](PF6)2 is a local molecular process similar to that in aqueous

[FeII(bpy)3]2+, with effects specific to the crystal environment primarily manifesting long

after the relaxation to the HS state is complete.

iii

Acknowledgements

I must first and foremost thank my advisor R. J. Dwayne Miller. Dwayne’s enthusiasm

in the pursuit of scientific endeavors has been inspirational in my development as a

scientist. He has a reassuring optimism even when things go awry and an unwavering

belief in his students’ capabilities. Throughout my long and sometimes tumultuous PhD

work, he never seemed to doubt my ability to carry this work through to completion.

That has made all the difference.

All the students past and present who have passed through the Miller group during

my years here should be acknowledged for their support and camaraderie. I would like

to extend particular thanks to a number of individuals who played an important role

in bringing this work to fruition. My close friend and collaborator Nelson Liu has been

constant source of support, both theoretical and emotional, throughout my PhD work.

I’m not sure how I would have gotten through it without him. I would also like to thank

former graduate students Alexei Halpin and Philip Johnson. During their time in the

group, they were always available to provide guidance when needed. The excellent quality

of their work inspired me to stay focused in my own efforts. Etienne Pelletier should

also be acknowledged for providing great technical advice during the development of my

lab. I thank Olivier Paré-Labrosse for constructive discussions that helped guide the

interpretation of my results. Yifeng Jiang has been a close collaborator during the final

few years of this work, and contributed the ultrafast electron diffraction data that appears

in the final chapter. Patrick Rui also collaborated on experiments done in Hamburg and

contributed to the development of the apparatus used in this work in the early days.

Finally, Alessandra Picchiotti was a great friend and host during my visits to Germany,

and made my time there very enjoyable.

The Miller group’s postdocs and research associates have been a great help throughout

the years. I was particularly lucky to have worked directly with Valentyn Prokhorenko

during the early years of my PhD work. It was from him that I first learned the particulars

iv

of working in an ultrafast laser lab. His relentless pursuit of exceptionally clean data

provided the benchmark that I tried to attain in developing my own setup. Even after

he moved to the German side of the Miller group in Hamburg, he continued to provide

invaluable advice throughout the development of the experimental apparatus used in

this work. Research associate Gustavo Moriena has been consistently understanding

and helpful, and has always made sure that the larger aspects of our shared lab space

were kept under control. Postdocs Amy Stevens, Samansa Maneshi, and Henrike Müller-

Werkmeister have been very supportive in recent years, both scientifically and personally.

Prior to the arrival of the current team of postdocs, Arash Zarrine-Afsar, Francis Talbot,

Germán Sciaini, and Ryan Cooney helped keep me motivated and on track.

Thanks must be extended to contributors outside of the Miller group. Working from

the European XFEL, Wojciech Gawelda provided the samples used for the experiments

in this thesis, and provided valuable insights in interpreting my data. Former Miller

group postdoc and frequent collaborator at the University of Toronto Cheng Lu did the

local sample preparation necessary to make the experiments presented herein possible. I

would also like to thank my advisory committee, University of Toronto professors John

Wei and Dvira Segal, as well as McGill University professor Patanjali Kambhampati who

acted as the external examiner at my final PhD defense.

Finally, my deepest thanks go to my family and friends. Jessica Gahunia has been a

constant source of love and support over the last few years. Her perseverance and work

ethic in her own studies has influenced my own habits and kept me focused throughout

the writing process. I would also like to thank my sister and brother-in-law Dawn and

Geoff Dinnes and my nephews Nathan and Taylor who I look forward to watching grow

in the coming years. Most importantly, my parents John and Heather Field have always

encouraged me in my academic endeavors, and have always been there to keep me positive

and grounded even in the toughest times. I could not have done it without them.

v

Contents

1 Spin Crossover in Condensed Matter 1

1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.1 Ultrafast Measurement . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.2 Spin Crossover . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.1.3 [FeII(bpy)3]2+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.2 Magnetic Characterization of Spin States . . . . . . . . . . . . . . . . . . 7

1.2.1 Magnetic Susceptibility . . . . . . . . . . . . . . . . . . . . . . . . 7

1.2.2 Mössbauer Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . 9

1.3 UV-Vis spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.3.1 UV-Vis Absorption Spectrum of [FeII(bpy)3]2+ . . . . . . . . . . . 12

1.3.2 Time-Resolved UV-Vis Studies of [FeII(bpy)3]2+ . . . . . . . . . . 13

1.4 Ultrafast Time-Resolved X-Ray Studies . . . . . . . . . . . . . . . . . . . 17

1.5 Time-Resolved Studies of Related Spin Crossover Crystals . . . . . . . . 23

1.5.1 Cooperative Lattice Effects . . . . . . . . . . . . . . . . . . . . . 25

1.6 Overview and Thesis Outline . . . . . . . . . . . . . . . . . . . . . . . . 27

2 Spin Crossover Theory 29

2.1 Electronic Structure of [FeII(bpy)3]2+ . . . . . . . . . . . . . . . . . . . . 29

2.2 Thermal Spin Crossover . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

2.3 Photoinduced Spin Transitions . . . . . . . . . . . . . . . . . . . . . . . . 35

vi

2.3.1 Internal Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . 36

2.3.2 Intersystem Crossing . . . . . . . . . . . . . . . . . . . . . . . . . 38

2.3.3 Application to [FeII(bpy)3]2+ . . . . . . . . . . . . . . . . . . . . . 40

3 Experimental Setup 43

3.1 Femtosecond Laser System . . . . . . . . . . . . . . . . . . . . . . . . . . 43

3.2 Second Harmonic Generation . . . . . . . . . . . . . . . . . . . . . . . . 45

3.3 Pulse Shaping and Characterization . . . . . . . . . . . . . . . . . . . . . 47

3.3.1 4F Acousto-Optic Pulse Shaper . . . . . . . . . . . . . . . . . . . 48

3.3.2 Limitations of Acousto-Optic Pulse Shapers . . . . . . . . . . . . 52

3.3.3 Frequency-Resolved Optical Gating . . . . . . . . . . . . . . . . . 54

3.4 Broadband Supercontinuum Generation . . . . . . . . . . . . . . . . . . . 58

3.5 Transient Absorption Spectrometer . . . . . . . . . . . . . . . . . . . . . 62

3.5.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

3.5.2 Data Collection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

3.5.3 Beam Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

3.5.4 Group Velocity Dispersion Correction and Estimation of Instru-

ment Response Function Duration . . . . . . . . . . . . . . . . . . 68

4 Ultrafast Dynamics of Single Crystal [FeII(bpy)3](PF6)2 71

4.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

4.2 Experimental Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

4.2.1 Sample Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . 72

4.2.2 Measurement Conditions . . . . . . . . . . . . . . . . . . . . . . . 73

4.3 Linear Absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

4.4 Transient Absorption Data . . . . . . . . . . . . . . . . . . . . . . . . . . 76

4.4.1 Linearity of the TA Signal . . . . . . . . . . . . . . . . . . . . . . 76

4.4.2 TA of Aqueous [FeII(bpy)3]2+ . . . . . . . . . . . . . . . . . . . . 78

vii

4.4.3 TA of Single Crystal [FeII(bpy)3](PF6)2 . . . . . . . . . . . . . . . 79

4.5 Method of Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

4.6 Results of Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

4.6.1 Aqueous [FeII(bpy)3]2+ . . . . . . . . . . . . . . . . . . . . . . . . 84

4.6.2 Single Crystal [FeII(bpy)3](PF6)2 . . . . . . . . . . . . . . . . . . 88

4.7 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

5 Conclusions and Future Work 102

5.1 Development of the Experiment . . . . . . . . . . . . . . . . . . . . . . . 102

5.2 Summary of [FeII(bpy)3]2+ Results . . . . . . . . . . . . . . . . . . . . . . 103

5.2.1 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

5.3 Future Work on [FeII(bpy)3](PF6)2 . . . . . . . . . . . . . . . . . . . . . 106

5.3.1 Ultrafast Electron Diffraction . . . . . . . . . . . . . . . . . . . . 106

5.3.2 Time-Resolved Magneto-Optic Kerr Effect Spectroscopy . . . . . 108

5.4 Future Applications of the Experimental Setup: Coherent Control . . . . 109

5.4.1 Candidate System: ZW-NAIP . . . . . . . . . . . . . . . . . . . . 110

5.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

Bibliography 114

viii

List of Figures

1.1 a) Unit cell of the LS [FeII(bpy)3](PF6)2 crystal. The atoms are coloured

as follows: orange - iron, blue - nitrogen, black - carbon, purple - phos-

phorous, green - fluorine. b) Structures of the LS (blue) and HS (red)

[FeII(bpy)3]2+ ion, showing the elongated Fe–N bonds in the HS state. . . 7

1.2 The product χMT (where χM is the molar magnetic susceptibility and T

is temperature) as a function of temperature in [Fe(btz)2(NCS)2] (left),

[Fe(phen)2(NCS)2] (middle) and [Fe(dppz)2(NCS)2]pyridine (right). Re-

produced from [14] with permission of The Royal Society of Chemistry. . 8

1.3 a) Mössbauer spectrum of [Fe(ptz)6](BF4)2 at various temperatures, show-

ing a thermal spin transition around 136 K. The blue shaded areas are due

to the LS complex, while the red shaded areas are due to the HS complex.

