Electron transfer in a supramolecular complex of zinc chlorin carboxylate anion with Li⁺@C₆₀ affording the long-lived charge-separated state

A supramolecular complex was formed between zinc chlorin carboxylate (ZnCh^-) and lithium ion-encapsulated [60]fullerene (Li ⁺@C₆₀) by an electrostatic interaction in benzonitrile (PhCN). The binding constant was determined to be 7.7 × 10⁴ M^-1. No fluorescence quenching of ZnCh^- was observed upon addition of Li⁺@C₆₀, indicating that no electron transfer (ET) from the singlet excited state of ZnCh^- (¹[ZnCh ^-]*) to Li⁺@C₆₀ occurred. In contrast, the transient absorption band due to triplet excited state of ZnCh^- (³[ZnCh^-]*) was efficiently quenched by ET from ³[ZnCh^-]* to Li⁺@C₆₀ to produce the charge-separated (CS) state, [ZnCh^-]^•+/ Li⁺@C₆₀^•-, with the rate constant of k_ET = 5.3 × 10⁴ s^-1. The charge-recombination dynamics was monitored by the decay of the transient absorption band at 1035 nm due to Li⁺@C₆₀ ^•-. The lifetime of the CS state was determined to be 170 μs. The spin state of CS state was triplet determined by EPR measurements at low temperature. The reorganization energy (λ) and electronic coupling term (V) of ET and back electron transfer (BET) were determined from the temperature dependence of k_ET and k_BET to be λ = 0.46 ± 0.02 eV and V = 0.095 ± 0.030 cm^-1 for ET and λ = 1.26 ± 0.04 eV and V = 0.066 ± 0.010 cm^-1 for BET based on the Marcus theory of nonadiabatic electron transfer. Such small V values result from the small orbital interaction between ZnCh^- and Li ⁺@C₆₀ moieties to afford the long-lived CS state.