Transportation of Nano/Microparticles via Photoinduced Crawling of Azobenzene Crystals

The stimulus‐driven motion of microscale objects on solid surfaces is a promising process to realize the manipulation of microdroplets, which has applications in fields areas such as material accumulation and sensing at a micro level. Light is an effective external stimulus for object manipulation because it can provide contactless spatial and temporal control. In this study, a new method is devised to transport nano/micromaterials on a glass surface by exploiting the photoinduced crawling motion of crystals. 4‐(methylamino)azobenzene (4‐MAAB) crystals are used to transport nano/micromaterials via photoirradiation from a single visible‐light source without using expensive equipment such as lasers and positioning devices. Nano/micromaterials mixed with the 4‐MAAB crystals are successfully transported with the crawling 4‐MAAB crystals. The nano/micromaterial transport is tracked using the fluorescence from the nano/micromaterials embedded into the 4‐MAAB crystals. In situ time‐resolved X‐ray diffraction measurements are also conducted for mechanistic analysis. This study offers new development paths in fields such as microfluidics and microrobotics.


Introduction
The manipulation of microscale objects has applications in fields such as material accumulation, chemical reactions, and sensing in narrow spaces. [1] In particular, the manipulation of microdroplets has been extensively studied to analyze the motion of microscale objects driven on solid surfaces. To move a microdroplet, its wettability must be continuously varied by modifying the microstructure of the surface, [2] chemical composition, [3] and charge density gradient. [4] However, wettability cannot be flexibly altered because the direction of movement of the droplet is fixed by the surface gradient. Droplet motion can be driven by electrowetting, [5] which changes the wettability of the surface depending on the applied voltage between the electrode and the droplet. The droplet can move along any directed path in this method. However, this approach involves the fabrication of complex electrode patterns on the surface. and can cause electrolysis of the substrate or medium owing to the high applied voltage (≈100 V). This warrants the development of methods that enable the manipulation of microscale objects on unmodified solid surfaces.
Microsized organic crystals can crawl on unmodified glass under light stimulation. [6] A continuous irradiation of 3,3′-dimethylazobenzene crystals with both UV and visible light (365 and 465 nm, respectively) causes them to crawl on glass surfaces. The crawling motion is achieved using light-emitting diodes (LEDs) or mercury lamps, which ensures the emission of UV and visible light from different directions. Moreover, azobenzene crystals can climb the walls of glass surfaces, indicating that the crawling occurs horizontally as well as vertically. Light is a promising external stimulus for object manipulation because it can provide contactless spatial and temporal control and eliminates the requirement of expensive equipment such as lasers and positioning devices for realizing crawling motion. Therefore, the photoinduced crawling of crystals could be leveraged to transport materials on solid surfaces.

