Photoinduced Shape Changes of Mixed Molecular Glass Particles

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Photoinduced Shape Changes of Mixed Molecular Glass Particles Containing Azobenzene-Based Photochromic Amorphous Molecular Materials Fixed in Agar Gel Hideyuki Nakano, Ryota Ichikawa, Hiroyasu Ukai, and Ayame Kitano J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b03651 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 17, 2018

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The Journal of Physical Chemistry

Photoinduced Shape Changes of Mixed Molecular Glass Particles Containing Azobenzene-based Photochromic Amorphous Molecular Materials Fixed in Agar Gel Hideyuki Nakano,* Ryota Ichikawa, Hiroyasu Ukai, and Ayame Kitano Department of Applied Chemistry, Muroran Institute of Technology, Mizumoto-cho, Muroran, Hokkaido 050-8585, Japan. ABSTRACT: It has been found that mixed molecular glass particles exhibit photoinduced shape changes, elongating their shapes to form string-like structures similar to single molecular glass particles of azobenzene-based photochromic amorphous molecular materials. Furthermore, interestingly, the addition of 15 mol% of photochemically inert 4,4',4"-tris[3methylphenyl(phenyl)amino]triphenylamine enhanced the phenomenon relative to single particles of 4-[bis(4methylphenyl)amino]azobenzene. The present mixed systems allowed to elucidate the effects of Tg and of the apparent photochromic reactivity independently by changing the mixing ratio of suitable materials. It has been clearly demonstrated that increase in apparent photochromic reactivity enhanced monotonically the photomechanical elongation of the particles. On the other hand, it has been found that increase in Tg was favorable for the present photoinduced elongation of the particle while the effect of Tg became saturated at sufficiently high Tg values.

Azobenzene-based molecules and polymers are attractive materials with a variety of functions. Due to their vivid color, they have been widely used as coloring agents for textiles and foods.1 Some of them have been known to change their colors in response to their surrounding environments and such azobenzene-based materials have been used as pH-indicators2–3 and other color changeable materials.4–7 Azobenzene-based molecules and polymers with electron donor and accepter groups are of interest as promising candidates for use in second harmonic generation due to their non-linear optical properties.8–9 Rigid azobenzene structure is also important as the core of liquid crystalline materials.8,10–11 In addition, the azobenzene-based materials often exhibit photochromism based on photoinduced trans–cis and cis–trans isomerization reactions.12–14 Due to their large changes in molecular structures accompanied by photochromic reactions, azobenzene-based materials have great potential as candidates for photomechanical systems with large movements and/or structural changes. In an early stage, photoalignment of liquid-crystalline systems, including azobenzenedoped liquid crystalline systems and alignment-control system with azobenzene-modified command surfaces, have been demonstrated.15 Subsequently, photomechanical bending motions of azobenzene-based liquidcrystalline polymer films and fibers,16–19 and photoinduced surface relief grating (SRG) formation for which azobenzene-based polymer films20–26 were used have been reported. Related phenomena such as photomechanical deformations of azobenzene-based polymer spheres and particles, photoinduced vitrification and fluidization for azobenzene-based molecular crystals, and photoinduced oscillatory motion of a noncovalent assembly of oleic acid

and an azobenzene derivative have been reported.27–33 Photoinduced changes in surface structures of selfassembled monolayer films including azobenzene moiety have also been investigated in nanoscale.34 The authors have performed several parts of a series of studies on creation of amorphous molecular materials, namely low-molecular-mass organic materials that readily form amorphous glasses at temperatures above room temperature.35–39 We have later created photochromic amorphous molecular materials and investigated their photochromic reaction properties in their amorphous states.40–46 In addition, we have found that azobenzenebased photochromic amorphous molecular materials exhibited several kinds of photomechanical behaviors related to photoinduced mass transport.47–57 For example, it has been demonstrated that SRG formation of azobenzene-based amorphous molecular materials are induced by coherent irradiation of laser beams and that the SRG forming ability depends upon polarization direction of incident beams, p-polarized beams being preferable to spolarized ones.47–49,51 Subsequently, we have fabricated molecular fibers of azobenzene-based amorphous molecular materials by melt-spinning method and the fibers exhibited photoinduced bending motions. Interestingly, the bending direction of the fibers could be controlled by altering the polarization direction of incident beams. That is, polarized beams with polarization directions parallel and perpendicular to the fiber axis induced bending in the direction opposite and toward the light source, respectively.50,54 In addition, we have demonstrated photoinduced mass flow at the surface of amorphous films,52 and photoinduced movements of glass fragments placed on substrate upon angled irradiation of p-polarized laser

