Photoswitchable Liquid‐to‐Solid Transition of Azobenzene‐Decorated Polysiloxanes

Having external control over fundamental properties of polymers, such as their physical state, is a crucial yet challenging design criterion for smart materials. Liquifying polymers through photochemical events has significantly advanced various research lines. However, the opposite process of solidifying a polymer that is intrinsically in a liquid state reversibly with light is unattained. Herein, the light‐controlled liquid‐to‐solid transition of polysiloxanes is reported, which are decorated with a small number of azobenzene‐functionalized ureidopyrimidinone (Azo‐UPy) pendants. The UPy moieties toggle between intra‐ and intermolecular hydrogen bonding via trans→cis photoisomerization of the azobenzene. This transformation on the molecular level leads to the formation of strong supramolecular cross‐links, which, in turn, results in the macroscopic solidification of the material. The photoswitching event enables the post‐synthetic tailoring of the polymers’ mechanical properties, thus providing an alternative to the addition of plasticizers or hardeners. Moreover, the adhesion strength of the photochromic material increases by a factor of 6 upon exposure to UV light. In situ illumination during rheological measurements reveals the delicate interplay between wavelength dependent penetration depth and photoswitching efficiency. This conceptually new (de)bonding on demand strategy paves the way for creating light‐responsive materials with exciting applications in temporal adhesion, recycling, lithography, and material processing.


Introduction
Next-generation materials that allow external and on demand modification of their macroscopic properties have gained considerable attention to meet the challenges of modern society. [1][2][3][4] The use of light as a non-invasive stimulus has numerous benefits due to its exquisite precision in terms of space, time, and energy. [5] Consequently, photoresponsive polymers have extensively been studied as their physicochemical properties can be adjusted post-synthetically without the addition of further reagents such as plasticizers or hardeners. [6][7][8][9] In particular, the incorporation of photoswitchable molecules into materials provides ample opportunities to remote-control their functions and features. [10] Photoswitches are small organic molecules that undergo reversible transformation between their stable and (meta)stable isomers upon exposure to light. [11][12][13][14][15] By rational design, the geometrical and electronic changes on the molecular (photoswitch) level can trigger pronounced macroscopic effects. [16][17][18][19][20][21][22] In recent years, comprehensive research has been dedicated to developing smart polymeric materials that become processable at room temperature by photochemical processes. [23,24] Wu et al. observed in their seminal work on linear azobenzene-decorated polyacrylates a reversible solid-to-liquid transition through the isomerization of the photochromic pendants. [25] By photoswitching the azobenzenes from the stretched trans state to the bent cis isomer, the glass transition temperature T g of the polymer and thus its adhesion strength were reduced notably. This photomelting phenomenon has been conceptually expanded to crosslinked materials, [26] alternating polymers, [27] as well as telechelic oligomers. [28] Multistage photomodulation of T g was achieved by Aprahamian and co-workers by illuminating bistable hydrazone photoswitches with different colors of light. [29] These photochromic polymers have found wide-spread application as self-healing materials [30] and photoactuators. [26] Another feature of great interest is (de)bonding on demand for temporal adhesion, [31][32][33] in which non-covalent interactions play an integral role. [34,35] Low molecular weight photoswitches have also been discovered to undergo reversible solid-to-liquid phase transitions, [36] yet lack material processability and mechanical integrity as opposed to photoswitches anchored to an amorphous polymer support.
To maximize the effect of the photoswitching event on the bulk properties, a high content of photoswitches is incorporated in these polymeric materials. On the one hand, the large quantity of dyes renders the materials optically dense, which strongly limits the penetration depth of light into the material and restricts the overall photoswitching efficiency. [24] To bypass this obstacle, the photochromic polymers are often dissolved in organic solvents, switched in solution, and subsequently dried. [29,30] While this workaround is beneficial to achieve the desired effect, it hampers applications in bulk. On the other hand, the high amount of crystalline photochromic molecules generally causes the materials to be solid in their native state and thereby reduces the design space for advanced materials. Hence, novel strategies to achieve large macroscopic effects upon illumination with light at a low number of integrated photoswitches are desired. [37] This would allow for the creation of photoresponsive polymeric materials that are liquids in their thermodynamically stable state and solidify upon light illumination. This conceptually new approach makes the development of innovative adhesion, lithography, and material processing technologies accessible.
