Thermal response and thermochromism of methyl red-based copolymer systems – coupled responsiveness in critical solution behaviour and optical absorption properties

Abstract

Until now, only limited experimental knowledge and sparse theoretical treatment about the mechanisms of thermochromism of azo dyes in solution has been available. Especially the coupling of thermoresponsiveness of polymers with the inherent thermochromism of azo dyes is attractive to enhance the optical response for applications like polymeric optical pH- and temperature-dual sensors. To elucidate the different mechanisms contributing to the thermochromism of such azo chromophores, we synthesised monomers based on the constitutional isomers of the common pH indicator methyl red. The ortho-isomer was copolymerised with hydrophilic monomers and the photocrosslinker benzophenone acrylamide, with the resulting copolymers being converted to networks by irradiation with UV-light and yielding hydrogels after swelling with water. N-Isopropylacrylamide was used as comonomer to introduce thermoresponsiveness in the polymers in form of a lower critical solution temperature (LCST) behaviour. Three different dye systems with varying protonation states were investigated by temperature-dependant UV-vis spectroscopy: as monomers in solution, as part of copolymers in solution, and as photocrosslinked hydrogels. Consequently, we were able to identify the four different mechanisms of vibronic thermochromism, thermo-solvatochromism, thermo-perichromism and thermo-halochromism. Their interplay was investigated by choosing appropriate combinations of solvents, acid and comonomers. The LCST behaviour of the N-isopropylacrylamide copolymers could be exploited to strongly influence thermochromism, providing insight into the mechanisms of critical solution behaviour of polymers and thermochromism alike. The experimental data suggest that various thermochromic mechanisms act simultaneously and mutually influence each other, specifically with thermo-solvato- and thermo-perichromism affecting thermo-halochromism. These effects are best described by the terms thermo-solvato-halochromism and thermo-peri-halochromism. Notably, on the basis of the identified thermochromic mechanisms prevailing in the monomer solutions, the behaviour of the more complex polymer systems can be elucidated, and consequently, the distinct properties of the dye in combination with polymer-inherent phenomena can be deduced. To our knowledge, this is the first comprehensive study to harmonise the understanding of the different thermochromic mechanisms in azobenzene, their mutual action, and the strong influence of thermoresponse on thermochromism.