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beam.52,53 These phenomena were also found to depend upon polarization direction of the incident beam, spolarized beam being unable to induce such phenomena. We have also reported the photomechanical effects for hybrid materials depending upon the polarization direction of incident bams.55–57 All these phenomena were explained by anisotropic mass transport induced in the direction parallel to the polarization direction of incident laser beam; however, the materials in these cases were placed in somewhat anisotropic environments that might affect anisotropic photomechanical behaviors. Detailed mechanisms of all these phenomena depending upon the polarization direction of the incident beam have not been clarified yet. Most recently, we have investigated photomechanical shape changes of glass particles of azobenzene-based amorphous molecular materials fixed in agar gel which provided isotropic environment.58 It was found that the irradiation with linearly polarized laser beam induced shape changes of the particles to form string-like structure, and it was confirmed by three dimensional observation that the elongation direction of the particles was parallel to the polarization direction of the incident beam. The phenomena were explained by the photoinduced vibration and/or transport of molecules parallel to the polarization direction of the incident beam to generate particles’ force to push the surrounding gel away. It was suggested that both glass-transition temperature (Tg) of the material and photochromic reactivity in amorphous state affected the phenomena; that is, increase in Tg was favorable while introduction of bulky substituents at both ends of azobenzene moiety was undesirable for the photoinduced elongation due to reduction of photochromic reactivity in the amorphous state. Thus, these effects of Tg and reactivity seemed to relate complexly with each other and it is of interest and of importance to clarify the effects of "Tg" and "reactivity" independently, that will provide important information for clarifying the mechanism of all the above-mentioned phenomena observed for azobenzene-based photochromic amorphous molecular materials regarding photoinduced anisotropic mass transport. In the present study, we have investigated mixed molecular glasses containing azobenzene-based photochromic amorphous molecular materials. It is thought that the photoinduced shape changes of the mixed glass particles allow to elucidate effects of Tg and the reactivity independently by changing the mixing ratio of suitable materials. Not only the azobenzene-based photochromic amorphous molecular materials, 4-[bis(4methylphenyl)amino]azobenzene (BMAB),45 4[phenyl(biphenyl-4-yl)amino]azobenzene (PBAB),59 and 4-[bis(9,9-dimethylfluoren-2-yl)amino]azobenzene (BFlAB),45 but also photochemically inert amorphous molecular materials, 1,3,5-tris[3methylphenyl(phenyl)amino]benzene (m-MTDAB)36 and 4,4',4"-tris[3-methylphenyl(phenyl)amino]triphenylamine (m-MTDATA),35 (Figure 1) were used and photoinduced

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shape changes of several kinds of mixed particles fixed in agar gel have been investigated.

N

N N

N

N

N

PBAB

BMAB

N N

N

BFlAB N N N N

N N

m-MTDAB

N

m-MTDATA

Figure 1. Chemical structures of the materials used in the present study.

EXPERIMENTAL SECTION Materials. Azobenzene-based photochromic amorphous molecular materials, BMAB, PBAB, and BFlAB, and photochemically inert amorphous molecular materials, m-MTDATA and m-MTDAB, were prepared by the methods described in our previous papers.35–37,45,59 Agar was purchased commercially (Kanto Chemical Co., Inc.) and used without further purification. Sample Preparation. Two materials with a predetermined ratio (total amount: ca. 4 mg) were completely dissolved in small amount of THF (ca. 1 mL) and the solution was dropcast onto hot glass substrate (ca. 70-80 °C) to remove the solvent immediately. Resulting glassy lumps were broken with a spatula at room temperature to obtain mixed particles in appropriate size. Agar was dissolved in deionized water (2.5 mg cm–3) by heating at ca. 90 °C and the solution was poured into the transparent glass cell with a path length of 2 mm. The mixed particles obtained from the process described above were dispersed into the solution in the cell, which was gradually cooled at ambient atmosphere, followed by storing in refrigerator (ca. 3 °C) to obtain the sample of the particles fixed in agar gel. Measurements and Apparatus. Differential scanning calorimetry (DSC) for mixed glasses was carried out by means of a Seiko DSC6220. In order to monitor photoinduced shape changes of the mixed glass particles, the


