Understanding a Thermoemissive ESIPT-Based Solid-State Off–On Switch as a Dual-Channel Chemosensor in Solid and Solution Phases: Detailed Experimental and Theoretical Study

The excited-state intramolecular proton transfer (ESIPT)-assisted aggregation induced emission enhancement (AIEE) in an organic moiety (4-[(2,4-dihydroxy-benzylidene)-amino]-1,5-dimethyl-2-phenyl-1,2-dihydro-pyrazol-3-one (HL)) has been established based on detailed experimental and theoretical studies by synthesizing and characterizing two imine-based compounds, 4-[(2,4-dihydroxy-benzylidene)-amino]-1,5-dimethyl-2-phenyl-1,2-dihydro-pyrazol-3-one (HL) and its supporting analog 4-[(2-methoxy-benzylidene)-amino]-1,5-dimethyl-2-phenyl-1,2-dihydro-pyrazol-3-one (L′). HL is nonemissive in the solution state, whereas in the solid state, it shows a bluish-green emission (λ_ex: 360 nm) at room temperature, and on heating, it becomes nonemissive at ∼90 °C with a quick thermo “off–on” reversible phenomenon for a long period. The characterization studies including the single-crystal X-ray diffraction study explained the emissive feature of HL due to close packing and aggregation through multiple intermolecular H-bonding, C–H···π, π–π interactions, and nonemissive characteristics feature of HL in the twisted-keto form at elevated temperature. The theoretical support of ESIPT and water-mediated ESPT of HL is obtained by computing structural and energy parameters of enol, keto, and transition states in ground and excited states using density functional theory (DFT) and time-dependent DFT (TDDFT) methods. To detect the molecular packing in the crystalline phase, all of the inter- and intramolecular noncovalent interactions (NCIs) were computed using the noncovalent interaction-reduced density gradient (NCI-RDG) method along with Bader’s quantum theory of atoms-in-molecules (QTAIM). Additionally, in both solid and aqueous phases, HL was found to perform as a dual-channel sensing probe for Al³⁺ and Zn²⁺ ions by tuning the λ_ex and the pH of the aqueous medium. The present study will open a new way to explore novel organic luminescent solids as an off–on switch using external thermal stimuli.