Substituent Positioning Controls the Trade-Off between Charge Transfer and ESIPT Kinetics in Disubstituted HBT Derivatives

The photophysical properties of six disubstituted 2-(2'-hydroxyphenyl)-benzothiazole (HBT) derivatives (diHBT1-6) are systematically investigated using density functional theory (DFT) and TD-DFT with the CAM-B3LYP/TZVP and B3LYP/TZVP levels and RI-CC2/def2-TZVP level. Ground- and excited-state geometries, vertical excitation and emission energies, frontier molecular orbitals, hole-electron distributions, and potential energy curves along the excited-state intramolecular proton transfer (ESIPT) coordinate are analyzed to elucidate the interplay between substituent positioning, intramolecular charge transfer (CT), and excited-state intramolecular proton transfer (ESIPT) behavior. We reveal a previously unrecognized structure-property relationship: emission wavelength and ESIPT barrier are inversely tunable through substituent topology. Specifically, meta-electron-withdrawing group(meta-EWG)/para-electron-donating group(para-EDG) configurations (diHBT3 and diHBT6) maximize CT character, yielding the longest emission wavelengths, while simultaneously elevating the forward ESIPT barrier to >4.5 kcal/mol. In contrast, ortho-EWG/para-EDG isomers (diHBT2 and diHBT5) preserve locally excited (LE) character and exhibit ultralow ESIPT barriers (<0.7 kcal/mol) at the cost of blue-shifted emission. This decoupling of spectral tuning from proton transfer energy landscape, governed by the spatial alignment of donor and acceptor moieties, establishes a generalizable design paradigm for HBT-based fluorophores. Our findings provide a rigorous theoretical framework for rationally engineering emission color and ESIPT behavior independently, enabling precise optimization of fluorescent probes for bioimaging and optoelectronic applications.