Development of a novel colorimetric thermometer based on poly(N-vinylcaprolactam) with push–π–pull tricyanofuran hydrazone anion dye

Abstract

Thermochromic poly(N-vinylcaprolactam-co-tricyanofuran hydrazone) [poly(VC-co-TCFH)] gel labeled with a halochromic chromophore was developed using traditional free radical polymerization. A novel vinyl-containing tricyanofuran hydrazone (TCFH) halochromic chromophore bearing a hydrazone sensor group was generated via an azo-coupling procedure of a tricyanofyran (TCF) heterocycle with the diazonium chloride salt of 2-hydroxy-4-nitroaniline comprising an allyl group. The molecular structures of the TCFH chromophore and related intermediates were proved by different techniques, including FT-IR, mass spectroscopy (MS), ¹H/¹³C NMR and elemental analyses (C, H, N). This chromophore showed a pronounced solvatochromic activity in various organic solvents of different polarities. pH sensing activity was also monitored at different pH values. Both solvatochromism and pH-responsiveness can be ascribed to the delocalization of charge on TCFH. Poly(VC-co-TCFH) acted as a thermochromic sensor device generating immediate colorimetric changes from yellow to orange, red, wine, violet, and purple with increasing temperature of aqueous poly(VC-co-TCFH) solution. Such colorimetric changes are proportionally interrelated with the temperature increase from 33 to 47 °C due to a phase change in an aqueous solution of poly(VC-co-TCFH) copolymer. The poly(VC-co-TCFH) copolymer composed of N-vinylcaprolactam (VC) and push–π–pull hydrazone type tricyanofuran chromophore monomers functions as a temperature-driven chromogenic thermometer. The morphology of the gel nanostructure was monitored by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Mechanistic studies accounting for solvatochromism and halochromism of TCFH as well as thermochromism of poly(VC-co-TCFH) are proposed.