Reconfigurable Materials Based on Photocontrolled Metal–Ligand Coordination

Photoresponsive materials have attracted growing interest because of their potential applications in materials science, such as photoswitches, photopatterning, information storage, and so on. However, there are some challenges for photoresponsive materials for certain applications: 1) Only a few photoresponsive surfaces are transformed into multiple states after photoreactions to adapt to changing environmental conditions; 2) Photoresponsive materials may not function properly in cold environments, especially for gels. To address these problems, we have recently developed photoresponsive materials based on ruthenium (Ru) complexes. Such Ru complexes showed a photoinduced ligand substitution under visible light irradiation. Reconfigurable surfaces that can adapt to environmental changes and photoresponsive organohydrogels that function effectively at sub‐zero temperatures have been fabricated using photoresponsive Ru complexes. Herein, it is demonstrated that based on photocontrolled Ru–ligand coordination, reconfigurable surfaces can be modified for user‐defined functions via visible light irradiation and that photoresponsive gels can function even at –20 °C. As a perspective, Ru‐containing photoresponsive complexes could open up pathways for a variety of applications.

DOI: 10.1002/aisy.202000112 Photoresponsive materials have attracted growing interest because of their potential applications in materials science, such as photoswitches, photopatterning, information storage, and so on. However, there are some challenges for photoresponsive materials for certain applications: 1) Only a few photoresponsive surfaces are transformed into multiple states after photoreactions to adapt to changing environmental conditions; 2) Photoresponsive materials may not function properly in cold environments, especially for gels. To address these problems, we have recently developed photoresponsive materials based on ruthenium (Ru) complexes. Such Ru complexes showed a photoinduced ligand substitution under visible light irradiation. Reconfigurable surfaces that can adapt to environmental changes and photoresponsive organohydrogels that function effectively at sub-zero temperatures have been fabricated using photoresponsive Ru complexes. Herein, it is demonstrated that based on photocontrolled Ru-ligand coordination, reconfigurable surfaces can be modified for user-defined functions via visible light irradiation and that photoresponsive gels can function even at -20 C. As a perspective, Ru-containing photoresponsive complexes could open up pathways for a variety of applications.
the dynamic bond between Ru and the thioether reforms spontaneously. [16,28,30,31] Among these photoreactions based on Ru-ligands, the photosubstitution of Ru-thioether attracts much more attention due to its dynamic and reversible properties. In these research news, we present two applications of reconfigurable materials based on a visible light-controlled Ru-thioether coordination reported by our group. [30,31] 2. Reconfigurable Materials Based on Photocontrolled Ru-Thioether Coordination

