Mechanistic Study of Initial Quenching Process in Photocatalytic α-Keto Acids Radical Acylation:A General Strategy for Enhancing Quantum Efficiency
The photocatalytic decarboxy-lation of α-keto acids to gener-ate acyl radicals under mild conditions represents a novel strategy in organic synthesis.However,the quantum effi-ciency of this process has been underexplored,limiting its practicality.To improve quan-tum efficiency,detailed analy-sis of mechanisms and kinetic data for key steps are essential.In this work,using time-re-solved 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 concentra-tion.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.
Chemical kinetics and dynamicsTime-resolved spectroscopyElectron transfer quenchingPhotocatalytic reaction
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