Theoretical Study on the Photophysical Mechanism for the Detection of Biothiols Based on the 1,3,4-Thiadiazole Fluorescence Probe.

The photophysical mechanism of the 1,3,4-thiadiazole-based probe N, N-dimethyl-5-(2-((7-nitrobenzo[c][1,2,5]oxadiazol-4-yl)oxy)phenyl)-1,3,4-thiadiazol-2-amine (L-C) for biothiol (RSH) detection has been theoretically investigated. Initially, significant intramolecular charge transfer (ICT) is observed in L-C, with a charge-transfer distance of 4.87 Å. The energetically lower acceptor LUMO (-3.74 eV) compared to the donor (-1.42 eV) facilitates a photoinduced electron transfer (PET) process, which effectively quenches the fluorescence of L-C. Following 7-nitro-1,2,3-benzoxazole (NBD) thiolysis by RSH, the generated intermediate L-A undergoes ground-state twisting with a low barrier (0.399 kcal/mol) to form the L-A-Enol conformer. This process is consistent with the significantly strengthened intramolecular hydrogen bond from -2.024 to -10.285 kcal/mol. Upon photoexcitation to the first singlet (S1) state, L-A-Enol undergoes excited-state intramolecular proton transfer (ESIPT) to its keto form with a barrier of 3.162 kcal/mol. Finally, the keto form returns to the enol form via a ground-state intramolecular proton transfer (GSIPT) process with a low barrier of 0.705 kcal/mol, completing the photophysical cycle. Notably, the calculated enolic emission peak (419 nm) is close to the experimental peak (415 nm), while the keto form exhibits weak fluorescence at 539 nm. These results demonstrate that the experimentally observed fluorescence enhancement is mainly due to the enol form, rather than the keto emission. These theoretical calculations reveal the detailed detection mechanism of L-C toward RSH via combined PET and ESIPT processes, offering insights that differ from previously established explanations.