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The stimulus-driven motion of microscale objects on solid surfaces is a promising process to realize the manipulation of microdroplets, which has applications in fields areas such as material accumulation and sensing at a micro level. Light is an effective external stimulus for object manipulation because it can provide contactless spatial and temporal control. In this study, a new method is devised to transport nano/micromaterials on a glass surface by exploiting the photoinduced crawling motion of crystals. 4-(methylamino)azobenzene (4-MAAB) crystals are used to transport nano/micromaterials via photoirradiation from a single visible-light source without using expensive equipment such as lasers and positioning devices. Nano/micromaterials mixed with the 4-MAAB crystals are successfully transported with the crawling 4-MAAB crystals. The nano/micromaterial transport is tracked using the fluorescence from the nano/micromaterials embedded into the 4-MAAB crystals. In situ time-resolved X-ray diffraction measurements are also conducted for mechanistic analysis. This study offers new development paths in fields such as microfluidics and microrobotics.
macromolecules. [21,22] These discoveries have led to applications related to diverse smart materials, controllable adhesion, [13,[17][18][19][20] photoresists, [9] gas storage, [11] solar thermal fuels, [12,23,24] selfhealing, [22] the switching degradation of biopolymers, [25] and swimming crystals. [14] 3,3′-Dimethylazobenzene crystals exhibit a crystal-to-liquid phase transition under photo-irradiation, wherein its trans isomer in the solid state isomerizes to its cis counterpart under UV irradiation, resulting in melting. Moreover, the liquid-state cis isomers recrystallize to the solid state via thermal backisomerization or photoisomerization under visible-light irradiation. Photoinduced crawling of the crystals occurs when light is obliquely irradiated on a glass substrate, [6,26] leading to a position-dependent variation in the light penetration depth within the same crystal. [27] Consequently, both melting-and crystallization-dominated positions coexist in the same crystal. This nonequilibrium state of the phase transition is considered the driving force of the crawling motion.
Simple photoirradiation systems should be designed to utilize the phase-transition-based photoinduced crawling motion for transporting materials on solid surfaces. However, the crawling motion of 3,3′-dimethylazobenzene induced by both UV and visible-light irradiation from opposite directions is realized using complex optical systems. Ideally, the photoinduced crawling should be achieved using a single light source. The benefits of using a single light source over multiple sources are costeffectiveness, a simple system, and a better control of motion. Recently, our group found that 4-(methylamino)azobenzene (4-MAAB) crystals exhibit crawling motion under irradiation from only a single light source. [28] The crystals exhibit negative phototaxis, similar to living organisms, because they move away from the light source. In addition, 4-MAAB is suitable for transporting nano/micromaterials because their crawling motion can be induced even using white light. In 4-MAAB, the electron-donating amino group at the para position lowers the energy barrier for thermal back-isomerization. Therefore, even when the trans isomer photoisomerizes to its cis counterpart, the cis isomer thermally isomerizes to the trans isomer within only 1 s at room temperature. Consequently, photoisomerization and thermal back-isomerization are continuously repeated under sustained irradiation (Figure 1a). Owing to the oblique incidence of light, the trend of crystal melting and recrystallization is not uniform. Although this non-equilibrium state possibly drives the crawling motion, the detailed mechanism remain unclear.
In this study, 4-MAAB crystals were used to demonstrate the transportation of nano/micromaterials driven by simple continuous photoirradiation from a single visible-light source. Fluorescent quantum dots (QDs) and fluorescent silica particles were selected as the nano-and micromaterials for transport. In this strategy, 465-nm-wavelength light was employed as a stimulus to realize the crawling and excite the fluorescent particles. After both nano/micromaterials were individually mixed with the 4-MAAB crystals, they were successfully transported along with the 4-MAAB crystals exhibiting light-induced crawling motion. The transportation of the nano/micromaterials was demonstrated by observing the fluorescence from the nano/ micromaterials incorporated into the 4-MAAB crystals. Furthermore, in situ time-resolved X-ray diffraction (XRD) experiments were performed on the crawling 4-MAAB crystals to obtain www.advmatinterfaces.de mechanistic insights. The movement of a bright diffraction spot in a 4-MAAB crystal was observed in correspondence with the crawling motion.

