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-dinitro-1,8-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.