Investigations on the Multiple Excited State Intramolecular Proton Transfer Processes and Twisted Intramolecular Charge Transfer/Twisted Intramolecular Charge Shuttle States of a Fluorescence Sensor Based on Schiff Base

Excited state intramolecular proton transfer (ESIPT), twisted intramolecular charge transfer (TICT), and twisted intramolecular charge shuttle (TICS) play fundamental roles in the photophysical process of dyes and sensors. In this paper, four ESIPT processes, two TICT states, and two TICS states are observed on the excited state potential energy surface of a turn-on sensor based on Schiff base with the aid of DFT and TDDFT. All these TICT/TICS states are dark states and responsible for the weak fluorescence of the sensor. Interestingly, these TICT and TICS states are generated under intrinsically different mechanisms. The isomerization of the C═N bond of the Schiff base leads to two TICS states, while the isomerization of the adjacent C–C bond leads to two TICT states. Transition states, energy barriers, and rate constants for all of these dynamic processes are obtained to clarify their relationship and evaluate their chances of happening. It is demonstrated that among the four dark states, the TICT-1 state is the global minimum with the lowest energy barrier and highest reaction rate. This state is the major factor that induces fluorescence quenching of the sensor. Besides, the Zn²⁺ sensing mechanism is clarified after getting a clear picture of the photophysical process.