Transportation of Fluorescent QDs
The nano/micromaterials that can be transported via the lightinduced crawling motion of the crystals need to be observable under optical microscopic examination. Although the direct observation of nanomaterials using optical microscopy is difficult, the presence of nanoparticles can be confirmed by monitoring their fluorescence. The absence of noticeable emission from the 4-MAAB crystals aided the detection of the fluorescence of the nanoparticles. In this study, fluorescent red-emitting QDs were introduced to the 4-MAAB crystals as markers for tracking the transportation driven by lightinduced crawling. Because the red emission at ≈660 nm did not overlap with the absorption of the 4-MAAB-crystal-based thin film (Figure 1b), the examination of the QDs in the crawling 4-MAAB crystals was facilitated. CdSe@ZnS QDs dispersed in toluene were mixed with the 4-MAAB crystals that were melted on a hot plate at 95 °C. The molten 4-MAAB was then cooled to room temperature to solidify it. The crystalline solid was ground into a fine powder and deposited onto the surface of an unmodified glass substrate, which was considered hydrophilic as it exhibited a water contact angle of ≈23°. The mixing ratio of the 4-MAAB crystals and QDs was ≈10:1 by weight. Blue LED light (465 nm) was used to photoisomerize 4-MAAB and excite the QDs at an intensity of 200 mW cm −2 . Note that the white backlight of the microscope was not used in these experiments to allow the observation of the fluorescence from the QDs. The rear side of the glass substrate was irradiated with an LED light that was obliquely incident at an incident angle of 60°. Irradiation with an LED light increased the temperature in the 4-MAAB crystals. For example, the temperature of a 4-MAABcrystal thin film with a thickness of ≈25 µm increases from 24 to 36 °C, as shown in Figure S1 (Supporting Information), which is considerably lower than the reported melting point of the 4-MAAB crystals (87 °C). [28] As shown in Figure 1c-f (Movie S1, Supporting Information), the red-emitting crystals exhibited crawling motion. The red emission originated from the QDs because the crystals were illuminated only with blue LED light. Therefore, the negative phototactic behavior of the 4-MAAB crystals enables the transportation of nanomaterials on an unmodified glass surface. The average transport velocity of the four crystals shown in Movie S1 (Supporting Information) is ≈5.1 ± 2.1 µm min −1 . To analyze the motion, the travel distance was defined as the lateral distance traversed by the rear edge of the crystal. In our previous study, we demonstrated that the average velocity of the crawling motion of 4-MAAB crystals without QDs was 3.4 ± 1.7 µm min −1 under the light-irradiation conditions identical to those in this experiment. [28] The presence of QDs did not significantly change the crawling motion.
Because each crystal differed in shape and size, the velocity varied considerably. For example, the equivalent diameters of the crystals shown in Figure 1f are ≈66 and 125 µm, respectively, and their corresponding velocities are 2.2 and 5.3 µm min −1 .
These results indicate that differently sized crystals realized material transportation and that the distinct red emission from the QDs enabled this observation. Red-as well as green-colored regions were observed in the images during the transportation of the QDs, the latter possibly being generated by the blue LED light transmitted through the 4-MAAB crystals. Despite the color difference, both red and green regions are part of the 4-MAAB crystals containing the QDs. Therefore, this color difference indicates that the red regions contained relatively more QDs compared to the green region, that is, the QDs were localized in each crystal. The localized QDs successfully enabled the visualization of the material transportation.
To find out how QDs are transported, we focused on the changes in the brightness of the red emission from the QDs during the motion of the crystals. Dark red emission was observed at the start of the photoirradiation (t = 0 min; Figure 1c), and it brightened at 25 min (Figure 1d), darkened at 50 min, and brightened again at 60 min. The brightness increased and then decreased repeatedly, which was presumably not caused by the irradiation-induced degradation of the QDs, as they were coated with chemically stable ZnS. The updown movement of the QDs with the crawling motion of the 4-MAAB crystals was hypothesized to be responsible for the fluctuating emission. The QDs were assumed to be localized within a 4-MAAB crystal toward its top, away from the glass substrate ( Figure 1g). The morphology of the crystal observed from the side represented a simplified hemisphere ( Figure S2, Supporting Information). When the QDs were on top of the crystal, blue light was mostly absorbed by the 4-MAAB before it reached the QDs (Figure 1g). Therefore, the resulting emission was weak because the QDs could not be readily excited. When the QDs moved downward in the crystal and closer to the light source, the QDs were excited before the blue light was sufficiently absorbed by the 4-MAAB (Figure 1h). Consequently, the QDs excited by the blue light exhibited a bright red emission. Therefore, the repetitive vertical movement of the QDs may have led to the corresponding changes in the brightness of the emitted light (Figure 1g-i). The 0.54-µm-thick 4-MAAB crystalline film exhibited an absorbance of 1.55 at a wavelength of 465 nm (Figure 1b), indicating that its transmittance was ≈3%. Therefore, the 4-MAAB crystal could act as a long-pass filter for excitation light.
Model experiments were then performed to demonstrate that the 4-MAAB crystals inhibited the emission of the QDs. A QD layer was deposited onto a glass substrate, and ≈25-µm-thick 4-MAAB crystalline film was subsequently formed. Figure 2a shows the microscopic image of the front side of the film; i.e., the surface of the 4-MAAB film opposite the QD layersubjected to blue LED illumination from the rear side (see schematic in Figure 2c). The film exhibited red emission (Figure 2e) because the blue LED light directly excited the QDs without being absorbed by the 4-MAAB film. Subsequently, the sample was flipped, and the rear of the film was illuminated with blue LED light and subsequently observed (see image and schematic in Figure 2b,d, respectively). The film did not exhibit any perceptible red emission but showed weak green transmitted light at certain locations (Figure 2f). The QDs were not sufficiently www.advmatinterfaces.de excited, because the 4-MAAB film absorbed most of the blue LED light. These results demonstrate that the position of the QDs relative to the 4-MAAB crystals strongly affected the emission, and that the QDs indicated the lateral as well as the vertical positions. Therefore, the positions of the QDs may have changed during transportation (Figure 1 g-j).
Although nanomaterials were transported on a glass surface, the transported matter was a mixture of 4-MAAB and the nanomaterial. In material transport, the carrier must be removed and only the target material should be present at the desired location (Figure 2g). The 4-MAAB crystals were successfully removed from the substrate used for QDs transportation by immersing it in methanol. In these experiments, the QDs were transported using the 4-MAAB crystals on a glass substrate whose surface was hydrophobized with hexamethyldisilazane (HMDS) (Figure 2h,i). The water contact angle of the hydrophobic glass was ≈88°. The 4-MAAB crystals were removed by immersing the glass substrate in methanol after the QDs were transported. 4-MAAB was effectively removed from the substrate owing to its solubility in methanol (Figure 2j). However, the QDs dispersed in 4-MAAB crystals did not disperse in methanol, which is a polar solvent, because they were protected by octadecylamine, a type of hydrophobic alkylamine. Therefore, the QDs carried by the crystals remained on the hydrophobic glass substrate. The emission of the remaining QDs was observed under UV irradiation (Figure 2k), which confirmed the presence of only the transported nanomaterials on the substrate.

