Indigo and its synthetic derivative, thioindigo, are common dyes that have been used for colouring art pieces and textiles since ancient times. Beyond displaying vibrant colours, these chromophores feature interesting photochemical properties that can be utilized for applications in material science and biomedicine. Thioindigo, in particular, is a well-known photoswitch that can reversibly interconvert between the cis- and trans- isomers under photonic fields, enabling a 180º rotation of the aromatic structure around the central C=C bond (Scheme 1A).  However, despite the discovery of thioindigo’s photochromism over 100 years ago, reports on its applications in soft matter materials are scarce. The major shortcoming of these compounds is their very low solubility in all conventional solvents, preventing their chemical modification and incorporation into soft matter materials.

We thus set out to design a chemical approach to incorporate thioindigo into soft matter materials, enabling the light-induced tuning of their properties via the photoswitching of the thioindigo component. To our delight, we were able to readily synthesize a thioindigo bismethacrylate linker which can be used in polymerization and crosslinking (Scheme 1B).

Scheme 1. (A) Thioindigo moieties undergo reversible photoisomerization enabled by green and blue light, leading to a spatial conformational change of the attached molecular payload by a 180º rotation around the C=C bond. (B) A thioindigo bismethacrylate that can undergo free-radical thiol-ene polymerisation with a PEG-dithiol forming step-growth polymer with thioindigo moiety incorporated in the polymer backbone.

Photochemistry is currently undergoing a precision revolution, with critical impacts on photoresponsive material design, such as the hydrogels in our current study. One of the most fascinating findings by our team is that the absorbance of spectrum of a chromophore – here a photoswitch – contains little to no information which wavelengths lead to the most effective photochemical transformation. In fact, most chromophore show very little alignment of their photochemical reactivity with the absorbance spectrum with the reactivity often being red-shifted. The photochemical reactivity can be probed via so-called action plots using tuneable laser systems, and we have recorded action plots for our thioindigo species here as well. Perhaps not surprisingly, the action plot indicates the typical misalignment of reactivity with absorptivity and has allowed us to identify the optimum photochemical switching conditions.

The low solubility of the thioindigo linker prevented us from using it directly in polymer crosslinking for fabrication of light responsive polymeric materials. Furthermore, we found that the compound did not readily undergo free radical polymerization in chloroform at low concentrations, neither participate in base-catalysed thiol-Michael addition. After further screening, we discovered the methacrylate functions can participate in free radical thiol-ene reactions, enabling incorporation of the thioindigo moiety into a water soluble poly(ethylene glycol) main chain by step growth polymerization (Scheme 1B). The resultant polymer allows us to investigate the photoswitching properties of the embedded thioindigo in a range of solvents with various polarity, including water. Critically, we observed the photoisomerization of thioindigo in aqueous environment, suggesting potential applications in biomaterials space. Indeed, thioindigo-containing polymer displays thermoresponsive behaviour, forming physically crosslinked hydrogel at 37 ᵒC. The storage modulus of such a hydrogel can be decreased or increased by irradiation of green light and blue light, respectively. When incorporated into chemically crosslinked PEG-based hydrogels, the photoswitching of thioindigo function enables substantial changes in the stiffness of the hydrogels (Figure 1). Specifically, the hydrogels can be softened with green light (λ_max = 540 nm) and stiffened with blue light (λ_max = 470 nm). This change in stiffness is due to the reversible transition of the polarity between the trans- and cis- isomers, as our chemical quantum calculations indicate the cis- isomer is more polar than the trans- isomer.

Figure 1. (A) Representative scheme of photoswitching of the inserted thioindigo isomers with different dipole moments within hydrogel structure (top), and images of GelT5 in water at 37 ºC following green (λ_max = 540 nm) or blue (λ_max = 470 nm) LED light irradiation for 30 min, displaying expansion and contraction. (B) Rheological data of the photoswitching of thioindigo-containing hydrogel under alternating green and blue light irradiations.

The user-defined control of the hydrogels’ stiffness is particularly interesting for studies in cell biology, since the changes in mechanical play an important role in directing cell growth and movement. Also known as mechanotransduction, this stiffness sensing communication takes place in a continuous feedback cycle. Our preliminary testing of human embryonic kidney cells grown on thioindigo containing hydrogels confirms that the light-induced softening of the hydrogel substrate resulted in cells adopting a well-spread morphology as compared to cells grown on stiff substrate. We thus hope that the photoswitching properties of thioindigo will be utilized in next generation light responsive biomaterials, enabling new mechanistic understanding into how mechanical cues impact normal physiology and disease.