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

Abstract

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.

Color‐Tunable Multifunctional Excited‐State Intramolecular Proton Transfer Emitter: Stimulated Emission of a Single Dye

“…As presented for 1,4-dioxane/ water mixtures, the emission profile of HBT 1 is dominated by K* species when aggregated. Such behavior is often encountered in HBX derivatives, [51,52] due to the beneficial Restriction of Intramolecular Motion (RIM). As such, ACQ of deprotonated form is competing with AIEE of K* species, making it possible to apply HBT 1 as solid-state emitter, which is further confirmed by QY of powder in KBr pellet.…”

Balancing the Push–Pull Effect on the Synthesis and Fluorescent Properties of New ESIPT Dyes for Thin Film Applications

“…In terms of ESIPT molecules, very few studies have been performed to analyze the effect of temperature or other extrinsic conditions on the emission properties of thin films. In particular, temperatures above room temperature were reported to induce a change in the FL of the ESIPT emission, but these studies were performed either in solution or the bulk solid state (i.e., powder or crystal). − To the best of our knowledge, the effect of thermal annealing on ESIPT thin films has not been studied, which, together with morphological analysis, is of relevant interest for different applications such as optoelectronic or photovoltaic devices.…”

Combined Experimental and Theoretical Studies on the Rubbing-Induced Fluorescence Behavior of a Luminophore in the Solid State

“…The most exciting and accurate among all other available techniques is the use of solid-state fluorescent sensors, which can be used to conceal hidden messages. This can be achieved by mechanochromic, photochromic, and thermochromic fluorescent compounds. − Upon external stimuli (pressure, temperature, and heat), these compounds respond swiftly by turn off/on fluorescence. The mechanism of action mostly depends on the coherent aggregation of molecules or losing coherency.…”

“…This can be achieved by mechanochromic, photochromic, and thermochromic fluorescent compounds. 8 − 10 Upon external stimuli (pressure, temperature, and heat), these compounds respond swiftly by turn off/on fluorescence. The mechanism of action mostly depends on the coherent aggregation of molecules or losing coherency.…”