3. Design of New Azobenzene Crystals for Actuation
Molecular crystals provide a platform for various functionalities, as organic molecules can form diverse structures with limited constituents. The strategy used to functionalize a crystal relies on the crystal design, specifically, the molecular structure and intermolecular interactions. The molecular architecture provides the basis for new crystal structures expressing specific functions, although it is challenging to predict crystal structures from molecular structures. This section reviews several works describing photomechanical azobenzene crystal construction, according to the crystal design.
3.1. Chirality Induction
Dozens of photomechanical crystals, not of only azobenzenes but also of other photochromic compounds, have been reported to bend under photoirradiation. The bending motion is a mode of actuation resulting from elongation or contraction of the irradiated surface. In contrast to bending, there have been few reports on twisting of the crystal, which is another mode of actuation [34
]. The twisted shape has chirality, because the right-handed twist does not overlay its mirror image, i.e., the left-handed twist. Thus, photoisomerization in chiral photochromic crystals may lead to macroscopic deformation with chirality, such as a twisting motion.
Based on this concept, the photomechanical motion of thin crystals of compound trans
was revealed [38
]. In this study, a thin plate crystal was obtained by sublimation. The crystal had a (001) face with the b
-axis as the longitudinal axis, along which intermolecular NH---O hydrogen bonding chains formed in the crystal. When the front (001) face was irradiated with UV light, the crystal bent away from the source with slight left-handed twisting (Figure 6
). The crystals, which showed bending with the slight twist, returned to their initial shape within 2–3 min after turning off the UV light. When the back (00-1) surface was irradiated, bending motion with a twist was observed, similar to that of the (001) face (Figure 6
This bending accompanying a twist is caused by elongation of the irradiated surface along the orthogonal direction. At the molecular level, trans-to-cis photoisomerization of compound trans-(S)-11 leads to contraction of the a-axis and elongation of the b-axis, based on the optimized molecular geometries of cis-(S)-11. This change in molecular structure occurs mostly on the irradiated surface, and the probability of trans-to-cis photoisomerization decreases along the thickness direction due to the attenuation of UV light. This is why cis-(S)-11 molecules are formed in the crystal along a gradient from the irradiated surface. The strain induces elongation along the b-axis and contraction along the a-axis, resulting in bending away from the light source with a twist.
3.2. Crystallization with Multi-Molecular Components
Another strategy is to design photomechanical azobenzene crystals with multi-components. One notable crystal engineering technique is co-crystallization. A co-crystal is defined as a crystal consisting of at least two molecules. Co-crystals can be obtained by tuning intermolecular interactions, for instance hydrogen bond and halogen bond interactions. To induce intermolecular interactions between two components, it is of key importance to design one component as a donor and the other as an acceptor of the expected intermolecular interaction. To date, there have been numerous reports on co-crystallization techniques, and the functions of co-crystals, where the number of combinations of two components is nearly infinite and affords diverse crystal structures [39
The crystals of fluorinated azobenzenes 12
-form exhibit photomechanical bending due to cis
photoisomerization upon visible light irradiation [42
]. Photomechanical azobenzene co-crystals 17
were developed by constructing halogen bonds between halogenated azobenzene as a donor and a pyridine compound as an accepter [44
]. The halogen I- or Br- at the para
-position of the electron-deficient perfluorphenyl group is highly polarizable and works as a donor, forming the linear interaction between an accepter of the halogen bond, pyridine. The halogen bonding motif results in a zig-zag crystal structure, with alternating alignment of the donor and acceptor molecules, for example at a pitch of 20 Å in the case of the co-crystal of 17
). Here, the co-crystal consists of cis
-azobenzene, despite the lower stability of the cis
-isomer compared with the trans
-isomer; the cis
thermal back-isomerization of fluorinated azobenzene is very slow, allowing retention of the cis
-isomer during recrystallization.
