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Mechanistic Insights into Photo-Induced Bond Dissociation of a 2-(4-Nitrophenyl)-1H-indole-3-ylmethyl (NPIM) Derivative: A Case Study on Quantum Spin-Manipulation for Uncaging

Year: 2026

Type: article

Abstract: Photolabile protecting groups (PPGs) are indispensable tools for spatiotemporally controlled release of reactive functional groups. Recently, we proposed a quantum spin-manipulation (QSM-1) strategy that employs triplet ground-state cations to render ion-pair recombination spin-forbidden, thereby enhancing the uncaging quantum yield. In this study, the photodissociation mechanism of a 2-(4-nitrophenyl)-1H-indole-3-ylmethyl (NPIM) photocage bearing a benzoate leaving group was investigated to evaluate the effectiveness of a triplet excited-state-mediated QSM pathway (QSM-2). Spectroscopic analyses revealed that the excited state of NPIM possesses a relatively small singlet-triplet energy gap (ΔEST = 5.3 kcal mol-1), enabling efficient intersystem crossing (ISC) from the intramolecular charge-transfer singlet excited state to the triplet excited state. Oxygen-quenching experiments demonstrated that photodissociation proceeds predominantly from the triplet excited state. Time-resolved absorption spectroscopy identified the singlet excited state (τ = 1.6 ns), triplet excited state (τ = 2.3 μs), and NPIM radical intermediate (τ = 1.5 ms). In situ infrared analysis directly detected benzoic acid, an oxidized NPIM product, and CO2 generated from the benzoyl radical, supporting a homolytic bond-cleavage mechanism. Ultrafast time-resolved infrared spectroscopy of the NPIM chromophore revealed formation of a twisted intramolecular charge-transfer state that promotes ISC by reducing ΔEST. Density functional theory calculations further indicate that C-O bond homolysis from the triplet excited state is energetically feasible and is driven by relief of excited-state antiaromaticity. These results establish that uncaging of NPIM derivatives proceeds through triplet-state homolysis and demonstrate the validity of the QSM-2 strategy for designing efficient photochemical release systems.

Source: Journal of the American Chemical Society

DOI: 10.1021/jacs.6c13942