Photoinduced mechanical motions of biferrocene-containing

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Photoinduced mechanical motions of biferrocene-containing pseudorotaxane crystals Kai-Jen Chen, Ann Chen Tan, Chi-Hsien Wang, Ting-Shen Kuo, Pei-Lin Chen, and Masaki Horie Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01169 • Publication Date (Web): 07 Dec 2018 Downloaded from http://pubs.acs.org on December 8, 2018

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Photoinduced mechanical motions of biferrocenecontaining pseudorotaxane crystals† Kai-Jen Chen,† Ann Chen Tan,† Chi-Hsien Wang,† Ting-Shen Kuo,‡ Pei-Lin Chen§ and Masaki Horie*,†

†Department

of Chemical Engineering, National Tsing Hua University, 101, Sec. 2,

Kuang-Fu Road, Hsinchu, 30013, Taiwan. E-mail: [email protected] ‡Department

of Chemistry, National Taiwan Normal University, No. 88, Section 4,

Tingzhou Road, Taipei 11677, Taiwan. §Instrumentation

Center, National Tsing Hua University, 101, Sec. 2, Kuang-Fu Road,

Hsinchu 30013, Taiwan.

Abstract: This study presents photoresponsive, dynamic pseudorotaxane crystals composed of axle molecules containing biferrocene or ferrocene groups threaded through


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a dibenzo[24]crown-8 ether ring. A biferrocene-containing pseudorotaxane crystal is used for photomechanical conversion under 445-nm laser irradiation and provides a lifting force that is 2900 times the weight of the crystal itself.


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Stimuli-responsive molecules have been designed and used to develop molecular machines such as molecular lifts,1 motors,2-4 muscles5,

6

and actuators.7,

8

Interlocked

molecules, such as rotaxanes and catenanes, have achieved efficiently controllable mechanical motion,9-12 but only few of them have been characterised and observed in crystal state.13-15 Compared to liquid-crystal,16, 17 polymer-film18-20 or hydrogel materials,21, 22

the closely packed and absolutely ordered crystalline materials would lead to more

efficient mechanical effects, with a faster spatial progression and hence a shorter recovering time, including shape changes (curling, bending, twisting)23-27 and locomotion (expansion, rotation, hopping, etc.)28-32 induced by heat and light. Therefore, molecular design and crystal engineering are essential for realising solid-state dynamic molecular systems from nanocomponents to macroscopic deformations.

Recently, we reported the rapid and reversible photoinduced expansion and contraction of crystals of ferrocene-containing pseudorotaxane 1 (Figure 1a),33, 34 which can be induced by turning on and off a 445-nm light source. In addition, we reported that pseudorotaxane crystals comprising photoresponsive ferrocenyl and azobenzene groups


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showed controllable bending motions induced by alternative 360 and 445 nm laser irradiation.35 These preliminary results prompted us to develop new photoresponsive complexes with improved photosensitivity and higher photomechanical conversion both at the molecular level and regarding macroscopic structural changes.

In this communication, we replaced ferrocene with biferrocene,36 which has a higher photoabsorption ability than ferrocene owing to its stronger conjugation effect (Figure 1).37, 38 We expect to observe an enhanced photomechanical deformation of the crystals by introducing a biferrocene moiety into the pseudorotaxanes. Additionally, a bulkier tetrabromo-substituted ring molecule was involved to modify the molecular packing structures in the crystal state. This may cause reduction of intra- and/or intramolecular π-π interactions of aromatic rings; expecting to provide macroscopic mechanical output of the crystals.39


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Figure 1. Schematic illustration of photoinduced structural changes in (a) ferrocene- and (b) biferrocene-containing pseudorotaxane crystals.

Scheme 1 shows the formation of the previously reported representative pseudorotaxane (1) and the new pseudorotaxanes (1Br, 2 and 2’Br), composed of ferrocene- or biferrocene-containing Axle 1, Axle 2 or Axle 2’ molecules with dibenzo[24]crown-8 ether (DB24C8) or tetrabromo-substituted DB24C8 (DB24C8-Br4) as a ring molecule. Single crystals exhibiting a light-yellow (for 1 and 1Br) or tangerine (for 2 and 2’Br) colour were obtained by mixing the axle and ring molecules in dichloromethane or chloroform, followed by diffusion of diethyl ether into the pseudorotaxane solution under ambient conditions (Scheme 1 and Figures S1-S22). For the crystallisation of 2 and 2’Br, a mixture of Axle 2 and Axle 2’ was used due to difficulties


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in the purification process, resulting in the formation of 2 with 1,3-connectivity and 2’Br with 1,1’-connectivity of the biferrocenyl groups, probably due to the preferable choice for organisation of the molecular packing.