All co-crystals of 17
exhibit photomechanical bending under 532-nm laser irradiation, however, the deflection angle depends on the components and partners of the co-crystals. The most bendable co-crystal under weaker light intensity (5 mW/cm2
) was that of 17
). Upon light irradiation, the crystal bent away from the light source up to a deflection angle of 90°. The bent crystal did not return to its initial shape after the removal of irradiation, indicating irreversible photomechanical bending due to cis
photoisomerization upon visible light irradiation. When a cis
-isomer transforms to a trans
-isomer, the trans
-isomer forms a halogen bond with another pyridine, leading to a new co-crystal structure made from the trans
-isomer. The structural mismatch between the new daughter crystal and the mother co-crystal of the cis
-isomer produces the strain to bend away. The presence of a small amount of co-crystals in the trans
-isomer was confirmed experimentally via in-situ X-ray crystallographic analysis, which also revealed that the conversion process unfolded as a crystal-to-crystal process (Figure 7
Besides co-crystals, molecular machines such as rotaxanes are also composed of multi-components with an axial molecule and a ring molecule. Photo-reactive pseudorotaxane crystals of 23
were constructed with an axial element with azobenzene and a ring element with different substituents [45
]. The pseudorotaxanes with different substituents afforded crystal structures with differing intra- and intermolecular π–π distances and angles. All crystals of 23
bent upon UV (360 nm) and/or visible light (445 nm) irradiation due to trans
photoisomerization, although the bending angle varied with the crystal structure.
4. Other Mechanical Responses of Azobenzene Crystals
Besides bending and twisting behaviors, some azobenzene crystals reportedly exhibit movement, that is, locomotion, on a substrate surface upon encountering an external stimulus. Locomotion of a material from one position to another is potentially useful for the transfer of compounds, and to penetrate small spaces. Effectively, locomotive molecular crystals may work as small robots in some environments. Two locomotive features of azobenzene crystals are described in the following.
First, crawling locomotion occurs due to a change in the physicochemical properties of the crystal caused by trans
]. When rhombus-shaped crystals of 3,3′-dimethylazobenzene (26
) were irradiated by UV light from one direction, and simultaneously by visible light from the opposite direction, the crystals moved, i.e., crawled, very slowly on the glass surface in the direction of visible light irradiation (Figure 8
). Phenomenologically, crawling locomotion of the crystal originates from the melting and crystallization of opposite surfaces via light irradiation. The crystals of 26
, whose melting temperature is ca.
51–54 °C, melt upon UV irradiation at room temperature, due to the depression of the melting point induced by trans
photoisomerization. On the back side, visible light irradiation causes cis
photochemical back-isomerization, resulting in recrystallization of trans
. This suggests that the retraction caused by UV-light-induced melting, and crystal growth caused by visible-light-induced recrystallization, are likely responsible for crawl locomotion on the glass surface. Recently, the crawling motion of crystals of 27
was achieved by using only visible light [47
]. Such solid-to-liquid transitions induced by photoisomerization also enable swimming on water surfaces [48
] and multi-directional bending of thin crystals of compound 10
Second, thermal locomotion is induced by structural phase transitions in trans
crystals, which is also expressed as photomechanical motion, as described previously [50
]. In this study, the thermal structural phase transition was unexpectedly found to be a reversible process at 145 °C upon heating and cooling. This phase transition proceeds in a single-crystal-to-single-crystal manner, without collapse of the single crystal. Due to the phase transition, the length of the b
-axis, the longitudinal direction of a plate-like crystal, decreases by 0.3% at temperatures higher than 145 °C, and returns to the initial length at lower temperatures. Here, an azobenzene molecule in a unit cell changes its conformation slightly but maintains trans
-form during the phase transition. The decrease of, and return to the original, crystal length lead to bending motion at the phase transition, due to the temperature gradient along the thickness direction. Upon heating, the phase transition starts at the lower side of the low-temperature (LT) phase crystal, and the crystal bends due to shortening of the crystal length. With additional heating, the whole crystal completes the phase transition to a high-temperature (HT) phase. The HT phase crystal returns to the LT phase upon cooling through additional bending motion, as the transition to the LT phase occurs at the upper side cooled by the surrounding air. In turn, this leads to elongation of the crystal at the upper side. Thus, the crystal exhibits bending during both the heating and cooling cycles.
