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Reversible Laser-Induced Bending of Pseudorotaxane Crystals Shao-Chi Cheng, Kai-Jen Chen, Yuji Suzaki, Yoshitaka Tsuchido, Ting-Shen Kuo, Kohtaro Osakada, and Masaki Horie J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10998 • Publication Date (Web): 23 Dec 2017 Downloaded from http://pubs.acs.org on December 23, 2017

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Journal of the American Chemical Society

Reversible Laser-Induced Bending of Pseudorotaxane Crystals Shao-Chi Cheng,† Kai-Jen Chen,† Yuji Suzaki,‡ Yoshitaka Tsuchido,‡ Ting-Shen Kuo,§ Kohtaro Osakada,‡ and Masaki Horie*,† †

Department of Chemical Engineering, National Tsing Hua University, 101, Sec. 2, Kuang-Fu Road, Hsinchu, 30013, Taiwan ‡ Laboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan § Department of Chemistry, National Normal University, No. 88, Section 4, Tingzhou Road, Taipei, 11677, Taiwan

Supporting Information Placeholder ABSTRACT: This study investigated the dynamic photoresponse of pseudorotaxane crystals with azobenzene and ferrocenyl groups in the axle component. X-ray crystallography showed pseudorotaxanes with a methylazobenzene group and a dibromophenylene ring in the cyclic component to exhibit twisting of the trans-azobenzene groups at torsion angles of 17° and 38°, respectively. Repeated alternating laser irradiation of the crystals at 360 and 445 nm produced bending of 20–30° in opposite directions, with no evidence of decay. Under 445-nm irradiation, bending took place within 0.3 seconds. A crystal of non-substituted pseudorotaxane showed bending of only 2° under 360-nm irradiation due to multiple π-π interactions between the planar trans-azobenzene groups. The pseudorotaxane crystals have two chromophores, bent rapidly and reversibly on irradiation at rates depending on the molecular structure.

Rotaxanes and pseudorotaxanes consist of axle and ring molecules and the interlocking pattern of these molecules allows the ring to move along the axle under the control of external stimuli, in a process known as shuttling.1-7 Therefore, they are considered to be promising candidates for use in molecular machines and as switches. However, while the dynamic performance of rotaxanes in a solution has been extensively studied,8-10 few reports have examined their structural control in the solid state. When arranged as an oriented film, rotaxanes can act as molecular machines and can be used to convert photoenergy or chemical energy into work at the film surface.11-13 The unidirectional control of the motion of a droplet on a rotaxanecoated surface by application of photoirradiation has been reported.11 Another study demonstrated an artificial molecular muscle, operated by shuttling rotaxanes in a monolayer film and detected by the cantilever of an atomic force microscope.12 Shuttling has also been used to control electrical conductivity, creating high-density molecular memory on a crossed electrode.13 Fewer studies have examined the behavior of rotaxanes when in the crystalline state.6,14,15 The rotaxane species have an ordered alignment owing to the crystallographic symmetry, and in the course of phase transition, a macroscopic change in their physical properties may be observed. The molecular structure can also be observed directly, and single-crystal X-ray crystallography can reveal the source of the motion at the molecular level. In recent papers, we reported photo- and thermally induced crystal-to-crystal phase transition of a [2]pseudorotaxane comprising a ferrocenylmethyl(4-methylphenyl)ammonium axle and a dibenzo[24]crown-8 ether (DB24C8) ring.16,17 Thermal phase transition occurred at 128 °C and was accompanied by the shortening of the c-axis (5%) and elongation of the b-axis (7%). Laser irradiation at 445 nm caused the molecular alignment and

shape of the crystals to change in a much less anisotropic manner and more rapidly than when thermal induction was used. The photomechanisms involving photochemically active organic compounds have been intensively investigated.18-20 As shown in Figure 1, the current study applied azobenzene groups to the pseudorotaxanes and investigated the effect of laser irradiation on the crystal structure. Azobenzene-containing molecules have been intensively studied as they are known to undergo cis-trans isomerization when driven by specific wavelengths of light or heat.21-26 Their high isomerization efficiency and reversibility makes their compounds suitable in a wide range of applications, including optical data storage23 and as photoresponsive biomaterials.24 A number of research groups have reported achieving isomerization of the azobenzene group when in the solid state, producing macroscopic mechanical motion.21,25-27 For example, single crystals made up of the azobenzene group have demonstrated reversible bending motion when exposed to photoirradiation.26,27 In this communication, we report rapid and highly reversible bending of the crystals of these pseudorotaxanes and discuss the relationship between molecular structure and mechanical output. We expected that the molecular motion of the azobenzene group would be controlled by the ring molecule, whereas the ferrocenyl group would act as a photosensitizer.

Figure 1. Schematic illustration of cis-trans isomerization of azobenzene-containing pseudorotaxanes and photoinduced bending of a crystal.

