Photoresponsive polymers with multi-azobenzene groups

Azobenzene is a switchable compound that exhibits reversible trans – cis photoisomerization. Photoresponsive polymers have been prepared by incorporating azobenzene groups into polymers. Most of the reported azo-benzene-containing polymers (azopolymers) have only one azobenzene group in the repeat unit of the polymers (mono-azopolymers). To improve photoresponsiveness, multi-azobenzene groups have been covalently or non-covalently introduced into the repeat units of azopolymers (multi-azopolymers). In this Perspective, we introduce photoresponsive multi-azopolymers. We summarize the preparation of such polymers and high-light their potential applications in photoactuation, photo-patterning, and photoinduced birefringence. The innovative potential, open questions, and remaining challenges in this ﬁ eld are also discussed.


Introduction
Azobenzene is a photoswitchable compound. 1 Light irradiation can induce reversible trans-to-cis isomerization. 2,3 This property enables the use of light to control a variety of properties of azobenzene-containing materials, such as their polarity, 4,5 solubility, 5,6 melting points, 7-9 glass transition temperature (T g ) values, [10][11][12] viscosity, [13][14][15] wrinkles, [16][17][18] morphology, [19][20][21][22][23] mechanical properties, 24,25 etc. In the past, the photophysical and photochemical properties of azobenzene derivatives have been extensively studied. Substituents strongly influence the spectral characteristics and photoswitching behaviors of azobenzene derivatives. Rau divided azobenzenes 26,27 into three types ( Fig. 1): (1) azobenzene-type compounds bearing alkyl, aryl, halide, carbonyl, amide, nitrile, ester, and carboxylic acid substituents. The π-π* and n-π* absorption bands of trans azobenzene are located in the UV range and visible range, respectively. UV irradiation induces trans-to-cis isomerization (Fig. 2a). The half-life of cis isomers is usually from hours to days. Visible light irradiation  or thermal relaxation induces cis-to-trans isomerization (Fig. 2a); (2) aminoazobenzene-type compounds having at least one amino or hydroxyl group located ortho or para to the azobenzene group. The π-π* absorption band of the trans isomer is bathochromically shifted to 400-450 nm, partially overlapping with the n-π* absorption band. The half-life of cis isomers becomes shorter, usually from milliseconds to seconds; (3) pseudo-stilbene-type compounds containing a push-pull substitution para to the azobenzene group. Their π-π* absorption band and n-π* absorption band are both located in the visible range, completely overlapping with each other. These types of azobenzene compounds have a fast rate for photoisomerization. The half-lives of cis isomers are usually from milliseconds to seconds.

Types of multi-azopolymers
We summarize a few types of multi-azopolymers in Fig. 3. There is still room for designing new multi-azopolymers based on polymer chemistry, supramolecular chemistry, and organic chemistry. From the polymer chemistry point of view, multiazobenzene groups can be introduced into polymer side chains, backbones, or both. Because of the diversity of polymer structures, new multi-azopolymers can be designed. From the supramolecular chemistry point of view, multiazopolymers can be prepared by non-covalently introducing multi-azobenzene groups into polymers. From the organic chemistry point of view, small-molecule multi-azobenzene groups can have different structures (see for example bisazobenzenes with flexible and conjugated linkers in Fig. 2a). Multi-azobenzene groups can be introduced into a repeat unit of polymers either in parallel or in series. In the following, we introduce different types of multi-azopolymers according to their structures.

