Theoretical study on the relationship between the position of the substituent and the ESIPT fluorescence characteristic of HPIP

The influences of the substituent base position on the excited state intramolecular proton transfer fluorescence properties were explored in 2-(2'-hydroxyphenyl)imidazo1,2-a-pyridine (HPIP) and HPIP's derivatives (5'Br-HPIP and 6'Br-HPIP). And the density functional theory (DFT) and time-dependent DFT (TD-DFT) methods were used to calculate the molecule structures. The calculated results showed that the influence of 5'Br-HPIP on the fluorescence intensity is stronger than that of 6'Br-HPIP. The fluorescence emission peak of 5'Br-HPIP occurred a blue shift compared with HPIP, and 6'Br-HPIP exhibited an opposite red shift. The change of the fluorescence emission peak was attributed to the decrease of the energy gap from 6'Br-HPIP to 5'Br-HPIP. Our work on the substituent position influence could be helpful to design and develop new materials.