Reproduced with permission from [42]. b) Logarithm of decay rates (kHL

in s-1) as a function of temperature in [Mn:Fe(0.05%)II(bpy)3](PF6). Black

circles are obtained from time-differential Mössbauer spectroscopy. White

circles are obtained from optical measurements. Reproduced from [33]

with permission of Springer. . . . . . . . . . . . . . . . . . . . . . . . . . 9

ix

1.4 a) Electronic potential energy surfaces of [FeII(bpy)3]2+ as a function of

Fe–N distance, as described in [24, 25]. b) Absorption spectrum of aque-

ous [FeII(bpy)3]2+ showing the contribution of various bands at different

wavelengths. Figures a) and b) reprinted from [17] by permission from

Macmillan Publishers Ltd.: Nature Chemistry, c© 2015. c) Polarized ab-

sorption spectrum of [Zn1-xFex(bpy)3](PF6)2 at 293 K using probe polar-

ization perpendicular to (π) and in the same plane as (σ) the molecular

trigonal axis of symmetry. Adapted from [13] with permission of Springer. 13

1.5 Transient absorption of aqueous [FeII(bpy)3]2+ in the UV (a) and visible

(b) as reported in [17]. Oscillations are clearly visible in both ranges

after photoexcitation. Reprinted from [17] by permission from Macmillan

Publishers Ltd.: Nature Chemistry, c© 2015. . . . . . . . . . . . . . . . . 16

1.6 a) Relaxation rate constants as a function of temperature for the HS state

of [FeII(bpy)3]2+ doped in various isostructural host lattices, [M(bpy)3](PF6)2

where M = Co (inverted filled triangle), Zn (filled diamond), Mn (filled tri-

angle), Cd (filled circle), and in the oxalate network [NaRh(ox)3][Zn(bpy)3]

(filled square) at ambient pressure, and for the Cd host lattice at 1 kbar

external pressure (open circle). Inset: Low temperature relaxation rate

constants as a function of unit cell volume. Reproduced with permission

from [34], c© 2002, Schweizerische Chemische Gesellschaft. b) Schematic

diagram showing the mechanism of the different HS→LS [FeII(bpy)3]2+

relaxation rates through the size of the host lattice’s unit cell (left) and

the lattice’s effect on the potential energy curves (right). Here, ∆E0HL is

the energy gap between the minima of the LS and HS state surfaces, and

∆QHL in the difference in the position of the minima of the two surfaces

along the reaction coordinate, Q. Reproduced from [13] with permission

of Springer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

x

1.7 a) Relative change in X-ray absorption at selected energies, along with

associated fits. The contributions of the MLCT state are shown in orange.

b) An example fit at E = 7121.5 eV. The top panel shows the contribu-

tions of the MLCT and HS states, along with the overall signal, with the

inset showing the change of the signal on a longer time scale. The middle

panel shows the populations of the MLCT and HS states derived from

the fit. The bottom panel shows the modeled change in the Fe–N radius,

noting a 50 fs phase shift of the oscillations (relative to directly excited

oscillations, shown in light blue) resulting from the convolution with the

modeled MLCT population. The shaded area shows the ensemble distri-

bution of Fe–N radii in the vibrationally hot sample, whose width decays

over time. Reproduced from [22] with permission of Dr. H. T. Lemke. . . 20

1.8 Cooperative effects in single crystal [Fe(phen)2NCS2] a) HS fraction as a

function of temperature, showing an abrupt transition at 183 K with a 2 K

hysteresis loop. Reproduced with permission from [64], c© 2003, published

by Elsevier Masson SAS. All rights reserved. b) Ultrafast photoswitching

monitored by transient optical transmission (OT) at 950 nm. c) HS frac-

tion following photoexcitation. Arrows indicate rises caused by different

mechanisms. From left to right: ultrafast photoswitching, elastic lattice

expansion, and thermal switching from laser-deposited heat. d) Acoustic

phonons generated by elastic strain on the picosecond time scale, moni-

tored by transient reflection at 950 nm. Adapted from [62] with permission

of Dr. R. Bertoni. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

xi

2.1 The five 3d orbitals. The lobes contain 90% of the total electron probabil-

ity. The colour denotes the phase of the wavefunction. Orange is positive

and blue is negative. Nodal planes, where the electron probability is zero,

are also shown. Image from [65] used under Creative Commons by-nc-sa

3.0 license (http://creativecommons.org/licenses/by-nc-sa/3.0/). . . . . . 31

2.2 a) Separation of orbital energy levels resulting from ligand field splitting,

redrawn from [18]. b) Orbital occupancy in the LS and HS states, giving

and S = 0 (t2g6eg

0) LS state and a S = 2 (t2g4eg

2) HS state. . . . . . . . 32

2.3 a) The HS fraction as a function of temperature for various coupling

strengths, J2/RTc. Fixed model parameters are used to determine the

value of ∆G0. In this figure n is used in place of nH and J/kTc is used

in place of J2/RTc. Reproduced from [66] with permission of Springer. b)

Model of free energy as a function of the HS fraction at various temper-

atures when third-order interactions are considered. Tc = 120K in this

figure. Reprinted from [68], c© 1983, with permission from Elsevier. . . . 34

2.4 The deactivation mechanism of [FeII(bpy)3]2+ from 1MLCT to 5T2 pro-

posed in [25]. From [25], used with permission of Wiley-VCH. c© 2013

Wiley-VCH Verlag 17548 GmbH & Co. KGaA, Weinheim. . . . . . . . . 42

3.1 Schematic of the AOPS setup. G - Grating, SM - Spherical mirror, FM -

Folding mirror, AOM - Acousto-optic modulator. The magnitude of the

AOM Bragg angle, curvature of the spherical mirrors, and visual change

in colour across the spectrum are exaggerated for clarity. . . . . . . . . . 51

xii

3.2 a) Schematic of the TG FROG setup. RR - retroreflectors, BS1 - 30R/70T

fused silica beamsplitter, BS2 - 50R/50T fused silica beamslitter, FS -

Fused silica windows, OAPM - Off-axis parabolic mirror (f = 50 mm), FSS

- Fused silica sample, CL - Collimating lens (f =70 mm), AL - Achromatic

lens (f = 70mm). b) Phase matching diagram and beam geometry at

OAPM (bottom) and CL (top). . . . . . . . . . . . . . . . . . . . . . . . 56

3.3 Measured FROG traces (left column) with corresponding retrieved spec-

tral profiles (middle column) and temporal profiles (right column). Both

intensity and phase profiles (blue and green lines, respectively) are shown.

The center column also shows the spectral intensity and phase profiles ex-

pected from the applied pulse shaping mask (black and red dashed lines

respectively). a) TL pulse. b) Positively chirped pulse. c) Pulse with

sinusoidally modulated phase. d) Double pulse. . . . . . . . . . . . . . . 57

3.4 Schematic of supercontinuum generation setup. HWP - Half-wave plate,

PBS - Polarizing beam splitting cube, QWP1 - 800 nm quarter-wave plate,

BBO - β-barium borate crystal for SHG, F1 - 400 nm bandpass filter,

QWP2 - 400 nm quarter-wave plate, FM - Flippable mirror, L - Lens (f

= 100 mm), CaF2 - Rotated CaF2 window, OAPM - Off-axis parabolic

mirror (f = 101.6 mm), AQWP - Achromatic quarter-wave plate (for UV

or Vis range), F2 - Colour filter (350 nm cutoff or 335-610 nm bandpass

filter). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

3.5 Typical spectra of the 400 nm and 800 nm pumped white light continua

used for the experiments presented. The sharp cutoff of the 400 nm

pumped WL spectrum at 350 nm is due to the low pass filter used to

remove the residual 400 nm pump light. . . . . . . . . . . . . . . . . . . 62

xiii

3.6 Schematic of the TA spectrometer. BS1 - 30R/70T fused silica beam-

splitter, BS2 - 10R/90T fused silica beamsplitter, P - Polarizer, HWP -

Half-wave plate, FS - Fused silica window, SM - Spherical mirror (f = 250

mm), PD - Photodiode, SP - Spectrometer. . . . . . . . . . . . . . . . . . 64

3.7 Beam profiles of the focused pump, visible probe, and UV probe at the

sample position measured by knife-edge. The horizontal (blue) and ver-

tical (red) profiles are shown for each beam, with the circles showing the

measured data points, and the solid lines showing the fits. The extracted

1/e2 diameters are shown in the legends. . . . . . . . . . . . . . . . . . . 67

3.8 Example of time-zero delay correction using measured cross-phase modu-

lation in deionized water. Positive changes in absorption are shown in red,

and negative changes are shown in blue. . . . . . . . . . . . . . . . . . . 68

3.9 Measured cross-phase modulation in deionized water with associated fits.

Data are shown for the UV (top) and visible (bottom) ranges, with pos-

itive changes in absorption shown in red, and negative changes shown in

blue. The rightmost column shows the fitted Gaussian width across all

wavelengths. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

4.1 Measured absorption spectra of aqueous [FeII(bpy)3]2+ and single crystal

[FeII(bpy)3](PF6)2 samples used in the presented TA experiments. The

dashed line shows the division between the data measured using the UV

and visible range supercontinuua. . . . . . . . . . . . . . . . . . . . . . . 75

4.2 UV TA spectrum of aqueous [FeII(bpy)3]2+ at 5 ps for various pump flu-

ences (left) along with linear fits at selected wavelengths (right). . . . . . 77

