A solar actuator based on hydrogen-bonded azopolymers for electricity generation

Max Planck Institute for Polymer Resea Germany. E-mail: wusi@mpip-mainz.mpg.d New York University Abu Dhabi, PO Box 12 E-mail: pance.naumov@nyu.edu Department of Chemistry, Zhejiang Sci-Tech Province, China Department of Chemistry and Molecular En Shanghai 200241, China † Electronic supplementary informa 10.1039/c7ta11139h ‡ These authors contributed equally to th Cite this: J. Mater. Chem. A, 2018, 6, 3361

which stabilizes liquid crystal phases, while the cis-azobenzene is not a liquid crystal mesogen and usually destabilizes the liquid crystal phases. 29 Thus, trans-to-cis photoisomerization of azobenzene groups in LCEs can induce liquid-crystal-toisotropic phase transition, which results in reshaping of the LCEs that manifests as mechanical motion. 1 Additionally, the photoisomerization of azobenzene has been widely utilized to control the state, morphology, and size of polymer matrices. [30][31][32][33][34] Since the trans-to-cis isomerization is induced by UV light, the UV light is normally the rst choice as a stimulus to actuate azobenzene-based LCEs. 1 Lasers at visible wavelengths, which can induce trans-cis-trans cycling of azobenzene groups in LCEs have also been used. 19 Since the sun is a cost-free source of energy, using sunlight is a comparatively much more energyefficient approach to photoactuation. Because visible light comprises the majority of the solar spectrum, azotolane derivatives responsive to wavelengths that are red-shied relative to the absorption of conventional azobenzenes were prepared and introduced into LCEs. [35][36][37] It was shown that-aer being concentrated by lenses and ltered to a suitable wavelength range-sunlight can indeed induce bending of such azotolanecontaining LCEs. 35,36 Recently, tetra-ortho-substituted azobenzenes that can be switched solely by visible light were developed. [38][39][40][41][42][43][44][45][46] Kumar et al. 47 fabricated ortho-uoroazobenzene-containing LCEs and showed that non-concentrated sunlight induces chaotic oscillations with relatively small amplitudes. While sunlight-induced motion has already been demonstrated, the development of solar actuators with controlled and enhanced motions for efficient solar energy conversion remains a challenge.
In this work, by using a fundamentally different approach from that used in photoactuating LCEs, we succeeded in developing a new actuator that is driven by sunlight. Unlike ordinary LCEs, the new photoactive material does not require alignment of chromophores and crosslinking, making the solar actuator better suited for implementation in devices. Importantly, we also demonstrated that sunlight-induced bending of the new actuator is efficient enough to generate electricity. The actuator (F-Azo@AG) was fabricated by introducing 2,2 0 ,6,6 0 -tetrauoro-4,4 0 -diacetamidoazobenzene (F-Azo) into agarose (AG) as the matrix (Fig. 1). The absorption spectrum of F-Azo overlaps with the solar spectrum better than the azobenzene (Fig. S1 †) and its photoisomerization can be induced with sunlight. AG was used as the matrix based on previous results showing that when combined with photoswitchable molecules such as 1,4-bis(para-hydroxystyryl)benzene or azobenzene derivatives, AG is a mechanically compliant matrix that can be efficiently actuated. 20,22 The hydrogen bonding network of AG provides both moderate mechanical strength and soness required for photoactuation. Furthermore, the F-Azo@AG lm was coupled to a polyvinylidene diuoride (PVDF) lm that acts as a piezoelectric transducer (Fig. 1). PVDF is a dielectric material capable of generating electric current when subjected to external mechanical force. [48][49][50] Upon irradiation with sunlight, the bending of the F-Azo@AG lm drives bending of the PVDF piezoelectric transducer, resulting in generation of electricity.
F-Azo was synthesized by coupling 2,6-diuoro-4-bromoaniline followed by substitution with acetamide (for details, see the ESI, Fig. S2 †). 39 The trans isomer of F-Azo has a p-p* absorption band in the UV region and an n-p* band in the visible region (Fig. 2b). Thus, F-Azo can absorb sunlight from the UV region to around 600 nm in the visible range. When compared to azobenzene, F-Azo has a more intense and red-shied absorption band in the visible range ( Fig. S1 †), making F-Azo a better suited chromophore for sunlight harvesting.
