Self-Propelled Motion of a Camphor Disk on a Photosensitive

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Self-Propelled Motion of a Camphor Disk on a Photosensitive Amphiphilic Molecular Layer Satoshi Nakata, Kyoko Nasu, Yasutaka Irie, and Sayaka Hatano Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b04285 • Publication Date (Web): 26 Feb 2019 Downloaded from http://pubs.acs.org on March 6, 2019

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Langmuir

Self-Propelled Motion of a Camphor Disk on a Photosensitive Amphiphilic Molecular Layer

Satoshi Nakata,* Kyoko Nasu, Yasutaka Irie, and Sayaka Hatano

Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan.

* Corresponding author. Tel. & Fax: +81-824-24-7409; E-mail: [email protected]

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ABSTRACT We have studied the self-propelled motion of a camphor disk on a 2,2’-bis(2-chlorophenyl)4,4’,5,5’-tetraphenyl-1,2’-biimidazole (o-Cl-HABI) molecular layer, which was developed on water, as a photomechanical sensing system. The o-Cl-HABI dimer changed to its monomeric form upon UV light irradiation and as a result, the surface pressure of the o-Cl-HABI molecular layer decreased. The reciprocating motion of a camphor disk in the absence of UV light irradiation was observed at A = 45 Å2 molecule-1, of which the surface pressure was ~10 mN m-1. Random motion was observed under UV light irradiation at A = 45 Å2 molecule-1, of which the surface pressure was ~5 mN m-1. Therefore, the nature of motion of a camphor disk changes depending on the photosensitivity of the o-Cl-HABI molecular layer. We have discussed the mechanism of the change in the motion of the camphor disk in relation to the photoreaction of the o-Cl-HABI molecular layer with surface pressure acting as the driving force.

KEYWORDS. Self-propelled motion, reciprocating motion, surface pressure, photoreaction of o-ClHABI.

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INTRODUCTION The development of self-propelled objects is important industrially and medically to transport materials or the objects to a target place in a narrow space.1-3 Most self-propelled objects exhibit random motion or unidirectional motion, the direction of which is determined by the initial asymmetry of the object or the direction of an external force.1-5 In contrast, bacteria exhibit characteristic motion, such as chemotaxis and collective motion.6-8 There have been several reports on the photosensation, phototaxis, and photomanipulation of self-propelled objects.9-16 For example, surface tension acts as the driving force for the motion of a self-propelled object that is sensitive to light irradiation, such as in the photoreaction from hydrophobic

benzoquinone

to

hydroquinone.9

hydrophilic

As

another

example,

the

photoisomerization of an amphiphilic azo compound changes the surface pressure.10-14 Therefore, light irradiation can induce the characteristic motion of a camphor disk acting as a self-propelled object on an azo derivative molecular layer;10 deforms vesicles, which are composed of amphiphilic azo compound;11 or realizes photomanipulation.12-14 However, the differences in the surface pressure between the cis- and trans-isomers of an azo-compound are not very large. Consequently, another substance with a large difference in surface pressure upon light irradiation is necessary to enhance the photosensitivity of the self-propelled motion. In this study, we introduce an imidazole dimer, 2,2’-bis(2-chlorophenyl)-4,4’,5,5’tetraphenyl-1,2’-biimidazole (o-Cl-HABI), used to photochemically control the nature of selfpropelled motion. Two 2,4,5-triphenyl imidazolyl radical (TPIR) derivatives, which are imidazole monomers, are produced from a single o-Cl-HABI molecule upon UV light irradiation (Scheme 1).1720

The obtained TPIR can be thermally reverted into o-Cl-HABI in the absence of UV light

irradiation. The surface pressure of the o-Cl-HABI molecular layer on water decreases upon UV light 3 ACS Paragon Plus Environment

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irradiation. A camphor disk was placed on the o-Cl-HABI molecular layer developed on water, which displayed characteristic motion (e.g., reciprocating motion) that was sensitive to UV light irradiation. We have discussed the relationship between the change in the motion of the camphor disk and the photoreaction of the o-Cl-HABI molecules from the viewpoint of the surface pressure acting as the driving force. The present system is significant to design the novel self-propelled systems which exhibit characteristic motion depending on the light irradiation and mechanical force.

Scheme 1. The chemical structures of o-Cl-HABI and TPIR.15, 16 Two TPIR molecules are produced upon UV light irradiation of a single o-Cl-HABI molecule, which are thermally reverted to the o-ClHABI molecule in the absence of UV light irradiation.

