Thermochromism and photomotion of azobenzene-containing polymers and hydrogels

This work describes the synthesis, and thermochromic and photoactuation properties of azobenzene-containing polymers and hydrogels.
For these polymers, monomers based on the azo dye methyl red were synthesised with varying linkages between the chromophore and the polymerizable group. These monomers showed distinct thermochromic behaviour in solution in dependence on protonation state and solvent.
By determining the thermochromism of the single dyes, it was possible to elucidate the thermochromic effects of the corresponding copolymers.
The parent dye methyl red was functionalised with either an alcohol, a primary amine, or a secondary amine, yielding an ester, secondary amide, or tertiary amide attached to the aromatic ring, respectively. These exhibited largely varying thermochromic behaviours. It was possible to identify thermo-halochromism, thermo-solvatochromism, and thermo-solvatohalochromism for the ester, which provided the foundation to understand thermochromic effects and their interplay in non-thermoresponsive and lower critical solution temperature type thermoresponsive copolymers and hydrogels, which could be obtained through photocrosslinking. Both polymers in solution and gels strongly influence thermo-halochromism of this azo dye. Secondary amide functionalities on two ends of a tether between chromophore and polymerizable unit lead to intricate effects on thermo-halochromism, disturbing the simple equilibrium between neutral and protonated form by hydrogen bonding motifs in dependence on the tether length. A tertiary amide in ortho-position to the azo bridge results in a distinct shift in the tautomeric equilibrium between ammonium and azonium form of the protonated azo dye compared to other pseudostilbenes. Thermo-tautochromism occurs and acidic hydrolysis of the tertiary amide is catalysed through intramolecular hydrogen bonding at low temperatures, accompanied by an intense change of colour. The structure and hydrogen bonding capabilities of the corresponding copolymer or gel dictate the temperature at which hydrolysis becomes dominant.
When the azo dye-containing hydrogels were exposed to laser light instead of temperature as a trigger, intense photomotion could be observed. In thermoresponsive hydrogels with a cloud point close to room temperature, fast collapse occurred over a large area, which can be attributed to local photothermal effects. By changing the liquid medium to isopropanol instead of water, the thermoresponse can be switched off and the photomotion changed in scale to smaller but faster actuation. The same behaviour is inherent to non-thermoresponsive hydrogels. In this way, two different modes of photoactuation can be exploited by choice of the polymer matrix.