Control of photomechanical crystal twisting by illumination direction

Feb 16, 2018 - Photomechanical molecular crystals have been investigated as mesoscopic photoactuators. Here, we report how the photomechanical twistin...
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Control of photomechanical crystal twisting by illumination direction Daichi Kitagawa, Hajime Tsujioka, Fei Tong, Xinning Dong, Christopher J. Bardeen, and Seiya Kobatake J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 16 Feb 2018 Downloaded from http://pubs.acs.org on February 16, 2018

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Journal of the American Chemical Society

Control of photomechanical crystal twisting by illumination direction Daichi Kitagawa,† Hajime Tsujioka,† Fei Tong,‡ Xinning Dong,‡ Christopher J. Bardeen,*,‡ and Seiya Kobatake*,† †

Department of Applied Chemistry, Graduate School of Engineering, Osaka City University, 3-3-138 Sugimoto, Sumiyoshiku, Osaka 558-8585, Japan. ‡ Department of Chemistry, University of California, 501 Big Springs Road, Riverside, CA 92521, USA

Supporting Information Placeholder ABSTRACT: Photomechanical molecular crystals have been investigated as mesoscopic photoactuators. Here, we report how the photomechanical twisting of 1,2-bis(2-methyl-5-phenyl-3thienyl)perfluorocyclopentene (1a) crystals depends on illumination direction. The ribbon-like crystal of 1a could be successfully prepared by a sublimation method. The ribbon crystal exhibited reversible photomechanical crystal twisting upon alternating irradiation with ultraviolet (UV) and visible light. Moreover, changing the UV illumination direction with respect to the crystal resulted in different twisting modes, ranging from helicoid to cylindrical. Control of photomechanical crystal deformation by illumination direction provides a convenient and useful way to generate a variety of photomechanical motions from a single crystal.

development of these materials. We recently found that a diarylethene derivative, 1,2-bis(2-methyl-5-phenyl-3thienyl)perfluorocyclopentene (1a, Scheme 1) can be crystallized in different shapes depending on crystal growth method: slow evaporation of organic solvent yields large block-like crystals, while sublimation yields ribbon-like crystals. Moreover, the ribbon shaped crystals exhibit photomechanical twisting. In this paper, we have investigated the effects of illumination direction on the photomechanical twisting. Control of the illumination angle of the incident light can tune the twisting mode between a helicoid type and a cylindrical helix type. Our results may lead to new and possibly useful modes of photomechanical motion of molecular crystals. F F

Photomechanical materials convert photon energy into mechanical motion. The use of light can achieve precise spatial control without the need for direct physical contact. They are promising candidates as actuators in the future.1 Especially, photomechanical molecular crystals show some advantages over polymer-based materials, including faster response time,2,3 higher Young's modulus,4 and an ordered crystal structure which can be easily characterized by X-ray diffraction techniques.5-7 Multiple researchers have prepared various photomechanical crystals that show different motions such as contraction,2,8,9 expansion,10,11 bending,11-34 fragmentation,8,35-38 and twisting.19,39,40 One challenge is to harness these crystals to do more complex work via controlled shape changes. Examples of using crystal shape to control motion include recent work by Lu et al., on the shape-dependent photoinduced bending, curling, rolling, and salient behaviors of styrylbenzoxazole derivatives.41 Another example by Bardeen et al. demonstrated that branched microcrystals of one anthracene derivative could show light driven ratchet-like rotation.42 The crystal structures prepared by slow pH-reprecipitation method generate a "defective" crystal which results in a chiral shape that can lead to the unidirectional rotation by twisting of its branch arms. In this previous work, light illumination was carried out evenly and no effort was made to investigate the effect of illumination conditions in detail. Our group has reported the effect of illumination conditions such as wavelength and power on photomechanical crystal deformations like bending.22,24 Controlling more complex shape changes using light exposure conditions is important for the further

F F

F F F F

CH3 S H3C 1a

UV Vis.

S

F F

F F CH 3

S H3C

S

1b

Scheme 1. Molecular structure change of diarylethene used in this work

Figure 1. Optical microphotographs of (a) block shape crystals prepared by slow evaporation from n-hexane and acetone mixed solution of 1a and (b) ribbon shape crystals prepared by sublimation of 1a. Scale bar is 1 mm.

