Detection mechanism and solvent effects of the 3-hydroxy-2-(4-(pyrrolidin-1-yl)phenyl)benzo[g]quinolin-4(1H)-one (PBQ) probe

Hydroxy-2-(4-(pyrrolidin-1-yl)phenyl)benzo[g]quinolin-4(1H)-one (PBQ) is a ratiometric fluorescent probe based on excited-state intramolecular proton transfer (ESIPT). PBQ-1 is the reaction product following its exposure to phosgene. Density functional theory (DFT) and time dependent density functional theory (DFT) have been used to study the excited state dynamics of PBQ and PBQ-1 in different solvents. The results show that the reaction of PBQ with a transition from charge-transfer excitation to local excitation before and after the reaction. It becomes more difficult for PBQ in the excited state to transfer proton with increasing solvent polarity. The product PBQ-1 undergoes a molecular structure twist, and the angle of twisting decreases with increasing solvent polarity, resulting in a lower degree of rotational freedom of the hydroxyl group (5-OH) at the 5th carbon position, which makes it more susceptible to ESIPT reactions. Therefore, PBQ-1 is more susceptible to ESIPT as solvent polarity increases. Our theoretical calculations also elucidate the cause of the blue shift of PBQ fluorescence and the impact of the twisting intramolecular charge transfer phenomenon on the solvent effect. Furthermore, our study provides the theoretical guidance for the designing probe based on excited state intramolecular proton transfer.