Enhancing photoredox catalysis by suppressing back electron transfer with the aid of a spin catalyst.

While organic dye-based photoredox catalysis provides a sustainable platform for inert bond activation, its efficiency remains limited by detrimental back electron transfer (BET) processes. Herein, we present a spin catalysis strategy that addresses this challenge by manipulating the spin kinetics of radical ion pairs (RIPs) using Gd-DOTA as a spin catalyst. In photocatalytic hydrodechlorination of methyl 4-chlorobenzoate, this approach achieved a remarkable spin catalysis effect (SCE) of 70%, accompanied by a 25-fold acceleration in reaction kinetics (65% conversion in 25 min 640 min without spin catalyst). The generality of SCE was demonstrated across diverse substrates spanning varied functional groups and halides (Cl/Br/I). Through integrated time-resolved spectroscopic measurements and density functional theory calculations, we established a quantitative kinetic model revealing that the Gd(iii) center promotes spin conversion of RIPs from singlet to triplet states, thereby effectively suppressing BET to enhance forward reaction flux. This work pioneers the integration of spin catalysis strategy into photoredox systems, offering both a mechanistic framework for spin-state manipulation in reaction engineering and a transformative kinetic approach to boost catalytic efficiency beyond current thermodynamic consideration solely based on redox properties.