Table of Contents

Title Page

Preface

Chapter 1: Introduction

1.1 An Overview of the ICT Process
1.2 Experimental and Theoretical Studies of the ICT Process
1.3 Applications of ICT Molecules
References

Chapter 2: Brief History of ICT Molecules

2.1 Introduction
2.2 Background of Studies on Charge Transfer
2.3 A Brief Review of ICT Process in Commonly Studied Organic Molecules
2.4 Structure of the ICT State: Twisted or Not?
2.5 Through-Space Charge Transfer
2.6 Charge Transfer in Inorganic Complexes
2.7 Electron Transfer in Biomolecules
References

Chapter 3: New Theoretical and Experimental Techniques for Studying the ICT Process

3.1 Introduction
3.2 Computational Studies on ICT
3.3 Theoretical Treatment of ICT Rates
3.4 Experimental Methods for Studying ICT Process
References

Chapter 4: Medium Effect on ICT Process: Theory and Experiments

4.1 Introduction
4.2 Some Theories and Models of Solvation
4.3 Effect of Solvent Polarity, Viscosity, and Temperature in the ICT Process
4.4 Studies of Solvation in Some ICT Molecules
4.5 Effect of Hydrogen Bonding on ICT
4.6 Resonance-Assisted Hydrogen Bonding (RAHB)
4.7 Studies of ICT in Solvent Mixture and Confined Media
4.8 Studies of ICT in the Solid State
References

Chapter 5: Nonlinear Optical Response of ICT Molecules

5.1 Introduction
5.2 The NLO Response to Electric Field
5.3 Theoretical Calculation of NLO Response of ICT Molecules
5.4 Studies of Two-Photon Absorption
5.5 Third-Order NLO Response of ICT Molecules
5.6 Experimental Studies on Nonlinear Optical Response
5.7 Studies of NLO Molecular Switches
References

Chapter 6: Recent Technological Applications of ICT Molecules and Prospect of Designing New Molecules

6.1 Introduction
6.2 Application of ICT-Based Molecules
References

Chapter 7: Summary and Outlook

7.1 Introduction
7.2 Summary of Studies of ICT
Further Readings

Index

End User License Agreement

List of Illustrations

Chapter 1: Introduction

Figure 1.1 The intramolecular charge-transfer processes (a) in a (poly)ene system and (b) in an aromatic donor–acceptor molecule. Donor (D) to acceptor (A) charge transfer occurs in the excited state through the π-electron bridge to form an ICT state of higher dipole moment.
Figure 1.2 Schematic representation of intramolecular charge transfer (ICT) process in a model system. In the given scheme, excitation leads the molecule to the S₂ state from which it reaches the S₁-LE state through internal conversion. The ICT state is accessible through the S₁-LE state. Emission form both the LE and ICT states leads to dual emission from the molecule.
Figure 1.3 The free energy curves of the ground (S₀) and first excited (S₁) states of betaine-30. The vibrational levels and possible ET mechanism are also shown.
Figure 1.4 Potential energy curves along the reaction coordinate, q, for the reactant and product states. The curves a, b, and c represent the normal, barrier-less and inverted Marcus regions, respectively (see text for details).
Figure 1.5 The chemical structure of QTCP and QBTP.
Figure 1.6 (a) Possible light-induced motions of the HTI derivatives, as proposed by Dube et al. (b) The chemical structures of the HTI photoswitches Z-1 to Z-4.
Figure 1.7 Schematic representation of excited-state deactivation mechanism of HTI derivatives: (a) Z-4 (in all solvents) and Z-1 (in cyclohexane, CH₂Cl₂ and DMSO).
Figure 1.8 (a) Chemical structure of the HTI derivatives Z-1 to Z-8 studied by Dube and coworkers; (b) crystal structures of this Z-1 to Z-8.
Figure 1.9 Chemical structures of the molecules studied by Turro and coworkers. The molecules were ordered on the basis of expected degree of twist along the triazole C—N bond as well as their donor/acceptor character.
Figure 1.10 The ground- (S₀) and first-excited state (S₁) optimized structures of compounds 1 and 2, studied by Ghosh and coworkers
Figure 1.11 Chemical structures of π-conjugated donor–acceptor molecules studies by Champagne and coworkers.
Figure 1.12 Simplified Jablonski diagrams for (a) phosphorescence- and (b) TADF-based emitters. The difference between the spectral distributions of phosphorescence (dashed curve) and TADF emissions (solid curve) are shown in (c). Important rate processes are indicated using arrows. In the Figure R, NR, F, P, ISC, RISC, and ΔE_ST stand for radiative, non-radiative, fluorescence, phosphorescence, intersystem crossing, reverse ISC and singlet–triplet splitting energy, respectively.