Here, we present the first example of a light-induced and reversible liquid-to-solid transition of azobenzene-containing polymers based on supramolecular bonding and debonding (Figure 1a). We employ an ortho-ester substituted azobenzene moiety covalently linked to an ureidopyrimidinone group (Azo-UPy), which was previously devised by Zhang and coworkers. [38] Photoisomerization to the cis-azobenzene unlocks the Azo-UPy motif, which forms strong homodimers based on quadrupole hydrogen-bonding. [39] This on demand light-induced modification was used to steer the formation of supramolecular dimers in solution. Inspired by this approach, we set out to exploit the Azo-UPy group as a photoswitchable cross-linking unit attached to linear polysiloxanes, serving as a liquid polymer platform, and examine its photoswitching behavior in bulk. In the liquid dormant state, the binding site of the Azo-UPy group is intramolecularly blocked by the ester group of the trans-azobenzene. Upon photoisomerization using UV light, the supramolecular polysiloxane network solidifies -a process that is reversible by regenerating the trans-azobenzene through irradiation with light of a longer wavelength or heat. We demonstrate the potential of this lightgated liquid-to-solid transition by remote-controlling the me- chanical properties and adhesion abilities of the photochromic material. Based on in-depth rheological ( Figure 1b) and mechanical experiments, we critically analyze the importance of choosing the optimal color of light to maximize the desired macroscopic effect.

Results and Discussion
To evaluate the effect of cross-linking density on the macroscopic properties, we functionalized commercially available poly(dimethylsiloxane-co-methylhydrosiloxane) with different ratios of Azo-UPy and 1-hexene via hydrosilylation to create a polymer library (P0-P4, Figure 2a). While P0 is solely decorated with hexyl side chains, thereby serving as a control polymer sample, P1-P4 incorporate a small number of azobenzene pendant groups (1-4 units per polymer chain on average). The detailed synthesis of Azo-UPy and the functionalized polysiloxanes P0-P4 as well as their characterization by NMR and FTIR spectroscopy are described in the Supporting Information (Figures S1-S16, Supporting Information). As shown by differential scanning calorimetry (DSC), all polymers P1-P4 exhibit an amorphous character without an observable T g in the measured temperature range ( Figure S17, Supporting Information).
While Zhang and coworkers extensively studied related photoswitchable UPy groups in solution, [38] small structural . c) A 10 μm thin film of P4 was exposed to an alternating sequence of 365 and 530 nm light until the respective photostationary states were obtained (t irr = 2 min each). The recorded absorption values at 390 nm are plotted as a function of the number of switching cycles to display the reversibility and fatigue resistance of the photochromic system. FTIR spectra of the N-H urea stretch region (2900-3500 cm −1 ) of d) Azo-UPy and e) P4 in the bulk state after illumination with various wavelengths. The spectral changes indicate a decrease in the concentration of intramolecular hydrogen bonds between the ester and urea group in the Azo-UPy moiety upon light-induced trans→cis isomerization. All FTIR spectra are normalized at 3080 cm −1 .
