Photomechanical Bending of Molecular Crystals

Abstract

Creation of mechanical crystals has been the focus of my study on solid-­state photoreactions of molecular crystals for 20 years. Our search of photomechanical crystals, based on the operating principle that crystals should bend via reversible photochromic reactions, found that typical photochromic crystals composed of azobenzene, salicylideneaniline, and furylfulgide bend upon photoirradiation. When plate-like microcrystals of trans-4-(dimethylamino)azobenzene were irradiated at 365 nm, the crystals quickly bent away from the light. Thirty seconds after the light was extinguished, the crystal returned to its initial flat shape. To our knowledge, trans–cis photoisomerization of azobenzene chromophores has not been observed in the crystalline state, but the occurrence of photoisomerization near the crystal surface was confirmed. Plate-like microcrystals of a salicylideneaniline compound could be repeatedly bent and straightened over 200 cycles by alternately irradiating with UV and visible light, respectively. Microcrystals of furylfulgide exhibited the same behavior. X-ray crystallographic analyses before and after photoirradiation showed that the mechanism of bending in salicylideneaniline and furylfulgide crystals involved molecular-level shape changes that induced macroscale mechanical motion. Synthetic molecular machines that convert photoenergy directly into mechanical work through molecular structural changes are attractive for both basic research and many potential applications. These photomechanical crystals will contribute to the advancement of molecular machinery.

1 Introduction

Crystals have a well-deserved reputation for rigidity. The attractive forces that trap molecules into three-dimensional solids also keep the atoms relatively immobile. In recent decades, however, a large number of solid-state reactions have been observed in molecular crystals [1, 2]. Bimolecular photoreactions in the crystalline state have also been observed [3, 4], and the relationship between crystal structure and these reactions has been elucidated. Crystalline state photoreactions usually induce molecular motion in the crystal lattice, thereby suggesting the possibility of motion in bulk crystals. It has been my goal to make photomechanical crystals for 20 years.

Recently, molecular machines based on the conversion of molecular motion to macroscale mechanical motion have attracted interest from both basic researchers and applications engineers. Such machines are amenable to remote operation with light and do not require wire connections. Supramolecular machines such as shuttles and motors have been reported, but the motion was only spectroscopically detected [5, 6] and not linked to macroscale mechanical motion in bulk materials. Macroscale mechanical motion of molecular materials has been observed in gels [7] and nematic elastomers [8–13], in which a photoinduced order–disorder phase transition acted as the driving force. Irie and co-workers [14–18] first reported rapid, reversible bending in rod-shaped microcrystals of photochromic diarylethene upon photoirradiation, thereby linking molecular-level shape changes to macroscale mechanical motion in crystals. Bardeen and co-workers [19–23] demonstrated irreversible and reversible shape changes in rod-shaped nanocrystals of anthracene carboxylates by photodimerization. Since then, several photomechanical crystals have been reported [24–26], revealing potential opportunities for synthetic molecular machinery [27].

We started studying photomechanical crystals several years ago. Initially, our work focused on known, typical photochromic crystals in order to identify the characteristics required for photomechanical motion in crystals. Since then, several photomechanical crystals have been found, based on the operating principle that crystals should bend via reversible, photochromic reactions. This chapter introduces recent studies of photomechanical bending in crystals of azobenzene [28, 29], salicylideneaniline [30], and furylfulgide [31].

2 Photomechanical Bending of Azobenzene Crystals

Azobenzenes are typical chromophores that undergo trans–cis photoisomerization and are incorporated into various photofunctional materials. Azobenzene-containing elastomer films bend upon photoirradiation, and this effect has been applied in the fabrication of a light-driven plastic motor [8–12] and a high-frequency, light-driven polymer oscillator [13]. This section describes plate-like microcrystals of trans-4-(dimethylamino)azobenzene (trans-1a) [28] and trans-4-aminoazobenzene (­trans-1b) [29] that exhibit reversible bending upon irradiation with UV light (Fig. 2.1).

