Supramolecular tetrad of subphthalocyanine-triphenylamine-zinc porphyrin coordinated to fullerene as an "antenna-reaction-center" mimic: formation of a long-lived charge-separated state in nonpolar solvent

We report here the formation of a long-lived charge-separated state of a self-assembled donor-acceptor tetrad, formed by axial coordination of a fulleropyrrolidine appended with an imidazole coordinating ligand (C ₆₀Im) to the zinc center of a subphthalocyanine-triphenylamine-zinc porphyrin (SubPc-TPA-ZnP), as a charge-stabilizing antenna reaction center mimic in toluene. The subphthalocyanine and triphenylamine entities, with their high-energy singlet states, act as an energy-transferring antenna unit to produce a singlet zinc porphyrin. The formation constant for the self-assembled tetrad was determined to be 1.0×10 ⁴M ^-1, suggesting a moderately stable complex-formation. The geometric and electronic structures of the covalently linked SubPc-TPA-ZnP triad and self-assembled SubPc-TPA-ZnP:C ₆₀Im tetrad were examined by using an ab initio B3LYP/6-31G method. The majority of the highest occupied frontier molecular orbital was found over the ZnP and TPA entities, whereas the lowest unoccupied molecular orbital was located over the fullerene entity, suggesting the formation of the radical-ion pair (SubPc-TPA-ZnP ^·+:C ₆₀Im ^·-). The redox measurements revealed that the energy level of the radical-ion pair in toluene is located lower than that of the singlet and triplet states of the zinc porphyrin and fullerene entities. The femtosecond transient absorption measurements revealed fast charge separation from the singlet porphyrin to the coordinated C ₆₀Im with a life-time of 1.1 ns. Interestingly, slow charge recombination (1.6 × 10 ⁵ s ^-1) and the long lifetime of the charge-separated state (6.6 μs) were obtained in toluene by utilizing the nanosecond transient measurements.