differences in our Azo-UPy model compound prompted us to analyze its photochromic behavior with UV/vis spectroscopy. To this end, a 3.8 × 10 −5 m solution of Azo-UPy in acetonitrile was illuminated with a 365 nm light-emitting diode (LED). During the course of irradiation, the typical spectral changes, which are accompanied with the trans→cis isomerization of the azobenzene, were monitored (the photostationary state, PSS, was reached after 90 s, Figure S18, Supporting Information). The reverse cis→trans isomerization was achieved either partially by green light irradiation ( irr = 530 nm) or fully by thermal relaxation at room temperature (t 1/2 = 19.6 h). The photoswitching efficiency was investigated by 1 H NMR spectroscopy in deuterated chloroform in dependence on the wavelength of light ( Figure S19, Supporting Information). We observed that the PSS decreased by raising the wavelength values of the LEDs, yielding conversions to the cis isomer of 41% (365 nm), 32% (405 nm), 17% (455 nm), and 18% (530 nm). Next, we selected polymer P4 with the highest number of azobenzene units incorporated to perform a thin film illumination study (Figure 2b). The polymer displayed absorption maxima at 330 and 385 nm, which are attributed to the azobenzene moieties, and exhibited a photoswitching behavior comparable to the model compound Azo-UPy in acetonitrile solution. Under illumination with light, the azobenzene groups underwent trans→cis isomerization, as indicated by hypochromic spectral changes. Notably, the changes were the most pronounced when exposed to light with a wavelength of 365 nm and became less apparent with the increase in the LED's wavelength, which is consistent with the photoswitching efficiencies determined via NMR spectroscopy ( Figure S20, Supporting Information). The cis-azobenzene units within the polysiloxane environment showed faster thermal relaxation to their native trans states compared to those in an acetonitrile solution (t 1/2 = 8.3 h). The reversible and robust nature of the photochromic polymer was demonstrated by exposing the thin film of P4 alternatingly to 365 and 530 nm light for several cycles without signs of fatigue (Figure 2c).
A first indication that the unlocked Azo-UPy groups assemble and form intermolecular hydrogen bonds was obtained by DOSY NMR spectroscopy ( Figures S21 and S22, Supporting Information). After reaching a PSS of 40% through UV light illumination, polymer P4 in CDCl 3 solution displayed a decrease in diffusion coefficient from 1.9 to 1.1 × 10 −6 cm 2 s −1 due to the formation of a supramolecular polymer network. The fact that the small molecule Azo-UPy showed a similar effect under comparable conditions, along with the characteristic downfield shift of the N-H signals, strengthens the evidence for supramolecular dimerization ( Figures S23 and S24, Supporting Information). To demonstrate that dimerization also occurs in the condensed phase, we recorded spectral changes via FTIR spectroscopy experiments that account for photoisomerization of azobenzene units in Azo-UPy and P4. As displayed in Figure 2d,e, the N-H urea stretch vibration at 3275 cm −1 , which is generally observed for hydrogen bonded urea moieties, [40] diminishes upon irradiation with light. This spectral change indicates that photoisomerization causes the concentration of intramolecular hydrogen bonds between the ester and urea group present in the trans isomer of Azo-UPy to decrease. It should be noted that a distinct vibration at 3275 cm −1 is rarely observed for UPy dimers due to the disparate molecular structure. Instead, a light-induced increase in intensity of the N-H urea bending vibration at ≈1600 cm −1 was recorded, which is typical for UPy dimerization ( Figure S25, Supporting Information). [41] The photoisomerization and formation of UPy dimers in Azo-UPy upon illumination with various wavelengths of light was further supported by the emergence of a free C=O ester stretch vibration at 1720 cm −1 ( Figure S25, Supporting Information).
