Mechanistic Study of Initial Quenching Process in Photocatalytic α-Keto Acids Radical Acylation: A General Strategy for Enhancing Quantum Efficiency^†

The photocatalytic decarboxylation of α-keto acids to generate acyl radicals under mild conditions represents a novel strategy in organic synthesis. However, the quantum efficiency of this process has been underexplored, limiting its practicality. To improve quantum efficiency, detailed analysis of mechanisms and kinetic data for key steps are essential. In this work, using time-resolved emission and absorption spectroscopy, we conducted a mechanistic study focusing on the excited-state properties of representative photocatalysts and their quenching efficiencies during the initial quenching process (Ir(dFCF₃ppy)₂(dtbbpy)⁺ (IrIII), Eosin Y (EY), Rose Bengal (RB), and 4CzPN). Our findings revealed that RB is active in its triplet states (³RBH*), with lifetimes of 103 ns (in air) and 3.4 μs (in anaerobic conditions), while EY and 4CzPN are active in their singlet states (¹EYH* and ¹4CzPN*), with lifetimes of 2.9 ns and 5.1 ns, respectively. We measured the second-order rate constants for quenching by electron transfer from α-keto acids: ¹EYH*, 2.3 × 10⁹ (mol/L)⁻¹·s⁻¹; ³RBH*, 3.2 × 10⁸ (mol/L)⁻¹·s⁻¹; ¹4CzPN*, 2.8 × 10⁸ (mol/L)⁻¹·s⁻¹. With our previously reported data for IrIII, we established the quenching efficiency relationships for these photocatalysts with α-keto acids concentration. Our steady-state chromatography experiments determined the quantum efficiencies for consumption of α-keto acids (IrIII > RBH > EYH > 4CzPN), correlating these efficiencies with the initial quenching process. The results suggest that IrIII/RBH under anaerobic conditions could be optimal for high quantum efficiency. This study provides a foundation for designing new photocatalytic α-keto acid radical acylation systems with enhanced quantum efficiency.