4.3 UV TA spectrum of single crystal [FeII(bpy)3](PF6)2 at 5 ps for various

pump fluences (left) along with linear fits at selected wavelengths (right). 78

4.4 Aqueous TA data in the UV (top) and visible (bottom) ranges, on short

(-1–4 ps, left) and long (4–850 ps, right) time scales. . . . . . . . . . . . . 79

xiv

4.5 Single crystal TA data in the UV (top) and visible (bottom) ranges, on

short (-1–4 ps, left) and long (4–850 ps, right) time scales. . . . . . . . . 80

4.6 Single crystal TA data in the visible range from 4–850 ps for samples with

varying optical densities. The measured optical density at 533 nm is shown

for each of the samples. Each of the data sets shown are normalized for

better comparison of the apparent oscillatory behavior on this time scale. 81

4.7 Global fits of the TA data for aqueous [FeII(bpy)3]2+ in the UV (top) and

visible (bottom) ranges, on short (-1–4 ps, left) and long (4–850 ps, right)

time scales. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

4.8 Decay-associated spectra used in the fits shown in figure 4.7. The DAS

are shown for the short (left) and long (right) time delay scans in the UV

(top) and visible (bottom). . . . . . . . . . . . . . . . . . . . . . . . . . . 86

4.9 Kinetic traces of TA of aqueous [FeII(bpy)3]2+ at selected wavelengths with

associated fits. Short (left) and long (right) time delay scans are shown. . 87

4.10 Residuals of the global fits of the aqueous [FeII(bpy)3]2+ TA data in the

UV (top) and visible (bottom) ranges, on short (-1–4 ps, left) and long

(4–850 ps, right) time scales. . . . . . . . . . . . . . . . . . . . . . . . . . 88

4.11 a) Spectrally resolved FT of aqueous [FeII(bpy)3]2+ fit residuals after t =

300 fs in the UV (top) and visible (bottom) ranges. The FT in the visible

range is multiplied by 10 for visual clarity. b) Spectrally integrated FT in

the UV (top) and visible (bottom) ranges. . . . . . . . . . . . . . . . . . 89

4.12 Global fits of the TA data for single crystal [FeII(bpy)3](PF6)2 in the UV

(top) and visible (bottom) ranges, on short (-1–4 ps, left) and long (4–850

ps, right) time scales. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

4.13 Decay-associated spectra used in the fits shown in figure 4.12. The DAS

are shown for the short (left) and long (right) time delay scans in the UV

(top) and visible (bottom). . . . . . . . . . . . . . . . . . . . . . . . . . . 91

xv

4.14 Comparison of visible-range long time delay DAS used in the fits of the

single crystal [FeII(bpy)3](PF6)2 TA data with the GS absorption and its

second derivative. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

4.15 Kinetic traces of TA of single crystal [FeII(bpy)3](PF6)2 at selected wave-

lengths with associated fits. Short (left) and long (right) time delay scans

are shown. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

4.16 Residuals of the global fits of the single crystal [FeII(bpy)3](PF6)2 TA data

in the UV (top) and visible (bottom) ranges, on short (-1–4 ps, left) and

long (4–850 ps, right) time scales. . . . . . . . . . . . . . . . . . . . . . . 94

4.17 a) Spectrally resolved FT of single crystal [FeII(bpy)3](PF6)2 fit residuals

after t = 300 fs in the UV (top) and visible (bottom) ranges. The mag-

nitude of the FTs in the UV and visible ranges are on the same scale. b)

Spectrally integrated FT in the UV (top) and visible (bottom) ranges. . . 95

4.18 a) Spectrally resolved FT of single crystal [FeII(bpy)3](PF6)2 fit residuals

for the visible-range long time delay data shown in figure 4.6. The optical

density of each sample at 533 nm is inset. b) Corresponding spectrally

integrated FT for each sample. In each case, the fitted peak corresponds

to the apparent wavelength-modulated oscillations visible in that data. . 97

4.19 Experimental phonon periods as a function of measured optical density at

533 nm in multiple single crystal [FeII(bpy)3](PF6)2 samples. . . . . . . . 98

xvi

5.1 Preliminary UED data of [FeII(bpy)3](PF6)2 a) Ground state (LS) electron

diffraction pattern. A section of the data is obscured by a beam blocker

put in place to block the residual undiffracted electron beam. b) Kinetic

SVD components of measured changes following excitation with a 400 nm,

70 fs pulse. The sum of the 2nd and 3rd SVD components is also shown.

c) Average change in the diffraction pattern corresponding to the sum

of the 2nd and 3rd SVD components between 2 and 35 ps. The size of

the circles represents the intensity of the diffraction spot at that position.

The colour represents the percent change of the diffraction spot with red

being positive and blue being negative. d) Simulated HS-LS difference

diffraction pattern. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

5.2 a) Ground state absorption spectrum of ZW-NAIP, showing multiple tran-

sitions as elucidated in [136]. The molecular structure is inset. b) Proposed

photocycle of ZW-NAIP showing the contributions of various processes to

the TA spectrum. Adapted from [137] with permission of The Royal So-

ciety of Chemistry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

5.3 a) Preliminary GVD-corrected TA spectrum of ZW-NAIP in methanol. b)

Kinetic traces of TA at selected wavelengths. ∼60 cm-1 Oscillations are

clearly visible in the 434 nm trace. . . . . . . . . . . . . . . . . . . . . . 111

xvii

List of Abbreviations

AOM Acousto-optic modulator

AOPS Acousto-optic pulse shaper

AQWP Achromatic quarter-wave plate

AWG Arbitrary waveform generator

BBO β-Barium borate

CCD Charge coupled device

CPM Cross-phase modulation

CW Continuous wave

DAQ Data acquisition card

DAS Decay-associated spectrum

ESA Excited state absorption

EXAFS Extended X-ray absorption fine structure

FM Folding mirror

FROG Frequency-resolved optical gating

FT Fourier transform

FWHM Full width at half maximum

GA Global analysis

GSB Ground state bleach

GVD Group velocity dispersion

HS High-spin

HWP Half-wave plate

xviii

IC Internal conversion

IR Infrared

IRF Instrument response function

ISC Intersystem crossing

LBO Lithium triborate

LCM Liquid crystal modulator

LIESST Light-induced excited spin state trapping

LMCT Ligand-to-metal charge-transfer

LS Low-spin

MC Metal-centered

MLCT Metal-to-ligand charge-transfer

Nd:YLF Neodymium:yttrium lithium fluoride

Nd:YVO4 Neodymium:yttrium orthovanadate

OAPM Off-axis parabolic mirror

OT Optical transmission

PBS Polarizing beam splitting cube

PSA Product state absorption

QWP Quarter-wave plate

REGEN Regenerative amplifier

SCO Spin crossover

SHG Second harmonic generation

SE Stimulated emission

SO Spin-orbit

SQUID Superconducting quantum interference device

SVD Singular value decomposition

TA Transient absorption

TG Transient grating

xix

Ti:Sapph Titanium:sapphire

TL Transform limited

TMC Transition metal complex

TR MOKE Time-resolved magneto-optic Kerr effect spectroscopy

UED Ultrafast electron diffraction

UV Ultraviolet

Vis Visible

WLG White light generation

XANES X-ray absorption near edge structure

XES X-ray emission spectroscopy

ZW-NAIP Zwitterionic photoswitch based on an N-alkylated indanylidene

pyrroline Schiff base framework

xx

Chapter 1

Spin Crossover in Condensed Matter

1.1 Introduction

1.1.1 Ultrafast Measurement

Throughout the history of science, the advancement of technology has been a driver of

new discoveries. As these advances push the limits of what we are able to observe, we

gain a deeper and more complete understanding of the world around us.

To fully understand the dynamics of chemical reactions and other molecular transi-

tions, it is necessary to resolve their behavior on the relevant time and length scales.

These scales are, respectively, the femtosecond (10-15 s) time scale and angstrom (10-10

m) length scale [1]. Typical molecular transitions occur in tens to thousands of femtosec-

onds, and can involve motions as small as a fraction of an angstrom. In recent decades,

a number of techniques have become available to directly probe changes in the nuclear

and electronic properties of molecules on these scales.

Advances in laser technology have allowed shorter and shorter time scales to be

probed. Specifically, these improvements are the result of developments allowing very

short laser pulses to be produced. Currently, laser pulses as short as a few femtosec-

onds can be generated with relative ease on tabletop setups [2], and recent advancements

1

Chapter 1. Spin Crossover in Condensed Matter 2

continue to push this limit into the attosecond regime [3]. Shorter pulses improve the

achievable time resolution by reducing the duration of the interaction between the pulses

and the material under study.

