Photoinduced electron transfer in a charge-transfer complex formed between corannulene and Li⁺@C₆₀by concave-convex π-π interactions

A charge-transfer (CT) complex was formed between corannulene (C₂₀H₁₀) and lithium ion-encapsulated [60]fullerene (Li⁺@C₆₀) with the binding constant K_G = 1.9 × 10 M^-1 by concave-convex π-π CT interactions in benzonitrile at 298 K, exhibiting a broad CT absorption extended to the NIR region. Femotosecond laser excitation of the C₂₀H₁₀/Li⁺@C₆₀ CT complex resulted in the singlet chargeseparated (CS) state, 1(C₂₀H₁₀^•+/Li⁺@C₆₀^•-), which decayed with the lifetime of 1.4 ns. Nanosecond laser excitation of Li⁺@C₆₀ resulted in intermolecular electron transfer (ET) from C₂₀H₁₀ to the triplet excited state of Li⁺@C₆₀ [³(Li⁺@C₆₀)∗] to produce the triplet CS state ³(C₂₀H₁₀^•+/Li⁺@C₆₀^•-). The distance between two electron spins in the triplet CS state was estimated to be 10 Å 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 μs in benzonitrile at 298 K. The temperature dependence of the rate constant of BET afforded the reorganization energy (λ = 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.