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The Journal of Physical Chemistry sample cell was placed on the stage of the optical microscope (Optiphot X2, Nikon) at room temperature (ca. 21– 22 °C) and the sample was irradiated with laser beam (488 nm, CYAN-488-100 CDRH, SpectraPhysics Inc.) with an output power of 15 mW (ca. 1.7 Wcm–2) from the bottom of the sample cell through a polarizer as shown in Figure 2. Relative length of the long axis (RL) of the particles was defined as the ratio of the length of the elongated particle after irradiation relative to the diameter of the particle before irradiation. For collection of RL data, more than 9 particles in approximately spherical shape with diameters of 5–15 µm were selected randomly. Since their photoinduced elongations seemed to be saturated within 60 min in all cases, the growth of RL values upon irradiation were monitored for 60 min in the present study.

Figure 2. Schematic experimental set-up for monitoring the photomechanical behaviors.

Figure 3. DSC curve of the mixed glass of BMAB (85 mol%) –1 and m-MTDATA (15 mol%). Scan rate: 5 °Cmin .

The present mixed particles exhibited photomechanical shape changes as observed for single molecular glasses systems.58 For example, a mixed particle of BMAB (85 mol%) and m-MTDATA (15 mol%) fixed in the agar gel exhibited such photomechanical shape change upon irradiation with linearly polarized laser beam as shown in Figure 4. In the case that the sample particles were irradiated, the shapes of the particles were gradually and drastically changed, being elongated parallel to the polarization direction of the incident laser beam to form string-like structure. Such photoinduced elongation seemed to be saturated after irradiation for within ca. 60 min. Behaviors of the particles were also shown in Supporting Movie. The resulting structures were maintained at room temperature after stopping the irradiation.

RESULTS AND DISCUSSION Figure 3 shows a DSC curve of the resulting mixed glass sample of BMAB (85 mol%) and m-MTDATA (15 mol%). Observation of just one glass transition phenomenon has suggested that the molecules were mixed homogeneously without phase separation. Tg of the sample was found to be 31 °C, being between those of BMAB (27 °C) and mMTDATA (75 °C). All the mixed samples with different ratios in the present study were also obtained as homogeneously mixed glasses suggested by their DSC curves (Figures S1-S4 in Supporting Information). Tgs of the samples were almost linearly related to the mixing ratio as described below.

Figure 4. Photoinduced shape changes of mixed glass particles of BMAB (85 mol%) and m-MTDATA (15 mol%) in agar gel upon irradiation with linearly polarized laser beam (488 nm, 15 mW). a) Before irradiation. b) After 60 min irradiation. The arrow indicates polarization direction of the incident beam. Scale bar: 20 µm.

It is of interest that the addition of 15 mol% of photochemically inert m-MTDATA enhance the phenomenon relative to single particles of BMAB. Figure 5 shows change in the relative length of the long axis (RL) of the particles during irradiation with a laser beam with an intensity of 15 mW for single glass particles of BMAB and for mixed glass particles of BMAB (85 mol%) and mMTDATA (15 mol%). It has been found that RL values gradually increased and finally saturated within ca. 60 min. It is to be noted that the rate of photoinduced elongation for the mixed particles was considerably faster than that for the single BMAB particles was. The RL values after 60 min irradiation for the mixed particles of BMAB (85 mol%) and m-MTDATA (15 mol%) was 3.9±0.9,



being greater than 2.0±0.4, that for single particles of BMAB. This result suggests that the increase in Tg of the particle by mixing with m-MTDATA with a higher Tg promotes photomechanical elongation of the particles. It is noteworthy that not only photochromic BMAB molecules but also photochemically inert m-MTDATA molecules were shown to move in the particles to elongate their shapes. The result was consistent with photoinduced SRG formation observed for mixed film of BMAB and m-MTDATA, suggesting that the photochemically inert molecules moved together with photochromic ones.37 Figure 6. a) RL values after 60 min irradiation of mixed particles of BMAB and m-MTDATA with different ratios. b) Tgs of their mixed samples.