Polymers with non-conjugated multiazobenzene groups
Non-conjugated azobenzene groups mean that two or more azobenzene groups are connected via non-conjugated flexible linkers. Non-conjugated multi-azobenzene groups can be introduced into polymers in series. For example, Aida and coworkers studied a polymer with three azobenzene groups in series in the side chain of each repeat unit (Fig. 4). 93 They synthesized a methacrylate monomer with three azobenzene groups and prepared polymer 1 through free radical polymerization (Fig. 4a, m = 3). Because of the long side chains, polymer 1 is like a brush. The polymer chains of 1 were oriented homeotropically to the substrates via one-step hot-pressing (Fig. 4b).
The hot-pressed film exhibited a rapid and reversible photomechanical response under the irradiation of UV and visible light ( Fig. 4c and d). 74,94 In contrast, the control sample, polymer 2 (Fig. 4a, m = 1), which had only one azobenzene group in each repeat unit, did not show the alignment behavior by hot pressing. These results indicate that the multi-azopolymer forms ordered structures more easily via hot pressing than the mono-azopolymer does. On the basis of these interesting findings, the authors demonstrated a design principle for polymers that form 2D homeotropic orders. 95 Akiyama et al. prepared oligo-dendrimer type molecules, multiazobenzene sugar-alcohol derivatives, [96][97][98] in which the multi azobenzene groups were connected in parallel with each other. These kinds of molecules showed photochemically reversible solidliquid transitions and could be applied to reworkable adhesives.
In 2013, Zhu and Wang reported polymers with two and three azobenzene groups in parallel in the polymer side chains. 99 These polymers showed reversible photoisomerization. Furthermore, birefringence was induced by LPL irradiation. Multi-azobenzene moieties in each repeat unit of the polymer resulted in larger birefringence.
Kim et al. also prepared polymers with azobenzene groups in parallel in the polymer side chains (Fig. 5). 100 Different from Zhu and Wang's strategy, polymer 3 was synthesized via ring opening metathesis polymerization (ROMP). Free-standing films were prepared. The backbone and azobenzene side chains of polymer 3 in the film were perpendicular and parallel to the layer normal, respectively. When the film was exposed to UV light, it quickly bent away from the light source due to trans-to-cis isomerization (Fig. 5b-d). After subsequent visible light irradiation, the film recovered to its original state because of cis-to-trans back isomerization. Photoinduced bending and unbending were fully reversible (Fig. 5e). Further investigations showed that such types of multi-azopolymers exhibit photoisomerization 101 and photoinduced chirality, 102 and can be used as photoresponsive semiconductors with good photo-fatigue resistance. 103 Polymers with conjugated multiazobenzene groups Some azo groups were connected via conjugated linkers. Conjugated multi-azobenzene groups have been introduced into polymer side chains (Fig. 6a). Polymer 4 was prepared by Meng and Natansohn through free radical polymerization. 90 Compared with 5 that has mono-azobenzene groups (Fig. 6b), 104 4 showed larger photoinduced birefringence because of the intrinsic higher aspect ratio of the bis-azobenzene group (Fig. 6c and d). In addition, 4 showed a slower rate of photoinduced birefringence and a higher birefringence stability than 5, which is because of the lower mobility of the bis-azobenzene group.
In addition, polymethacrylates with similar structures have been synthesized and their photoresponsive properties, such as photoisomerization, 91,105-112 photo-alignment, 106,107,110    photoinduced birefringence, 91,105,106,[108][109][110]113 information storage, 112 and the induction of helical chirality, 111,114,115 have been investigated. The rate of photoinduced birefringence for the bis-azopolymers was slower than that for monoazopolymers, 90,91 while the birefringence values for bisazopolymers were higher and the birefringence stability for bis-azopolymers was better than those for monoazopolymers. 90,91,108,109 However, there was an exception, where both the rate and the level of photoinduced birefringence for the bis-azopolymers were better than those for mono-azopolymers. 105 In addition to multi-azopolymers with a poly(meth)acrylate backbone, multi-azopolymers with other types of backbones have also been studied, such as polymethacrylamides showing photoinduced birefringence 116 and polyurethane anionomers showing photoisomerization. 117 Besides photoinduced birefringence, multi-azopolymers are also suited for photoinduced SRGs. In an early example, Wang et al. reported the inscription of SRGs on polymers with conjugated bis-azobenzene groups on polymer side chains. 118 Recently, Zhang et al. synthesized poly(aryl ether)s 6, 7, 8, and 9 ( Fig. 7) via polycondensation. 119 Polymers 6, 7, 8, and 9 had high glass transition temperature (T g ) values and good thermal stability. 119 These polymers also showed photoisomerization (Fig. 7b) and photoinduced SRGs under the irradiation of interference beams (Fig. 7c and d), which have potential applications in optical data storage.
Besides polymers with conjugated multi-azobenzene groups on side chains, polymers with conjugated multi-azobenzene groups on polymer backbones were also reported. Wu et al. reported a series of main-chain type bis-azopolyurethanes (10 and 11) (Fig. 8). 120 The mono-azopolyurethane (12) was also studied for comparison. Polymers 10, 11, and 12 had T g values at 182°C, 167°C, and 143°C, respectively. 120 All of them showed photoinduced birefringence (Fig. 8b). Polymers 10 and 11 displayed smaller birefringence than polymer 12 due to the bent V shape of the bis-azobenzene groups that did not generate large birefringence. Wu et al. quantitatively investigated the stability of birefringence with parameter E n , where E n is defined as the ratio of birefringence conserved for a very long time to saturated birefringence. A higher value of E n meant higher stability. They found that the birefringence of 10 and 11 showed higher stability than that of 12 at high tempera- tures, even close to T g (Fig. 8c), which was attributed to the following two reasons: (1) the bulky bis-azobenzene groups with lower mobility enhanced the stability of the orientation; (2) the special architectures of V-shaped bis-azobenzene groups might result in higher relative stability of the photoinduced birefringence. The SRG formed in polymer 11 could be only partly erased by heating (Fig. 8d and e), which was distinct from thermally erasable SRGs reported in other literature studies. 121,122 Based on this work, the authors further investigated photoinduced birefringence and SRGs in polyurethane elastomers with bis-azobenzene groups in the hard segment. 123 Another type of main-chain conjugated multi-azopolymer, which contained azobenzene crown ether moieties, has been studied by Ardeleanu et al. 124,125 This type of multi-azopolymer also showed good thermal stability and reversible photoisomerization behavior.
In addition, a novel organo-phosphonate with bisazobenzene moieties in the main chain has been reported by Homocianu et al. 126 The new structure endowed the polymers with reversible dual stimuli response, such as light and pH.