Subnanometer spatial resolutions have been achievable through X-ray crystallography

since as early as 1913 [4], owing to the wavelength of X-rays, which ranges from 0.01 to

10 nm. Similar resolutions have been achievable with electron microscopes since the

1950s with recent advancements allowing for spatial resolutions well under an angstrom

[5, 6]. This resolution is made possible by the effective wavelength (i.e., the de Broglie

wavelength) of the electrons, which is a function of their kinetic energy and is well below

an angstrom for kinetic energies as low as 1 keV.

Time-resolved optical experiments allow various molecular processes to be character-

ized depending on the wavelength of the probing light, and measurement technique used.

The most common of these is the “pump-probe” technique, whereby a sample is excited

by an intense “pump” pulse and subsequently measured using a weaker “probe” pulse

after a known time delay. Variations of this technique allow different processes to be

observed. Transitions between electronic states can be followed by monitoring changes in

absorption of light in the ultraviolet (UV) and visible (Vis) spectral ranges. Fluorescence

decays can be observed by monitoring gated emitted light from an excited sample. Vibra-

tional dynamics can be characterized by monitoring changes in the absorption of infrared

(IR) light. Further information can be obtained from higher-dimensional spectroscopies

in both the UV-Vis and IR ranges, where sequences involving more pulses can be used

to directly observe couplings between electronic/vibrational states and to separate the

contributions of homogeneous and inhomogeneous broadening on the spectrum [7,8].

In recent years, a number of pump-probe techniques have become available that enable

measurements with both high temporal and spatial resolution. Ultrafast X-ray diffrac-

tion, which uses X-ray probe pulses from sources such as synchrotron and free-electron

laser radiation, allows for changes in the electron density of molecules to be observed,

Chapter 1. Spin Crossover in Condensed Matter 3

which is a proxy for their nuclear structure. Ultrafast electron probe pulses, generated

by shining femtosecond UV laser pulses on a charged photocathode, can similarly be

used for time-resolved diffraction experiments [1]. In contrast to X-rays, electrons scat-

ter primarily from a sample’s atomic nuclei via the Coulomb potential, and thus provide

a direct probe of nuclear structure. Time-resolved electron diffraction has a number of

advantages over X-ray diffraction, as it is orders of magnitude less expensive to develop,

and can be built on a table top. However, additional factors must be taken into account,

such as the effect of the electrons’ Coulombic repulsion on the achievable pulse duration,

the intensity or effective brightness of the electron source, and the requirement of a high

vacuum over the propagation volume of the electrons [1].

1.1.2 Spin Crossover

The focus of this thesis is on a molecular transition known as a spin crossover (SCO). This

refers to a transition whereby the spin multiplicity of a molecule is switched from low-

spin (LS) to high-spin (HS) or vice-versa by a change in pressure or temperature, or by

photoexcitation. This phenomenon was first reported in 1931 by Cambi and Szegö, who

observed the behavior in tris(N,N-dialkyldithiocarbamatoiron(III)) complexes [9]. SCO

represents an example of molecular multistability, where a molecule has multiple stable

or metastable electronic states depending on the environmental conditions. As such,

SCO systems have been studied as potential candidates for a number of applications,

such as magnetic data storage, solar power generation, optical displays, and molecular

switches [10]. Various SCO complexes have displayed properties that are conducive

to such applications, including high efficiency, reversibility, chemical stability, and very

fast response times. Additionally, SCO has biological relevance, as the active site of

certain porphyrins, contained in proteins such as hemoglobin and myoglobin, can be

considered to undergo SCO during the detachment of O2 and the other simple ligands

they transport [11].

Chapter 1. Spin Crossover in Condensed Matter 4

SCO is commonly observed in various transition metal complexes (TMCs, also called

coordination complexes). These complexes consist of a metal center called the coordina-

tion center, surrounded by bound molecules or ions, called ligands. In particular, SCO

has most commonly been observed in TMCs whose central metal atom is a first-row

transition metal, although this is not a strict requirement [12].

SCO is possible because of the energetic structure of the metal atom’s orbitals in

the presence of the field generated by the ligands. In simple terms, external perturba-

tions change the electronic configuration of the metal’s d orbitals, which changes the spin

multiplicity. In general, this change in the electronic configuration is known to be accom-

panied by structural changes. In particular, the metal-ligand bonds shorten or lengthen

due to greater bonding or anti-bonding character of the d orbitals occupied [13]. This is

also known to be associated with a change in the molecular volume and, in the case of

SCO crystals, unit cell volume [12].

Thermal spin transitions in LS ground state complexes occur in systems where ther-

mal energy is sufficient to overcome the energy gap between the spin states. As the

temperature increases, the thermal energy is converted to successively higher vibrational

energy levels until a crossing point is reached. Once the gap has been overcome, the

higher energy spin state is entropically favorable, because of the higher spin multiplicity

and density of vibrational states [14].

Direct spin transitions induced by photoexcitation are dipole-forbidden, as the spin

number is not conserved. Such transitions therefore proceed via relaxation from higher-

lying excited states. Following photoexcitation, an electron is excited from the metal’s d

orbitals to the ligand orbitals (or vice-versa, depending on the system [15]), which puts the

molecule in a so-called metal-to-ligand charge-transfer state, or MLCT (in the opposite

case, this is called a ligand-to-metal charge-transfer state, LMCT). From there, the spin

transition occurs through a relaxation process involving a number of intersystem crossing

(ISC) and possibly internal conversion (IC) steps, which refer to radiationless transitions

Chapter 1. Spin Crossover in Condensed Matter 5

between electronic states having different or the same spin multiplicities, respectively.

Some of these steps may occur after the electron has returned to the metal’s d orbitals.

Such states are called “metal-centered” (MC). The specific states in the relaxation process

depend on the SCO complex under study.

Following the spin transition, SCO complexes typically relax back to the ground

state on a longer time scale. At cryogenic temperatures, the photoinduced spin state

can become “trapped” for long periods of time, as the molecules do not possess enough

energy to overcome the barrier between the initial and final spin states. This is called

“light-induced excited spin state trapping” or LIESST. In some cases, the trapped state

can also be pumped back to the initial state by irradiating at a different wavelength, a

process called reverse-LIESST [16].

1.1.3 [FeII(bpy)3]2+

Some SCO systems do not have accessible thermal transitions, because the energy gap

between the minima of the LS and HS potential energy surfaces is too large. In these

cases, spin transitions are only possible via photoexcitation. Studying such systems offers

unique experimental challenges. Determining the spectral and structural characteristics

of the LS and HS states is less straightforward than in the opposite case, where these

properties can be unambiguously determined simply by carrying out experiments at

temperatures below and above the spin transition temperature.

One such system is iron(II) tris(2,2’-bipyridine) ([FeII(bpy)3]2+), which is the main

experimental focus of this thesis. This system undergoes a LS→HS transition upon pho-

toexcitation by near-UV and visible light [11]. It has been the focus of many studies and

is sometimes considered the archetype of photoinduced SCO transitions [17]. It serves as

an excellent model system for the complex interplay between electronic and structural

degrees of freedom in SCO transitions [18]. As this system lacks a thermal transition,

its spectral and structural properties must be determined through time-resolved meth-

Chapter 1. Spin Crossover in Condensed Matter 6

ods that can probe the HS state at very short times after it has been populated by

photoexcitation.

A noteworthy aspect of the [FeII(bpy)3]2+ complex is the rate at which the HS state

is formed following photoexcitation. It has been argued that this transition is complete

within 50 fs [17]. The associated elongation of the central Fe–N bonds is believed to

occur on a similar time scale [19–22]. This extremely fast transition rate has provoked a

debate about the relaxation pathway that leads to the final HS state, with inconsistencies

in the literature regarding precisely which intermediate states are involved [17,23–25].

Previous studies on this system have investigated its ultrafast dynamics in either

aqueous samples [17–21, 23, 26–32], [FeII(bpy)3]2+ complexes doped in isostructural host

crystals [33–37], or in one case, powder crystals of [FeII(bpy)3](PF6)2 [38]. In this thesis,

SCO is investigated for the first time in single crystals of [FeII(bpy)3](PF6)2 with mea-

surements on aqueous [FeII(bpy)3]2+ serving as a control experiment for the effects of

the crystal environment and possible collective effects. The single crystal environment

is of great interest for measurements that directly probe changes in the molecular struc-

ture such as time-resolved X-ray and electron diffraction. Studying this phase may also

provide insights into the behavior of the complex in the solid state environment, which

would be required for its potential applications. The unit cell of the LS [FeII(bpy)3](PF6)2

crystal and the structure of the LS and HS states of the [FeII(bpy)3]2+ ion are shown in

figure 1.1. The structures shown are reconstructed from the results in [39] and [40].

In this chapter, the literature on SCO will be reviewed, with a focus on [FeII(bpy)3]2+.

Methods that directly measure the molecular spin state will be discussed, along with

results identifying the HS state of the [FeII(bpy)3]2+ complex as the metastable state fol-

lowing photoexcitation. This will be followed a review of results on [FeII(bpy)3]2+ from

various optical and X-ray spectroscopies, with particular attention on time-resolved spec-

troscopies. The effects of crystal environments on SCO dynamics will also be discussed,

along with results on related crystalline SCO systems.

