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 10⁷–10⁸s^–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 H₂at 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 H₂while 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.