Theoretical exploration in the substituent effect on photophysical properties and excited-state intramolecular proton transfer process of benzo[a]imidazo[5,1,2-cd]indolizines

In this work, we explore the substituent effect, which is a tunable parameter for the excited-state intra-molecular proton transfer (ESIPT) process, on the photo-physical properties and ESIPT process of benzo[a]imidazo[5,1,2-cd]indolizines via theoretical methods. The calculations in geometric parameters, O–H stretching vibration frequencies, hydrogen bonding energies and energy curves along with the proton transfer path demonstrate the substitution exhibits different effects on the hydrogen bonding behaviors in the ground and excited state, thereby on the ESIPT process. In addition, the simulated absorption and emission spectra are in agreement well with the experimental results. Compared to those for the analogs bearing fluorine and methyl substitutions, the relatively higher quantum yield and the blue-shift emission wavelength of methoxy group substituted compound have been attributed to the less extent of charge transfer in the excited state.