Sensing mechanism of fluorescent sensor to Cu2+ based on inhibiting ultra-fast intramolecular proton transfer process

Abstract

A novel and efficient chemosensor 1 for detecting Cu2+ has recently been developed. However, the photophysical properties of chemosensor 1 and its response mechanism to Cu2+ are still unclear. Herein, the density functional theory and the time-dependent density functional theory approaches are implemented to investigate the excited state behavior of chemosensor 1 and its sensing mechanism for Cu2+ is revealed. Through constructing the potential energy curve with the dihedral angle of hydroxide radical as a variable, the irreversibility of the adjustment of the hydrogen proton direction is determined. This feature provides a favorable geometric configuration condition for the formation of intramolecular hydrogen bond. Moreover, the reduced density gradient analysis and topological analysis are performed to visualize the hydrogen bond strength, it is found that the hydrogen bond is enhanced in first singlet excited state (S1) compared with that in ground state (S0). The chemosensor 1 has only a low potential barrier in the S1 state, indicating that it could undergo an ultra-fast excited state intramolecular proton transfer (ESIPT) process. Furthermore, the reaction sites of chemosensor 1 and Cu2+ is theoretically predicted by the electrostatic potential analysis and the coordination mode of 1 + Cu2+-H+ is confirmed. Thus, we verify that the deprotonation inhibits the ESIPT behavior and leads to fluorescence quenching to achieve the recognition of chemosensor 1 to Cu2+. In addition, the binding energy of Cu2+ with chemosensor 1 is greater than that of Mg2+ and Zn2+, the high selectivity of chemosensor 1 to Cu2+ is illustrated. Our investigation clarifies the sensing mechanism of chemosensor 1 to Cu2+ based on inhibiting ultra-fast ESIPT process, which provides a theoretical basis for the development of new metal ion sensors.