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 S0 and S1 states were analyzed. The absorption and fluorescence spectra were calculated, and the potential energy curves in both S0 and S1 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.