Figure 5. The change in the relative length of the long axis (RL) of the particles during irradiation with a laser beam with an intensity of 15 mW. a) Single glass particles of BMAB. b) Mixed glass particles of BMAB (85 mol%) and mMTDATA (15 mol%).

Figure 6 shows RL values after 60 min irradiation of mixed particles of BMAB and m-MTDATA with different ratios together with their Tgs. In the region of relatively low ratio of m-MTDATA (less than 15 mol%), the RL values increased with an increase in ratio of m-MTDATA. The result was caused by the increase in Tg accompanied with an increase in ratio of m-MTDATA. However, in the region of higher m-MTDATA ratio, RL decreased with an increase in ratio of m-MTDATA. This is presumably because the apparent photochromic reactivity per molecule lowered with a decrease in ratio of photochromic BMAB molecule. Thus, the present phenomena of photoinduced elongation of the mixed particles were affected by both Tg and the apparent photochromic reactivity per molecule. The fact is consistent with the phenomena observed for single systems of azobenzene-based amorphous molecular materials.57

It is thought that the mixed systems have an advantage in providing a suitable system to elucidate effects of Tg and reactivity independently by selecting appropriate two materials. Next, mixed particles of BMAB and BFlAB were investigated. In this system, BMAB and BFlAB were assumed to have similar reactivity since these materials exhibited similar photochromic properties in their amorphous films while their Tgs were considerably different from each other.45 Therefore, changing the ratio of BMAB and BFlAB may alter Tg without apparent photochromic reactivity per molecule changed. Figure 7 shows RL values after 60 min irradiation of mixed particles of BMAB and BFlAB with different ratios together with their Tgs. As shown in Figure 7b, Tg of this system increases almost linearly with an increase in ratio of BFlAB. It has been found that the RL values increased with an increase in ratio of BFlAB in the region of BFlAB ratio of less than ca. 50 mol% while on the other hand the RL value was almost constant in the region of higher BFlAB ratio. The result indicated that the increase in Tg was favorable for the present photoinduced elongation of particles while the effect of Tg was saturated at Tg higher than ca. 50-60 °C.

Figure 7. a) RL values after 60 min irradiation of mixed particles of BMAB and BFlAB with different ratios. b) Tgs of their mixed samples.

Next, combinations of photochromic and photochemically inert amorphous molecular materials with similar


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The Journal of Physical Chemistry Tgs have been investigated. In these systems, apparent photochromic reactivity per molecule is able to be controlled without changing Tg by altering the mixing ratio of the materials. Figure 8 shows RL values after 60 min irradiation of mixed particles of PBAB and m-MTDAB with different ratios together with their Tgs. As shown in Figure 8b, Tgs of the mixed materials were almost constant in the whole range of mixing ratio at ca. 46-47 °C. It has been found that the RL values were governed by mixing ratio. In the case that the ratio of m-MTDAB was 75 mol%, photoinduced elongation of the mixed particles were scarcely observed. When the ratio of photochromic PBAB increased, the RL value increased and seemed to reach maximum at ca. 75 mol% of PBAB (i.e., ca. 25 mol% of m-MTDAB). The fact suggested that the RL values increased with an increase in apparent photochromic reactivity of the material. It has been found that the RL value after 60 min irradiation for 0 mol% of m-MTDATA was almost equal to that for 25 mol%. The authors presume that this was caused by saturation of the effect of reactivity. However, Tg of the system was ca. 47 °C, which was lower than the saturation temperature of the effect of Tg described above, and it might have influenced the results. Thus, the mixed particles of photochromic amorphous molecular material, BFlAB, and photochemically inert amorphous molecular material, m-MTDATA, were also investigated as shown in Figure 9. The Tg decreased with an increase in ratio of m-MTDATA as shown in Figure 9b due to Tg of m-MTDATA lower than that of BFlAB; however, Tgs of all samples were higher than the saturation temperature, which made the difference in the effect of Tg between different mixed particles negligible. As shown in Figure 9a, the RL values increased monotonically with a decrease in ratio of m-MTDATA, i.e., with an increase in photochromic BFlAB. Thus, the increase in apparent photochromic reactivity per molecule enhanced the photoinduced elongation of the particles monotonically.

