Tadashi Mori - Circularly Polarized Luminescence of Isolated Small Organic Molecules

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Circularly polarized luminescence (CPL) is one of the optical properties of chiral materials that measures the intensity difference between the left and right circularly polarized spontaneous emission from an intrinsically chiral fluorophore or fluorophore in a chiral environment. Therefore, CPL can be considered as an emission analog of circular dichroisms (CD). While tremendous efforts have been devoted to characterize the observed CD based on the structure of the molecule, successfully generating the so-called structurechiroptical property relationships, such study on the CPL has been hitherto rather limited, due primarily to the instrumental limitations. As the CPL measurement becomes more accessible, the related study has been more concerned, particularly in this decade. Because the CPL responses are fundamentally based on the (emissive) excited state of the fluorophore, this property can be also used to effectively probe the configurational and/or conformational features of a chiral molecule in its electronic excited states. The degree of circular polarization is usually quantified by the dissymmetry factor, i.e., the relative intensity difference of left and right circularly polarized absorption (in CD) or emission (in CPL), respectively. Theoretically, the degrees of these are characterized by the intensity and orientation of electric and magnetic transition dipole moments of the relevant transition. These features are not easily deduced from the molecular structure alone, but the quantum chemical calculations, especially with the ansatz equivalent or superior to the cost-efficient time-dependent density functional theory, are ordinarily respectable for accurately reproducing these values and better understanding of the relationship, although a reliable yet accessible general calculation is still in the developing stage. The differences between CPL and CD (of the emissive state) are often small, as the structural difference between the thermally equilibrated ground state and the excited state, particularly of fluorescent -systems, is relatively minor, and can also be significant in some cases, where nature of emission state, their structural relaxation in the excited state, effect of forbidden state, vibrational coupling, as well as excitonic coupling may play a substantial role. A complete understanding of all these and other factors are definitely required in order to fully understand the CPL characteristics of the chiral substances. While a number of investigations have addressed and disclosed some of these issues, more studies on the appropriate model systems are certainly anticipated to better comprehend the structureproperty relationships for the CPL response.

This book comprehensively collects leading research on recently advanced CPL of small organic molecules, mostly in their isolated states, providing a status quo of the CPL chemistry that will be useful for scientists, researchers, and engineers in general, and is particularly useful for photophysical chemists, organic chemists, supramolecular chemists, spectroscopists, chemical engineers, and others in chemistry-related fields. As compared to lanthanide-based fluorophores, the studies on the organic molecule-based CPL behavior and preparations of CPL-responsive organic molecule are in emerging phase with the relatively smaller dissymmetric factors, but has been extensively attracted in recent years, since chiral organic molecules are potentially more useful in practical applications, as their fluorescence intensity, wavelength, and degree of dissymmetry can be rationally modulated through structural modifications.

This book begins with a short introductory chapter on molecular CPL, and the following chapters of this book consist of detailed descriptions of state-of-the-art advancement of all convincing CPL-responsive organic molecules, classified by the type of inherent chirality as axial chirality in biaryls (by Imai), planar chirality in cyclophanes (by Morisaki), helical chirality in helicenes (by Crassous and Hasobe), distorted chirality in dipyrromethene-related dyes (by Hall and de la Moya Cerero), and other relatively new and unique chiral molecules (by Mori). The book also covers future applications in areas such as advanced imaging and information technologies (by Nakashima). Most of the studies described here are rather focused on the structureproperty relationship of relatively simple molecules investigated in non-aggregated form in solution, and also includes some recent progresses on supramolecular behaviors in hostguest interaction and aggregation formation (by Haino, Abbate, and Liu). The extended photophysical processes such as excimer formation as well as delayed fluorescence are also thought-provoking in terms of the CPL responses, which are also addressed in this book (by Lacour and Pieters). Last but not least, this book also highlights the recent development of commercially available CPL instrument (by Suzuki) as well as time-resolved CD spectroscopy (by Araki), to facilitate the further development and future design of CPL molecules. In order to make each contribution complete in itself, there is some inevitable overlap among the chapters.

Reports and studies on CPL materials, especially of organic systems, are rapidly increasing. Accordingly, it is difficult to cover all aspects of this ongoing active research in one book. However, this book on the CPL of isolated organic molecules serves the purpose of providing valuable information and some insights into the industry, academics, and researchers who are searching for current state of this emerging CPL research area, hoping that the book will stimulate further academic and applied research and promote the industrial applications of organic molecule-based CPL-response materials.