Fig. 2.1

Photoisomerization of azobenzene 1

2.1 4-(Dimethylamino)azobenzene

Microcrystals of trans-1a were prepared by sublimating crystalline powders in a glass tube oven under vacuum or in air. Plate-like microcrystals (several hundred micrometers in length) grew on the edges and surface of the glass plate after heating to a temperature near the melting point (114 °C) for several hours (Fig. 2.2a, b). X-ray diffraction (XRD) measurements revealed three sharp peaks (Fig. 2.2c), which were assigned to the 0 0 2, 0 0 4, and 0 0 6 peaks on the basis of consistencies with crystallographic data [32]. The top surface of the plate-like microcrystals was identified as the (0 0 1) face with its longitudinal direction along the a-axis, based on comparisons with plate-like bulk crystals, which exhibited a (0 0 1) face along the a-axis.

Fig. 2.2

Plate-like microcrystals of trans-1a grew on the edges (a) and surface (b) of a glass plate by sublimation and condensation. The scale bars are 500 mm. (c) X-ray diffractogram of trans-1a microcrystals before irradiation. (d) The 0 0 4 peak intensity decreased to 93% upon irradiation at 365 nm for 5 s and then increased when the irradiation was stopped. (e) X-ray diffractogram of trans-1a microcrystals were measured under constant UV irradiation. Peak heights decreased and peak widths expanded, but no new peaks appeared

Figure 2.3a shows a piece of a plate-like microcrystal (525 × 280 × 5 mm³) whose lower portion was fixed to the glass surface and whose upper portion was free. Upon UV irradiation (365 nm, 5 mW/cm²) with a light-emitting diode (LED) at the (0 0 1) surface from the right rear, the crystal quickly bent away from the light source, reaching a maximum deflection after 0.5 s (Fig. 2.3b). The maximum deflection angle was quite large, leading to the formation of a ~180° semicircle along the b-axis. When illumination was stopped, the crystal returned to its initial flat shape after 30 s (Fig. 2.3a). Figure 2.3c shows the same crystal when irradiated from the right (Fig. 2.3d) and left sides (Fig. 2.3e), which induced bending in opposite directions.

Fig. 2.3

The (0 0 1) face of a plate-like trans-1a microcrystal (a) before and (b) after UV irradiation from the right rear. The same crystal is shown (c) before and after UV irradiation from the (d) right and (e) left sides. The scale bars are 200 mm

A narrow, plate-like crystal (70 × 5 × 1 mm³) with one end fixed to another crystal was subjected to repeated bending (Fig. 2.4a). As described above, comparisons with bulk crystals showed that the (0 0 1) plane was oriented perpendicular to the surface of the glass plate with the longitudinal direction along the a-axis. When subjected to UV irradiation at the (0 0 1) surface, the crystal bent away from the light source, reaching a maximum deflection of 5 mm after 0.2 s, as measured at the end of the crystal (Fig. 2.4b). The degree of deflection along the a-axis was considerably smaller than that along the b-axis (Fig. 2.3b, d, e). The bent crystal returned to its initial straight form 3.8 s after the illumination was stopped. This reversible bending was observed over 100 cycles of alternating UV irradiation (2 s) and darkness (5 s) (Fig. 2.4c).

