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. [ABSTRACT FROM AUTHOR]