Chapter 1. Spin Crossover in Condensed Matter 7

Figure 1.1: a) Unit cell of the LS [FeII(bpy)3](PF6)2 crystal. The atoms are coloured asfollows: orange - iron, blue - nitrogen, black - carbon, purple - phosphorous, green - fluorine. b)Structures of the LS (blue) and HS (red) [FeII(bpy)3]

2+ ion, showing the elongated Fe–N bondsin the HS state.

1.2 Magnetic Characterization of Spin States

In characterizing the spin states of a molecular complex, direct observations of its mag-

netic properties are most ideal. Such measurements provide an unambiguous probe, but

are generally limited in their time resolution. Therefore, such experiments are of little

utility in characterizing intermediate and transient states in SCO systems. However,

other methods, such as optical spectroscopies, can be used in conjunction with magnetic

methods to characterize other properties of the LS and HS states. These can in turn be

used as indirect probes of the spin states, often having significant advantages such as an

improved time resolution and signal-to-noise ratio.

1.2.1 Magnetic Susceptibility

The most straightforward way to detect changes in a system’s spin state is by direct mea-

surement of its magnetic susceptibility. In solid samples, superconducting quantum in-

terference device (SQUID) magnetometers are most commonly used for this purpose [12].

Chapter 1. Spin Crossover in Condensed Matter 8

Figure 1.2: The product χMT (where χM is the molar magnetic susceptibility and T is tem-perature) as a function of temperature in [Fe(btz)2(NCS)2] (left), [Fe(phen)2(NCS)2] (middle)and [Fe(dppz)2(NCS)2]pyridine (right). Reproduced from [14] with permission of The RoyalSociety of Chemistry.

These magnetometers use superconducting circuits to detect very small magnetic fields,

as low as a few attotesla [41]. However, they have very low time resolution. For example,

measurements with the aforementioned sensitivity require several days of accumulation

time, and as such are not useful in determining transient spin states. Nonetheless, these

and similar devices are very useful for identifying changes in the magnetic states of SCO

complexes having thermal transitions, as the different spin states can be populated in-

definitely, provided the temperature is correctly controlled.

Figure 1.2 shows examples of the thermal spin transition in three solid SCO com-

pounds, [Fe(L)2(NCS)2] where L = 2,2’-bithiazoline (btz), 1,10-phenanthroline (phen)

and dipyrido[3,2-a:2’3’-c]phenazine (dppz), characterized by measurements of the mag-

netic susceptibility. Using this method, the spin transition temperature can be deduced,

along with certain characteristics of the transition (i.e., slope, hysteresis, states at in-

termediate temperatures, etc.). In the solid state, the precise nature of the thermal

transition is determined by intermolecular cooperativity. This behavior arises from elas-

tic interactions resulting from the change in volume upon spin transition, leading to

internal pressure in the solid samples [14]. Gradual thermal transitions (such as in figure

Chapter 1. Spin Crossover in Condensed Matter 9

1.2, left) are observed in samples where there is little cooperativity, whereas abrupt tran-

sitions, and those with hysteresis (such as in figure 1.2, middle and right, respectively)

are indicative of highly cooperative systems [12].

1.2.2 Mössbauer Spectroscopy

The spin state of SCO compounds can also be directly measured by Mössbauer spec-

troscopy. This technique uses the recoilless absorption and emission of gamma rays from

solid crystals, along with the Doppler shift induced by accelerating the absorber/emitter

through a range of velocities, to measure very small differences in transition energies [42].

In the case of SCO, this is applied to measure the hyperfine splitting of spin sub-states

in an external magnetic field. An example of the Mössbauer spectra of a spin crossover

complex [Fe(ptz)6](BF4)2 (ptz = 1-propyltetrazole), showing a thermal LS→HS transi-

tion from a singlet to a doublet, is shown in figure 1.3a.

While this technique can be applied to measure stable spin states, as in the case of

magnetic susceptibility measurements, it has the advantage of allowing for time-resolved

Figure 1.3: a) Mössbauer spectrum of [Fe(ptz)6](BF4)2 at various temperatures, showing athermal spin transition around 136 K. The blue shaded areas are due to the LS complex, while thered shaded areas are due to the HS complex. Reproduced with permission from [42]. b) Logarithmof decay rates (kHL in s

-1) as a function of temperature in [Mn:Fe(0.05%)II(bpy)3](PF6). Blackcircles are obtained from time-differential Mössbauer spectroscopy. White circles are obtainedfrom optical measurements. Reproduced from [33] with permission of Springer.

Chapter 1. Spin Crossover in Condensed Matter 10

measurements, although the time resolution is typically low (nanoseconds) compared to

that achievable by other methods [42, 43]. Nonetheless, this allows for the signatures

of the spin state that are observable by other methods to be identified in cases where

there is no thermal transition. For example, correlating the decay times of the HS

state measured by Mössbauer spectroscopy to those measured optically allows for the

spectral signatures of the HS state to be identified. This has been used to identify the

optical signatures belonging to the HS→LS relaxation in [FeII(bpy)3]2+ doped in a an

isostructural [Mn(bpy)3](PF6)2 host lattice [33]. The decay rates measured by Mössbauer

and optical spectroscopies are compared in figure 1.3b. The agreement between the two

is strong evidence that the decaying state observed by optical spectroscopy is the HS

state.

1.3 UV-Vis spectroscopy

UV-Vis spectroscopy is used to investigate optical transitions between electronic states.

As the name suggests, the energies of the photons involved in such transitions typically

fall in the UV-Vis range of the electromagnetic spectrum.

The bands of a sample’s absorption spectrum show transition energies from the elec-

tronic ground state to higher-lying excited states, the nature of which can be deduced

based on theoretical calculations or by comparison to other results. Such spectra are

acquired by measuring the absorption of light by a sample over a range of wavelengths.

UV-Vis transient absorption (TA) spectroscopy is an example of a pump-probe tech-

nique. This time-resolved method is used to monitor changes in a sample’s absorption

following photoexcitation. TA spectra are typically collected as follows: A pulse reso-

nant with a particular transition is used to initiate a photochemical or photophysical

process in a sample. After a known time delay, a second pulse, which is most commonly

a supercontinuum pulse, is used to measure the instantaneous change in the sample’s

Chapter 1. Spin Crossover in Condensed Matter 11

absorption. The time delay is then varied to produce a map of the sample’s changes in

absorption as a function of time after photoexcitation.

A number of signals arising from changes in the sample’s electronic state can appear

in TA spectra. Ground state bleach (GSB) is a negative change in absorption arising

from the depletion of molecules in the ground state following excitation. Stimulated emis-

sion (SE) can also manifest as a negative change in absorption because of the increased

(emitted) light reaching the detector. Excited state absorption (ESA) and product state

absorption (PSA) are positive changes in absorption arising from the arrival of the sample

in an excited state or a product state following photoexcitation, caused by the different

absorption spectra of those states. Signals arising from molecular vibrations and other

processes such as vibrational cooling can also manifest themselves if they lead to changes

in transition energies or probabilities between electronic states. Finally, in the time in-

terval where the pump and probe pulses are coincident, cross-phase modulation (CPM)

results from the time-dependent change in the sample’s refractive index induced by the

pump pulse, which modulates the probe’s phase, effectively changing its spectrum [44].

The major results of this thesis were obtained using TA spectroscopy.

Fluorescence, which results from radiative transitions from an excited state to a lower

energy state, also typically involves photons with energies in the UV-Vis range. A fluo-

rescence spectrum can be collected by continuously irradiating a sample into an excited

state, collecting the emitted light, and measuring its spectrum. Time-resolved observa-

tion of the fluorescence is also possible via fluorescence upconversion. In this method,

which is another example of a pump-probe technique, the sample is excited with a reso-

nant laser pulse. After a known time delay, the light emitted from the sample is mixed

with a “gate” pulse in a nonlinear crystal. The spectrum of the resulting converted light

is then measured over a range of time delays between the two pulses. This method is

used to observe the fluorescence decay of excited states over time, which can give insight

into the number and nature of states involved [11].

Chapter 1. Spin Crossover in Condensed Matter 12

1.3.1 UV-Vis Absorption Spectrum of [FeII(bpy)3]2+

A representation of the electronic potential energy surfaces of [FeII(bpy)3]2+ as a function

of Fe–N distance are shown in figure 1.4a. The surfaces shown are based on calculations

in [24, 25]. The MC states are labeled by Mulliken symbols of the form XYZ. Here, X is

the spin multiplicity. The term Y is either A, E, or T, where A refers to a non-degenerate

state (i.e., one wavefunction corresponds to this state) that is symmetric upon rotations

about the principal axis of rotation, E refers to a doubly degenerate state, and T refers

to a triply degenerate state. The subscript Z can be either 1 or 2 which refers to states

that are symmetric or anti-symmetric with respect to π rotations perpendicular to the

principal axis of rotation, respectively. Additionally, the subscripts u and g are sometimes

used to refer to states that are anti-symmetric (ungerade) and symmetric (gerade) upon

inversion through a center of symmetry, respectively.

