Large Reorganization Energy of Pyrrolidine-Substituted Perylenediimide in Electron Transfer

Abstract

Excited-state dynamics of an electron-donating, pyrrolidine-substituted perylenediimide-C60 linked dyad have been investigated by means of time-resolved transient absorption spectroscopy and fluorescence lifetime measurements. By the picosecond transient absorption measurements at a selective excitation of the perylenediimide moiety, a charge-separated state has been successfully detected in polar solvents (i.e., benzonitrile, pyridine, and o-dichlorobenzene), demonstrating the occurrence of photoinduced electron transfer from the perylenediimide to the C60 moiety. In contrast, in nonpolar solvents (i.e., toluene), singlet−singlet energy transfer takes place from the perylenediimide to the C60, followed by intersystem crossing to the C60 excited triplet state and subsequent triplet−triplet energy transfer to yield the perylenediimide excited triplet state. Rate constants of the charge recombination in the polar solvents are found to be comparable to or even larger than those of the charge separation, which is in sharp contrast with electron transfer behavior in typical donor-C60 linked systems. A reorganization energy (0.86 eV) of the perylenediimide-C60 linked dyad obtained in the polar solvents is significantly larger than those of similar porphyrin-C60 linked dyads (0.51−0.66 eV) in which both have comparable edge-to-edge distances between donor and acceptor. The large reorganization energy of the perylenediimide-C60 linked dyad relative to the porphyrin-C60 linked dyads results from a relatively large conformational change in the pyrrolidine groups at the perylenediimide moiety accompanied by one-electron oxidation. This agrees with the fact that charge recombination to the ground state rather than the excited triplet state of the perylenediimide moiety is predominant in benzonitrile, irrespective of the lower energy level of the excited triplet state than that of the charge-separated state