Mechanistic Insights into the Fluorescence Behavior of 2-(2-(1,3-Dioxoisoindolin-2-yl)phenyl)benzothiazole (PHBT) and Its Hydrazine-Induced Products: A DFT/TD-DFT Study

In this study, the fluorescence quenching mechanism of the hydrazine probe 2-(2-(1,3-dioxoisoindolin-2-yl)phenyl)benzothiazole (PHBT) and the distinct luminescence behaviors of its reaction products 2-[2-(2-hydrazinocarbonylbenzamido)-5-methylphenyl]benzothiazole (CHBT), 2-(2-amino-5-methylphenyl)benzothiazole (AMBT), and the AMBT-based derivative 2-(2-acetamido-5-methylphenyl)benzothiazole (ACBT), were systematically investigated via density functional theory/time-dependent DFT (DFT/TD-DFT) calculations. The results indicate that the fluorescence quenching of PHBT arises from a synergistic donor-type photoinduced electron transfer (d-PET) and twisted intramolecular charge transfer (TICT) mechanism. Upon photoexcitation, the system initially populates a bright LE state (S3), followed by a d-PET process that transfers an electron from the donor to the acceptor, leading to a dark CT state (S1). This CT state subsequently undergoes TICT, ultimately resulting in fluorescence quenching via nonradiative decay. For CHBT, AMBT, and ACBT, excited-state hydrogen bond strengthening was verified through IR and reduced density gradient (RDG) analysis, with the order CHBT > ACBT > AMBT. Potential energy curve scans demonstrate that CHBT and ACBT can undergo ESIPT with low energy barriers (4.20 and 5.54 kcal/mol, respectively), yielding tautomer-form emission. In contrast, AMBT exhibits a higher barrier (8.93 kcal/mol) and remains in the normal form, resulting in normal form-based fluorescence. FMOs and hole–electron analysis further confirm that all three compounds exhibit LE states, and the charge transfer degree (D index) follows CHBT < ACBT < AMBT, which is negatively correlated with the hydrogen bond strengthening trend. This work provides deep insights into the fluorescence switching mechanism and elucidates the electronic and protonic processes governing the luminescence of hydrazine-derived probes.