Scheme 1. Formation of pseudorotaxanes and optical micrographs of their crystals.


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Figure 2 shows molecular structures of the pseudorotaxanes obtained by singlecrystal X-ray crystallography. 1 and 2 form a triclinic crystal system with Z (molecules/unit cell) = 2 (Figures 2a and 2b and Table S1). Substituting ferrocene by biferrocene led to several different interactions. 2 had unit-cell axes a = 11.1 Å, b = 12.3 Å and c = 23.1 Å, which were +8%, +10% and +19% longer than those of 1. As a result, the cell volume of 2 (3082 Å3) was 40% higher than that of 1 (2196 Å3), leading to a 5% lower density of 2 (1.31 g/cm3) compared to 1 (1.38 g/cm3).

Next, the molecular structures are compared: both 1 and 2 were stabilised by intraand intermolecular π–π interactions between aromatic rings A and B and A and A’, respectively. For 2, the distances A–B and A–A’ were observed to be 3.79 and 3.98 Å (Table S2), respectively, which is +1% and +3% longer than the corresponding values for 1. Such longer π–π distances in 2 are probably due to the presence of biferrocenyl groups, which occupy a large space in the cavities of the crown ether rings, expanding the unit cell. According to our previous studies, π–π interactions significantly influence


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macroscopic crystal motion,33, 34 and therefore, aromatic rings with long π–π distances are expected to provide flexibility to the molecular motions in the crystal state.

On the other hand, both 1Br and 2’Br show a monoclinic crystal system with Z = 4 (Figures 2c and 2d and Table S1). They have higher densities compared to 1 and 2 (i.e. 1.67 g/cm3 for 1Br and 1.66 g/cm3 for 2’Br). 1Br and 2’Br are stabilised by the intramolecular π–π interactions corresponding to A–B (i.e. 3.92 Å for 1Br and 3.94 Å for 2’Br) as well as the intermolecular π-π interactions between two adjacent DB24C8-Br4 molecules (B–C’ distance = 3.93 Å for 1Br and 3.77 Å for 2’Br) (Table S3). The newly observed B–C’ interaction is probably due to the presence of rigid and planar Brsubstituted catechol groups. Furthermore, Figures S23 and S24 and Tables S4 and S5 summarise the geometric contacts among the hydrogens of the axle component and the oxygen atoms of ring molecule and the centroid of aromatic ring C. 1Br and 2’Br tend to show shorter geometric contacts than 1 and 2 within the sum of the van der Waals’ radius of O-H (< 2.72 Å) and C-H (< 2.90 Å). Such shorter geometric contacts may be


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disadvantageous in molecular flexibility though the longer intramolecular π–π interaction may be advantageous.


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Figure 2. Molecular structures of (a) 1, (b) 2, (c) 1Br and (d) 2’Br, obtained by singlecrystal X-ray crystallography. The hydrogen atoms in all the structures as well as the chloroform molecules in 2 are omitted for clarity.

Next, the optical birefringence (Δn) of single crystals of the pseudorotaxanes was measured on the top surface of the crystals using a cross-polarised microscope. The Δn value of 2 was observed to be 0.038, which is significantly lower than that of 1 (0.070). Similarly, the Δn of 2’Br was lower than that of 1Br (i.e. 0.029 for 1Br and 0.015 for 2’Br). To correlate the Δn values with molecular alignment, the packing structures of 1, 2, 1Br and 2’Br were projected along the (001) plane, which corresponds to the top surface of the crystals (Figure S25). Since the Δn values are related to the averaging of the structural anisotropy inside the crystal, the angles between aromatic rings (A, B and C) and the (001) plane were compared. We found that the angles between aromatic ring A and the (001) plane decreased progressively the order 73° (for 1) > 57° (for 2) > 44° (for 1Br) > 43° (for 2’Br). This order is consistent with the trend observed for the Δn values (namely,


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0.070 for 1 > 0.038 for 2 > 0.029 for 1Br > 0.015 for 2’Br). The same rule is applied to the angles between aromatic rings B or C and the (001) plane.

