Computational Insights into Sensing Mechanism for Al3+ in a New Acylhydrazone Fluorescent Probe Based on Excited-State Intramolecular Proton Transfer (ESIPT) and Twisted Intramolecular Charge Transfer (TICT)

The work explored the fluorescent properties of probe N′-(2, 4-dihydroxy-benzylidene)pyridine-3-carbohydrazide (HL) and its sensing mechanism for the Al3+ ion in detail. HL has two competing deactivation processes: ESIPT and TICT. Upon light-excitation, only one proton can transfer, and the SPT1 structure is generated. The SPT1 form is highly emissive, which is inconsistent with the colorless emission observed in the experiment. Then a nonemissive TICT state was obtained by rotating the C–N single bond. The energy barrier of the TICT process is lower than that of the ESIPT process, which indicates that probe HL will decay to the TICT state and quench the fluorescence. When Al3+ is recognized by probe HL, strong coordinate bonds are formed between HL and Al3+, and then the TICT state is prohibited, and the fluorescence of HL is turned on. Al3+ as a coordinated ion can effectively remove the TICT state but cannot influence the photoinduced electron transfer (PET) process of HL.