DYNAMICS OF PHOTOINDUCED ELECTRON-TRANSFER IN A ROTENOID-PORPHYRIN-DINITRONAPHTHALENEDICARBOXIMIDE MOLECULAR TRIAD

An electron acceptor moiety based on the 4,5-dinitro-1,8-naphthalenedi carboximide system has been prepared and used as the basis for synthes is 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 tran sfer to generate in high yield a charge-separated state consisting of the porphyrin radical cation and imide radical anion. In the triad, th is C-P.+-NIm(.-) state decays in part by a second electron transfer fr om the carotenoid to yield a final C.+-P-NIm(.-) charge-separated stat e. In benzonitrile, this stale is formed with a quantum yield of 0.33 and has a lifetime of 430 ns. The 4,5-dinitro-1,8-naphthalenedicarboxi mide moiety is conveniently synthesized and undergoes facile and rever sible one-electron reduction. The NIm(.-) ion has a readily observable spectroscopic signature in the visible. In contrast to a series of cl osely related triads reported by other investigators, the triad studie d here shows no evidence for photoinduced electron transfer from the c arotenoid first excited singlet state.