Reconfigurable Multifunctional Surfaces Based on Ru-Thioether Photosubstitution
As already introduced, Ru-thioether complexes are capable of reversible ligand photosubstitution between two states by visible light irradiation or in dark conditions. In aqueous solution, visible light irradiation breaks the bond between thioether and the Ru backbone and the ligand is replaced by H 2 O. If the solution is placed in the dark with free thioethers, the thioethers spontaneously coordinate with the Ru backbone. More importantly, reversible and dynamic photoreactions on the surface allow multiple states in the photosubstitution process.
In 2018, our group reported on a reconfigurable surface modified with Ru-thioether, which is controlled by visible light. [30] Compared with the conventional reported photoreactions on surfaces, the Ru-thioether-modified reconfigurable surfaces show reversible and dynamic properties under light control. Inspired by the replacement of different bits by a screwdriver, the aqueous Ru compound [Ru(tpy-COOH)(biq)(H 2 O)](PF 6 ) 2 (Ru-H 2 O) was first grafted onto a substrate that can be defined as a "multibit screwdriver." Different functional thioethers acting as "bits" can be coordinated with Ru-H 2 O. These "bits" could be dynamically controlled with visible light to modify the reconfigurable surface with several functional states. The bits are removed from the screwdriver by photosubstitution using visible light. In the dark, another set of bits is automatically attached to the screwdriver by a thermal substitution process (Figure 2a,b). In this way, a number of surface applications such as surface pattern rewriting, protein adsorption control, and reversible surface wettability have been achieved based on visible lightcontrolled Ru-thioether coordination.
To illustrate the concept of reconfigurable properties of Ru-H 2 O-modified surfaces, fluorescein isothiocyanate (MeSC 2 H 4 -FITC) and rhodamine B isothiocyanate (MeSC 2 H 4 -RhB), both containing thioether groups, were synthesized to provide the ability to rewrite the surface patterns. MeSC 2 H 4 -FITC and MeSC 2 H 4 -RhB are fluorescent under different irradiation wavelengths. First, the Ru-H 2 O-modified surface was immersed in the aqueous solution of MeSC 2 H 4 -FITC in the dark, whereby the color of the surface was transferred from black to green under the fluorescence microscope, demonstrating a fluorescent surface (Figure 2c,d). When the surface under a photomask was irradiated with green light, the exposed area changed back to the original Ru-H 2 O (black) due to photocleavage (Figure 2e). Finally, when the substrate was immersed in the aqueous solution of MeSC 2 H 4 -RhB in the dark, the red area appeared under the fluorescence microscope (Figure 2f ). The whole process successfully demonstrates the rewriting property of the Ru complexmodified surface.
In addition, surface wettability could be reversibly controlled by photosubstitution. By preparing a Ru-H 2 O-modified surface on a porous silica coating, two functional thioether ligands could be coordinated onto the substrate. 2-(Methylthio)ethanol (MTE) could modify the substrate to become hydrophilic. (3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluorodecyl)(methyl) sulfane (HFDMS) was thus able to make the surface hydrophobic by photosubstitution. Figure 1g shows the changes in surface wettability by measuring the contact angle. A Ru-MTE-modified substrate had a water contact angle of 27 AE 2 . After green light irradiation, the hydrophilic MTE was cleaved off, and HFDMS was able to coordinate to the Ru-H 2 O-modified surface in the dark. We could observe that the water contact angle increased dramatically to 154 AE 2 . Thus, we can tune the reconfigurable surface wettability with photosubstitutions of dynamic Ru-thioether bonds.
Not only organic small molecules but also polymers are suitable ligands for the fabrication of reconfigurable surfaces. Manipulable protein adsorption surfaces can be modified by Ru complexes controlled by visible light. By addition of poly(ethylene glycol)-modified thioether (MeSC 2 H 4 -PEG) to the Ru-H 2 O-modified surface a protein-resistant surface was produced because PEGylation has a high resistance to protein adsorption. When the surface is immersed for 2 h in the solution of fluorescence-labeled bovine serum albumin (BSA), it still shows no fluorescence, which is a strong evidence for a protein adsorption-resistant surface. When exposed to visible light through a photomask, exposed parts of the surface coordinated with MeSC 2 H 4 -PEG were cleaved. In addition, the surface was immersed again in BSA solution, whereby the exposed Ru-H 2 O (black) regions captured the fluorescence-labeled BSA via electrostatic interactions (Figure 2h). The Ru-H 2 O-modified surface was converted from protein resistant to protein adsorptive. In addition, a number of photoresponsive biomaterials require a deep penetration of light into the tissue. We were able to show that red light successfully penetrated a 4 mm piece of pork tissue and photoactivated the Ru-MeSC 2 H 4 -PEG-modified surface, thereby converting it into protein adsorptive. Thus, a functional manipulating protein adsorption surface with Ru-thioether complexes was created.  www.advancedsciencenews.com www.advintellsyst.com