Figure 1.4b shows the UV-Vis absorption spectrum of aqueous [FeII(bpy)3]2+. The

bands in the visible are due to the absorption of the MLCT manifold, while further to the

UV very strong absorption results from the transition of electrons from the bipyradine

ligand’s π orbitals to its π∗ orbitals. The triplet (3T1 and3T2) and quintet (

5T2) states

do not contribute to the absorption spectrum, as optical transitions to these states are

forbidden because of their different spin multiplicities. However, the triplet states may

be involved in the relaxation from the MLCT state to the HS 5T2 state.

The absorption spectrum of doped single crystals of [Zn1-xFex(bpy)3](PF6)2 (which

is similar to what would be expected for the pure crystal, as [Zn(bpy)3](PF6)2 does

not significantly absorb in this spectral range) is very similar to that of the aqueous

complex. The same absorption bands are present and have been attributed to the same

MLCT transitions [13, 35]. In contrast to the aqueous case, the absorption spectrum

has polarization dependence owing to the specific molecular orientations in the crystal.

Figure 1.4c shows this absorption spectrum for polarizations perpendicular to and in the

same plane as the molecular trigonal axis of symmetry. The bands are sharper than

Chapter 1. Spin Crossover in Condensed Matter 13

Figure 1.4: a) Electronic potential energy surfaces of [FeII(bpy)3]2+ as a function of Fe–N

distance, as described in [24, 25]. b) Absorption spectrum of aqueous [FeII(bpy)3]2+ showing

the contribution of various bands at different wavelengths. Figures a) and b) reprinted from[17] by permission from Macmillan Publishers Ltd.: Nature Chemistry, c© 2015. c) Polarizedabsorption spectrum of [Zn1-xFex(bpy)3](PF6)2 at 293 K using probe polarization perpendicularto (π) and in the same plane as (σ) the molecular trigonal axis of symmetry. Adapted from [13]with permission of Springer.

in the aqueous case due to reduced inhomogeneous broadening in the highly structured

crystal environment.

As the spin transition of [FeII(bpy)3]2+ cannot be induced thermally, there is some

difficulty in determining the absorption spectrum of the HS state. Nonetheless, the

optical signatures of the HS state have been inferred by comparison to related compounds

[13,26,45], and by correlating the decay time of the HS state determined by Mössbauer to

decay times measured in optical experiments [13,33]. The primary features of the HS-LS

UV-Vis difference spectrum are increased absorption from the HS state in the UV near

300 nm, and decreased absorption in the visible range, due to the bleach of the LS state.

1.3.2 Time-Resolved UV-Vis Studies of [FeII(bpy)3]2+

A number of studies, using TA and fluorescence upconversion on aqueous [FeII(bpy)3]2+

have been published in recent years [17,26,27]. The primary goal of this research has been

to elucidate the details of the photocycle leading to the LS→HS transition. A particular

Chapter 1. Spin Crossover in Condensed Matter 14

point of interest is the extremely fast formation of the HS state via relaxation from the

MLCT manifold. In addition, as there is a significant energy gap between the minima

of the MLCT manifold and the HS state, these studies have discussed the mechanisms

for storage and/or dissipation of excess energy following the complex’s arrival in the HS

state [17, 26].

Fluorescence upconversion data were reported by Gawelda et al. [27]. The data pro-

vided evidence of the decay of the 1MLCT state into the 3MLCT state within 20 ± 5

fs via ISC, followed by a subsequent decay of the 3MLCT state within 150 fs. This was

inferred by fitting kinetic traces of the emission bands to exponential decays convolved

with the instrument response in different spectral regions, and noting the delayed ap-

pearence of a second emission band during the decay of the primary emission. It should

be noted, however, that both time scales are shorter than or comparable to the reported

instrument response time of the experiment (∼110 fs). A subsequent study showed no

excitation wavelength dependence in the spectra or decay rates of the fluorescence [29].

The same study also reported the first TA measurements on aqueous [FeII(bpy)3]2+,

with detection in the visible range (350–640 nm) [27]. By fitting the measured data

with a sum of exponentially decaying spectra, three relevant spectral components were

found, with (116 ± 10) fs, (960 ± 100) fs, and (665 ± 35) ps decay times. The short-

est component, whose lifetime was comparable to the effective time resolution of the

experiment, was assigned to the absorption of the MLCT states by comparison of the

associated spectrum to the absorption of the reduced [FeII(bpy)3]+ complex. The spec-

trum of the longest component was very similar to the ground state absorption spectrum

and was thus attributed to the recovery of the LS state from the HS state. However,

a persistent ESA was found on the red side of the spectrum that was suggested to be

caused by quintet-quintet absorption of the HS (5T2) state. The intermediate component

was speculated to be caused by MC states. Both the persistent ESA on the red side of

the spectrum and the observed 960 fs component were later attributed to the effects of

Chapter 1. Spin Crossover in Condensed Matter 15

solvated electrons caused by the very high excitation fluence used in this study (>35

mJ/cm2) [18, 30].

A related study focused on TA in the UV range [26]. Strong ESA in the UV range

after photoexcitation was observed, which is the expected signature of the HS state. This

interpretation is consistent with the visible range results, as the UV absorption decay was

found to have the same lifetime as the recovery of the GSB signal. The rise time of the

HS ESA, corresponding to the decay of the MLCT states, was found to be 130 fs (which

was again comparable to the time resolution of the experiment). Additionally, spectral

components with (1.1 ± 0.2) ps and (3.4 ± 1.2) ps time constants were attributed to

vibrational cooling of the “hot” molecule after its arrival in the HS state. A particularly

important finding of this study was the existence of strong 130 cm-1 oscillations in the UV

range, overlapping the HS ESA. This was attributed to the excitation of a low-frequency

wave packet on the HS state surface, excited impulsively through the elongation of the

Fe–N bonds in the LS→HS transition. The results presented suggest that the excess

energy between the minima of the MLCT and HS states is stored as vibrational energy

which is then dissipated on the time scale of several picoseconds.

The measurements performed in the aforementioned TA studies were repeated with

improved time resolution and an appropriate excitation fluence [17]. In this study, the

decay rate of the MLCT states into the HS state was found to be

Chapter 1. Spin Crossover in Condensed Matter 16

Figure 1.5: Transient absorption of aqueous [FeII(bpy)3]2+ in the UV (a) and visible (b) as

reported in [17]. Oscillations are clearly visible in both ranges after photoexcitation. Reprintedfrom [17] by permission from Macmillan Publishers Ltd.: Nature Chemistry, c© 2015.

oscillations, which is equal to the reported formation time of the HS state. Additionally,

oscillations were observed in the visible range TA data, which were not observed in [27],

most likely as a result of the much higher fluence (causing effects unrelated to the spin

crossover photocycle) and reduced signal-to-noise ratio in that study. The oscillations

were nonetheless attributed to coherent wavepacket motions on the HS state surface based

on a number of observations. Firstly, the observed modes were strongest in the region of

HS ESA. Secondly, there is some evidence of HS ESA in the visible range, overlapping

the GSB, which could explain the observation of oscillations in the visible range. Finally,

the damping time of the oscillations were found to be wavelength dependent, with shorter

damping times in the visible range. This is consistent with relaxation into a vibrationally

“hot” HS state, where the motion of the wave packet along the reaction coordinate

initially samples configurations farther from the minimum of the HS state surface. The

wavelength dependence of oscillations in TA measurements due to wave packet motion

is established in literature [51,52].

Although recent studies have focused on aqueous [FeII(bpy)3]2+, a number of earlier

Chapter 1. Spin Crossover in Condensed Matter 17

studies (one of which was previously alluded to in section 1.2.2) have investigated the

decay of the HS state of [FeII(bpy)3](PF6)2 doped in isostructural host lattices. These

studies used flash photolysis, which is a precursor to TA spectroscopy. Although the

method used is somewhat different, both techniques provide the same information in

principle, albeit with much lower time-resolution in the case of flash photolysis.

Using this method, the relaxation of the HS state of [FeII(bpy)3](PF6)2 doped in

various isostructural crystal host lattices [M(bpy)3](PF6)2, where M=Co, Zn, Mn and

Cd has been characterized over a range of temperatures [34, 36]. It was found that

the logarithm of relaxation rate constant of the HS state was linearly proportional to the

volume of the host lattice’s unit cell (figure 1.6a, inset). This is the result of the increased

volume of the HS state. As the size of the the host lattice’s unit cell is decreased, the

driving force of the of the HS→LS relaxation is increased, which destabilizes the HS

state. This can be explained as an increase in the energy gap between the HS and LS

states, as shown in figure 1.6b. This influence of the lattice is called chemical pressure.