UV–Vis

absorption

spectra

of

the

pseudorotaxanes

were

obtained

in

dichloromethane solution (Figure 3). We found that both 1 and 1Br exhibited the maximum absorption at a wavelength (λmax) of 433 nm (Figure 3a), whereas biferrocenylgroup-containing 2 and 2’Br showed λmax at 453 nm (Figure 3b). These absorptions are due to the metal-to-ligand charge transfer (MLCT) of the organometallic ferrocenyl or biferrocenyl groups. The spectra of 2 and 2’Br showed a red-shift compared to those of 1 and 1Br. Furthermore, the molar absorptivity (ε) of 2 and 2’Br (ε453 = 810 M-1 cm-1) was significantly higher than that of 1 and 1Br (ε433 = 125 M-1 cm-1). These differences are due to the stronger conjugation effect in the biferrocenyl group as compared to the ferrocenyl group.37, 38 In particular, a higher ε value is expected to enhance the photosensitivity in the photomechanical conversion of crystals. Based on these results, 445-nm laser light was employed to follow the photomechanical experiments.


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Figure 3. UV–vis spectra of pseudorotaxanes in a dichloromethane solution.

Figure 4a shows typical reflection images of a crystal of 2 observed using a confocal laser microscope. Crystal expansion and contraction were observed upon turning on and off a 445-nm laser (Figure 4a and Figure S26). This procedure was repeated over 1,000 times and no damage was detected (Figure S27). Importantly, irradiation of ferrocene single crystals with a laser (445 nm) led to rapid decomposition and melting.33 The laser-power dependence of the change in relative area of the (001) facet, which is the top face of the single crystals, is shown in Figure 4b. Pseudorotaxanes containing a biferrocenyl group show a higher sensitivity to the 445-nm laser light as


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compared to those containing a ferrocenyl group (2 > 1 and 2’Br > 1Br). For instance, 2 achieved a +1.2% area change whereas 1 was only able to reach +0.35% at the same laser power of 10 mW, despite having the same ring component. In a comparison between pseudorotaxanes containing DB24C8 and DB24C8-Br4, the former complexes showed a significantly larger area change (1 > 1Br and 2 > 2’Br).


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Figure 4. (a) Reflection micrographic images of a crystal of 2 without/with 445-nm laser irradiation at 125 mW. (b) Laser-power dependence of the change in the relative area of the (001) facet of the single crystals induced by focused 445-nm laser irradiation.

In our previous report, we related the photoinduced molecular structural changes of 1 to the macroscopic three-dimensional expansion of a crystal.33 According to the single-crystal X-ray crystallography of 1, the ferrocenyl group was activated by photoirradiation leading to the three-dimensional expansion of the crystal. However, similar analysis was inherently difficult for 2 due to degradation of single crystals during long-time photoirradiation. Therefore, the powder X-ray diffraction (XRD) patterns for 2 were collected with/without 445-nm laser irradiation (Figure 5a). The pattern obtained with irradiation shows different peaks compared with that obtained without irradiation this indicates a change in the molecular structure of 2. To analyse the evolution of the diffraction peaks with irradiation, we calculated differential patterns between the pattern with laser on and off (Figure 5b). In the differential pattern, the peaks at 2θ = 7°, 15° and


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20° show significant changes. The peaks corresponding to (010) and (01-3), which were determined from the computational calculation of single-crystal data, decrease in intensity, and the peak corresponding to (115) increases in intensity in the pattern obtained with irradiation. Figure 5c displays the molecular structures related to the (010), (01-3) and (115) planes. The decreased (010) and (01-3) planes are close to the vertical packing direction of biferrocene groups. Contrastingly, the increased (115) plane is close to two catechol rings of DB24C8. These results imply that the decrease in (010) and (01-3) with irradiation is possibly caused by an activation of biferrocene groups, meanwhile the increase in the (115) is possibly caused by the formation of intermolecular ordered structure of DB24C8.


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Figure 5. (a) Powder XRD patterns of 2 without/with 445-nm laser irradiation. (b) Differential XRD pattern between laser on and off. (c) Molecular packing structures of 2 with Miller indexes (010), (01-3) and (115) and their parallel planes.


