Long-Lived Charge-Separated States in Self-Assembled TiO₂ Photoanodes Incorporating a Spin-Transition Cobalt Complex

Improving the efficiency of photoelectrocatalytic cells relies on precise control of interfacial electron transfer rates to favor the generation of long-lived charge-separated states. Achieving efficient forward electron transfer while suppressing charge recombination remains a central challenge. In this study, we investigate a cobalt-based transition metal complex that undergoes charge transfer-induced spin crossover (CTISC) as a strategy to modulate interfacial charge dynamics in dye-sensitized photoelectrochemical architectures. Ultrafast and nanosecond transient spectroscopy were used to quantify electron injection and dye regeneration. Open-circuit voltage decay measurements were employed to assess the lifetimes of injected electrons under operating conditions. Density functional theory (DFT) calculations were used to estimate the inner-sphere and outer-sphere reorganization energies associated with the redox processes. The results demonstrate that the large inner-sphere reorganization energy associated with spin-state change significantly prolongs charge-separated lifetimes. These findings highlight the potential of spin-state-mediated reorganization as a design principle for suppressing charge recombination and improving the performance of dye-sensitized photoelectrochemical systems.