Photoinduced Crawling of the Microparticle-Containing 4-MAAB Crystals
To demonstrate the variety of materials that could be transported using the 4-MAAB crystals, the transport of microscale fluorescent silica particles (≈3 µm in diameter) was investigated. Silica particles dispersed in water were mixed with molten 4-MAAB; the water was completely evaporated using a hot plate at 95 °C. The mixture was subsequently cooled to room temperature, yielding a crystalline solid. The mixing ratio of the 4-MAAB crystals and silica particles was ≈4:1 by weight. The crystalline solid was finely ground to obtain silica-particleincorporated 4-MAAB crystals, which were then deposited on an unmodified glass substrate. The 4-MAAB crystals were then irradiated with blue LED light from the rear side of the substrate at an intensity of 100 mW cm −2 . Figure 3a shows a brightfield image of the crystals, which was acquired immediately after the start of the blue-light irradiation, and Figure 3c shows the corresponding fluorescence image. Crystals 1, 2, 3, and 4 are indicated in Figure 3 using orange, green, magenta, and purple circles, respectively. Crystals 1 and 3 clearly showed fluorescence, whereas crystals 2 and 4 were dark. The peak wavelength

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of the excitation light for observing fluorescence was 546 nm, and the fluorescence peak wavelength was 585 nm, which did not overlap with the absorbance spectrum of the 4-MAAB crystals. The fluorescence and bright-field images revealed the red fluorescence of the silica-containing crystals.
The transport of the silica microparticles was monitored using bright-field microscopy, which revealed the negative phototaxis of the fine crystals (Figure 3a,b; Movie S2, Supporting Information). Because the fluorescent silica microparticles exhibited weaker emission than that of the QDs, the fluorescence due to the scattering of the LED light (465 nm) for material transportation was not observed. Therefore, after the microsized silica particles were transported, the fluorescence observations were conducted again at the same position on the glass substrate. Crystals 1 and 3 exhibited fluorescence both before and after the transportation (Figure 3c,d), indicating that the fluorescent silica microparticles were successfully carried by the crawling 4-MAAB crystals. The presence of the fluorescent silica microparticles in crystals 2 and 4, which did not exhibit fluorescence, could not be confirmed (Figure 3c,d). However, this result proved that fluorescent crystals 1 and 3 contained fluorescent microparticles.
In these experiments, fine crystals were observed to be reasonably uniform in size at ≈10 µm, which facilitated the analysis of the behavior of each particle during transportation. In Figure 4a-d, the X-axis represents the time elapsed after the start of the photoirradiation, whereas the left and right Y-axes represent the velocity of the motion and the total distance traveled by each crystal, respectively. The velocity of each crystal corresponds to the distance traveled per minute, with the travel distance defined as the displacement in the horizontal direction with respect to the screen. As shown in Figure 4a, crystal 1 remains almost stationary for 20 min before crawling and exhibits an unstable velocity during the crawling motion. After 50 min, the crystal exhibits a rod-like shape and moved along a curve (Movie S2, Supporting Information). In contrast, crystals 2 and 3 do not stop crawling during the photoirradiation (Figure 4b,c). Additionally, crystal 4 stops six times during the irradiation-induced crawling motion.
The crawling motion of crystal 1 may have been affected by the change in the position of the silica particles. A change in the shape of crystal 1 can be observed in the fluorescence image (Figure 3d), indicating that the position of the fluorescent silica particles inside the crystal is altered. A silica particle is evidently present at the rear end of crystal 1, which appears as a tail tip (Figure 3b; Movie S2, Supporting Information). The silica particles fixed on the glass surface possibly pin the crystal motion. Because each crystal is not considerably larger than the silica particles (diameters of 10 and 3 µm, respectively), the possibility of the silica particles contacting the bottom of the glass surface is high.
In contrast to crystal 1 that stopped completely, crystal 3 did not stop during its motion. Additionally, the nonfluorescent crystal 4 paused six times during motion. The average velocity of each crystal was similar in the range of 1.4-1.9 µm min −1 (Table S1, Supporting Information). Herein, the average velocity is the average of the respective velocities shown in Figure 4 by the total time. The periods when the particles were stationary were not included in the velocity calculations. These results suggested that the presence of microparticles did not adversely affect the light-induced crawling of the crystals, although pinning by the particles altered the "smoothness" of the motion. This property of the crawling motion could facilitate the transportation of materials.

