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
(λex: 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 λex 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.