Substituent-induced modulation of competing dual-acceptor hydrogen bond in ESIPT-type HBT derivatives

The 2-(2'-hydroxyphenyl) benzothiazole derivatives (HBT-R) featuring a dual-acceptor hydrogen bond (H-bond) have attracted our attention due to their unique structure and potential applications. There are two distinct acceptor sites located on either side of the central H-bond donor, thereby giving rise to the formation of inter-acceptor competition. In this study, quantum chemical calculations are employed to elucidate that substituents possessing distinct electronic properties modulate H-bond orientations, photophysical characteristics, and excited-state intramolecular proton transfer (ESIPT) in a dual-acceptor H-bond system. The origins of the competition in H-bonds and their effect on the distinct photophysical phenomena were revealed. The relaxed potential energy curves and molecular dynamics conducted for HBT-R indicate that electron-donating groups improve the efficiency of the ESIPT process in molecules with O-H···N H-bond. However, these groups hinder the ESIPT process in molecules with an opposite H-bond orientation. This work is expected to provide comprehensive insights into the mechanisms of substituent-induced modulation of competing dual-acceptor H-bond to inspire the design of more advanced luminescent materials.