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One of the challenges in the field of molecular machines is to transduce molecular motions into functional work at a macroscopic level. Therefore, we measured the mechanical force of crystals of 1 and 2 in response to laser irradiation at 445 nm (Figure 6). A microforce analyser composed of two gauges on a cantilever beam was used, and a probe detector was carefully placed on the centre of a crystal surface (Figures 6a and 6b). A 12.8 μg crystal of 2 was irradiated using a 445-nm laser with increasing laser power from 155 to 270 mW. The transformation of light energy into mechanical work along the depth direction shows a positive relationship with the laser power. A maximum force of 37.2 mg was achieved upon irradiation at 270 mW. This force is equal to 2900 times the crystal weight (Figure 6c). Under the same conditions, a crystal of 1 provided only 1600 times the crystal weight (23.6 mg force/14.7 μg crystal, Figure 5d).33 Further attempts to increase the laser power remained fruitless and accompanied by the quick disintegration of crystals in both 1 and 2.


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Figure 6. (a) Illustration of the force detection accompanied by 445-nm laser irradiation (on/off). (b) Optical micrographs of top and side views of the force detection. Time dependences of the lifting weight of (c) 2 and (d) 1 induced by 445-nm laser irradiation at various laser powers.

Herein, we discuss the relationship between molecular packing structures and mechanical motions induced by light irradiation. It is essential to have sufficient free space for efficient molecular motions in a crystal40-42. Therefore, the longer π–π distances in 2


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(compared to 1), which result from the substitution of ferrocene by biferrocene, are advantageous for molecular motions in the crystal state. In particular, the aromatic rings forming π–π interactions are expected to act as movable components. They can be thermally activated and expanded by photoabsorption of the biferrocenyl group, leading to a distinct macroscopic deformation of the crystals. Furthermore, the lower Δn value of 2 compared to 1 suggests the presence of less aligned aromatic rings, which tend to provide the flexibility required for mechanical motion in the crystal state. However, compared to 1 and 2, the tetrabromo-substituents in the rings of 1Br and 2’Br lead to a weaker photoinduced crystal deformation at the macroscopic level. Despite the tetrabromo-substituents derived longer π–π distances and less aligned packing of aromatic rings with lower Δn value than those of 1 and 2, the dense crystal packing, limited void space and the shorter axle-ring contacts prevent the total molecular motion.

In conclusion, a distinct deformation of pseudorotaxane crystals can be achieved by replacing ferrocene with biferrocene, which has a higher photosensitivity at 445 nm. A single-crystal X-ray crystallographic analysis suggests that the longer intra- and


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intermolecular distances resulting from the presence of the biferrocenyl group may favour macroscopic mechanical motion and improve the sensitivity of the crystals to 445-nm laser irradiation. This carries the remarkable note that a lower energy is required to achieve the same work performed by the biferrocene-containing pseudorotaxane. The development of pseudorotaxane crystals is a pioneering step in the study of solid-state molecular motions. We expect that such materials could be used as stimuli-responsive mechanical components in molecular actuators and switches.

This work was financially supported by Ministry of Science and Technology Taiwan.

Supporting

information

is

available:

detailed

synthetic

procedures,

characterisations, single-crystal X-ray crystallographic data, and photoirradiation experiments.

References:


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(40) Weiss, R. G.; Ramamurthy, V.; Hammond, G. S., Photochemistry in organized and confining media: a model. Acc. Chem. Res. 1993, 26, 530-536. (41) Natarajan, A.; Mague, J. T.; Venkatesan, K.; Arai, T.; Ramamurthy, V., VolumeDemanding Cis−Trans Isomerization of 1,2-Diaryl Olefins in the Solid State. J.

Org. Chem. 2006, 71, 1055-1059. (42) In‐Hyeok, P.; Raghavender, M.; Hyeong‐Hwan, L.; Evania, M. C.; Sheng, Q. H.; Sung, L. S.; J., V. J., Formation of a Syndiotactic Organic Polymer Inside a MOF by a [2+2] Photo‐Polymerization Reaction. Angew. Chem. Int. Ed. 2015, 54, 73137317.


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For Table of Contents Use Only, Photoinduced mechanical motions of biferrocene-containing pseudorotaxane crystals

Kai-Jen Chen, Ann Chen Tan, Chi-Hsien Wang, Ting-Shen Kuo, Pei-Lin Chen and Masaki Horie*

We report photoresponsive pseudorotaxane crystals composed of biferrocene-containing axle threaded through a dibenzo[24]crown-8 ether ring, providing a lifting force 2900 times the weight of the crystal itself.


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Photoinduced mechanical motions of biferrocene-containing

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