Spin Manipulation Effect in Photodeprotection Reaction Using Triplet Ground-State Cation

Photolabile protecting groups (PPGs), which temporarily mask functional groups (LGs) and regenerate their activity upon light irradiation of PPG-LGs under reagent-free neutral conditions, have been widely utilized in diverse research fields ranging from organic synthesis to neuroscience. However, under conditions where diffusion of the ion pair consisting of PPG+ and LG- is unfavorable, the recombination reaction becomes dominant, making efficient photodeprotection difficult. If triplet ground-state cations could be generated, the recombination process would become spin-forbidden, thereby enhancing photodeprotection efficiency. As a proof of concept, we newly designed a PPG capable of generating the triplet ground-state cation 2-(4-methoxyphenyl)-1H-inden-1-ylium cation (T-PMI+). The generation process and dynamics of T-PMI+ were investigated by ultrafast transient absorption spectroscopy, while its reactivity was elucidated through chemical trapping experiments by benzyl bromide, molecular oxygen, and ferrocene. This study demonstrates spin manipulation as a new strategy for boosting the efficiency of photodeprotection reactions. Notably, a remarkably high deprotection selectivity of 92% was achieved.