Photoswitches are light-absorbing molecules capable of reversible isomerization, finding applications in actuators, optics, molecular motors, and photobiology. Azobenzenes are widely used, particularly in biosciences due to their geometric changes upon *trans*/cis-isomerization, enabling photoregulation of mechanical functions in biological settings. However, azobenzenes have limited spatial control due to the low rotational barriers around the C-N bond, resulting in a wide distribution of *cis*-isomer geometries. This limitation stems from the isomerization mechanism involving rotation around the C-N=N-C dihedral angle, rather than inversion of the C-N=N in-plane bending angle. Similar limitations exist in other photoswitches like fulgimides or stilbenes. In contrast, (thio)indigo derivatives offer exceptional spatial control through a 180° rotation around the central C=C bond during *trans*/cis-isomerization. The blue-shifted absorption of the *cis*-form compared to the *trans*-form results from non-bonding interactions between sulfur/nitrogen and oxygen atoms, unlike sterically-induced distortions in azobenzene and stilbene-based photoswitches. This makes (thio)indigos excellent candidates for molecular machinery with robust spatial control over attached payloads. Despite their discovery over a century ago and extensive study of their photoisomerization, applications in soft matter materials have been limited, mostly focusing on dye-doping of liquid crystal displays and solid supports, or as a molecular tweezer at micromolar concentrations. The major limitation is the low solubility of (thio)indigos in most organic solvents and polymers, preventing covalent incorporation or compatibilization into polymeric materials. Their rigid, planar structure promotes aggregation, reducing mobility and reactivity. While modifications like alkyl or siloxane side chains have been introduced to reduce aggregation, their use remains limited to dopants in liquid crystals or polymer films. Attempts to create water-soluble adducts via sulfonic acid residues resulted in only micromolar solubility, with higher concentrations leading to aggregation and spectral broadening. Therefore, effective solubilizing strategies are crucial for their use in aqueous environments and applications in chemical and mechanical biology, and smart soft matter materials. This study pioneers a strategy to integrate thioindigo functionality into polymer main chains, enabling investigation of photoswitching in various solvents, including water.

The existing literature extensively covers the photochromic properties and isomerization mechanisms of thioindigo derivatives. However, their applications in soft matter materials have been hampered by their poor solubility in common solvents and polymers. Previous research efforts have focused on incorporating thioindigo as dopants in liquid crystal displays and solid supports or utilizing them as molecular tweezers for metal ion capture and release, but these approaches have limitations. The challenge of achieving high concentrations of thioindigo in aqueous solutions for biological applications has also been a significant hurdle. Studies on improving their solubility have yielded limited success, with water-soluble adducts achieving only micromolar concentrations before significant aggregation occurs. Therefore, the development of novel strategies for integrating thioindigo into polymer backbones is crucial to unlocking its full potential in soft matter materials and biological applications.

