Mechanisms of photodecarboxylations for alkane synthesis: a theoretical study

Abstract Photochemical reactions are crucial in pharmaceutical and high-level chemical production. Ruthenium polypyridine complexes (e.g. [Ru(bpy)3]2+) have garnered significant attention as photoredox catalysts in photodecarboxylations, particularly when using phthalimide-based esters as precursors. This study investigates photodecarboxylation pathways for alkane synthesis using [Ru(bpy)3]2+, by examining the photophysical, photoredox and electrochemical properties of the elementary reactions via the density functional theory (DFT) and time-dependent DFT methods. For the first time, photoinduction was shown to correlate with the Bailar twist angle and its distortion in the complexes, and the S0→S1 vertical excitation promotes the S1→T1 intersystem crossing and reduction of [Ru(bpy)3]2+. These findings confirm that the photoredox catalytic cycle is thermodynamically more favourable than the redox cycle in the S0 state, and additional UVA irradiation could enhance the photoredox catalytic cycle. Redox potential energy surfaces revealed that the Bailar twist torque/resisting forces are key driving factors behind the photoinduced processes. In the S0 state, the rigid structure of [Ru(bpy)3]2+ generates strong Bailar twist resisting forces, which hinder efficient metal-to-ligand charge transfer and single-electron transfer. Theoretical results validated the hypothesized photodecarboxylation pathways. The theoretical strategies, methods and findings presented in this study can serve as guidelines for future theoretical and experimental studies aimed at optimizing or designing photodecarboxylation processes for efficient alkane synthesis.