Supramolecular multi-azopolymers
In the above two sections, multi-azobenzene groups were introduced into polymers covalently. In contrast, multi-azobenzene groups can also be introduced into polymers via supramolecular interactions. Supramolecular multi-azobenzene groups can be introduced into polymers in series or in parallel.
One of us designed supramolecular bis-azopolymers by connecting a side-chain azopolymer with azobenzene small molecules via hydrogen bonding (Fig. 9a). 80 Compared with the mono-azopolymer (13, Fig. 9a), the supramolecular polymers 13 (14) x showed larger photoinduced birefringence and better stability when the content of 14 increased (Fig. 9b). These results are attributed to higher concentrations of azobenzene groups, a higher aspect ratio of the chromophores, and more ordered structures based on chromophore-chromophore interactions. In addition, by using a newly designed laser direct writing system (Fig. 9c), the supramolecular polymer 13(14) 1.0 was used for four-dimensional optical recording by integrating the polarization and the intensity of the laser and the planar two dimensions (Fig. 9d), which had an information density 20 times that of a normal DVD. Fig. 9(e-j) show the polarized optical microscopy images of the microstructure at different angles. These types of supramolecular bis-azopolymers have better solubility and processability than the covalent bisazopolymers.
Priimagi et al. reported another type of supramolecular bisazopolymer by connecting poly(4-vinylpyridine) (P4VP) with bis-azobenzene small molecules via hydrogen bonding. [127][128][129] They prepared SRGs on the supramolecular bis-azopolymers. Furthermore, Vapaavuori et al. compared the photoresponsive properties of supramolecular polymers with bis-azobenzene and mono-azobenzene groups (Fig. 10). 92 Supramolecular multi-azopolymers showed better photoinduced birefringence (Fig. 10c). Supramolecular bis-azopolymers 15(17) 0.5 exhibited more efficient photoinduced SRG formation (Fig. 10d). A few investigations based on similar supramolecular azopolymers were reported. [129][130][131][132] Besides hydrogen-bonded supramolecular multi-azopolymers, supramolecular multi-azopolymers based on other supramolecular interactions such as host-guest interactions 133 and ionic interactions 84 have also been investigated. These polymers formed supramolecular hydrogels with multi-stimuli responsive properties 133 and were used for photoinduced SRGs. 84 The above-mentioned examples showed that supramolecular multi-azobenzene groups can be introduced into polymers in series, while del Barrio et al. demonstrated supramolecular polymers that had hydrogen-bonded multi-azobenzene groups in parallel (Fig. 11). 134 The photoinduced orientations of these supramolecular multi-azopolymers were studied (Fig. 11a-c). The supramolecular multi-azopolymers showed large and stable photoinduced birefringence even at low contents of the azobenzene compounds (Fig. 11b-d), which were superior to the systems previously reported. 135 Additionally, other studies based on hydrogen-bonded polymers with supramolecular multi-azobenzene groups in parallel have been reported. Photoinduced anisotropy, 136 orientation, 136,137 or birefringence 136,137 and the photoinduced deformation 138 of these supramolecular multi-azopolymers have been investigated.

Conclusions and perspective
We introduced multi-azopolymers that exhibit interesting photoresponsive properties. Both side-chain and main-chain multi-azopolymers have been investigated. The photoisomerization of multi-azobenzene groups resulted in more types of isomers. Because multi-azobenzene groups usually have a larger aspect ratio than mono-azobenzene groups, they exhibit larger photoinduced birefringence. Multi-azobenzene groups can be introduced into polymers via supramolecular interactions. This supramolecular strategy may simplify the route for the preparation of multi-azopolymers. 139 Moreover, the composition of supramolecular complexes can be adjusted easily by changing the ratio of azobenzene components and polymer matrices.
Although multi-azopolymers have been extensively studied, we believe that the full potential of multi-azopolymers has not been realised yet. Some key areas in photoresponsive polymers remain to profit from designing multi-azopolymers. Some challenges with regard to multi-azopolymers should be addressed to take this field forward.
First, multi-azobenzene groups, especially the conjugated ones, have poorer solubility than their mono-azobenzene counterparts, which makes the synthesis and processing of multi-azopolymers more difficult. Decoupling the conjugation of multi-azobenzene groups via supramolecular interactions may improve solubility and address the issue of processing.
Second, almost all the reported multi-azopolymers have linear architectures. A field that awaits exploration is the combination of interesting photoresponsiveness of multi-azobenzene groups with a variety of polymer topologies, such as stars, branches, and dendrimers. Such a design will result in new photoresponsive polymers.
Fourth, multi-azobenzene small molecules with heterogenic azobenzene groups showed orthogonal photoswitching by light with different wavelengths. 85,142,143 Such heterogenic multi-azobenzene groups can be switched between more than 2 states. Multi-azopolymers with heterogenic azobenzene groups have not been explored yet and can be controlled with light of different colors.

Conflicts of interest
There are no conflicts to declare.