When diarylethene 1a was recrystallized from an organic mixed solvent of n-hexane and acetone using a slow evaporation method, block-shaped crystals could be obtained as shown in Figure 1a.43,44 On the other hand, we found that the ribbon crystals appeared after sublimation as shown in Figure 1b. The details of the sublimation method are described in the Supporting Information. To determine the molecular orientation in the crystal, powder X-ray diffraction (PXRD) measurements were performed.

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Figures 2a and b show PXRD pattern of the ribbon crystal and the pattern calculated from single-crystal X-ray crystallographic data of the block crystal of 1a. Single-crystal X-ray crystallography of the block shape crystal of 1a showed a monoclinic crystal system and a space group of P21/c (Table S1).43 As shown in Figure 2a, only diffraction peaks corresponding to the (002) and (006) Miller planes were observed for the ribbon. This result means that the flat surface of the ribbon is the (001) face. Figure 2c shows optical microphotographs of the ribbon crystal. The corner angles of the crystal were 122°, 116°, and 122°. Figure 2d shows the molecular packing in the unit cell viewed from the (001) face, and Figure 2e illustrates the crystal shape viewed from the (001) face. Calculated corner angles in the illustration are 122°, 116°, and 122°, respectively. These angles are consistent with the single crystal structure, which means that the block crystal and the ribbon crystal are the same polymorph. The long axis and short axis of the ribbon crystal correspond to the b-axis and a-axis in the unit cell, respectively.

Molecular orientation of 1a in the crystal viewed from (001) face. (e) Illustration of crystal shape and corner angles viewed from (001) face.

We then investigated the photomechanical behavior of crystalline 1a. Large block shaped crystals usually have widths on the order of 0.1 mm. Although they undergo photochromic reaction from the open-ring isomer 1a to the closed-ring isomer 1b,43,44 they do not exhibit any photomechanical motion due to their large bending inertia. On the other hand, when UV irradiation (365 nm light) was carried out on the ribbon crystal, it exhibited photomechanical twisting accompanying the color change from colorless to blue as shown in Figure 3, Figure S1, and Video S1. Moreover, upon irradiation with visible light, the blue color disappeared and the crystal returned to its initial straight shape. This photomechanical behavior is ascribed to the reversible photochromic reaction of diarylethene molecules in the crystalline phase. In fact, the photomechanical crystal twisting could be repeated more than 10 cycles.

Figure 3. Photoreversible crystal twisting of the ribbon crystal of 1a. UV irradiation was carried out from left side on the image. The ribbon crystal was fixed to the glass capillary by glue. Scale bar is 300 µm.

Figure 2. (a) PXRD pattern of a ribbon crystal of 1a and (b) the pattern calculated from single crystal X-ray crystallographic data of 1a. (c) Optical microphotograph of a ribbon crystal of 1a. Scale bar is 100 µm. (d)

Twisting is a commonly observed photomechanical response of the ribbon morphology.19,39,40 It is usually assumed that the twist period is determined solely by internal strain generated at the interface of the reactant-product bilayer. If generating photoproduct on one side of the ribbon is the only requirement for twisting, then one would naively expect that the final twist shape should be the same for different illumination conditions. To see if this was true, we first investigated whether the twist direction could be controlled by varying the illumination conditions. Figure 4 shows the photomechanical twisting behavior of the ribbon crystal when UV irradiation was carried out from four different directions (5 sec and 26.3 mW cm−2 for each panel). Magnified images are also shown in Figure S2. When the ribbon crystal was irradiated with UV light from upper back to front as shown in Figure 4a, it twisted into a left-handed helix. On the other hand, the ribbon crystal twisted into a right-handed helix when it was irradiated with UV light from lower back to front as shown in Figure 4b. Moreover, when the UV irradiation was carried out from front to back as shown in Figures 4c and d, the ribbon crystal twisted in the righthanded helix and the left-handed helix, respectively, which means that the ribbon crystal tends to twist toward the incident light. These results suggest that changing the direction of the incident light induces extra photoproduct on one side or the other of the crystal ribbon which would bias the twist. Once the twist starts in one direction, it will continue in that direction. It is kind of like symmetry breaking in physics: a small bias will send the system to one or the other outcomes.

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Figure 4. Photoinduced twisting behavior of the ribbon crystal of 1a in different direction depending on the angle of the incident light. Scale bar is 300 µm.

