Theoretical Insights Into the Effect of π-Conjugation Extension on Photophysical Properties and ESIPT Behavior of a Novel Benzimidazole-Based Fluorophore.

The π-conjugation extension can precisely regulate the ESIPT process by reconstructing the conjugated size of the molecular skeleton. In this work, a detailed theoretical study on the fluorescence properties and excited state intramolecular proton transfer (ESIPT) processes of a novel fluorophore of 3'-(1H-benzo-[d]-imidazol-2-yl)-4' -hydroxy-[1,1'-biphenyl]-4-nitrile (Bibc-OH) and its derivatives (Bibc-OH-1, Bibc-OH-2, Bibc-OH-3 and Bibc-OH-4) has been performed via density functional theory (DFT) and time-dependent DFT (TD-DFT) methods, and the influence of π-conjugation extension on photophysical features and ESIPT mechanisms has been demonstrated. The experimental absorption and emission wavelengths of Bibc-OH has been reproduced, indicating that the calculation level used in this work is very reliable. The intramolecular hydrogen bonds (IHBs) become stronger upon photoexcitation, which is favorable to the ESIPT process. It can be found that linear extension and anthracene ring extension cause a red shift in the absorption peak, enol and keto fluorescence peaks of Bibc-OH, while naphthalene ring extension causes a red shift in the absorption peak and enol fluorescence peak of Bibc- OH, and a blue shift in the keto fluorescence peak. Furthermore, the ESIPT behavior can be modulated by π-conjugation extension. Both linear extension and fused ring extension increase the ESIPT energy barrier and inhibit the ESIPT process. The anthracene ring extension has the strongest inhibitory effect on the ESIPT process.