Synergistic Regulation of ESIPT Behavior of Novel Benzoxazole-Based Fluorophore by Π-Conjugated Sites and Fused-Ring Extensions: a TD-DFT Study

The effects of different π-conjugation extension modes on the photophysical properties and excited state intramolecular proton transfer (ESIPT) behavior were systematically explored via density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. It can be clearly found that the intramolecular hydrogen bonds (IHBs) become stronger upon photoexcitation, which is favorable to the ESIPT process. However, all π-conjugation extension modes weaken the IHB, and then hinder the ESIPT process, but the degree of inhibition of ESIPT varies with different π-conjugation expansion modes. Fused naphthalene ring has a more significant inhibitory effect on ESIPT process. Furthermore, the fused naphthalene ring results in a more pronounced red shift of the enol and keto fluorescence peaks compared to the fused benzene ring. While at the same fused ring, the π-conjugation extension at the 2,3- and 1,2-position causes the fluorescence peak to shift the most and the least towards the red, respectively.