Excited-State Charge Transfer and Photocatalytic Hydrogen Evolution Mediated by CdSe Quantum Dot-Ferrocene Dyads

We studied two aspects of the photoinduced charge-transfer reactivity of donor–acceptor dyads consisting of CdSe quantum dots (QDs) functionalized with 6-(ferrocenyl)hexanethiolate (FcC6S–). First, we used steady-state and time-resolved emission measurements to characterize the dynamics and mechanisms of photoinduced hole transfer from CdSe to FcC6S–. Within mixed dispersions of CdSe QDs and 6-(ferrocenyl)hexanethiol (FcC6SH), both band-edge and trap-state emission from CdSe were quenched dynamically. At the highest concentrations of FcC6SH, band-edge emission was quenched by 97.5%, and trap-state emission was quenched by 89.5%. Lifetimes of band-edge and trap-state emission decreased by 7- and 3-fold, respectively. Dynamic quenching of emission reveals that holes were transferred from both the valence band-edge and trap states of CdSe QDs to adsorbed FcC6S–. Rate constants of hole transfer were on the order of 107–108 s–1. Second, we characterized redox reactions initiated by CdSe-to-FcC6S– hole transfer. Photoelectrochemical experiments revealed that CdSe-FcC6S– dyads promoted the reduction of H+ to H2, or the hydrogen evolution reaction (HER), under white-light illumination. CdSe-FcC6S– dyads with low loadings of FcC6S– promoted oxidative photocurrents, wherein lactic acid was oxidized by the dyads and H+ was reduced to H2 at a platinum counter electrode (CE). CdSe-FcC6S– dyads with the highest amounts of FcC6S– exhibited enhanced photostability and promoted the light-initiated HER via two distinct mechanisms. At relatively positive applied potentials, lactic acid was oxidized by CdSe-FcC6S– dyads while H+ was reduced at the CE. At more negative potentials, CdSe-FcC6S– dyads reduced H+ to H2 while lactic acid was oxidized at the CE. Finally, in photochemical experiments, CdSe-FcC6S– dyads promoted HER in the absence of an applied potential or external circuit. Our results provide insights into the mechanisms of light-initiated hole transfer from QDs to ferrocene derivatives and reveal that photoinduced charge separation can enable photocatalytic HER, a fuel-forming reduction half-reaction.