After gaining knowledge about the photoswitching behavior of polymer P4 in both solution and bulk, we aimed to probe the effect on the macroscopic properties. Initially, we performed rheological measurements on P0 and the photochromic polymers P1-P4 in the linear dormant state and cross-linked dynamic state to determine the most significant change in moduli ( Figures  S26-S30, Supporting Information). For P1-P4, light-induced isomerization was initially conducted in dichloromethane solution to maximize the switching efficiency prior to measuring the cross-linked dynamic state in the bulk. Interestingly, the increasing moduli observed in the frequency sweeps of P1-P4 prior to illumination compared to P0 suggest that weak secondarystacking is already present between the Azo-UPy groups, [42] yet still all samples retain a liquid-like character at low frequencies. Subsequently, upon switching with a 365 nm LED, polymers P1-P4 displayed a remarkable increase in their elastic character due to cross-linking by Azo-UPy dimerization. Consequently, the average relaxation times derived from the G'/G'' intersection of P2, P3, and P4 increased with a factor 10, 15, and 60, respectively. Polymer P1 did not yet exhibit a crossover point in the measured frequency range. In case of P4, the liquid-like region shifts outside of the measured frequency range and the increased relaxation time in combination with the increased modulus effectively result in a liquid-to-solid transition in the lower frequency range (0.5-30 rad s −1 , Figure 3a,b). This rheological change is indicative of transient network formation and results in rubberlike behavior [43][44][45] at typical deformation time scales. We attribute this remarkable difference to efficient cross-linking resulting from the pronounced mobility of the photoswitchable groups within the fluid-like polysiloxane matrix and a highparameter, which forces the Azo-UPy motifs together. [46] On the contrary, P1-P3 kept their fluid-like properties, most likely due to lack of network formation. The insufficient network formation for P1-P3 can be rationalized based on the aforementioned photoisomerization yields of roughly 30-40%, which render 1 in 3 cross-link points active. Hence, only P4 is pushed into the range of a fully interconnected network structure and is therefore selected for further in-depth rheological and mechanical measurements. By thermal cis→trans isomerization of the illuminated polymer P4 over a period of 16 h, we showed the reversibility of this system transitioning from a transient network back to the initial liquid dormant state (inset Figure 3b). Photographs visualize the large difference in physical appearance of polymer P4 before and after illumination with UV light supporting the liquid-tosolid transition (Figure 3; Figure S31, Supporting Information).
The previous measurements on the cross-linked samples were performed after photoswitching in solution and subsequent removal of the solvent. Illuminating the photochromic polymers in solution generally increases the desired effect and has been established as a standard protocol in literature to bypass the high optical density of the bulk material. [29,30] To go one step further, we illuminated polymer P4 in situ during rheological measurements to explore the bulk photoswitching behavior. For this purpose, we used a rheometer modified with a transparent window enabling illumination through the bottom plate while measuring (Figure 4a). The subsequent time sweep at a constant frequency (10 rad s −1 ) provided us with three consecutive cycles of 365 nm light illumination followed by cooling and thermal relaxation. All three cycles are almost identical which is characteristic for a nondestructive and reversible switching process (Figure 4b). However, the change in moduli is less significant compared to that of the samples measured after switching in solution, hinting toward the influence of optical density hampering the penetration of the light through the sample.
This motivated us to utilize light with longer wavelengths at which the extinction coefficient of the material is lower and thus the penetration depth of the light is larger. Compared to the extinction coefficient at 365 nm, the values decrease by a factor of 1.2 at 405 nm, 1.9 at 455 nm, and 10.8 at 530 nm ( Figure  S32 and Table S1, Supporting Information). Accordingly, LEDs emitting at these four wavelengths were successively applied in the in situ illumination rheological measurements and the tan values were recorded to assess the liquid-to-solid transitions. Tan > 1 indicates behavior that is dominated by viscous flow, i.e., liquid-like behavior, whereas tan < 1 indicates that elastic contributions dominate the response resulting in solid-like or elastic behavior at a particular frequency. Compared to UV light (tan = 0.98), the illumination with visible light resulted in lower tan values of 0.96 for 405 nm, 0.93 for 455 nm, and 0.82 for 530 nm at 10 rad s −1 , hence inducing a more solid-like character of the polysiloxane network (Figure 4c). Even though the photostationary state of Azo-UPy is the highest at 365 nm and decreases with longer wavelengths, the largest macroscopic effect and strongest solidification was achieved with green light when irradiated in bulk. This experiment showcases that a more pronounced penetration depth counterbalances or even surpasses a larger PSS in regard to the desired light-induced effect in bulk. In addition, we performed amplitude-modulated frequency-modulated atomic force microscopy (AM-FM AFM) to probe the viscoelastic properties of a thin film of polymer P4 at the microscopic and nanoscopic scale. The tan distributions after 365 nm light illumination and thermal relaxation closely match with the tan changes observed in the rheological measurements indicating the absence of locally microphase segregated areas (Figure 4d; Figure S33, Supporting Information).