When a plate-like crystal with thickness gradient along the length was repeatedly heated and cooled near the phase transition temperature on a silanized glass, it was noted that the crystal “walked” in the manner of an inchworm due to repeated bending and straightening (Figure 9
). In another case, a thin plate-like crystal moved much faster by rolling on a glass surface after a single heating or cooling process. In the cases of both walking and rolling, the locomotion is induced by the asymmetric shape of the crystal. Walking occurs according to the thickness gradient, where one side is thicker than the other, leading to unidirectional movement via bending and straightening. Rolling occurs according to the width gradient, and leads to loss of balance during bending and then flipping.
These examples suggest that the mechanical responses of molecular crystals can be diversified by a photoisomerization-induced phase change from the crystal form to a melted form, and by thermal structural phase transitions. The mechanical function of the structural phase transition can be combined with the photomechanical response of photochromic crystals. Combining the structural phase transition with photoisomerization has resulted in multiple mechanical motions of crystals [51
5. Possibilities for Future Applications
So far, we have reviewed the mechanical responses of azobenzene crystals. The actuation mechanism is interesting from a purely scientific standpoint. However, it is important to remember that mechanically responsive crystals also show great potential as smart actuators and will find appropriate applications as the field advances.
A straightforward example of implementation is provided by photomechanical bending of azobenzene crystals, applied to the gripper of a micropipette (Figure 10
]. Here, the micropipette was constructed using the arm of an azobenzene compound 22
nanowire on the left side, and an additional fixed polystyrene nanowire arm on the right side. When the pipette arms were irradiated by UV light, the azobenzene nanowire bent towards the light source, owing to the transparency of polystyrene. The bending of the azobenzene nanowire led to the grabbing of a small particle. This is one of many possible applications of azobenzene crystals while other potential applications include microelectromechanical systems (MEMS), soft robots, flexible devices, medical catheters, aerospace devices, and so on. Recent advances in 3D and 4D printing technology could be further improved by mechanically responsive molecular crystals, owing to inherently advantageous properties such as light weight and softness [54
However, there are still some problems limiting the application of azobenzene crystals. The first concerns the difficulty of increasing the maximum force at actuation, and the second concerns the difficulty of fabricating the desired shape. Regarding the first one, mechanically responsive molecular crystals typically generate a maximum stress in the range of 1–50 MPa at actuation [50
]. These values are generally 10–100 times larger than the maximum stress of typical human muscle (0.3 MPa) [58
]. To lead larger stress to larger output force, multiple pieces of crystals need to be integrated. Also, it is challenging to fabricate molecular crystals with the desired shape and size.
The hybridization of crystals and polymers is a promising approach to overcome the limitations discussed above [59
]. When molecular crystals are incorporated into a connective polymer, the hybrid material will be more flexible and easier to control, in terms of size and shape, due to the properties of the polymer. In addition, such hybrid materials should respond faster and generate a larger force than polymers, owing to the inherent advantages of molecular crystals. Thus, the hybrid strategy of combining a polymer with a mechanical crystal, expected to be realized in the near future, should enhance the maximum force capability and simplify the fabrication process of actuator materials.
This paper reviewed the current state-of-the-art of photomechanical azobenzene crystals. Photomechanical responses result from simple trans-cis photoisomerization, but lead to a wide array of photomechanical behaviors varying by actuation mode (bending or twisting), bending magnitude, response speed, and relaxation time. Tuning azobenzene substituents and employing crystal engineering techniques, such as co-crystallization, have crucial roles in diversifying the photo-responsiveness and functionality of azobenzene crystals. Although there are still challenges ahead, photomechanical crystals show promise as novel actuators, with possible applications in soft robots, and medical and flexible devices.