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EXPERIMENTAL SECTION Camphor (Wako Pure Chemical Industries, Ltd., Osaka, Japan) was used without any further purification. 2,2’-Bis(2-chlorophenyl)-4,4’,5,5’-tetraphenyl-1,2’-biimidazole (o-Cl-HABI) was purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). Chloroform was purchased from Nacalai Tesque, Inc. (Tokyo, Japan).

To prepare the o-Cl-HABI molecular layer on water, water was first distilled and then purified using a Millipore Milli-Q filtering system (Direct-Q, Millipore SAS, France; pH = 6.3 and resistance = 18.2 MΩ). o-Cl-HABI was dissolved in chloroform and the resulting chloroform solution was dropped onto the water phase using a microsyringe. The volume of 0.32 mM o-ClHABI solution was 50 L, i.e., the amount of o-Cl-HABI was 1.62 ×10-8 mol. The surface pressure () – surface area (A) isotherm of the o-Cl-HABI molecular layer was measured using a surface pressure meter (Kyowa Interface Science Co. Ltd., HMB, Saitama, Japan) at 293 ±1 K. Compression of the o-Cl-HABI molecular layer was started 5 min after the addition of the chloroform solution to the water surface to allow the chloroform to evaporate. The surface area of the water phase decreased from 210 to 21 cm2 (= 215 to 22 Å2 molecule-1) at a rate of 18.9 cm2 min-1 (= 19.4 Å2 molecule-1 min-1) to give the -A isotherm for the molecular layer. A camphor disk (diameter: 3 mm, thickness: 1 mm, mass: 5 mg) was obtained using an FTIR pellet die set. When A reached a specific value (e.g., 45 Å2 molecule-1), the compression was stopped, and the camphor disk was placed on the o-Cl-HABI molecular layer developed on water. The motion of the camphor disk was monitored using a digital video camera (HDR-CX590, SONY, Japan; minimum time-resolution, 1/30 s) and the images were analyzed with image processing software (ImageJ, National Institute of Health, Bethesda, MD, USA). A UV lamp (Sankyo Denki, 5 ACS Paragon Plus Environment

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Co., Ltd., Kanagawa, Japan, ES-27BLB, 27 W) was used to irradiate the o-Cl-HABI molecular layer from above (wavelength: 365 nm; light intensity: 144 mW cm−2). To monitor the camphor motion in the absence of UV irradiation, the water surface was irradiated with red light. At least three experiments were performed to confirm the reproducibility of our results.

RESULTS

1. Surface pressure–surface area isotherms of the o-Cl-HABI molecular layer

Figure 1 shows the surface pressure ()–surface area (A) isotherms measured for the o-ClHABI molecular layer with and without UV light irradiation. Under compression, the  value was zero with and without UV light irradiation at A >60 Å2 molecule-1. However, the  value in the absence of UV light irradiation was higher than that measured under UV light irradiation at A ≤ 60 Å2 molecule-1.

Figure 1. The -A isotherms of the o-Cl-HABI molecular layer developed on water with (grey line) and without (black line) UV irradiation. The chemical reaction of o-Cl-HABI under UV light irradiation is shown in Scheme 1. Here, A was calculated from one o-Cl-HABI dimer molecule. 6 ACS Paragon Plus Environment

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Figure 2 shows the time-variation of the surface area (A) (Fig. 2a) and surface pressure ( (Fig. 2b) of the o-Cl-HABI molecular layer with and without UV light irradiation. In this experiment, A decreased from A = 210 Å2 molecule-1 upon compression with or without UV light irradiation. When A reached 45 Å2 molecule-1 (td = 0) it remained at this A value at td >0. The  values at td = 0 with and without UV light irradiation were 11 and 26 mN m-1, respectively. These values are similar to those of the -A isotherms at A = 45 Å2 molecule-1 with and without UV light irradiation, respectively (Fig. 1). The  value decreased to 5 and 10 mN m-1 at td = 200 s with and without UV light irradiation, respectively.

We performed another experiment in which the o-Cl-HABI molecular layer was not irradiated with UV light at td < 0 and then subsequently irradiated with UV light at td ≥ 0 (see the dotted line in Fig. 2b). In this experiment, the  value decreased to 8 mN m-1 at td = 200 s.

