Context <p>In recent years, fluorescent probe molecular technology has received increasing attention. However, the mechanism by which fluorescent probe molecules detect ions still requires further study. This research explores the sensing mechanism of the 1,3,4-thiadiazole derivative 2-(5-(4-(diethylamino)phenyl)-1,3,4-thiadiazol-2-yl) phenol (L-E) as a highly selective fluorescent probe for Cu²⁺. The current theoretical calculations propose a detection mechanism that has not yet been observed experimentally. Analyses of structural parameters, interaction region indicators, infrared spectra, absorption and fluorescence spectra, electron distribution, and potential energy curves indicate that following the addition of Cu²⁺, the fluorescence quenching of L-E is due to twisted intramolecular charge transfer (TICT) rather than excited state intramolecular proton transfer (ESIPT). Cu replaces H to form the complex L-E-Cu, which inhibits the ESIPT process and facilitates the TICT process, leading to fluorescence quenching. This research elucidates novel mechanisms underlying metal ion detection and recognition, providing invaluable guidance for the development of advanced fluorescent probes.</p> Methods <p>The structures of the studied compounds in ground (S₀) and excited (S₁) states were optimized using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) at the B3PW91/6-311G(d) level. All calculations were performed in the gas phase with singlet spin symmetry. Infrared spectra, absorption and fluorescence spectra, electron distribution analysis, and potential energy curves were all calculated using the same functional and basis set. Electron distribution and IRI analyses were performed using the Multiwfn program, and the results were visualized with Gnuplot and VMD software.</p>