Theoretical investigations on the excited-state intramolecular proton transfer in the solvated 2-hydroxy-1-naphthaldehyde carbohydrazone

The vast applications of 2-hydroxy-1-naphthaldehyde-derived systems in the sensors originate from their unusual excited-state intramolecular proton transfer (ESIPT) fluorescence in the molecules. The mechanism of ESIPT fluorescence in the solvated 2-hydroxy-1-naphthaldehyde carbohydrazone (HNLSC) system was investigated by time-dependent density functional theory (TDDFT) calculation. The solvation stabilized both ground state and excited state in the enol form, and the medium intramolecular interaction ensured the bond break, bond-forming and proton transfer in the conversion from excited enol form to keto form. ESIPT reaction from the enol form to the keto form had a low barrier of 2.54 kcal mol in the cyclohexane solvation, and all the calculated emission was consistent with the experimental findings. Moreover, the disaggregation of excited enol form was favoured instead of the IPT conversion to keto form, vanishing the specific ESIPT pathway in the protic surroundings. Our research can give a meaningful insight into the two kinds of fluorescence spectroscopy in the HNISC system found by experimental measurement and be potential guidance to the application of 2-hydroxy-1-naphthaldehyde-derived systems in the development of new-type sensors, nonlinear optical materials and biochemical probes.