The researchers synthesized a polymerizable thioindigo bismethacrylate linker (*trans*-1) on a gram scale using a facile protocol that did not require column chromatography. Despite methacrylate and ester substitutions, *trans*-1 was insoluble in most polar organic solvents, only dissolving in chloroform up to 5 mM. UV-Vis spectroscopy and NMR analysis were performed on chloroform solutions. Irradiation at λmax = 540 nm induced a decrease in the 540 nm absorbance and the appearance of a blue-shifted peak at 490 nm, indicating *trans*-to-*cis*-isomerization, reversible with irradiation at λmax = 490 nm. This photoswitching was repeatable for over 10 cycles without significant photodegradation. A tuneable laser system and action plot analysis were used to determine the optimal wavelengths for photoisomerization. The maximum quantum yields were 0.18 and 0.62 for *trans*-to-*cis* and *cis*-to-*trans*-isomerization, respectively, both at 530 nm. The highest *cis*/ *trans*-ratio (72.5%) for *trans*-to-*cis*-isomerization was achieved at ~550 nm, while the highest *trans*/ *cis*-ratio (94.5%) for *cis*-to-*trans*-isomerization was at 400 nm. The optimal wavelengths were determined to be 540-550 nm and 450-470 nm for *trans*-to-*cis* and *cis*-to-*trans*-isomerization, respectively. The *cis*-isomer thermally reverted to the *trans*-isomer with a half-life of 24 minutes at ambient temperature. Free radical thiol-ene chemistry was used to integrate thioindigo into a polymer structure. Poly(ethylene glycol) (PEG) with thiol end groups (PEG44-(SH)2) was used as a water-soluble precursor, and step-growth polymerization with *trans*-1 yielded the thioindigo-containing polymer P1. P1 was soluble in a wide range of polar solvents. Using LED light sources at 540 nm and 470 nm, the photoswitching of P1 was studied in various solvents by UV-Vis spectroscopy. The efficiency of photoswitching decreased with increasing solvent polarity, with complex interactions and aggregation observed in highly polar solvents. Aqueous solutions of P1 (≥5 wt%) exhibited thermoresponsive gelation, with the hydrogel's storage modulus decreasing under green light irradiation and recovering under blue light irradiation. This photoswitching of the storage modulus was repeatable for several cycles. The hydrogel's modulus also spontaneously recovered in the dark at 37 °C. Computational calculations (M06-2X/def2-TZVP) were employed to investigate the photoisomerization process. The *trans*-isomer was calculated to be more stable than the *cis*-isomer by approximately 0.4 eV, with longer λabs (472 nm vs 444 nm). The *trans*-isomer also had a lower dipole moment (4.39 Debye) than the *cis*-isomer (5.87 Debye). Calculations showed that the energy barrier for *trans*-to-*cis* transformation was 0.423 eV, and for *cis*-to-*trans* was 0.37 eV. Protic solvents such as methanol were found to hinder photoswitching by forming hydrogen bonds with thioindigo, increasing the energy barrier to rotation. To create hydrogels, a polymer precursor P2 was synthesized from *trans*-1 and PEG12-(SH)2 via thiol-ene polymerization. P2 was crosslinked with PEG448-(propiolate)4 in DMF via thiol-propiolate addition. The organogels were then treated with water to form swollen hydrogels with varying thioindigo content (0.55-5.5 wt%). The hydrogels' optical and physical properties were characterized. The hydrogel with the highest thioindigo content (GelT5) showed changes in stiffness in response to temperature and light irradiation. Rheological measurements showed the modulus decreased under green light and increased under blue light. The photoswitching of the hydrogel's stiffness was also repeatable. Cell viability assays (Annexin V/PI staining) using HEK-293T cells and human PBMCs demonstrated good biocompatibility of the hydrogels and the photoswitching process.

The study successfully synthesized a polymerizable thioindigo bismethacrylate linker (*trans*-1) and incorporated it into polymer main chains, solving the solubility issue of thioindigo. Action plot analysis determined the optimal wavelengths for photoisomerization (540-550 nm for *trans*-to-*cis* and 450-470 nm for *cis*-to-*trans*). The resulting thioindigo-containing polymers (P1) exhibited reversible photoisomerization in various organic solvents and water. The incorporation of thioindigo into hydrogels enabled visible light-induced modulation of hydrogel stiffness, with the storage modulus decreasing under green light and recovering under blue light. Computational calculations provided insights into the photoisomerization mechanism and the influence of solvent polarity. Importantly, both the thioindigo-containing hydrogels and the photoswitching processes showed good biocompatibility with cells.

This work addresses the long-standing challenge of utilizing thioindigo's advantageous photoswitching properties in soft matter materials. The successful covalent integration of thioindigo into polymer main chains and hydrogels represents a significant advancement. The detailed photochemical action plot analysis provided precise optimal wavelengths for efficient photoisomerization, improving upon previous methods that relied solely on absorption spectra. The observed decrease in photoswitching efficiency with increasing solvent polarity is explained by the computational findings, demonstrating the hindering effect of hydrogen bonding in protic solvents. The ability to modulate hydrogel stiffness using visible light opens up exciting possibilities for applications requiring precise and reversible control of material properties. The excellent biocompatibility of the hydrogels further enhances their potential for biological and biomedical applications.

This study successfully overcomes the solubility limitations of thioindigo, enabling its integration into polymer main chains and hydrogels. The precise determination of optimal wavelengths for photoisomerization, the demonstration of reversible photoswitching in various solvents, and the light-induced modulation of hydrogel stiffness are significant contributions. The biocompatibility of the hydrogels further broadens their potential applications. Future research could explore the use of these materials in various biomedical applications, such as drug delivery, tissue engineering, and biosensors. Investigating different polymer architectures and thioindigo derivatives to further enhance photoswitching efficiency and tailor material properties would also be valuable.

The study primarily focused on PEG-based hydrogels. Investigating the applicability of this approach to other polymer systems would be beneficial. The thermal back-isomerization of the *cis*-isomer needs further optimization to enhance the long-term stability of the *cis*-state in applications requiring persistent modulation of material properties. A more comprehensive investigation of the long-term biocompatibility and potential toxicity at high concentrations is also warranted for future applications.