Chapter 2: Brief History of ICT Molecules

Figure 2.1 A simplified version of the Jablonski diagram showing a molecule undergoing different photophysical processes after photoexcitation. The photophysical processes are discussed in the text.
Figure 2.2 Jablonski diagram showing a molecule undergoing (a) ICT process and (b) IPT process after photoexcitation. Some other photophysical processes are also shown.
Figure 2.3 The normal (a) and quinoid (b) forms of a D-π-A molecule, where the spacer (π) is a benzene ring.
Figure 2.4 The chemical structure of locally excited (LE), TICT, PICT, RICT, WICT and partially twisted ICT (pTICT) states of ABN derivatives. R = —H, ABN and R = —CH₃, DMABN.
Figure 2.5 Structure of (a) TMABN and (b) MIN.
Figure 2.6 The potential energy surfaces (PESs) for intramolecular charge-transfer process. The photophysical processes involved are discussed in the text.
Figure 2.7 Structure of NMC6, NTC6, ABN, DMABN, PIPBN, PYRBN, and BPB (left). The twist angle (θ) is defined as the average of the left (τ_l) and right (τ_r) torsional angles. The potentials and dynamics for DMABN (left).
Figure 2.8 A schematic representation of photophysics of DMABN, an ICT molecule, in polar solvent
Figure 2.9 Molecular structures of 2,3,5,6-tetrafluoro-4-aminobenzonitriles studied by Zachariasse and coworkers.
Figure 2.10 (a) Optimized structures of excited-state minima of ABN. Some important bond lengths are shown in angstroms. (b) Schematic representation of excited-state reactions of ABN (relative energies are not in scale).
Figure 2.11 Schematic representation of excited-state reactions of ABN-4F (relative energies are not in scale).
Figure 2.12 Some of the donors (1–2 and 1′–2′) and acceptor (a–d) units of the polyenes and carotenoids studied by Lehn and coworkers.
Figure 2.13 (a) Chemical structure of Rhodamine B and (b) structure of P4C, P5C, and P6C.
Figure 2.14 Structure of C152, C153, and C481.
Figure 2.15 Structure of Oxazine 750.
Figure 2.16 Structure of (a) PTZ-TRZ; (b) phenothiazine core electron-donor and oxazole-acceptor molecules studied by Ghosh and coworkers and (c) PTZ-10-AnP (A), PTZ-10P-AnP (B), and PTZ-3-Anp (C).
Figure 2.17 Structure of TTHBI-Ph, TTHBI-PhCF₃, TTHBI-PhOMe, and TTHBI-PhNPh₂.
Figure 2.18 Structure of the molecules studied by (a) Wolf, (b) Pereira, (c) Saha and coworkers. (d) DMAPPDE molecules studied by Jana et al. (A = —CN, —CO₂Et and —CO₂H) and (e and f) tetrahydropyrene derivatives were studied by Gopidas and coworkers.
Figure 2.19 Structure of the molecules (A–C) studied by Carlotti et al., along with the reference molecule (R).
Figure 2.20 A typical electronic energy diagram of 9AAECA; keto-enol equilibrium and excited-state processes. The absorption wavelengths are reported in nanometer. Acr stands for acridine.
Figure 2.21 (a) The ICT process in D-A-D molecule studied by Toyota and coworkers. (b) The expanded π-conjugated system in this molecule.
Figure 2.22 Structures of DA and DAD molecules studied by Bai and coworkers.
Figure 2.23 Left: Structures of FPP and PP. Right panel shows the (a) TICT and (b) PICT forms of Nile red.
Figure 2.24 A conceptual representation on how frontier orbital interactions determine the energy gap of the twisted or planar state.
Figure 2.25 The structures of tetrafluorene molecules.
Figure 2.26 The structures of the molecular dyads (1–5) studied by Fortage et al. along with their model compounds. The donor (D) and acceptor (A) units are also shown (left). The forward electron transfer (charge separation) and back electron transfer (charge recombination) processes are shown in the right upper panel. The right lower panel shows the schematic presentation of energy levels in dyads (1–5), where GS is the ground state, Exc. and Rel. are excitation, relaxation processes, respectively, and ET is electron transfer process.
Figure 2.27 The structures of the organic and coordination compounds studied by Adamo and coworkers (left). In the right upper panel, the mechanism of direct and stepwise electron transfer processes are shown, while the right lower panel shows the potential energy diagram of these two pathways.
Figure 2.28 Schematic representation of the energy levels and energy-transfer pathways between the peridinin and chlorophyll-a in the PCP complex. Excitation at 535 nm is shown as a double arrow. The wavy arrows and the dashed arrow represent intramolecular relaxation processes and the long-lived chlorophyll-a fluorescence, respectively. The energy-transfer channels are shown as solid arrows (S₂ to Q_x channel, 25%; S₁/ICT to Q_y channel, 63%), while the possible minor energy-transfer channel involving higher vibrational levels of the S₁/ICT state is shown using a dotted line. The corresponding time constant of the processes mentioned are also shown.