The significant change observed in macroscopic properties and elasticity upon illumination prompted us to examine the adhesive properties. Therefore, mechanical experiments were performed in lap shear mode on polymer P4 before and after illumination with different light sources. Herein, a thin layer of polymer P4 (d = 10 μm) was applied between two glass slides and illuminated from both sides to eliminate the effect of wavelength dependent penetration depth. Upon illumination with 365 nm light, a 6-fold increase in the initial lap shear strength compared to the pristine material was observed (Figure 5a; Figure S34, Supporting Information). This is a remarkable change in the adhesive response considering that the photoswitching process is intrinsically not quantitative and thus only a fraction of the four supramolecular cross-linking points gets activated. The fracture of the adhesive film occurring at high deformation rates further indicates the solidification of P4 upon illumination compared to the constant flow observed for the non-illuminated material ( Figure S35, Supporting Information). In addition, a 3-fold increase of the lap shear strength was realized with 405 nm light and a factor of two enhancement was observed for 455 and 530 nm light, respectively. This wavelength dependency is in accordance with the previously determined switching efficiencies.
Finally, we visually demonstrated the reversible bonding and debonding capabilities of these photoswitchable polymers. Hereto, the adhesive strength of polymer P4 between two glass slides was sequentially examined upon illumination with 365 and 530 nm light using counterweights. Initially, polymer P4 supports up to 50 g of weight before illumination (Figure 5b), while the maximum load could be increased to 1200 g after illumination with 365 nm light (Figure 5c). This notable change in adhesion strength showcases the large impact of non-covalent Azo-UPy dimerization. Eventually, on demand debonding of the two glass slides was realized by exposing the cross-linked polymer to 530 nm light for 180 s due to cis→trans isomerization (Figure 5d-f). Hereby, a temperature increase of 0.1°C was observed upon 530 nm light irradiation, confirming that the debonding process is exclusively driven by a photochemical effect and not photothermally induced.

Conclusion
Our study represents a novel strategy for altering the physical state of photochromic polymers from liquid to solid reversibly with light at ambient conditions. By anchoring azobenzene functionalized UPy groups to the backbone of polysiloxanes, we were able to turn the supramolecular network formation on and off. In stark contrast to related photochromic polymers in literature, we attached only a small number of photoswitchable pendants to the polymeric backbone (<1% of siloxane repeating units are functionalized with azobenzenes). This strategy allowed us to obtain liquid-like polymers with relatively low optical density, a prerequisite for effective photoswitching in bulk. Irrespective of the low photoswitch content and the non-quantitative trans→cis isomerization, the polymers' rheological, mechanical, and adhesion properties changed substantially upon light irradiation due to strong Azo-UPy dimerization. Polymer P4 with four Azo-UPy units incorporated showed the largest response to light as reflected by the distinct changes in storage and loss moduli and a 6fold increase in lap shear strength. Our study also accentuates the crucial trade-off between penetration depth and photoswitching efficiency depending on the color of light. Overall, our approach offers a significant departure from the widely studied solid-toliquid transition observed in photochromic polymers, as extensively documented in the literature. [25,26,31] The defining feature of our polymeric system is that it exhibits a liquid thermodynamically stable state and solidifies when converted to its metastable state with light. This conceptually new strategy makes exciting paths accessible to develop smart materials applied in (temporal) adhesion, 3D printing, and material processing technologies. www.advancedsciencenews.com www.afm-journal.de

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.