Flavin Takes Charge: CdS Nanorods Drive Light-To-Dark Electron Flow

Flavin cofactors are central mediators of biological redox chemistry, yet their interactions with abiotic photocatalysts remain poorly understood. Here, we show that colloidal CdS nanorods (NRs) drive efficient photoreduction of riboflavin (Rf) and flavin mononucleotide (FMN) while suppressing competing photodegradation pathways. Oxidation-triggered fluorescence recovery enables selective tracking of photoreduction, while electron paramagnetic resonance spectroscopy identifies the flavin semiquinone as the primary photoproduct. Steady state and time-resolved spectroscopy show contributions from both charge transfer (CT) and energy transfer (EnT) pathways. Within a semiquantitative analysis of the spectrally resolved CdS photoluminescence quenching responses, CT-associated quenching remains predominant for both flavins, while FMN additionally exhibits a smaller contribution consistent with EnT significant at low concentrations. Flavin addition shortens the CdS excited state lifetime while lengthening the flavin emission decay, consistent with donor-acceptor EnT. Quantum chemical calculations yield a Rf-CdS separation of 8.75 Å and 13.85 Å for FMN in the sampled free configurations, whereas a phosphate-directed bound FMN model consistent with its saturating quenching behavior yielded a distance of 5.98 Å. Despite stronger excited state quenching, FMN exhibits lower photoreduction yields and reduced stability. In contrast, Rf achieves greater accumulation of reduced species by mitigating these losses. Reduced flavins generated by CdS NRs further drive benzoquinone reduction in the dark, establishing a coupled light-driven and dark redox cycle. Together, these results reveal that strong surface interactions and excited state quenching do not necessarily maximize productive redox chemistry and establish design principles for directing semiconductor-driven redox chemistry in bioinspired hybrid systems.