Context <p>The excited-state intramolecular proton transfer (ESIPT) reaction of 9-phenyl-9H-carbazole substituted 2-hydroxyphenyl benzothiazole (HBT-PhCz) derivatives is studied based on TDDFT/B3LYP/TZVP theoretical level. To provide direct information on the effect of atomic electronegativity on the ESIPT reactions, the geometric parameters, infrared (IR) vibrational spectra, core-valence bifurcation (CVB) index coupling with predicting the hydrogen bonding energy (E_HB), vertical excitation behaviors, frontier molecular orbitals (MOs), and potential energy curves have been calculated for chalcogen-substituted HBT-PhCz derivatives (HBT-PhCz-O, HBT-PhCz-S, and HBT-PhCz-Se). The charge recombination caused by photoexcitation and energy gap between HOMO and LUMO orbitals further reveal the ESIPT trend. Potential energy curves of the three molecules under consideration are examined, with the conclusion being drawn that the ESIPT behaviors could be regulated by chalcogen element substitutions. In brief, we propose the ESIPT mechanism and spectral behavior of atomic electronegativity regulation for HBT-PhCz derivatives.</p> Methods <p>All molecular structures have been optimized using DFT and TDDFT methods with D3-B3LYP and TZVP levels by Gaussian 16 software. Vertical excitation simulations are based on TDDFT method with analyzing charge redistribution behaviors. Using the Multiwfn 3.8 software, the core-valence bifurcation (CVB) indexes are performed. Further, the potential energy surfaces have been constructed, based on which the transition state (TS) configurations are searched at the same theoretical level.</p>