Chapter 3: New Theoretical and Experimental Techniques for Studying the ICT Process

Figure 3.1 Two representative ICT molecules: (I) a linear polyene and (II) an aromatic donor–acceptor molecule.
Figure 3.2 The potential energy diagram showing the intramolecular charge-transfer process.
Figure 3.3 Chemical structures of the donor−acceptor biphenyls (I−III), investigated by Rettig and coworkers. The corresponding equilibrium ground-state twist angles (φ_g) obtained from AM1 calculations are also shown.
Figure 3.4 (a) The chemical structure of 4-[2-(4-dimethylaminophenyl)ethenyl]-1-methylpyridinium (HR) in its trans conformation. The angles φ₁, φ₂, φ₃, and φ₄ indicate four possible rotations inside the molecule (see text). (b) Energy of the ground (S₀), first (S₁), and second excited (S₂) states of HR as a function of rotation about the central double bond for different solvent dielectric constants. The numbers a|b|c on the top denote the bulk solvent dielectric constants, dielectric constants of the subshells, and the dielectric constants of the inner solute shells, respectively.
Figure 3.5 The molecular structure of DCM.
Figure 3.6 The chemical structure of para-julolidine malononitrile (JDMN). The S₀ → S₁ electronic transition dipole and the torsional angles that are involved in the S₁ electronic excited-state relaxation dynamics are also shown.
Figure 3.7 The chemical structures of the molecules studied by Dahl et al.
Figure 3.8 The diabatic states |L> and |R> used to define the model Hamiltonian.
Figure 3.9 A simple model depicting energetics of electron transfer on two intersecting parabolic (harmonic) potential energy surfaces.
Figure 3.10 Model potential for bridge-assisted long-range electron transfer.
Figure 3.11 In the upper panel: (a) The molecular structures of three chemical moieties of the triad: carotene (C), diaryl-porphyrin (P), and pyrrole-fullerene (C60). (b) The linear and bent conformations of the C-P-C60 dyad. In the lower panel: the HOMO and LUMO energies of linear and bent conformation of the dyads (in eV). The nature of the different electronic excitations (ππ*, CT1 and CT2) are also shown.
Figure 3.12 Absorption and fluorescence (dual emission from LE and ICT states) spectra of (a) DMABN in acetonitrile, (b) DMABN in ethanol, and (c) 4-(N-piperidinyl) aminobenzonitrile (P6C) in acetonitrile at 25 °C in arbitrary units. The changes in the spectral intensities due to photoproduct formation with increasing duration of controlled photoexcitation at 285 nm are also shown.
Figure 3.13 The potential energy surfaces of Auramine, as proposed by Palit and coworkers. The wavelength of the corresponding transitions (absorption, emission) are shown in the figure.
Figure 3.14 The potential energy surfaces of DMAC-A and DMAC-B. The authors found that the TICT process is feasible in DMAC-A, while no ICT reaction was detected in DMAC-B. The possible changes in structure of the aforesaid molecules due to ICT process are also shown.
Figure 3.15 Molecular structure of PDI/Ir complex dyads (1 and 2) and their respective ligands (3 and 4).
Figure 3.16 The excited-state deactivation pathway of DA1 in acetonitrile and cyclohexane involving the solvation and ICT relaxation (see text for details).
Figure 3.17 Diabatic and adiabatic representations of the photophysics of SBPa upon photoexcitation.
Figure 3.18 The ICT process in betaine-30.
Figure 3.19 Pictorial depiction of the photophysics of betaine-30 as obtained by Frontiera et al. (see text for details).
Figure 3.20 The upper panel shows the photoinduced charge-transfer process of betaine-30 (left) and DMANS along with their dipole moments in the ground and excited states. Lower panel shows the electric fields generated via charge transfer in DMANS and betaine-30 compared to that generated from photoexcitation of biased GaAs, while the direction of the applied field is the same in all the cases. The experimental configuration is also shown (right).