Figure 8. a) RL values after 60 min irradiation of mixed particles of PBAB and m-MTDAB with different ratios. b) Tgs of their mixed samples.

Figure 9. a) RL values after 60 min irradiation of mixed particles of BFlAB and m-MTDATA with different ratios. b) Tgs of their mixed samples.

It is to be noted that the plots of RL values after 60 min irradiation for BFlAB–m-MTDATA system as shown in Figure 9a became larger in comparison with those for PBAB–m-MTDAB system as shown in Figure 8a. For example, the plot for 50 mol% of m-MTDATA was 2.9±0.7 in Figure 9, which is larger than that for 50 mol% of mMTDAB in Figure 8 (2.4±0.3). This indicated that the increased Tg without the apparent photochromic reactivity changed was favorable for the present photoinduced elongation of the mixed particles. The result is consistent with the effect of Tg observed for single systems44 and for BMAB–BFlAB system described above. The present photomechanical elongation of the mixed particles is explained as follows. In the case that the particles are irradiated with a linearly polarized laser beam, photoinduced trans–cis and cis–trans isomerization reactions of the azobenzene-based molecules take place, resulting in softened particles. In addition, these molecules in the particles vibrate and/or are transported in the direction parallel to the polarization direction of the incident beam. As a result, particle force to push the surrounding gel away is generated in the direction parallel to the polarization direction, resulting in elongation of the particles. It has been clearly demonstrated that an increase in apparent photochromic reactivity per molecule enhanced monotonically the photoinduced elongation of the particles. The increase in the apparent photochromic reactivity, i.e., the frequency of trans–cis and cis–trans isomerization cycles facilitates vibration and/or transport of molecules to produce force to change their shapes. For effects of Tg, fluidity of particles at certain temperature upon irradiation was assumed to be reduced by increasing Tg of the particles. In the case that the glass particles with lower Tg was used, relatively high fluidity of the material inhibited the particles from elongation due to interfacial tension to shrink the particles. Thus, the increase in Tg was favorable for photoinduced elongation of mixed particles. Saturation of the effects of Tg at higher Tg values (> ca. 50-60 °C) suggested that the effect of inhibiting particles from elongation due to fluidity became negligibly small for the particles with sufficiently high Tg. This might indicate that apparent Tg of the par-



ticles of azobenzene-based photochromic amorphous molecular materials upon irradiation in the present conditions was ca. 30-40 °C lower than the Tg in the dark.

CONCLUSIONS Photomechanical shape changes of mixed glass particles containing azobenzene-based amorphous molecular materials fixed in agar gel have been investigated in the present study. It has been interestingly found that the addition of 15 mol% photochemically inert m-MTDATA enhanced photoinduced elongation of particles relative to the single particles of BMAB. The use of suitable mixed systems allowed to clearly demonstrate the effects of photochromic reactivity and of Tg. That is, the increase in photochromic reactivity enhances monotonically the photomechanical elongation of the particles and the increase in Tg was favorable for the present photoinduced elongation of the particles while the effect of Tg was saturated at sufficiently high Tg values (> ca. 50-60 °C). Although the mechanism of anisotropic elongation of the particles depending upon polarization direction of the incident beam has not been clarified, the present study has provided valuable information regarding a variety of photomechanical behaviors observed for azobenzenebased photochromic amorphous molecular materials.

ASSOCIATED CONTENT Supporting Information. The supporting movie shows the photoinduced shape changes of mixed glass particles of BMAB (85 mol%) and m-MTDATA (15 mol%) in agar gel upon irradiation with linearly polarized laser beam (488 nm, 15 mW) for 60 min, being shortened to ca. 8 sec. DSC charts for mixed samples were provided as Figures S1-S4. These materials are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Author Contributions Prof. H. Nakano, R. Ichikawa, H. Ukai, and A. Kitano contributed equally to this work.

ACKNOWLEDGMENT This work was supported by JSPS KAKENHI Grant Number JP26107006 in Scientific Research on Innovative Areas “Photosynergetics”.

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