Observation of the Crawling Motion of the Crystal using Time-Resolved XRD
To provide further insight into the crawling motion of the 4-MAAB crystal, time-resolved XRD experiments were performed for the in situ observation of the diffraction from the www.advmatinterfaces.de crawling crystal. The diffraction pattern of a 4-MAAB crystal crawling under photoirradiation was monitored using X-rays at the Synchrotron Radiation Facility (KEK, High Energy Accelerator Research Organization, PF-AR NW14A). The nano/ micromaterial-free 4-MAAB crystals were supported on an ≈0.02-mm-thick quartz substrate, as illustrated in Figure S3 in the Supporting Information. Both the X-rays and blue light were incident on the rear side of the substrate, with the former and latter being perpendicular and angled, respectively, with respect to the substrate. Movie S3 (Supporting Information) shows the crawling motion of the crystals under blue-light irradiation. Figure 5a shows the diffraction pattern acquired at the start of the X-ray irradiation and photoirradiation (t = 0 s). The diffraction patterns were collected by locally irradiating a fine

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crystal with X-rays possessing a beam size of ≈0.2 mm. Therefore, the obtained diffraction rings were blurry, in contrast to those obtained via powder XRD. However, the brightness of these diffraction rings did not change significantly during photoirradiation (Movie S4, Supporting Information), indicating that the 4-MAAB crystals did not melt completely even under photoirradiation. A bright diffraction spot appeared temporarily on one diffraction ring during the crawling motion of the crystals at t = 59.40 s (Figure 5c; Movie S4, Supporting Information). Notably, this spot moved along the diffraction ring and disappeared within a few seconds (Figure 5c-e; Movie S5, Supporting Information).
Typically, the movement of a diffraction spot in the XRD patterns indicates the rotation of a single crystal. Diffracted X-ray tracking (DXT) is a method based on time-resolved XRD, which is used to monitor the movement of a diffraction spot from a crystal. DXT, which was devised by Sasaki et al., [29,30] is widely used to observe the dynamic motion of membrane proteins, [31,32] supersaturated proteins, [33] and ion network domains. [34] A bright diffraction spot appears when the Bragg conditions are satisfied during the rotation of a single crystal. A further rotation of the single crystal changes the direction of the scattered X-rays, resulting in the movement of the diffraction points (Figure 5b). The bright diffraction spot disappears when the single crystal no longer satisfies the Bragg conditions owing to further rotation.
The 4-MAAB crystals used in this study were polycrystalline because they were prepared by cooling molten 4-MAAB. Therefore, one of the single crystals in the polycrystal probably corresponded to the experimentally observed bright diffraction spots. In contrast, the 4-MAAB crystals were partially melted by light irradiation during crawling motion. As mentioned earlier, the movement of the diffraction spot indicated rotation of a single crystal, which was possibly achieved by the flow of liquid 4-MAAB. This hypothesis is consistent with the changes in the brightness of the emission from the QDs during light-induced crawling (Figure 1c-f). The circulation of liquid 4-MAAB is a possible reason for the up-down movement of the QDs shown in Figure 1g-j.
Overall, these results indicate that the rotational motion was caused by the flow of liquid 4-MAAB, which circulated during the crawling motion, suggesting that the 4-MAAB crystals did not simply slide on the glass substrate. However, as the size and shape of the rotated 4-MAAB single crystal were unknown, detailed information on the direction and velocity of the flow could not be obtained. Further comprehensive investigations on the photoinduced crawling motion can be conducted by introducing probe crystals such as single-crystalline gold nanocrystals to 4-MAAB crystals, as they exhibit strong diffraction. Elucidating the associated mechanism using time-resolved XRD experiments will aid enhance the velocity of material transportation and stability of the crawling motion.

