Synthesis, spectroscopy, and semiempirical study of a novel porphyrin-flavin dyad

A new synthetic methodology for the preparation of donor-bridge-acceptor co mpounds incorporating a porphyrin donor and an all-aromatic oligo(diazaphen ylene) bridge is introduced. This approach allows the controlled preparatio n 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 interest ing 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 excita tion. Based on these premises, a spectroscopic, photophysical and semiempir ical study of the dyad la has been performed in three solvents of varying p olarity. The results demonstrate energy transfer from flavin to porphyrin w ith unit efficiency at high solvent polarity. In solvents of medium polarit y an additional internal conversion pathway is opened Following excitation of the flavin moiety. Spectroscopic, cyclovoltammetric, and semiempirical r esults all suggest that this pathway involves the intermediate population o f a short-lived charge-separated state.