Ultrafast charge transfer in excited electronic states and investigations into fundamental problems of exciplex chemistry: Our early studies and recent developments

Abstract

Among various excited state molecular interactions, photoinduced charge transfer (CT) and electron transfer (ET) are the most important fundamental ones underlying most problems in photophysical, photochemical and photobiological reaction processes. Among the excited state molecular interactions, the excited dipolar solute-polar solvent interactions are not typical CT or ET interactions. Nevertheless, this interaction is very important in the ultrafast ET reactions between donor (D) and acceptor (A) molecules, where the reaction rate is controlled by the solvent dynamics. When photoexcitation of solute molecule induces a large dipole moment in the excited solute in a polar solvent, one can observe a large fluorescence Stokes shift due to the solvation, for which the first quantitative theoretical formula was proposed and its experimental proof was given by Mataga et al. in 1955 [(a) N. Mataga, Y. Kaifu, M. Koizumi, Bull. Chem. Soc. Jpn. 28 (1955) 690; (b) N. Mataga, Y. Kaifu, M. Koizumi, Bull. Chem. Soc. Jpn. 29 (1956) 465.] and also by Lippert et al. [(a) E. Lippert, Z. Naturforsch. 109 (1955) 541; (b) E. Lippert, Ber. Bunsenges. Phys. Chem. 61 (1957) 962.] at the same time independently. Recent advances in the experimental methods for the measurement of time-resolved spectra with femtosecond (fs) laser spectroscopy have enabled us to reveal precise features of the dynamics of the fluorescence Stokes shift due to the ultrafast solvation of the large dipole moment induced by the photoexcitation. Of course, the problems of the excited state molecular interactions are not limited to the above-mentioned simple inter- and intramolecular photoinduced CT and ET reactions, including the solute–solvent interactions, but more complex cases can arise. Namely, the electronic structures of intermolecular exciplexes (EXs), of intramolecular EXs with non-rigid bridges between D and A groups, and of excited CT complexes appear to change due to solute–solvent interactions, a phenomenon that induces further geometrical structural changes, depending on the solvent polarities. This problem is very important for EX chemistry, i.e., for the elucidation of the mechanisms of photoinduced CT and subsequent chemical reactions in solutions and is closely related to the mechanisms of fluorescence quenching reactions in solution, which is a very important problem with a long history. We have started systematic investigations on EX chemistry, including this problem of the fundamental mechanisms of the fluorescence quenching reactions in solutions, during the period 1950–1970 and have carried out extensive work on the fundamental aspects of EX chemistry by means of nanosecond (ns), picosecond (ps), and fs laser spectroscopic studies and also some theoretical investigations [N. Mataga, M. Ottolenghi, in: R. Foster (Ed.), Molecular Association, vol. 2, Academic Press, London, 1979, p. 1; N. Mataga, Pure Appl. Chem. 56 (1984) 1255; J. R. Bolton, N. Mataga, G. McLendon (Eds.), Electron Transfer in Inorganic, Organic and Biological Systems, Advances in Chemistry Series, vol. 228, American Chemical Society, Washington, DC, 1991; N. Mataga, T. Okada, M. Masuhara (Eds.), Dynamics and Mechanisms of Photoinduced Electron Transfer and Related Phenomena, Elsevier, Amsterdam, 1992; N. Mataga, H. Miyasaka, Prog. React. Kinet. 19 (1994) 317; T. Kakitani, N. Matsuda, A. Yoshimori, N. Mataga, Prog. React. Kinet. 20 (1995) 347; N. Mataga, H. Miyasaka, Adv. Chem. Phys. 107 (1999) 431; N. Mataga, Pure Appl. Chem. 69 (1997) 729]. In this article, we discuss the results of our early studies on the fundamental aspects of EX chemistry and also those of our recent investigations, developing our early studies, under 15 different topical headings that are closely related to the most important essential problems of EX chemistry.