The effects of different heterocycles and solvents on the ESIPT mechanisms of three novel photoactive mono-formylated benzoxazole derivatives

The fluorescence behaviors and properties of three novel photoactive mono-formylated benzoxazole derivatives A-C are found to be affected by different heterocycles and solvents, as reported in a recent experiment (Rodembusch, et al., New J. Chem., 2016, 40, 2785). Unfortunately, the detailed excited-state intramolecular proton transfer (ESIPT) mechanisms of these compounds are lacking. In this study, we used density functional theory (DFT) and time-dependent DFT (TDDFT) methods to study the dynamic ESIPT processes of the three compounds A-C in two different surroundings (polar 1,4-dioxane and nonpolar dichloromethane solvents). The calculated absorption and fluorescence spectra were observed to mutually agree with the experimental data, which indicated that the TDDFT method we adopted was reliable. In addition, based on the analysis of bond lengths, bond angles and IR vibrational spectra in both solvents, it was confirmed that the intramolecular hydrogen bonds (HBs) of these compounds were strengthened in the S-1 state, which could promote the ESIPT reactions. Moreover, the frontier molecular orbitals (MOs) and the maps of the electron density difference between the S-0 and S-1 states displayed intramolecular charge transfer, which provided the probability of ESIPT reactions for the three compounds. Furthermore, based on the constructed potential energy curves, we revealed detailed dynamical ESIPT mechanisms of the compounds A-C. As a consequence, we found that the ESIPT processes were more likely to take place from A (8.48 kcal mol(-1)) B (5.36 kcal mol(-1)) C (0.75 kcal mol(-1)) compounds in the polar 1,4-dioxane solvent, whereas the sequence changed to B (4.01 kcal mol(-1)) A (1.30 kcal mol(-1)) C (1.15 kcal mol(-1)) in the nonpolar dichloromethane solvent. Additionally, it could be determined that the solvent polarity had a tremendous effect on compound A, whereas the effect on compound C was the smallest.