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Intramolecular Charge Transfer

Theory and Applications


Ramprasad Misra and S. P. Bhattacharyya






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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


Ramprasad Misra

S. P. Bhattacharyya