Figure 2. The time-variation of (a) the surface area per o-Cl-HABI molecule, A, and (b) surface 7 ACS Paragon Plus Environment

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pressure () with (ON, grey solid line) and without UV light irradiation (OFF, black solid line). td was defined as zero when A reached 45 Å2 molecule-1. The dotted grey line denotes the timevariation of  for the o-Cl-HABI molecular layer in the absence of UV light at td < 0 and with UV irradiation at td ≥ 0. The grey region between 140 and 160 s denotes the period of time used to analyze the motion.

To evaluate the degree of progress during the reaction from o-Cl-HABI to TPIR, and the reverse reaction, we measured the temporal change in the absorbance of a o-Cl-HABI solution in benzene with and without UV light irradiation, respectively (Figs. S1 and S2). We confirmed that the reaction from o-Cl-HABI to TPIR, and the reverse reaction, in the benzene solution are converged within 5 and 20 min at 293 K, respectively.

2. The characteristic motion of a camphor disk depending on UV light irradiation

In this experiment, a o-Cl-HABI molecular layer was developed on water, and then compressed from A = 210 Å2 molecule-1 with or without UV light irradiation. When A = 45 Å2 molecule-1, a camphor disk was placed on the o-Cl-HABI molecular layer (td = 0). The motion of the camphor disk was analyzed for 10 s at td ~150 s to compare the motion under the same temporal conditions (see grey region in Fig. 2). In the absence of UV light irradiation, a reciprocating motion, which followed the same trajectory, was observed (Fig. 3a). The value, L, was defined to evaluate the reciprocating motion (Fig. 3b), and the time-variation of L at  = 8/9 rad oscillated periodically with an almost constant amplitude (3 mm s-1) (Fig. 3c). To evaluate the periodicity of the oscillation, the time trace of L was Fourier transformed to the frequency domain. Here,  = 8/9 rad was selected in FFT when the amplitude was the maximum. A clear peak of 1 Hz was observed, as shown in Fig. 3d. 8 ACS Paragon Plus Environment

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The  values at the maximum and minimum amplitudes of L were 8/9 and 7/18 rad, respectively (Fig. 3e). The existence of the maximum and minimum (~ 0 mm s-1) amplitudes show that the motion of the camphor disk can be regarded as a one-dimensional reciprocating motion. The average value of three examinations was 3.0 ± 0.2 mm s-1.

Figure 3. The reciprocating motion observed for a camphor disk on a o-Cl-HABI molecular layer developed on water in the absence of UV light irradiation. The (a) outline of the camphor disk observed for 10 s, (b) definition of L and , (c) temporal trace of L at  = 8/9 rad, (d) power spectrum of FFT for (c), and (e) amplitude of the reciprocating motion depending on . A = 45 Å2 molecule-1 and td = ~150 s (see Movie S1).

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Under UV light irradiation, random motion was observed at A = 45 Å2 molecule-1, as shown in Fig. 4. The motion of the camphor disk did not exhibit the same outline (Fig. 4a). The timevariation of L was not periodically oscillated (Fig. 4b). There was no FFT peak (Fig. 4c). The error bars of the amplitude of L in Fig. 4d are significantly larger than those in Fig. 3e, and the amplitude was independent of  (Fig. 4d). Therefore, the motion was regarded as random motion.

Figure 4. The random motion observed for a camphor disk on a o-Cl-HABI molecular layer developed on water under UV light irradiation. The (a) outline of the camphor disk observed for 10 s, (b) temporal trace of L at  = 0 rad, (c) power spectrum of FFT for (b), and (d) amplitude of the motion depending on . A = 45 Å2 molecule-1 and td = ~150 s (see Movie S2).

We performed another examination on the self-propelled motion of a camphor disk on a o-

