Light-Induced Electron Transfer in Pyropheophytin−Anthraquinone and Phytochlorin−Anthraquinone Dyads: Influence of Conformational Exchange

Abstract

Novel types of donor−acceptor (DA) molecules, consisting of covalently linked pyropheophytin−anthraquinone (PQ and ZnPQ) and phytochlorin−anthraquinone (PCQ and ZnPCQ) dyads and their diastereomers were studied by means of steady-state and time-resolved fluorescence and absorption spectroscopies. Conformational properties were found to play an important role in understanding the observed photochemical behavior of the flexible diastereomers PQ1,2 and ZnPQ1,2 DA dyads. The more compact conformers undergo fast (in a few picoseconds or less) light-induced electron transfer (ET) from the singlet excited state. The most extended conformers, however, possess a separation between the donor and acceptor, which is too long to allow relaxation of the singlet exited state via ET. A fast conformational exchange between the open and folded conformers was observed in a variety of solvents and was found to be crucial for the observed electron-transfer properties of the flexible DA dyads studied. The more rigid DA systems, PCQ and ZnPCQ, exhibited electron-transfer properties that could well be elucidated in the frame of the Marcus theory.