Unveiling the effect of solvent polarity on the excited state intramolecular proton transfer and hydrogen bond mechanisms of DHP

In this present work, using density functional theory and time-dependent density functional theory, the mechanism of excited state intramolecular proton transfer (ESIPT) of 2-(4 '-diethylamino-2 '-hydroxyphenyl)-1Himidazo-[4,5-b] pyridine (DHP) in ether, 1-butanol, 1-propanol, and acetonitrile phases was systematically studied. The optimized geometry includes both normal and tautomeric structure. The fluorescence spectrum and absorption spectrum calculated based on geometric structure agree with the experimental data. The data of bond lengths and bond angles related to hydrogen bonds indicate that hydrogen bonds are strengthened in the S-1 state. Frontier molecular orbital analysis combined with Hirshfeld's method shows a redistribution of electron density in different solvents. The infrared vibrational spectra show that the hydrogen bond strength decreases with the increase of solvent polarity. Finally, through the analysis of the potential energy surface, it is found that the excited state intramolecular proton transfer becomes more and more difficult as the polarity of the solvent increases.