Efficient synthesis of porphyrin-iridium complex for enhanced cocatalyst-free photocatalytic hydrogen evolution

Abstract

A new porphyrin–iridium complex, T-Ir-ZnPF, is synthesized facilely through a one-pot, nucleophilic substitution reaction between the iridium (Ir) complex Ir-NH2 and zinc(II)-tetrakis(pentafluorophenyl)porphyrin ZnPF. In this porphyrin, the Ir-motif acts as a triplet energy donor, while the porphyrin moiety serves as a singlet energy acceptor. An efficient Förster resonance energy transfer from the Ir-motif to the porphyrin moiety enables exceptional light-harvesting capabilities in the broad ultraviolet–visible region, a longer photoexcited state electron lifetime, and a higher photoluminescent quantum yield for T-Ir-ZnPF compared to ZnPF without the Ir-motif. Moreover, T-Ir-ZnPF exhibits inhibition of aggregation-caused quenching, resulting in suppressed nonradiative decay channels and consequently long-lived photoexcited states. The cocatalyst-free homogeneous photocatalytic hydrogen evolution (PHE) system of T-Ir-ZnPF produces a PHE rate (ηH2) of 5.34 mmol g⁻¹ h⁻¹. Under the same photocatalytic conditions, ZnPF did not produce hydrogen, while Ir-NH2 delivered a very low ηH2 of 0.20 mmol g⁻¹ h⁻¹. Since the Stern–Volmer quenching constant of T-Ir-ZnPF is higher than those of ZnPF and Ir-NH2, the photoexcited reduced T-Ir-ZnPF species are formed more readily by gaining electrons from triethylamine. Subsequently, a direct and fast electron transfer from the reduced T-Ir-ZnPF to protons leads to a high cocatalyst-free PHE.