The photoisomerization was also quantitatively studied using 1 H NMR spectroscopy (Fig. 2c). The signals of F-Azo at 10.65, 7.50 and 2.12 ppm are ascribed to H 1, 2, and 3, respectively (Fig. 2a). Aer F-Azo was irradiated with green light (530 nm, 4.94 mW cm À2 ), three new signals of the cis isomer at 10.48, 7.33, 2.05 ppm appeared. Integration showed that $75% of the molecules exist as the cis isomer in the photostationary state. Subsequent irradiation with blue light (470 nm, 2.23 mW cm À2 ) reduced the cis content to $44% at the photostationary state. Because both trans and cis isomers absorb blue light, the concomitant trans-to-cis and cis-to-trans isomerization induced by blue light resulted in a mixture of the isomers. Complete cis-to-trans isomerization was accomplished by heating the sample at 90 C for 30 min. Interestingly, approximately 62% cis isomer was obtained when trans-F-Azo was irradiated with a solar simulator to a photostationary state, which indicated that sunlight can induce trans-to-cis isomerization of F-Azo.
Encouraged by this result, we introduced F-Azo into the AG matrix to prepare a solar actuator F-Azo@AG which contains $1.1 wt% F-Azo. When more F-Azo is introduced, it will crystallize in the AG matrix. As a result, the bending of the lm will be weakened. The F-Azo@AG lms were prepared by mixing F-Azo and AG in N,N-dimethyl formamide, casting the mixture on glass slides, and drying at room temperature over two days. Free-standing F-Azo@AG lms were obtained by peeling the lms off the glass substrates and stored in a desiccator. Firstly, the lm was measured using UV spectroscopy. As illustrated in Fig. S3, † the trans absorption decreased and the cis absorption increased simultaneously when the lm was illuminated by green light. The F-Azo@AG lm was also examined using a solar simulator. As shown in Fig. S4, † the trans absorption of the lm attenuated when it was exposed to the solar simulator. The results demonstrated that the trans-to-cis isomerization of F-Azo in the lm could be achieved under the irradiation of green light and solar simulator. The motion of a free-standing F-Azo@AG lm (20 mm Â 20 mm Â 40 mm) was studied by exposing the lm to light from a solar simulator ( Fig. 3a and Movie 1 †). The lm responded by bending along its centreline with small oscillations. The lm bends towards the sunlight and is thus phototropic. Aer irradiation for a few seconds, the lm reached maximal deection with only small amplitude oscillations. Subsequently, the lm was ipped and its reverse side was exposed to light from the solar simulator ( Fig. 3a(IV)). The lm unbent until it became nearly at (Fig. 3a(VI)), and then bent towards the sunlight (Fig. 3a(VII)). The result conrms that sunlight can indeed induce bending of the F-Azo@AG lm.
The mechanism of operation of AG-based actuators is qualitatively different from that of LCEs. It is well known that the orientation of liquid crystal mesogens controls the bending of LCEs. 1,3 In contrast, the F-Azo@AG lm was dark under polarized light, indicating that F-Azo is distributed homogeneously in the AG matrix and does not have any preferred orientation (Fig. S5 †).
The bending of AG-based actuators is usually affected by their shapes, imperfections in the fabrication process, and creases on the actuators. Previous work has shown that the shape of AG-based actuators is the primary factor that determines their bending performance. 20,22 To investigate these effects, the response to sunlight of F-Azo@AG lms with different shapes were studied. F-Azo@AG lms were cut into strips with a xed length and different widths (length, 7 mm; width, 1, 3, or 5 mm) and clamped using a support (Fig. 3b). Their bending performance under simulated sunlight (76.6 mW cm À2 ) was quantied ( Fig. 3c and Movies 2-4 †). The bending rate of the 1 mm-wide strip was the fastest and its tip deection was also the largest. As the width of the strip increased to 3 and 5 mm, the bending rate and tip deection gradually reduced. This result shows that the bending of F-Azo@AG strips is dependent on their shape. When the width increases, the aspect ratio decreases as the short side of the strip becomes close in length to the long side. The stress for bending along the longitudinal direction becomes comparable to that along the transverse direction. The strip has a tendency to bend and oscillate in both the longitudinal and the transverse directions. Although xing of one end of the 5 mm-wide strip using a clamp hindered its motion along the longitudinal  The bending performance of F-Azo@AG actuators can also be controlled by adjusting the light intensity. The bending performances of F-Azo@AG strip (7 mm Â 1 mm Â 40 mm) was studied by irradiation with simulated sunlight with different intensities (Fig. 3d and Movies 5-7 †). When the light intensity was increased, the bending rate and the tip deection also increased. This result shows that higher light intensity stimulates more pronounced optomechanical response.
