Study of excited-state relaxation mechanisms for two 1,8-naphthalimide-based derivatives detecting of hydrogen sulfide.

At present, theoretical calculations are widely used to explain the intrinsic mechanisms of chemical reactions. In this study, the sensing mechanisms of two 1,8-naphthalimide-based derivatives possessing long-chain substituents at the N-position have been theoretically investigated by density functional theory and time-dependent density functional theory. The cause of the fluorescence quenching arises from the strong coupling interaction between the charge transfer (CT) state and the locally excited (LE) state with similar energies. Upon photoexcitation, the probes preferentially convert to a structure possessing a distorted 1,8-naphthalimide fluorophore in the excited state via the minimum energy conical intersection (MECI) point between the LE and CT states and subsequently return to the ground state accompanied by no fluorescence. Through the analysis of the constructed linear interpolating internal coordinate pathway, we found that it is easy to reach the MECI point from the Franck-Condon point, and the sensing mechanism we obtained is consistent with the experimental phenomena. We hope that the mechanistic explorations in this paper will inspire the design of novel fluorescent probes in the future.