Photocontrolled Organohydrogels Based on Ru-Thioether Work Properly Below 0 C
Photoresponsive hydrogels based on polymer networks are well known and appealing for their tremendous applications in science and technology, such as 3D extracellular matrices fabrication, [32] cell adhesive regulation, [33] cargo release, [34] and actuator controlling. [35] However, most photoresponsive gels cannot operate below freezing temperature, because the frozen matrix inhibits photoreactions and structural changes in the gel. Furthermore, it is known that most photoresponsive hydrogels with their supramolecular interactions are only obtained in water. It is desirable to design and synthesize a hydrogel that can work in antifreeze solvents below the freezing point.
In 2020, our group reported for the first time about a reversible photosubstitution process below freezing point based on Ru complexes. [31] The photoresponsvie organohydrogel based on a Ru-containing polymer can operate even at À20 C. Figure 3a shows the schematic concept of photocontrolled organohydrogels based on Ru-thioether crosslinks. We used H 2 O/glycerol as a binary solvent for the antifreeze and Ru-thioether complexes as crosslinks, therefore we call the gel a metallopolymer organohydrogel. First, we prepared three different molar ratios of monomer concentrations to check for gelation. The results suggest that a sufficient amount of coordination bond between Ru and thioether is the key factor for gelation. With a suitable concentration ratio of coordination crosslinks, gel-to-sol transitions occurred under light irradiation at room temperature Figure 3. a) Scheme of photocontrolled organohydrogels based on Ru-thioether crosslinks. b) Organohydrogels with photoinduced reversible sol-gel transitions at 25 C and -20 C. c) Hydrogel of Ru-thioether at 0 C. d) Organohydrogel of Ru-thioether at -20 C. e) Free-standing organohydrogel for self-erasing patterns. f ) Self-erasing and rewritable patterns fabricated on an organohydrogel with different photomasks processed at 25 C and -20 C. Reproduced with permission. [31] Copyright 2020, Wiley-VCH.
www.advancedsciencenews.com www.advintellsyst.com and reversed to the gel state in the dark (Figure 3b). When the temperature dropped to À20 C, the gel-to-sol transitions persisted.
To demonstrate the photoswitchable mechanical properties of the reversible Ru-containing organohydrogels, we prepared control sample hydrogels with the same component ratio in H 2 O. Both hydrogel and organohydrogel worked perfectly at room temperature. When the temperature dropped to 0 C, the hydrogel immediately froze and showed no elastic properties. When the hydrogel was stretched, it became weak and brittle (Figure 3c). In contrast, even at À20 C, the organohydrogel still showed the highly elastic properties. When twisted, knotted, and stretched, it did not become brittle (Figure 3d).
Due to the reversible coordination of Ru-thioether, which induced color changes, we expected a reversible photopatterning based on the organohydrogel. Covering a piece of free-standing organohydrogel with a photomask could achieve this feature. On irradiation with green light, the exposed area of the organohydrogel cleaved the thioether ligand from the Ru backbone and transformed into Ru-H 2 O, which caused a color change and created a pattern. Afterward, the free-standing organohydrogel was placed in the dark at room temperature and the pattern disappeared, which was due to a coordination of the thioether ligand to Ru (Figure 3e,f ). In addition, we first reported on a self-erasable organohydrogel that functioned properly at À20 C. An array pattern was produced by green light irradiation through mask 2 at À20 C. Another three masks were also used for photopatterning by alternating light irradiation and dark transitions at the same temperature. In this way, we produced an antifreeze, reversible, and self-erasing photopatterning material by light control of a Ru-thioether-organohydrogel.

Conclusions
In summary, some ruthenium (Ru) complexes, especially Ru-thioether complexes, show dynamic and reversible properties during photosubstitution as ideal photoresponsive materials. These excellent advantages based on Ru-thioether complexes for multifunctional reconfigurable surfaces and antifreeze organohydrogels via light control have been developed. Ru-containing photoresponsive complexes could open up avenues for a wide range of applications. We believe that our work is a versatile method for adaptive surface functionalizations and provides fundamental knowledge of soft matter in cold environments.