At low temperatures (

Chapter 1. Spin Crossover in Condensed Matter 18

have been reported on aqueous [FeII(bpy)3]2+ [19–22, 31]. Although the details of this

technique are beyond the scope of this thesis, a brief description is as follows: The features

of an X-ray absorption spectrum result from the excitation of core electrons of particular

atoms of a given sample to the ionization threshold, which correspond to step-like “edges”

in the X-ray spectrum. The spectral features at energies slightly above the so-called K-

edge, which corresponds to the energy at which a 1s electron absorbs, are of particular

interest. These features are called the X-ray absorption near edge structure (XANES) and

at higher energies, extended X-ray absorption fine structure (EXAFS) and are sensitive

to the molecular structure local to the atom whose core electron is being excited. By

monitoring the change in absorption over time at particular transition energies following

photoexcitation, changes in the structure can be deduced by simulation of the X-ray

Figure 1.6: a) Relaxation rate constants as a function of temperature for the HS state of[FeII(bpy)3]

2+ doped in various isostructural host lattices, [M(bpy)3](PF6)2 where M = Co(inverted filled triangle), Zn (filled diamond), Mn (filled triangle), Cd (filled circle), and inthe oxalate network [NaRh(ox)3][Zn(bpy)3] (filled square) at ambient pressure, and for the Cdhost lattice at 1 kbar external pressure (open circle). Inset: Low temperature relaxation rateconstants as a function of unit cell volume. Reproduced with permission from [34], c© 2002,Schweizerische Chemische Gesellschaft. b) Schematic diagram showing the mechanism of thedifferent HS→LS [FeII(bpy)3]2+ relaxation rates through the size of the host lattice’s unit cell(left) and the lattice’s effect on the potential energy curves (right). Here, ∆E0HL is the energygap between the minima of the LS and HS state surfaces, and ∆QHL in the difference inthe position of the minima of the two surfaces along the reaction coordinate, Q. Reproducedfrom [13] with permission of Springer.

Chapter 1. Spin Crossover in Condensed Matter 19

absorption spectra of different molecular configurations [53]. In the particular case of

[FeII(bpy)3]2+, studies have used the XANES and EXAFS of the Fe K-edge to monitor

changes in the Fe–N bond length over time.

The earliest of these studies had low time resolution (∼100 ps), but was nonetheless

able to estimate the change in the Fe–N bond in the LS→HS transition [31]. The change

was found to be (0.20 ± 0.02) Å in agreement with crystallographic studies on static

structures of other Fe(II) SCO compounds [54, 55]. The measured decay time of the

excited state was also found to be ∼650 ps, in agreement with the decay of the HS state

found in optical measurements. More recent advances have greatly improved the time

resolution of these experiments, allowing the structure of the complex to be followed

during the earliest steps in the LS→HS transition [19–21]. These experiments were able

to reproduce the ∼0.2 Å Fe–N bond length change of earlier studies, and also gave a

time for the formation of the HS state of ∼150 fs in agreement with most optical TA

studies. Furthermore, by modeling the kinetics, these studies excluded the possibility of

intermediate 1,3T states from the LS→HS photocycle as suggested in some studies [24,25],

characterizing the full photocycle as a two-step ISC process (1MLCT→3MLCT→5T2).

Most recently, a study was reported with greatly improved time-resolution (∼25 fs)

and signal-to-noise compared to the aformentioned studies [22]. In addition to giving

refined values for the Fe–N bond length change and formation time for the HS state

(∼0.15 Å and 120 fs, respectively), the improved sensitivity allowed for the vibrations

of the Fe–N bond, as reported in the optical TA studies, to be monitored directly. The

frequency of the mode observed was (126 ± 3) cm-1, in agreement with those studies. A

distinction was made between the dephasing of apparent coherent molecular vibrations,

which decayed in (330± 10) fs and incoherent vibrational cooling, which decayed in (1.6±

0.1) ps. It should be noted that this study explicitly rejects the 50 fs formation time of the

HS state given in [17], finding better agreement with the data using a 120 fs time constant.

In spite of this, the model applied nonetheless reproduced the apparent 50 fs phase shift

Chapter 1. Spin Crossover in Condensed Matter 20

of the coherent oscillations reported in that study, which was instead reported to be the

result of a convolution of the modeled oscillatory dynamics with a 120 fs exponential

decay corresponding to the lifetime of the MLCT state. The 1MLCT→3MLCT→5T2

picture of the spin transition dynamics was supported in this study. Data at various

X-ray energies with associated fits, as well as an example fit showing the contributions

of various processes to the X-ray absorption signal, are shown in figure 1.7.

Time-resolved X-ray emission spectroscopy (XES) has also been used to study the

spin transition dynamics in aqueous [FeII(bpy)3]2+ [23]. In this case, the emission of

Kβ X-rays, corresponding to the emission of photons resulting from transitions of 3p

electrons to 1s holes created by the absorption of X-ray photons, is followed. As the

Figure 1.7: a) Relative change in X-ray absorption at selected energies, along with associatedfits. The contributions of the MLCT state are shown in orange. b) An example fit at E = 7121.5eV. The top panel shows the contributions of the MLCT and HS states, along with the overallsignal, with the inset showing the change of the signal on a longer time scale. The middlepanel shows the populations of the MLCT and HS states derived from the fit. The bottom panelshows the modeled change in the Fe–N radius, noting a 50 fs phase shift of the oscillations(relative to directly excited oscillations, shown in light blue) resulting from the convolutionwith the modeled MLCT population. The shaded area shows the ensemble distribution of Fe–Nradii in the vibrationally hot sample, whose width decays over time. Reproduced from [22] withpermission of Dr. H. T. Lemke.

Chapter 1. Spin Crossover in Condensed Matter 21

lifetime of the 1s hole is subfemtosecond, this constitutes an effectively instantaneous

probe. The spectrum of the emitted photons is sensitive to the spin multiplicity resulting

from the specific occupation of the 3d orbitals because of their interaction with the 3p

orbitals [23]. By fitting a sum of difference fluorescence spectra obtained by comparing

the X-ray emission spectra of molecules related to [FeII(bpy)3]2+ but having different spin

multiplicities, a number of kinetic models for the LS→HS relaxation were tested. The

best fit was obtained when including an intermediate 3T state, in contradiction to most

other recent experimental studies, suggesting instead a 1,3MLCT→3T→5T2 relaxation

cascade. In this picture, the MLCT and 3T states had lifetimes of (150 ± 50) fs and (70

± 30) fs, respectively.

One of the few existing ultrafast studies on crystalline [FeII(bpy)3](PF6)2 was reported

in [38]. In this study, X-ray powder diffraction was used. This technique uses the

coherent scattering of X-rays off the periodic crystal structure (particularly from the

atoms’ electrons) to produce diffraction patterns whose changes in time can be monitored.

By inverting the time-dependent diffraction patterns, changes in the electron densities in

real space can be deduced.

In this experiment, a 800 nm pump beam, with a reported intensity of 800 GW/cm2

was used at excite the sample via two-photon absorption. The same study also reported

an single-colour all-optical TA measurement, using a 400 nm pump beam and a 530

nm probe beam. The temporal shape of the detected changes (to the spatial electron

densities and absorptivity in the diffraction and TA measurements, respectively) after

excitation was found to be similar. However, the reported character of the charge-transfer

was unexpected in comparison to studies on the aqueous complex. In the aqueous case,

the excitation is believed to be local to individual [FeII(bpy)3]2+ molecules, with the

initial excitation transferring an electron from the iron’s d orbitals to a π* orbital on

the bipyradine ligands [24], and is believed to be accompanied by a ∼0.2 Å Fe–N bond

length change. On the other hand, in the X-ray diffraction paper, the assumption of

Chapter 1. Spin Crossover in Condensed Matter 22

a localized initial charge-transfer, along with the authors’ estimate of their fraction of

molecules excited (0.8%), led to multiple unphysical conclusions including a large change

in the distance between the bipyradine ligand and the central iron atom (∼1 Å) in spite

of a reported change in the Fe–N on the same order of magnitude as reported in the

aqueous case (∼0.15 Å), and a very large amount of transferred charge to the PF6-

counterion (32.5 e-) [38]. The suggestion that the PF6- counterion is involved in the

initial charge-transfer is an unexpected result in itself.

The authors rationalized their result by postulating that the initial charge transfer

involves many adjacent complexes via long-range unshielded Coulomb forces, leading to

an effectively larger excitation fraction than originally suggested. Experiments presented

in chapter 4 will show this interpretation is most likely incorrect, as the spectral signatures

associated with a localized charge transfer leading to SCO are in fact present in spectrally

resolved, ultrafast TA measurements of single crystal [FeII(bpy)3](PF6)2.

The anomalous results presented in the X-ray diffraction paper can be explained as the

result of an underestimated excitation fraction and/or ionization of the [FeII(bpy)3](PF6)2

complex due to the very high excitation intensity used. Indeed, it should be noted

that extremely high excitation intensities were used in most or all of the X-ray studies

discussed, which were very likely outside of the linear excitation regime, where the change

in sample absorptivity varies linearly with the fluence (or equivalently, the number of

excited complexes is linearly proportional to the number of pump photons). The X-

ray absorption studies discussed often used pump intensities in excess of 1 TW/cm2 at

excitation wavelengths resonant with the LS→MLCT transition [20,21,23] (other X-ray

studies [19, 31] did not state the intensity). This fact is also important in assessing the

results of reference [23], as the very high intensities used could be sufficient to ionize

the [FeII(bpy)3]2+ complex, which could lead to erroneous observations of the spin state

through molecular fragments formed in the excitation. In fact, a fluence dependence

measurement presented in that study showed that the excitation was not in the linear

Chapter 1. Spin Crossover in Condensed Matter 23

regime, as the change in transmission (which scales logarithmically with the change in

absorptivity) was found to approximately linear with the fluence at the excitation level

used.

