A research on excited-state intramolecular proton-transfer mechanism of a new chemosensor

Based on the density functional theory (DFT) and time-dependent density functional theory (TDDFT), the excited-state intramolecular proton-transfer (ESIPT) mechanism of a new reported chromophore by Kim et al. (Sensors Actuators B Chem 206:430–434, 2015) has been investigated theoretically. The calculated results of bond lengths and bond angles of hydrogen bond O–H···N, the infrared vibrational spectra and the hydrogen bonding energies all demonstrated that the intramolecular hydrogen bond is strengthened in the first excited state. It is no denying the fact that our calculated results reproduced the experimental absorbance and fluorescence emission spectra well, which demonstrates that the TDDFT theory we adopted is reasonable and effective. From the analysis of frontier molecular orbitals, it is reasonable to suggest that the intramolecular charge-transfer nature of the excitation prompts the proton transfer giving rise to an ESPT reaction. The constructed potential energy curves of ground state and the first excited state based on keeping the O–H distance fixed from 0.993 to 2.343 Å have been used to illustrate the ESIPT process. A relative lower barrier of 4.17 kcal/mol in the first excited-state potential energy curve proved the ESIPT mechanism.