Chapter 4: Medium Effect on ICT Process: Theory and Experiments

Figure 4.1 (a) Polar solvent cage around a dipolar solute, as proposed by Gutierrez and coworkers. (b) Model of dipole–dipole interaction between a polar molecule (solute) and two polar solvent molecules.
Figure 4.2 (a) The Lippert–Mataga plot for THHBI molecules (see text). (b) The change in absorption and emission maxima of the molecules with Reichardt's solvent parameters.
Figure 4.3 The chemical structure of DCVJ, CCVJ-TEG, DMABN, AMCA, coumarin 1, and coumarin 6.
Figure 4.4 Chemical structures of compounds (1–6), studied by Escudero (A). Schematic representation of PES of compounds (1–6). Energies are presented in eV, oscillator strengths of the transitions are presented in parenthesis and experimental values of φ are also reported (B).
Figure 4.5 The structure of DMABN, PBN and related molecules.
Figure 4.6 Schematic representation of photochemical reaction of PBN in gas phase and polar solvents.
Figure 4.7 Theoretically computed structures of (a) 1 : 3 PACO-H₂O cluster in the gas phase, (b) 1 : 3 PACO-H₂O cluster macrosolvated in water, and (c) the corresponding simulated absorption spectra (solid and dotted line, respectively).
Figure Scheme 4.1 Schematic representation of the processes involved during the ICT process (see text for details).
Figure 4.8 Schematic representation of the deactivation mechanism of FDMA as proposed by Bohnwagner and Dreuw. After photoexcitation to the bright S₂ state, the excited molecule may either relax to an S₁ state minimum S1_planar and fluoresce or it may decay to the ground state non-radiatively via internal conversion.
Figure 4.9 Schematic representation of the excited state relaxation pathways of thioflavin-T in chloroform and methanol. Although the locally excited molecule is likely to decay through the ICT-1 mechanism, the ICT-2 mechanism may be operational, depending on the solvent.
Figure 4.10 Schematic representation of the solvent effect on the ICT process in 3-aminoquinoline (see text for details).
Figure 4.11 The molecular structure and packing diagram of anthracene-o-carborane dyad studied by Chujo et al. (a and b in upper panel). The lower panel shows the energy levels of ground and excited states of the dyad variable dihedral angle (φ) as obtained from the quantum chemical calculations. The proposed absorption and emission processes in the dyad are also shown in this panel.
Figure 4.12 The chemical structures of TPA3TPAN and DTPA4TPAN.

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Preface

Over the last two decades, intramolecular charge transfer (ICT) molecules have been receiving wide-spread attention of scientists in view of their potential for technological applications in molecular electronics, solar cells, quantum optics, sensors, and so on. Organic ICT molecules in particular have been at the primary focus as they provide better stability and flexibility in design than the standard inorganic molecules. A large number of research publications dealing with different aspects of these molecules have been added to the literature over the recent years. We felt that a wide gap already exists between information available in books and monographs accessible to the graduate and Masters students as well as beginners in research and those available in specialized journals, and the gap is increasing by the day. A researcher interested in designing new organic nonlinear optical (NLO) material or ICT based sensor or molecule based materials for organic electronics, or a general reader curious to understand the ICT phenomenon more completely would welcome a book that bridges the that gap seems to exist. This book attempts to address the issue by providing a pedagogical description of the different stages and facets of ICT phenomenon and what goes behind the designing of ICT based materials for technological applications; what problems are to be taken care of and how to leverage theory and experiments in a specific context, and so on. In addition to an overview of the recent theoretical and experimental developments relating to ICT molecules and the ICT phenomenon the book contains a brief history of past efforts in this area. Charge transfer is a rather ubiquitous process in nature and a fundamental step in many chemical and biological processes, like photosynthesis and metabolism, for example. The ICT in conjugated π-electronic systems has attracted serious attention in view of the immense technological applications that the process has for example, in organic electronics and photovoltaics. Materials based on such molecules are potential candidates for organic light-emitting diodes (OLEDs), field effect transistors, dye-sensitized solar cells, and so on. In fact the main impetus to study electron transfer (ET) process in organic molecules and materials came from the urge to understand the mechanism of photosynthesis in plants and bacteria with the hope that the knowledge gained in the process would help the scientists in designing artificial photosynthetic systems for efficient conversion and storage of solar energy. The search for such systems is still on with a long way to go. Regular stocktaking is therefore essential in such a critical field of research.

