Computational insights into the photophysical processes of an acylhydrazone fluorescent probe based on conical intersection and sensing mechanism for Al3+ detection

Al³⁺ detection is crucial for food safety, environmental monitoring, and biological assays. This work investigated the fluorescent properties of the probe VN62 and its sensing mechanism for the detection of Al³⁺ ion in detail. The ground- and excited-state properties such as isomerization geometries and absorption spectra, as well as the photophysical processes of transitions in PES have been studied. There are few fluorescent S₁ states (PT*, TICT) of the VN62 molecule, and the conical intersections (CIs) have key importance in the fluorescent process. By comparing the radiative rate (k_r) and the rate of transition to CI, we deduce that these fluorescent states are lack of stability for light emission. Therefore, the fluorescence of probe alone can be quenched by CI pathways. After coordination with Al³⁺, the CI pathways will be suppressed, resulting in enhanced fluorescence. This sensing mechanism is different from the previously proposed mechanisms, such as the dark state of TICT or ESIPT. We anticipate that our research would shed some light on the design of turn-on sensors.