Efficient Intersystem Crossing in Tröger's Base Derived From 4‐Amino‐1,8‐Naphthalimide and Application as Potent PDT Reagent

Intersystem crossing (ISC) was observed for naphthalimide (NI)-derived Troger'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 Troger'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 mu s, 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 Troger's base was confirmed to be a potent photodynamic therapy reagent with HeLa cells (EC50=5.0 nm).