Dynamics of photoinduced electron transfer in a carotenoid - porphyrin - dinitronaphthalenedicarboximide molecular triad

An electron acceptor moiety based on the 4,5-dinitro-1,8-naphthalenedicarboximide system has been prepared and used as the basis for synthesis of a porphyrin - imide dyad (P-NIm) and a carotenoid - porphyrin - imide triad molecule (C-P-NIm). Excitation of the porphyrin moiety in either compound with visible light leads to rapid photoinduced electron transfer to generate in high yield a charge-separated state consisting of the porphyrin radical cation and imide radical anion. In the triad, this C-P_•+-NIm^•- state decays in part by a second electron transfer from the carotenoid to yield a final C^•+-P-NIm^•- charge-separated state. In benzonitrile, this state is formed with a quantum yield of 0.33 and has a lifetime of 430 ns. The 4,5-dinitro1,8-naphthalenedicarboximide moiety is conveniently synthesized and undergoes facile and reversible one-electron reduction. The NIm^•- ion has a readily observable spectroscopic signature in the visible. In contrast to a series of closely related triads reported by other investigators, the triad studied here shows no evidence for photoinduced electron transfer from the carotenoid first excited singlet state.