1.5 Time-Resolved Studies of Related Spin Crossover

Crystals

Although the literature on the ultrafast dynamics of crystalline [FeII(bpy)3](PF6)2 is lack-

ing, a number of related complexes in various crystalline states have been studied using

ultrafast measurements. These include iron SCO complexes doped into inert isostructural

host lattices [56], nanocrystals embedded in a polymer film [57,58], and thick (10-20 µm)

single crystals [59,60]. Although each complex displays slightly different dynamics, these

studies nonetheless give information about how SCO complexes behave in solid state

environments. The interpretation of these results is often more straightforward than in

the case of [FeII(bpy)3]2+, as many SCO complexes have a spin transition that can be

induced by changes in temperature/pressure, in addition to photoexcitation.

XANES has been used in conjunction with optical TA and transient reflectivity on

single crystals of the Fe(II) SCO complex [Fe(phen)2NCS2] [59, 60]. The optical range

over which it was possible to measure the TA was limited by the high absorptivity of the

samples resulting from the thickness of the single crystals (10-20 µm). As in the case

of [FeII(bpy)3]2+, the LS→HS conversion in [Fe(phen)2NCS2] was found to proceed via

fast ISC from initially excited MLCT states, which may proceed through intermediate

states, followed by relaxation into the HS state. The arrival in the HS state was found

to be accompanied by coherent 85 cm-1 bending and 113 cm-1 breathing modes, which

are sequentially activated after photoexcitation. The XANES measurements revealed a

∼0.2 Å Fe–N bond elongation that is believed to be complete within 100–200 fs, as in

other Fe(II) SCO complexes.

Chapter 1. Spin Crossover in Condensed Matter 24

Optical TA studies on the Fe(III) complex, [Fe(3-MeO-SalEen)2]PF6 were performed

on nanocrystals in a polymer film [57,58]. In contrast to the aforementioned single crys-

tal studies, this environment allowed for TA to be measured over a broad spectral range,

enabled by the sufficiently low sample absorptivity. The dynamics observed after pho-

toexcitation again reveal a similar relaxation cascade as that observed in [FeII(bpy)3]2+,

with a short lived (∼200 fs) LMCT state decaying into a vibrationally hot HS state.

The vibrational energy was found to dissipate in ∼1.6 ps, in good agreement with the

[FeII(bpy)3]2+ case [22]. Furthermore, significant 85 cm-1, 56 cm-1, and 35 cm-1 modes

were observed in the TA data. These oscillations were attributed to molecular modes

relevant to the LS→HS transition that are coherently activated during the displacive

LMCT→HS transition.

Finally, the HS→LS and LS→HS LIESST transitions were both characterized in

dilute [Zn1-xFex(ptz)6](BF4)2 single crystals at various temperatures [56]. The LS→HS

showed the now-familiar ultrafast (

Chapter 1. Spin Crossover in Condensed Matter 25

samples [60] where any collective effects would be absent. It should be noted that the

vibrational and structural dynamics of any given complex could in principle be affected

by the crystal environment used, and direct comparisons between solvated and crystalline

complexes are necessary to elucidate what differences, if any, can be attributed to these

environments.

1.5.1 Cooperative Lattice Effects

Cooperative effects in SCO crystals have been noted in a number of studies [61–63].

These effects have been observed both in thermal dependence studies [63], as previously

discussed, as well as in time-resolved optical and X-ray diffraction studies [61,62]. Strong

coupling between molecules in the crystal lattice resulting from the structural changes

associated with SCO have been identified as the cause of particular thermal behaviors.

This coupling has also been shown to result in features in TA spectra and X-ray diffrac-

tion patterns corresponding to a number of different processes on the nanosecond to

microsecond time scales.

Monoclinic and orthorhombic single crystal polymorphs of [(TPA)FeIII(TCC)]PF6

have been studied by two-colour TA spectroscopy and X-ray diffraction [61] with the

temperature held slightly below the LS→HS transition temperature. The photoinduced

spin transition in both polymorphs was found to follow the typical photocycle seen in

many other SCO systems, with initial excitation into a LMCT state, followed by arrival

in the HS state on the subpicosecond time scale. This process was found to be local to

single complexes, as has been observed in other crystalline SCO systems. However, on

longer time scales, two additional processes were found. Elastic effects caused by the

expansion of unit cells in the LS→HS transition was found to drive expansion of the

lattice on the 4–100 ns time scale. This was followed by slower thermalization of the

heat deposited by the laser on the 100 ns–100 µs scale, leading to a further increase of

the HS fraction by a factor of 5–10 compared to the initial population of the HS state

Chapter 1. Spin Crossover in Condensed Matter 26

by photoexcitation alone. The exact nature of the cooperative effects was found to be

dependent on both the crystal symmetry as well as the crystal size.

Similar behaviour has also been observed in the [Fe(phen)2NCS2] complex [62]. The

thermal transition shows an abrupt change around 130 K with a narrow 2 K hystere-

sis loop, suggesting a highly cooperative system [64]. The elastic and heating effects

described in the case of [(TPA)FeIII(TCC)]PF6 were again found to be present on the

nanosecond to microsecond time scale, with the heating effects being found to increase

nonlinearly as a function of excitation fluence. In addition to these effects, acoustic

phonons were observed using transient reflectivity. The elastic strain induced by the

Figure 1.8: Cooperative effects in single crystal [Fe(phen)2NCS2] a) HS fraction as a functionof temperature, showing an abrupt transition at 183 K with a 2 K hysteresis loop. Reproducedwith permission from [64], c© 2003, published by Elsevier Masson SAS. All rights reserved.b) Ultrafast photoswitching monitored by transient optical transmission (OT) at 950 nm. c)HS fraction following photoexcitation. Arrows indicate rises caused by different mechanisms.From left to right: ultrafast photoswitching, elastic lattice expansion, and thermal switchingfrom laser-deposited heat. d) Acoustic phonons generated by elastic strain on the picosecondtime scale, monitored by transient reflection at 950 nm. Adapted from [62] with permission ofDr. R. Bertoni.

Chapter 1. Spin Crossover in Condensed Matter 27

LS→HS transition was explained to be the generator of these phonons, which had pe-

riods on the order of hundreds of picoseconds. Figure 1.8 shows experimental results

highlighting each of these effects.

1.6 Overview and Thesis Outline

The goal of this thesis is to elucidate the effects of the crystal environment on the pho-

toinduced SCO transition in [FeII(bpy)3](PF6)2, particularly in comparison to aqueous

[FeII(bpy)3]2+. It will be shown that SCO in [FeII(bpy)3](PF6)2 single crystals is an ul-

trafast local molecular process very similar to that observed in liquid, with perturbations

caused by the crystal environment manifesting at longer times after excitation.

Chapter 2 outlines relevant theory that describes SCO in TMCs. Particular attention

will be given to the radiationless transition processes believed to be responsible for SCO

following excitation into the MLCT states. Computational results from literature that

describe plausible mechanisms of the ultrafast spin transition in [FeII(bpy)3]2+ will also

be discussed.

Chapter 3 gives an overview of the experimental apparatuses used in the experiments

presented, as well as the relevant theory involved in their operation. A great deal of the

work involved in producing this manuscript was in the development of this setup. With

the exception of the system providing the fundamental laser beam, which is a commercial

system, virtually all of the apparatuses used were designed, built and characterized by

the author.

Chapter 4 describes the major experimental results of this thesis, focusing on the

ultrafast dynamics of both aqueous [FeII(bpy)3]2+ and single crystal [FeII(bpy)3](PF6)2.

These dynamics are characterized using femtosecond TA spectroscopy, with the results

showing both critical similarities, as well as significant differences in the two environ-

ments. The analysis of the results presented focuses on characterizing the relaxation

Chapter 1. Spin Crossover in Condensed Matter 28

process involved in the LS→HS transition, identifying the molecular modes involved,

and in the single crystal case, investigating lattice effects on the picosecond time scale.

Chapter 5 will summarize the results obtained in this thesis. An overview of future

work on [FeII(bpy)3]2+, including some work currently in progress, will be provided. Other

experimental possibilities of the apparatus, in particular its applicability to coherent

control problems, will also be discussed.

Chapter 2

Spin Crossover Theory

This chapter outlines theoretical considerations related to the SCO process in both the

thermal and photoinduced cases. First, the molecular conditions that make SCO possible

in [FeII(bpy)3]2+ are discussed. Thermal spin transitions are then examined from a free

energy perspective, and it is shown that the nature of the spin transition curve is governed

by the degree of interaction between molecules in the system. Next, the mechanisms

responsible for photoinduced SCO, internal conversion (IC) and intersystem crossing

(ISC), are outlined and the conditions for ultrafast spin transitions are described. Finally,

the application of these considerations in computational studies on [FeII(bpy)3]2+ are

discussed.

2.1 Electronic Structure of [FeII(bpy)3]2+

[FeII(bpy)3]2+ is an example of a first-row TMC. In this c