Photoinduced Electron and Proton Transfer in a Molecular Triad

Abstract

A series of molecular triads, consisting of a porphyrin (P) covalently linked to a carotenoid polyene (C) and a naphthoquinone moiety (Q), have been prepared. Triad 1 features a quinone with an internally hydrogen-bonded carboxylic acid group. The photochemical properties of these molecules have been studied using steady-state and transient absorption and emission spectroscopies in three solvents: benzonitrile, dichloromethane, and chloroform. Each of the triads undergoes photoinduced electron transfer from the C-1P-Q singlet state to yield the charge-separated state C-P·+-Q·-. An electron transfer reaction from C to yield C·+-P-Q·- competes with fast electron-hole recombination in C-P·+-Q·-. Triad 1 produces the final C·+-P-Q·- state with the highest quantum yield of the series ( Φ ≥ 0.22), a factor of ca. 2 higher than for reference triads. Following the initial photoinduced electron transfer, a fast (k ~ 1012 s-1) proton shift from the carboxylic acid to Q·- generates the semiquinone, increasing the lifetime of P·+ and the yield of electron donation by C. In model P-Q dyads, the species P·+ is shown to be longer lived for quinones that feature an internal hydrogen bond. A thermodynamic model is proposed in which the increase in the lifetime of the P·+ moiety by the proton shift is attributed to the δ pK of the Q/Q·- couple, which dramatically lowers the driving force for electron-hole recombination.