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Scheme 1 summarizes the formation of [2]pseudorotaxanes 1-3 (see also Figures S1–S14 in the Supporting Information). They were produced in the anisotropic plate crystal form by dissolution of an equimolar mixture of dialkylammonium containing an azobenzene group and the ring molecule DB24C8, or its tetrabromo derivative DB24C8-Br4, in CH2Cl2, followed by crystallization by diffusion of Et2O vapor.

Scheme 1. [2]Pseudorotaxane formation

Figure 2 shows the molecular structures and intermolecular interactions, identified using single-crystal X-ray crystallography (see also Figure S15 and Tables S1 and S2 in the Supporting Information). The axle and cyclic components were formed by N─H⋯O hydrogen bonding and C─H⋯π interactions between the cyclopentadienyl group of ferrocene in the axle and an aromatic ring D of the catechol group in the cyclic component of the pseudorotaxanes. Each pseudorotaxane was stabilized by intra- and intermolecular interactions in a different manner. As shown in Figure 2a, pseudorotaxane 1 assumed a relatively planar trans-azobenzene conformation, in which aromatic rings A and B of the azobenzene group are slightly tilted each other with a plane angle of 4.2°. Intermolecular π-π stacking interactions were also observed between pairs of azobenzene groups (aromatic rings A-B’ and A’-B) at a distance of 4.5 Å. This suggests that the azobenzene groups acted to stabilize the crystal structure and restrict the motion of the molecules. We return to this below. In contrast, pseudorotaxanes 2 and 3 took a significantly twisted trans-azobenzene form. The plane angle between aromatic rings A and B was 38° for pseudorotaxane 2 and 17° for pseudorotaxane 3 (Figures 2b and 2c). Pseudorotaxane 2 exhibited intramolecular ππ interaction between the azobenzene and catechol groups of DB24C8 (aromatic rings B and C) at a distance of 3.8 Å, but no intermolecular π-π interaction (the distance of 4.9 Å between aromatic rings A and B’ was too great for π-π interaction to take place). Two azobenzene terminal groups of 3 exhibited intermolecular association between aromatic rings A and A’ at a distance of 4.0 Å. However, the bulky bromo substituents prevented other π-π interactions, for example between rings B and C or B and A’. Therefore, both of these pseudorotaxanes were expected to provide greater flexibility than pseudorotaxane 1 at the terminal azobenzene group in the crystalline state. A single crystal of 1 remained static when the top face of the crystal (Miller index: 001) was subjected to wide-field 445-nm laser irradiation at 90 mW for 25 sec (Figure 3a), but deformed slightly when this was followed by 360-nm irradiation at 90 mW, with a bend angle of +2° (Figure 3a (right)). In contrast, when one surface of a crystal of 2 (Miller index: 001) was irradiated, negative bending at an angle of -15° was observed under 445-nm irradiation at 90 mW for 25 sec. From this state, a positive bending of +20° was observed under 360-nm irradiation at 90 mW for 7 sec (Figure 3b).

Figure 2. Molecular structures of (a) 1, (b) 2, and (c) 3 derived using single-crystal X-ray crystallography. Intra- and intermolecular π-π distances and angles are shown in illustrations.

Figure 3. Optical micrographs showing photoinduced mechanical bending of crystals of (a) 1, (b) 2, and (c) 3 under wide-field laser irradiation at 360 nm and 445 nm (90 mW). Crystal size: 600 x 200 x 20 µm3 for 1, 900 x 400 x 30 µm3 for 2, and 1000 x 500 x 30 µm3 for 3. See also the Supporting Videos.


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When the 360-nm laser was turned off, the crystal spontaneously regained its former shape within a few minutes, whereas 445-nm irradiation caused the former shape to be regained within 0.3 sec. Similarly, 445-nm irradiation of one surface of a crystal of 3 (Miller index: 01-5) produced negative bending of -5°, and further 360-nm irradiation produced positive bending of +30° compared to the negative bending state (Figure 3c). When the 360-nm laser was turned off, this state was maintained for several hours, whereas the crystal quickly regained the negative bent shape under 445-nm irradiation within 0.3 sec. To quantify the rate and reversibility of crystal bending, the experiments were repeated more than 200 times (Figure 4). The crystal of 1, with its small bend angle, exhibited rapid up/down bending at a rate of less than 0.4 sec. In contrast, the crystals of 2 and 3, with their bend angles of 20° and 30°, respectively, exhibited up/down bending cycles of 7 sec and 30 sec, respectively. In both cases, the reverse process was much faster than the forward process, which took place in less than 0.3 sec.

isomerization of the trans-azobenzene group to cis. After irradiation at 445 nm (50 mW, 30 sec), isomerization of the cisazobenzene group to trans caused the absorption intensity at 324 nm to increase significantly. The changes in the UV-vis spectra of 1 and of the pseudorotaxane comprising tolyl and azobenzene groups are compared with each other (Figure S16 in the Supporting Information). The cis-to-trans photoisomerization of 1 was faster than that observed for its counterpart without ferrocenyl group. Pseudorotaxanes 2 and 3 also underwent similar photoisomerization when in CH2Cl2 solution. Crystal state trans-to-cis isomerization was next induced at 360 nm (50 mW, 30 sec), and the crystals were then dissolved in CH2Cl2 (Figure 5a-5c (iii)). Based on the absorption intensities, the rate of photoisomerization conversion in the crystal state was estimated to be 4% for 1, 28% for 2, and 40% for 3.

