Lewis Acid-Base Control of Excited-State Reactivity in an Iron(III) Photosensitizer

The use of iron-based photosensitizers in light-mediated electron transfer chemistry remains limited by their inherently short excited-state lifetimes. Here, we report the design, synthesis, and photophysical characterization of a novel iron(III) photosensitizer, [Fe(L^PhBMes)₂]⁺, in which the tridentate NHC ligand framework incorporates a Lewis acidic dimesityl boron moiety. Steady-state and time-resolved spectroscopy reveal that [Fe(L^PhBMes)₂]⁺ exhibits photophysical properties nearly identical to the widely studied [Fe(L^Ph)₂]⁺, yet demonstrates strikingly different reactivity upon binding small, strongly Lewis basic anions such as fluoride or hydroxide. Anion binding induces boronate formation, which subsequently triggers fast intramolecular electron transfer, leading to a reactive charge-separated species formed via irreversible bond cleavage. In-depth photostability measurements shed light on anion-concentration dependent degradation pathways. These findings represent the first integration of Lewis acidic triarylborane substituents into an iron(III)-based photosensitizer, establishing a platform for tuning excited-state dynamics together with exploiting static quenching strategies to bypass diffusion-limited electron transfer. This approach to selectively trigger excited-state charge separation and externally stimulated bond cleavage represents a promising avenue in targeted phototherapeutics application and drug release. Both strategies are potentially interesting to make iron-based photosensitizers more accessible for light-driven applications in the future.