Mechanistic Insights into Photo-Induced Bond Dissociation of A NPIM Derivative: A Case Study on Quantum Spin-Manipulation for Efficient Uncaging

Le résumé fourni par la source

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.

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