Unravelling the Competition between Internal Conversion and Intersystem Crossing in Twisted molecule 9-Phenylacridine by Femtosecond Time-resolved Spectroscopy

9-Phenylacridine (9-PA) is an important acridine-based medicine that has been proven to possess significant anticancer activity and can be used as a photodynamic therapy (PDT) agent. Meanwhile, the possible twisting of the C−C single bond at the C9 position after photo-excitation makes it a potential probe responsive to changes in the viscosity of living cells. However, the photophysical properties of 9-PA is poorly understood. In this study, we utilized femtosecond time-resolved spectroscopy combined with quantum chemical calculation methods to investigate the excited state dynamics of 9-PA in solutions with different viscosities. Notably, we demonstrated that the viscosity could strongly influence the deactivation pathway of the initially populated S₁ (ππ*) state of 9-PA. In low-viscosity solutions, the single bond at the C9 could twist after photo-excitation, leading to a conformation that shows efficient intersystem crossing. However, such process is suppressed in high-viscosity solutions, resulting a ~2.5 times higher internal conversion (IC) yield. A full picture of the excited state deactivation mechanism of 9-PA is proposed.