Substituent control of photophysical properties for excited-state intramolecular proton transfer (ESIPT) of o-LHBDI derivatives: a TD-DFT investigation

The substituted effect on excited-state intramolecular proton transfer (ESIPT) of o-LHBDI derivatives (4R-o-LHBDI) was investigated by DFT and TD-DFT methods. The structures of 4R-o-LHBDI (R: OH, NH2, CN, NO2, CF3) were fully optimized, and the H-bond distances, bond angles, and infrared spectra of the atoms involved in PT process in the S-0 and S-1 states were analyzed. The absorption and fluorescence spectra were calculated, and the potential energy curves in both S-0 and S-1 states were constructed. Moreover, the effects of different substituents on the ESIPT mechanism of 4R-o-LHBDI (R: OH, NH2, CN, NO2, CF3) were studied. The results indicate that ESIPT in the 4R-o-LHBDI is a little harder to proceed than that in o-LHBDI since the ESIPT barrier of 4R-o-LHBDI is slightly bigger than that value of o-LHBDI. When the substituent has stronger electron-withdrawing ability or weaker electron-donating ability, the ESIPT process has the smaller potential barrier. Graphical abstract