The contents of the book have been divided among seven chapters as detailed in the following. In the introductory chapter (Chapter 1) the basics of the ICT process has been discussed laying the foundation for the next five chapters (Chapters 2–6). The rather long history of evolution of the idea of ICT process and ICT molecules has been presented in Chapter 2 to place the modern developments in a proper perspective. The ICT molecules most frequently studied with steady state and time-resolved spectroscopic techniques in the UV–visible range; specially, time-correlated single photon counting (TCSPC) and fluorescence up-conversion techniques have been found widespread use for time-resolved studies on ICT. Two other useful techniques for probing ICT have been femtosecond transient absorption (TA) and resonance Raman spectroscopy. A relatively recent and novel experimental tool to follow the ICT dynamics is the terahertz (THz) spectroscopy. The idea behind the THz spectroscopy is simple. The ICT process involves movement of electronic charge from one end of the molecule to the other. If the charge is accelerated, electromagnetic (EM) radiation will be emitted. Assuming that the ICT occurs on a timescale of picoseconds, the frequency of the EM radiation will be in the THz range that can be detected and monitored. It is, as if, the moving charge “broadcasts” its own dynamics opening up a direct route to follow the ICT dynamics. We have devoted some space to this new mode of studying the ICT process – the so called THz spectroscopy.

A lot of theoretical calculations (modeling as well as calculations) on ICT molecules are now available in literature. The calculations have been mostly done at the Hartree–Fock (HF) level and of late increasingly at the level of the complete active space self-consistent field (CASSCF) method or CASSCF with second-order correction incorporated (CASPT2) along with variants of density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. Although majority of such calculations have been done at the adiabatic level, there has been a “paradigm shift” in the sense that theoreticians are increasingly looking into the very important role played by non-adiabatic factors in shaping the ICT process. The Chapter 3 is devoted to the relatively newer theoretical and experimental techniques that are being used to unravel the mystery of the ICT process and model ICT rates.

It is now firmly established that the ICT process is critically affected by the properties of the medium in which it occurs. Not only are the spectral signatures of the ICT molecules modulated by the polarity and hydrogen bonding abilities of the solvents, the ICT rate is also affected by thermal effects (temperature) and friction (solvent viscosity). Several ICT molecules have therefore been exploited as polarity and viscosity sensors. Such media effects on the ICT process have been elaborately discussed in Chapter 4. As mentioned already, the ICT molecules have technological relevance and are being explored for fabricating new molecules based NLO or halochromic materials, materials for solar cells, for OLEDs, for viscosity and polarity sensors, and so on. In view of the importance of the field and diversity of issues, we have devoted two consecutive chapters to discuss technological applications of ICT molecules (Chapters 5 and 6). The NLO phenomena with emphasis on hyperpolarizabilities that the ICT molecules are endowed with and several aspects of ICT based two-photon absorbing materials are dealt with in Chapter 5. Several issues with technological applications of ICT molecules in sensing, in OLEDs, and so on are examined in the penultimate Chapter 6 while Chapter 7 is devoted to consider future projection of research in ICT in backdrop of important developments that have already taken place.

This book is a review of studies of ICT process and related phenomena. We have reported results available in contemporary literature in good faith. Permissions for copyrighted materials have been duly obtained from the copyright holders. The book is primarily intended for the Masters and graduate (doctoral) students of chemistry and chemical physics. We hope that more specialized people too, will find the book useful. We have tried to strike a balance between experiment and theoretical aspects with the hope that it caters the need of the both theoreticians and experimentalists alike. In spite of our best efforts to make the compilation error-free, some unfortunate and unintended omissions might still be crept in. We sincerely regret any such blemishes.

We are happy to acknowledge the help and constant encouragement of colleagues and friends without which the book might not have seen the light of the day. A major part of the book was written while the authors were in the Indian Association for the Cultivation of Science (IACS). We thank all our colleagues in the Department of Physical Chemistry, IACS for providing the intellectual ambience needed for undertaking the project. We specially thank Prof. D.S. Ray and Prof. S. Adhikari for all the help and encouragements they extended to us. One of the authors (RM) wishes to thank his present mentor Prof. M. Sheves of the Weizmann Institute of Science for his constant support. RM is grateful to his parents, wife (Piyali), and other family members for their unfailing supports and encouragement. SPB wishes to express indebtedness to Bharati (wife), Rupsha (daughter), and Sayan (son) for their unfailing support and the colleagues in the Department of Chemistry, IIT Bombay for making his stay in the department (2012–2015) as Raja Ramanna fellow (DAE) fruitful.

August 21, 2017

Kolkata

Ramprasad Misra

S. P. Bhattacharyya

Intramolecular Charge Transfer

Theory and Applications

Preface