Efficient Intersystem Crossing in the Tröger's Base Derived From 4‐Amino‐1,8‐naphthalimide and Application as a Potent Photodynamic Therapy Reagent

Intersystem crossing (ISC) was observed for naphthalimide (NI)‐derived Tröger's base, and the ISC was confirmed to occur by a spin‐orbital charge‐transfer (SOCT) mechanism. Conventional electron donor/acceptor dyads showing SOCT‐ISC have semirigid linkers. In contrast, the linker between the two chromophores in Tröger's base is rigid and torsion is completely inhibited, which is beneficial for efficient SOCT‐ISC. Femtosecond transient absorption (TA) spectra demonstrated charge‐separation and charge‐recombination‐induced ISC processes. Nanosecond TA spectroscopy confirmed the ISC, and the triplet state is long‐lived (46 μs, room temperature). The ISC quantum yield is dependent on solvent polarity (8–41 %). The triplet state was studied by pulsed‐laser‐excited time‐resolved EPR spectroscopy, and both the NI‐localized triplet state and triplet charge‐transfer state were observed, which is in good agreement with the spin‐density analysis. The Tröger's base was confirmed to be a potent photodynamic therapy reagent with HeLa cells (EC50=5.0 nm). Tröger's base as photosensitizer: Efficient intersystem crossing (ISC) was discovered in naphthalimide‐derived Tröger's base NI‐TB, which provides a novel molecular structural motif for attaining efficient spin–orbital charge transfer ISC (SOCT‐ISC). The fully rigid, orthogonal structure is beneficial for efficient SOCT‐ISC. The ISC mechanism was confirmed by time‐resolved (TR) EPR spectroscopy. NI‐TB is a potent photodynamic therapy reagent towards HeLa cells (EC50=5.0 nm)