Figure 5. UV-Vis absorption spectra of (a) 1, (b) 2, and (c) 3 in CH2Cl2 solution at 25°C. (i) As-synthesized crystal dissolved in CH2Cl2. (ii) After irradiation at 360 nm (50 mW for 30 sec) in CH2Cl2 solution. (iii) Crystals dissolved in CH2Cl2 after irradiation at 360 nm (50 mW for 30 sec) in the crystal state. Figure 4. Induced bending of (a) 1, (b) 2, and (c) 3 crystals under alternating 360-nm and 445-nm wide-field laser irradiation at 90 mW. The changes observed in the crystal shape were related to isomerization of the azobenzene groups under laser irradiation. As shown in Figure 5a (i), in CH2Cl2 solution the UV-vis absorption spectrum of 1 showed maximum absorption (λmax) at a wavelength of 324 nm and much weaker absorption at 450 nm. This was attributed to the presence of azobenzene and ferrocenyl groups, respectively. Irradiation at 360 nm (50 mW, 30 sec) caused the absorption intensity at 324 nm to decrease significantly and a new peak to appear at 430 nm (Figure 5a (ii)). This was due to

We now consider the effect of the molecular structure of the pseudorotaxanes on the mechanical output. Pseudorotaxane 1 contains the thermally stable planar trans-azobenzene group, which exhibited little conformation change when in the crystal state under irradiation at either 445 nm or 360 nm. The azobenzene groups underwent multiple intermolecular π-π interactions, which strongly restricted molecular movement. In contrast, crystals of 2 showed significantly greater movement, with negative bending under 445nm irradiation and positive bending under 360-nm irradiation. Significantly, the crystal showed reversible spontaneous cis-totrans isomerization. These properties were attributed to the flexibility of the azobenzene group due to the absence of π-π interactions at the terminal group (the free aromatic ring A in



Figure 2b). Crystals of 3 showed similar bending motion under irradiation. However, the movement was slower than that of 2, and the reverse cis-to-trans process was not spontaneous, but required irradiation at 445 nm. These phenomena were attributed to the presence of intermolecular π-π interactions at the terminal azobenzene group (the partially restricted aromatic ring A in Figure 2c). The pseudorotaxane crystals comprise photoresponsive azobenzene and ferrocenyl groups. Combining axle and ring molecules with different substituents can modify the chemical structure of the pseudorotaxanes, providing structural diversity. The mechanical motion induced in each crystal by reversible cistrans photoisomerization is unique. Significant differences are produced by the molecular structure and packing of the crystals, which were revealed using single-crystal X-ray crystallography in this study. The bending of the crystals can be assisted by the ring molecule of pseudorotaxanes, which provide adjustability of molecular structural change, resulting in a degree of flexibility of the mechanical motion of the crystals. The crystals underwent cisto-trans reversal within 0.3 sec. Generally, azobenzene derivatives exhibit fast forward change and slow reversal.25,27 This may be due to the ferrocenyl group, which exhibits absorption at 445 nm, enhancing the rate of cis-to-trans isomerization and producing the characteristic “slow upward/fast downward” movement of the crystals. We expect these unique dynamic crystals, composed of interlocked molecules, to have applications as molecular actuators or as switches in photoresponsive mechanical devices. Pseudorotaxane 1 exhibited high-speed bending (0.3 sec for 1 cycle) with a small displacement, suggesting potential applications in on/off current switching in electric circuits.17,28 Pseudorotaxane 2, with the flexibility provided by its Me-substituent on the azobenzene group, exhibited spontaneous reversal of bending. This would simplify the design of systems using on/off laser switching at a single wavelength. Pseudorotaxane 3, which has tetrabromosubstituents on DB24C8, exhibited the greatest bending under photoirradiation. The cis-form was static at room temperature unless additional stimuli were applied. These crystals may have applications in multiple-state switches controlled by lasers of different wavelengths.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental details (PDF), crystal information files (CIF), movie (mp4)

AUTHOR INFORMATION Corresponding Author [email protected]

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This work was financially supported by Ministry of Science and Technology Taiwan and Frontier Research Center on Fundamental and Applied Sciences of Matters at National Tsing Hua University.

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ORCID Masaki Horie: 0000-0002-7734-5694

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT


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