Photoinduced Electron Transfer in a Charge-Transfer Complex Formed between Corannulene and Li+@C60 by Concave-Convex π-π Interactions

A charge-transfer (CT) complex was formed between corannulene (C20H10) and lithium ion-encapsulated [60]fullerene (Li+@C-60) with the binding constant K-G = 1.9 x 10 M-1 by concave-convex pi-pi CT interactions in benzonitrile at 298 K, exhibiting a broad CT absorption extended to the NIR region. Femotosecond laser excitation of the C20H10/Li+@C-60 CT complex resulted in the singlet charge-separated (CS) state, (1)(C20H10 center dot+/Li+@C-60(center dot-)), which decayed with the lifetime of 1.4 ns. Nanosecond laser excitation of Li+@C-60 resulted in intermolecular electron transfer (ET) from C20H10 to the triplet excited state of Li+@C-60 [(3)(Li+@C-60)*] to produce the triplet CS state (3)(C20H10 center dot+/Li+@C-60(center dot-)). The distance between two electron spins in the triplet CS state was estimated to be 10 angstrom from the zero-field splitting pattern observed by EPR measurements at 4 K The triplet CS state decayed to the ground state via intramolecular back electron transfer (BET). The CS lifetime was determined to be 240 ps in benzonitrile at 298 K. The temperature dependence of the rate constant of BET afforded the reorganization energy (lambda = 1.04 eV) and the electronic coupling term (V = 0.0080 cm(-1)). The long lifetime of triplet CS state results from the spin-forbidden BET process and a small V value.