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...

The fascinating field of thermo-halochromism of azo chromophores still astounds with unexplored facets nourished by the intricate relationship between molecular structure variations and their spectroscopic signatures. In this respect, we investigated the thermally dependent absorption behaviour of acrylamide derivatives of o-methyl red, characterised by two secondary amide linkages with hydrogen bonding-active protons in the pendant alkyl substituent. The systems were studied by a combination of UV-vis, derivative, and difference, as well as 2D-NMR (Nuclear Overhauser Effect Spectroscopy, NOESY) spectroscopy. These experiments show that the thermohalochromism is specifically influenced by hydrogen bonding interaction of the secondary amidoalkyl acrylamide side chain with the azobenzene core in dependence of the spacer length. Apparently, the substituent acts like a solvent, which is directly tethered to the chromophore and where the tether length determines the interaction by conformational freedom. We refer to this novel phenomenon as "H-bonding-controlled thermohalochromism".

The unique photomotion of azo materials under irradiation has been in the focus of research for decades and has been expanded to different classes of solids such as polymeric glasses, liquid crystalline materials, and elastomers. In this communication, azo dye-containing gels are obtained by photocrosslinking of non-thermoresponsive and lower critical solution temperature type thermoresponsive copolymers. These are analysed with light microscopy regarding their actuation behaviour under laser irradiation. The influences of the cloud-point temperature and of the laser power are investigated in a series of comparative experiments. The thermoresponsive hydrogels show more intense photoactuation when the cloud-point temperature of the non-crosslinked polymer is above, but closer to, room temperature, while higher laser powers lead to stronger motion, indicating a photothermal mechanism. In non-thermoresponsive gels, considerably weaker photoactuation occurs, signifying a secondary mechanism that is a direct consequence of the optical field-azo dye interaction.

The ammonium-azonium tautomerism of protonated push-pull-or dialkylaminoazobenzenes has been subject of research for single molecular dyes but not for the interesting architectures of polymers or gels. To address this gap in knowledge, two different types of polymer systems are devised. First, a 2-oxazoline of o-methyl red is developed for cationic ring-opening copolymerization with 2-methyl-2-oxazoline. Second, a piperazine acrylamide of o-methyl red is synthesized for free radical copolymerization with N-hydroxyethyl acrylamide and a benzophenone acrylamide. The resulting copolymer can be photocrosslinked and swollen in water to form a hydrogel. Investigation of the optical properties of these systems in response to temperature variation reveals unusual phenomena related to the stability and dynamics of the ammonium-azonium tautomerism in aqueous media. Reversible and irreversible thermochromic phenomena are found for completely protonated, partially protonated, and neutral states of the systems. Reversible thermochromism is linked to an ammonium-azonium tautomerism, while irreversible thermochromism is a consequence of hydrolysis at unusually low temperatures, apparently catalyzed by intramolecular hydrogen bonding motifs between the tertiary amide in ortho-position to the protonated azo bridge. Hydrolysis is strongly affected by the particular molecular structure of the copolymer. In the hydrogel, hydrolysis rates decrease by a reduced degree of conformational freedom.

Correction for ‘Thermal response and thermochromism of methyl red-based copolymer systems – coupled responsiveness in critical solution behaviour and optical absorption properties’ by Thorben Gwydion Jaik et al., Polym. Chem., 2022, DOI: 10.1039/D1PY01361K.