Synthesis, Spectroscopy, and Semiempirical Study of a Novel Porphyrin-Flavin Dyad

摘要

A new synthetic methodology for the preparation of donorbridge-acceptor compounds incorporating a porphyrin donor and an all-aromatic oligo (diazaphenylene) bridge is introduced. This approach allows the controlled preparation of photosynthetic model compounds with well-defined spacer structure and properties. The synthesis of the porphyrin-flavin dyads 1a-c is described to exemplify the strategy. This type of structure has a number of interesting spectroscopic and photophysical properties. The aromatic bridge results in a well-defined donor-acceptor distance; it can favor conjugation and at the same time be nonabsorbing in the visible and near-UV range. The choice of a flavin as the acceptor unit opens the way to a spectroscopic study by excitation of the acceptor in addition to the usual porphyrin donor excitation. Based on these premises, a spectroscopic, photophysical and semiempirical study of the dyad 1a has been performed in three solvents of varying polarity. The results demonstrate energy transfer from flavin to porphyrin with unit efficiency at high solvent polarity. In solvents of medium polarity an additional internal conversion pathway is opened following excitation of the flavin moiety. Spectroscopic, cyclovoltammetric, and semiempirical results all suggest that this pathway involves the inter mediate population of a short-lived charge-separated state.