Efficient electron transfer processes of the covalently linked perylenediimide-ferrocene systems: Femtosecond and nanosecond transient absorption studies

Abstract

Photoinduced electron-transfer processes of the newly synthesized rodlike covalent donor-acceptor molecules consisting of electron-donating ferrocenes (Fc) with electron-accepting perylenediimides (PDIs) with core-substituted cyano and pyrrolidine groups, forming Fc-PDI(CN) 2 dyad, Fc 2-PDI(CN) 2 triad, and Fc-PDI(Py) 2 dyad, have been investigated in benzonitrile. The geometric and electronic structures of the dyads and triad were probed by ab initio B3LYP/6-311G methods. The distribution of the highest occupied molecular orbitals (HOMOs) was on the ferrocene entities, while the distribution of the lowest unoccupied molecular orbitals (LUMOs) was on the PDI entities. Free-energy calculations verify that the light-induced processes from excited states of PDIs are exothermic. The excited state photochemical events are monitored by femtosecond and nanosecond transient absorption techniques. In benzonitrile, the quenching pathway involves fast and efficient charge separation from the ferrocenes to the excited PDIs. The finding that the lifetime of the Fc 2 +-[PDI(CN) 2] •- triad (59 ps) was found to be longer than that of the Fc +-[PDI(CN) 2] •- dyad (25 ps) in benzonitrile reflects the effect of the second ferrocene entity in stabilizing the radical ion pairs in the triad. In addition, photoinduced electron transfer in the Fc-PDI(Py) 2 dyad occurs via the drastic quenching of singlet state of PDI(Py) 2, resulting in the enhancement of triplet state of PDI(Py) 2 due to charge recombination of the radical-ion pairs. © 2010 American Chemical Society.