IngentaConnect Excited-state intramolecular H-atom transfer in nearly symmetrica...

Abstract:

H-atom transfer and proton transfer reactions, like electron transfer reactions, are of fundamental importance in both the physical and biological sciences. Hatom transfer and proton transfer reactions lie at the heart of acid-base chemistry and of a wide range of catalytic reactions in biological systems. Although much progress has been made in understanding electron transfer reactions through the combination of experimental and theoretical work, many aspects of excited-state H-atom and proton transfer reactions are poorly understood, in particular, the way in which the solvent or the intramolecular modes of the solute couple to the reaction. We argue that hypericin and the hypocrellins undergo excited-state intramolecular H-atom transfer reactions. The hypericin and hypocrellin reactions are, relatively speaking, very slow, occurring in about 10 ps for hypericin and from 10-250 ps for hypocrellin A and may be explained in terms of a reaction coordinate that is dominated by intramolecular motions of the aromatic skeleton and the side chains. The observation of a 10 ps transient in hypocrellin A (which, like its analogue in hypericin, lacks a deuterium isotope effect) is essential in attaining a unified understanding of the hypericin and hypocrellin photophysics. Without this common 10 ps component, the photophysics of these two systems bear no similarities and are seemingly unrelated. Our assignment of intramolecular H-atom transfer to the excited-state kinetics has at times been controversial, owing largely to the mirror image symmetry between the absorption and emission spectra and to the absence of deuterium isotope effects. These topics are discussed in detail and we conclude that neither the absence of mirror image symmetry nor the presence of an isotope effect is a conditio sine qua non for a H-atom transfer reaction.

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Excited-state intramolecular H-atom transfer in nearly symmetrical perylene quinones: hypericin, hypocrellin, and their analogues