Conclusion
A new method for transporting nano/micromaterials on a glass surface using the photoinduced crawling motion of the crystals is presented herein. This differs from previous approaches adopted for manipulating droplets, which involve applying a chemical or surface charge gradient through surface modification. The method reported herein can introduce new techniques for achieving the microscale control of object movement. In particular, this study provides a novel approach to assemble target nanomaterials at desired positions on a solid substrate. Synchrotron XRD experiments revealed that a part of the melted crystals flowed during their crawling motion. Further investigation of the flow mechanism will improve the material transportation performance. The material transportation realized via the photoinduced crawling of the crystals is expected to open new avenues in diverse fields such as microfluidics, microrobotics, photoenergy transformation, and biotechnology.

Experimental Section
Materials: 4-MAAB was purchased from Tokyo Chemical Industry Co., Ltd., and purified by silica gel column chromatography and subsequent recrystallization from hexane. Octadecylamine-capped CdSe@ZnS QDs and HMDS were purchased from Sigma-Aldrich. Fluorescent silica microparticles with a diameter of ≈3 µm (sicastar-red F) were purchased from Micromod. The surfaces of the particles were unmodified.
Sample Preparation: A crystalline thin film was prepared using a liquid crystal cell with an ≈0.54-µm-sized gap for measuring the absorbance spectrum. Molten 4-MAAB was introduced to the cell via the capillary effect and cooled to room temperature for recrystallization. To investigate the effect of blue-LED-light absorption by the 4-MAAB crystals on the emission of the QDs, a QD layer was first prepared on a cover glass. A 50 µL dispersion of CdSe@ZnS (0.5 mg mL −1 in toluene) was added dropwise onto the glass substrate and dried under ambient conditions. Next, the 4-MAAB crystals were melted at 95 °C on the QD layer. After the melted 4-MAAB was enclosed with another cover glass, it was cooled to room temperature to prepare a crystalline film. A 25-µm-sized Kapton film was used as the spacer between the cover glasses.
The sample for time-resolved XRD measurements was obtained by sandwiching the molten crystal between a Kapton film and a quartz substrate, followed by recrystallization at room temperature. The quartz substrate, which was polished to a thickness of ≈0.02 mm, was purchased from ATOCK Inc. Several crystals remained on the substrate when the Kapton film was peeled off. Diffraction images were obtained using synchrotron X-rays incident on the crystals.
Equipment: A v-780 spectrophotometer (JASCO) was used to measure the absorbance spectrum of the 4-MAAB crystalline film. A blue LED (CCS Inc., 465 nm: HLV2-22BL-3 W) was used for photoirradiation. Light intensities were measured using a Newport 1917-R optical power meter with an 818-ST-UV photodetector. A brightfield epi-illumination system (BXFM, Olympus) was used to observe the crawling motion of the crystals and fluorescence from the CdSe@ZnS QDs. An objective lens with 20× magnification was used. An epifluorescence microscope (BX-51, Olympus) was used to observe the fluorescence of the silica microparticles. A dichroic mirror (Olympus, U-MNG) was used for the fluorescence observations. A high-pressure mercury lamp (USH-103OL) was used as the excitation source. Water contact angles were measured using a contact angle meter (Kyowa Interface Science Co., Ltd., DMe-210). An infrared (IR) camera (FLIR Systems Inc, FLIR i3) was used to photograph the temperature rise in the 4-MAAB crystal thin film under light irradiation. The 4-MAAB crystal was exposed to blue light for more than 30 s until thermal equilibrium was reached.
X-rays with an energy of ≈16.0 keV were used for the time-resolved XRD measurements. These experiments were performed at the High Energy Accelerator Research Organization (KEK) Photon Factory Advanced Ring (PF-AR) NW14A (Tsukuba, Japan). Time-resolved diffraction images were recorded using a photon-counting pixel detector www.advmatinterfaces.de (Pilatus 100 K, Dectris). Diffraction images were recorded every 50 ms, and the X-ray exposure time was 45 ms per image.

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