Time-dependent density functional theory (TD-DFT) study on the excited-state intramolecular proton transfer (ESIPT) in 2-hydroxybenzoyl compounds: significance of the intramolecular hydrogen bonding.

The excited-state properties of intramolecular hydrogen bonding (IMHB) in methyl salicylate (MS) and its effects on the excited-state intramolecular proton transfer (ESIPT) have been investigated using theoretical methods. From the geometric optimization and IR spectra in the ground and excited states calculated by density functional theory (DFT) and time-dependent DFT (TD-DFT) methods respectively, the IMHB is demonstrated to be significantly strengthened upon excitation to excited state S(1). Thereby, the ESIPT is facilitated by the excited-state IMHB strengthening since ESIPT takes place through IMHB. In addition, the absorption and fluorescence peaks of the S(1) state are also calculated using the TD-DFT method. It is noted that the calculated spectra are in good agreement with the experimental results, which has confirmed the ESIPT mechanism of MS first proposed by Weller. Moreover, other four 2-hydroxybenzoyl compounds forming strong IMHB are investigated to understand the effect of substituent R on the ESIPT process. We find that the hydrogen bond strength can be controlled by the inductive field effect of the substituent. Thus it is inferred that the ESIPT reaction can be facilitated by the inductive effect of electron-donating substituent.