Tunable keto emission of 2-(2′-hydroxyphenyl)benzothiazole derivatives with π-expansion, substitution and additional proton transfer site for excited-state proton transfer-based fluorescent probes: Theoretical insights

Electronic properties and excited-state intramolecular proton transfer (ESIPT) processes of 2-(2'-hydroxyphenyl) benzothiazole (HBT) and its derivatives with different chemical structures (pi-expansion, benzothiazole substituent and additional PT unit) have been theoretically investigated. The red-shifts of enol absorption peaks are found for all HBT derivatives regardless of chemical structures, whereas the blue-shifts or red-shifts of keto emission peaks are dependent on the orientation of pi-expansion and the additional PT unit compared to HBT, which are rationalized by frontier molecular orbitals. Among designed HBT derivatives, a HBT having two PT units connected with pi-expansion along the nodal plane is the best candidate for fluorescent probes because of its keto emission at 1159 nm with the largest Stokes shift. Moreover, potential energy curves and dynamic simulations confirm that the emission at much longer wavelength is from the di-keto species driven by double PT process. Based on this theoretical investigation, the keto emission of HBT derivatives in NIR region can be achieved by the combination of pi-expansion along the nodal plane and additional PT unit.