The detailed shape of the twist also depends on illumination conditions. We investigated the effect of the angle of the incident light on the photomechanical crystal twisting as shown in Figure 5. UV irradiation was carried out as shown in the image (30 sec and 26.3 mW/cm2 for each panel). Magnified images are also shown in Figure S3. When the ribbon crystal was irradiated with UV light from tip of the crystal (incident light angle = 0°), the ribbon crystal twisted into a helicoid shape (Figure 5a). On the other hand, when UV irradiation was incident on the ribbon at larger angles, it gradually transformed into a cylindrical helix shape (Figure 5b-g). The incident light angle determines the angle of twist. Eventually, when the ribbon crystal was irradiated with an angle of 90° as shown in Figure 5h, the crystal exhibited bending rather than twisting. In order to parameterize the variable twisting behavior, the crystal profile viewed from side (L) and height of the first helix from the glass capillary (H) were plotted against the incident light angle as shown in Figure 6. The actual length of the straight crystal is constant (ca. 1600 µm). When the incident light angle (θ) becomes larger, L and H became smaller and larger, respectively, due to the formation of a cylindrical helix. Katonis et al. demonstrated the ribbons that consist of liquid crystalline polymer can show curling, left-handed and righthanded helical twisting depending on the direction in which they are cut.45 In that case, they had to make a new material every time they wanted a different helicity. Using molecular crystals, we can tune the helicity simply by changing the light direction. We hypothesize that the sensitivity of the crystal shape change to light direction results from preferential excitation of differently oriented molecules within the crystal, generating an effective bilayer structure whose internal stress leads to helical ribbon deformations.6,19,39,40,46 A qualitative model that considers the two limiting cases is described below and in the Supporting Information. Briefly, a single crystal of 1a consists of two subpopulations with molecules oriented at angles of +20° and −20° with respect to the b-axis. Each molecule can generate stress both parallel and perpendicular to its long axis. The ability of a light beam to interact with each subpopulation depends on the direction of incidence relative to their transition dipole alignments. For example, if the light propagates parallel to the dipole, the molecule cannot be excited since the polarization will always be perpendicular to the dipole moment. If the light is incident perpendicular to the b-c edge (90°), it excites all molecules equally and they will generate stress both parallel and perpendicular to the ribbon long axis. The resulting stress tensor, aligned with the ribbon shape, can generate bending but has no diagonal component to initiate helical motion.

As the light is rotated, the two molecular orientations are no longer equally excited and a diagonal stress component leads to helicity. At very shallow angles (~0°), only one orientation of molecules is excited, because the other population has its transition dipole moment aligned close to parallel to the direction of light propagation. In this case, the stress tensor arises from only the subpopulation rotated at +20°, with both positive (expansive) and negative (compressive) components. These are close to the conditions for generating a pure twist.46 In other words, by exciting differently oriented molecules, the photoinduced strain tensor in the crystal (and thus the mode of photomechanical deformation) can be controlled by the direction of UV light irradiation. This model requires additional work to confirm it but does provide a framework for future experiments.

Figure 5. Different twisting motion, a helicoid and a cylindrical helix, depending on the angle of the incident light. Scale bar is 300 µm.

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(8) (9) (10) Figure 6. Plot of the crystal length viewed from side (L) and height of the first helix from the glass capillary (H) against the incident light angle (θ).

In conclusion, we have fabricated ribbon-shaped crystals of 1a by a sublimation method and investigated their photomechanical motion. The ribbon shape crystal exhibited photoreversible twisting upon alternating irradiation with UV and visible light. Both the twisting direction and mode (helicoid versus cylindrical) could be controlled by the direction of the incident light. These results indicate that the stress tensor on the crystal surface induced by photochromic reaction could be controlled by the illumination direction. While the detailed mechanism needs to be investigated further, the use of illumination angle to tune the mechanical response illustrates that photomechanical molecular crystals provide unique opportunities for the control of their motion.

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Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website. Magnified images of photomechanical crystal twisting, qualitative model for twisting, Illustration of two types of twisting: a helicoid and a cylindrical helix, strain tensor on the crystal surface, and preparation of the ribbon crystals of 1a by sublimation method (PDF). Movie of photomechanical crystal twisting: a helicoid and a cylindrical helix (Video S1-S2).

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AUTHOR INFORMATION

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Corresponding Author

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* E-mail: [email protected] * E-mail: [email protected]

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Notes The authors declare no competing financial interests.

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ACKNOWLEDGMENT

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This work was partly supported by JSPS KAKENHI Grant Number JP26107013, JP15K21725 in Scientific Research on Innovative Areas “Photosynergetics” (S.K.) and JSPS KAKENHI Grant Number JP16K17896 in Scientific Research for Young Scientists (B) (D.K.), and the National Science Foundation grant DMR1508099 (C.J.B).

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