Abstract
Thermochromic urea (U) organogel immobilized with a thermochromic tricyanofuran hydrazone (TCFH) chromophore was developed. Thermochromic TCFH chromophore bearing two nitro functional groups on a hydrazone recognition unit was synthesized via an azo-coupling reaction of tricyanofuran (TCF) heterocyclic moiety containing an active methyl group with the diazonium chloride salt of 2,4-dinitroaniline comprising two strongly electron-withdrawing nitro groups. The molecular structure of both intermediates and TCFH dye were characterized by several analytical methods, including 1H NMR, 13C NMR, IR, mass spectroscopy (MS), and elemental analysis. The thermochromic responsiveness could be attributed to the charge delocalization of TCFH as well as to the presence of an intramolecular charge transfer. The generated organogel displayed a thermoreversible sol–gel transition associated with color change. The origin of the monitored thermochromism is a conformational change of the tricyanofuran hydrazone backbone due to the temperature-driven deprotonation–protonation reversible process. The prepared urea–tricyanofuran hydrazone (UTCFH) thermometer acted as a diagnostic tool providing an instant color change between yellow, orange, red and purple upon changing the temperature of the UTCFH organogel in dimethyl sulfoxide (DMSO). This color change was proportionally correlated with increasing the temperature from 44 to 63 °C. The UTCFH organogel composed of urea and push-π-pull hydrazone type tricyanofuran chromophore immobilized physically in the urea organogel was found to function as a temperature-driven chromic thermometer. This chromogenic UTCFH organogel in DMSO displayed a phase transition at 41–48 °C. The morphological properties of the gel internal fibrous nanostructure (80–120 nm) were monitored by scanning electron microscopy (SEM). The colorimetric measurements were monitored by UV–Vis absorption spectroscopy. The chromogenic thermometer demonstrated a good reversibility without fatigue. The mechanism accounting for thermochromism of UTCFH organogel is proposed.
Highlights
The development of colorimetric sensors as environmentally monitoring early warning tools is highly interesting due to their ability for simple and fast naked-eye detection without the necessity for trained personnel, or any expensive or sophisticated instruments [1,2,3] colorimetric sensors have recently attracted more interest in the monitoring of various hazardous materials [4,5,6,7,8,9,10,11,12,13,14]
We report on the fabrication of a simple-structured thermochromic organogel-based colorimetric thermometer using urea as a thermoresponsive organogel and a tricyanofuran hydrazone as a thermochromic chromophore
In a wide range of chemical reactions, the active methyl, and methylene-containing compounds such as dicyanomethane or α-nitriles bearing carbonyl functional groups can interact with a base such as sodium ethoxide in order to undergo an azo-coupling with diazonium chloride salts of aromatic amines or to undergo a condensation reaction with aldehydes or ketones
Summary
The development of colorimetric sensors as environmentally monitoring early warning tools is highly interesting due to their ability for simple and fast naked-eye detection without the necessity for trained personnel, or any expensive or sophisticated instruments [1,2,3] colorimetric sensors have recently attracted more interest in the monitoring of various hazardous materials [4,5,6,7,8,9,10,11,12,13,14]. Hydrazone dyes are of high significance due to their physico-chemical properties and potential applications, such as sensors, nonlinear optics, and medical purposes This can be ascribed to their unique molecular structure, which is highly influenced by the type and location of substituents [15,16]. Tricyanofuran hydrazone dyes have significant characteristics, including photo- and thermal stability and comparatively high yield, as well as their capability to provide a wide range of colors depending on the substituents located on the arylhydrazone unit [23] They have been applied for use as antimicrobial agents and disperse dyestuffs for polyester fibers as well as solvatochromic, biochromic, metallochromic, vapochromic, thermochromic and halochromic sensors. Conjugation of the generated hydrazone anion with highly electron-pull moiety should result in potentially interesting vapochromic, metallochromic, solvatochromic, thermochromic, solvatofluorochromic, halochromic and biochromic dyes [21,22,23,24]
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