To understand the mechanism of sunlight-induced motility, an AG lm without F-Azo was irradiated with sunlight and its response was observed as a control (Fig. S6 and Movie 8 †). Irradiation of the AG lm induced very small oscillations (tip deection < 3 pixels). However, no bending was observed. This result conrms that the bending of the hybrid lms is due to the presence of F-Azo. The minor oscillations reect temporal instability which could be due to strains caused by photothermal effects. A comparison with earlier studies sheds some light on these effects. Although the motility of many azobenzene-based LCEs is due to a transition of the liquid crystal phase to the isotropic phase based on trans-to-cis photoisomerization, 18 photothermal effects cannot be excluded. For example, Zhao's group reported photoinduced contraction of LCEs originating from both photoisomerization of azobenzene and a photothermal effect. 15 Priimägi and collaborators demonstrated that LCEs doped with an azo dye showed photoinduced bending due to a photothermal effect. 51 To assess the photothermal effects, the F-Azo@AG lm was studied using an infrared thermometer. Irradiating a F-Azo@AG lm with simulated sunlight at 76.6 mW cm À2 increased the sample temperature by 3 C (Fig. S7 †). Our previous work has shown that direct heating of AG-based actuators of up to 6 C does not result in bending. 20,22 Importantly, irradiating the AG lm did not result in bending (Fig. S6 and Movie 8 †). Taken together, these results discredit the photothermal effects in F-Azo@AG as a major contributor to the motion of the lm.
We veried the effects of photoisomerization by irradiating F-Azo@AG lm with light of different wavelengths (Fig. 4, Movies 9 and 10 †). Irradiating the F-Azo@AG lm with green light (l ¼ 530 nm, 16.5 mW cm À2 ) for 18 s resulted in a bending angle of 52 ( Fig. 4a and Movie 9 †). The bent lm was unable to straighten to the initial state spontaneously, suggesting that bending is due to the existence of the cis form. In contrast, exposure of the F-Azo@AG lm to blue light (14.8 mW cm À2 ) for 81 s resulted in a bending angle of 15 ( Fig. 4b and Movie 10 †). The bending induced with blue light was slower and had smaller amplitude than when induced with green light. Although F-Azo can absorb more blue light than green light (Fig. 2b), the trans-to-cis isomerization is more efficiently induced with green light (Fig. 2c). The wavelength-dependent bending reveals that more efficient trans-cis isomerization leads to more efficient bending. These results also suggest that the photoisomerization plays an important role in the bending of F-Azo@AG, although the photothermal effect may synergistically induce some motion. The actual mechanism by which the hydrogen bonding network mediates the transfer of a momentum from the photoswitchable chromophores to the matrix within the reported AG-based actuators remains unclear and is a subject of ongoing research.
The most immediate application of sunlight-induced motions is conversion of solar light into another, useful form of energy. For this purpose, an F-Azo@AG lm was mechanically coupled with a PVDF piezoelectric transducer (Fig. 5a). A weight  was attached to the bottom of the F-Azo@AG lm, which stabilized the vibration of the generator. Tensile testing (Fig. S8 †) conrmed that F-Azo@AG lm was hard enough to transfer the photostress to the PVDF piezoelectric transducer. F-Azo@AG lm was also analyzed using a differential scanning calorimeter (DSC). As illustrated in Fig. S9, † no endothermic melting peaks could be observed in the curves before and aer light irradiation. The results demonstrated that there were no F-Azo crystals in the AG matrix, i.e. F-Azo was homogeneously dispersed in the AG matrix. When the lm was exposed briey to solar light, it instantly generated a bending movement that was transferred to the PVDF generator, resulting in an opencircuit alternating voltage signal. When the solar simulator was switched on in an interval of every 3 s, alternating voltage signals were generated with the same interval (Fig. 5b). To test if sunlight could drive the device for continuous generation of alternating voltage signal, the solar simulator was switched on for 6 s. A stable alternating voltage signal of 0.164 V was generated continuously for 6 s, indicating that the electricity generation could be accurately controlled by controlling the sunlight exposure time (Fig. 5c, dashed lines). A control experiment performed in the dark conrmed that the electricity generation is due to photoactuation (Fig. S10 †).

Conclusions
In summary, a novel solar actuator F-Azo@AG was fabricated by introducing sunlight-responsive F-Azo into the AG matrix. Sunlight can efficiently induce bending of the actuator. To demonstrate the use of F-Azo@AG for solar energy conversion, F-Azo@AG lm and a piezoelectric transducer were coupled into an electrical generator which converts the sunlight-driven motion of F-Azo@AG into electricity. The design principle for F-Azo@AG is fundamentally different from that of LCEs; notably, F-Azo@AG does not require prealignment of the chromophores and crosslinking, which facilitates the fabrication of devices. Compared with a humidity-driven electricity generator based on AG, 20,22 the solar actuator operates in a dry environment, which can be easily accomplished with simple electronics without hazards with short circuits. The main asset of the F-Azo@AG actuator, however, is its ability to harness solar energy. Within a broader context, this work provides a new strategy for designing solar actuators and demonstrates the application of actuators to solar energy conversion.

Conflicts of interest
There are no conicts to declare.