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Cl-HABI molecular layer developed on water under UV light irradiation at A = 45 Å2 molecule-1 (Fig. S3). In this experiment, UV light was not irradiated before placing the camphor disk on the water surface (td < 0). When A reached 45 Å2 molecule-1(td = 0), the UV light irradiation was started and the camphor disk placed on the surface within 5 s after td = 0. The time-variation of  is shown as a dotted line in Fig. 2b. A mixture of random and reciprocating motion was observed. Actually, the amplitude slightly depended on  and a broad FFT peak around 1.4 Hz was observed in Fig. S3. Subsequently, we examined the self-propelled motion of a camphor disk on a o-Cl-HABI molecular layer developed on water with or without UV light irradiation under higher compression (i.e., A = 40 Å2 molecule-1) to compare the effects of the o-Cl-HABI and TPIR molecular layers (Fig. 5, S4, and S5). Without UV light irradiation, no motion was observed (Fig. S5). On the other hand, reciprocating motion was observed upon UV light irradiation (Fig. 5). The  value observed at td = 0 for A = 40 Å2 molecule-1 with UV light irradiation was almost equal to that observed at A = 45 Å2 molecule-1 without UV light irradiation (~27 mN m-1). The maximum values observed for the amplitude and frequency of the reciprocation (amplitude: 3.1 ±0.4 mm s-1, frequency: 1.48 ±0.29 Hz) with UV light irradiation at A = 40 Å2 molecule-1 were similar to those without UV light irradiation at A = 45 Å2 molecule-1 (amplitude: 2.9 ±0.4 mm s-1, frequency: 1.39 ±0.37 Hz) (Fig. 3). In contrast, the minimum amplitude with UV light irradiation at A = 40 Å2 molecule-1 (amplitude: 0.8 ± 0.5 mm s-1) was clearly larger than that observed in the absence of UV light irradiation at A = 45 Å2 molecule-1 (amplitude: 0.2 ± 0.1 mm s-1).

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Figure 5. The reciprocating motion observed for a camphor disk on a o-Cl-HABI molecular layer developed on water under UV light irradiation. The (a) outline of the camphor disk for 10 s, (b) temporal trace of L at  = 0 rad, (c) power spectrum of FFT for (b), and (d) amplitude of the reciprocating motion depending on . A = 40 Å2 molecule-1 and td ~150 s.

DISCUSSION Based on our experimental results and the literature,5, 21, 22 we discuss the mechanism of the characteristic motion of a camphor disk on a o-Cl-HABI molecular layer developed on water, which is sensitive to UV light irradiation. Figure 1 suggests that the surface pressure for o-Cl-HABI molecular layer is clearly higher 12 ACS Paragon Plus Environment

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than that for TPIR molecular layer. In other words, the occupied area per o-Cl-HABI dimer molecule is clearly larger than that found per TPIR monomer. This means that the change in the angle of orientation observed for the TPIR molecular layer at the air/water interface under compression is clearly larger than that of the o-Cl-HABI molecular layer.17 In other words, the camphor disk can easily move on the TPIR molecular layer when compared with the o-Cl-HABI molecular layer due to the molecular mobility. The nature of the self-propelled motion of the camphor disk is determined by the surface pressure,5, 10, 21, 22 which changes due to the photosensitive o-Cl-HABI molecular layer studied in this research paper. No motion was observed at high surface pressure ( >10 mN m-1 at td ~ 150 s, see Fig. S5), which suggests that the surface pressure of the o-Cl-HABI molecular layer without UV light irradiation is higher than that of the camphor molecular layer. Therefore, the camphor disk cannot obtain the driving force require for motion. The mechanism of reciprocating motion for A = 45 Å2 molecule-1 without UV light irradiation is shown schematically in Fig. 6a. The camphor disk can move since the surface pressure of the camphor molecular layer is slightly higher than ~10 mN m-1 observed for the o-Cl-HABI molecular layer. However, the camphor disk cannot move further since the surface pressure of the oCl-HABI molecular layer is locally increased upon the collisions experienced between the camphor disk and o-Cl-HABI molecules. In general, the diffusion coefficient of a low weight molecular compound in water is ~ 5 ×10-10 m2 s-1, in other words, the migration length per second is 0.1 mm. This value suggests that the pure water surface on the trajectory of the camphor disk may keep due to the diffusion of the o-Cl-HABI molecules being significantly slower than the reciprocating motion of the camphor disk (~12 mm per second).21 That is, a one-dimensional water channel composed of oCl-HABI can be spontaneously made by the motion of the camphor disk. Thus, the direction of the 13 ACS Paragon Plus Environment

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camphor motion is reverted due to the inversion of the gradient of the surface tension around the camphor disk.5, 22, 23 In a similar way, the reciprocating motion is maintained. Characteristic features of motion will be designed in couple with spatio-temporal pattern composed of a molecular layer under nonequilibrium conditions.21 The direction of reciprocating motion under the experimental condition in Fig. 3 was ~ 8/9 rad for three examinations, i.e., the direction of reciprocating motion was similar to that of the Langmuir trough motion ( =  rad).

In other words, the vertical direction

of reciprocating motion was the angle of the minimum amplitude of reciprocating motion ( = 7/18 rad). This suggests that the packing of o-Cl-HABI molecular layer is strengthened in the vertical direction of the compression by the Langmuir trough. On the other hand, random motion is generated at a low surface pressure (