Fig. 2.4

Bending of a narrow, plate-like crystal of trans-1a, as shown by a comparison of images acquired (a) with and (b) without UV irradiation. The scale bar is 20 mm. (c) The reversible bending was repeatable over as many as 100 cycles

The crystals bend away from the light source due to a gradient in the extent of trans-to-cis photoisomerization as a function of light penetration. Photoisomerization and consequent elongation of the crystal at the irradiated surface along the a- and b-axes, with no changes at the non-illuminated rear surface, resulted in a bent macrostructure. However, only slight differences were observed in the absorption spectra (l _max = 400 nm) of the trans-1a microcrystals before and after UV irradiation. Therefore, the occurrence of photoisomerization could not be confirmed by absorption data. Direct evidence of the formation of cis-1a was obtained from nuclear magnetic resonance (NMR) measurements. The ¹H-NMR spectrum of trans-1a microcrystals that had been irradiated for 1 min was measured in benzene-d ₆. Singlet peaks were observed at 2.35 and 2.23 ppm, corresponding to the N(CH₃)₂ protons of the trans and cis isomers, respectively, with a 160:1 relative intensity [33]. This provided unambiguous evidence of trans-to-cis photoisomerization in the crystalline state. The estimated yield of the cis isomer was ~1% based on the decay of absorbance in benzene. With no UV irradiation, the cis isomer almost disappeared after 30 min as a result of cis-to-trans thermal isomerization. During UV irradiation, the melting point of trans-1a microcrystals (113–114 °C) decreased to 66–80 °C because of the coexistence of the cis isomer and returned to 113–114 °C when the irradiation was extinguished, further demonstrating the reversibility of isomerization. To our knowledge, trans–cis photoisomerization of azobenzene chromophores has not been observed in the bulk crystalline state because of the large geometric changes that would be required in such densely packed crystal lattices. However, we confirmed the occurrence of photoisomerization near the surface of trans-1a microcrystals.

The (0 0 1) top surface of the trans-1a microcrystal was smooth prior to irradiation, as observed by atomic force microscopy (AFM) (Fig. 2.5a). After UV irradiation for 5 s, uneven features appeared along the a-axis with a height of 10 nm and a relative roughness of 1% of the crystal thickness (900 nm) (Fig. 2.5b). The uneven features receded slightly after 1 h in the dark, but the initial smooth surface was not recovered. The relative XRD peak intensities for the microcrystals quickly decreased to 93% upon UV illumination for 5 s, likely because of both trans-to-cis photoisomerization and the deterioration of crystallinity (Fig. 2.2d). After the irradiation was stopped, the diffraction peaks increased to 97% of their initial intensities after 30 s and recovered completely after 15 min as a result of the cis-to-trans thermal isomerization and the recovery of crystallinity. However, no new peaks were observed in the diffractogram, even after continued irradiation (Fig. 2.2e). This suggests that the cis-1a molecules did not form a new crystalline phase, but may have instead adopted a fine polycrystalline or amorphous phase.

Fig. 2.5

AFM images of the (0 0 1) top surface of a trans-1a microcrystal (a) before and (b) after UV irradiation and illustrations of the molecular arrangements of the (c) (0 0 1) and (d) (0 1 0) faces before irradiation

In the trans-1a crystal, the planar molecules are arranged almost perpendicularly at the (0 0 1) face to form a herringbone structure along the a-axis (Fig. 2.5c) [32]. Upon UV irradiation, the planar trans-1a molecules undergo photoisomerization to cis-1a at the (0 0 1) crystal surface, resulting in an increased torsional conformation due to repulsion between the two phenyl planes. Crystalline cis-azobenzene molecules normally exhibit a dihedral angle of 64.26° between the two phenyl planes [34]. The trans-to-cis photoisomerization elongates the unit cell length along the b- and a-axes near the (0 0 1) crystal surface, giving rise to the uneven features. In contrast, since there is no photoisomerization in the absence of light, the unit cell dimensions remain constant at the non-illuminated surface, resulting in bending of the microcrystal. Furthermore, molecular interactions between the neighboring herringbone structures are weak, as seen in the packing arrangements at the (0 0 1) and (0 1 0) faces (Fig. 2.5c, d). Therefore, the uneven features along the b-axis are sharper than those along the a-axis (Fig. 2.5b). This leads to a drastic bending motion along the b-axis (Fig. 2.3) relative to a small crystal bending observed along the a-axis (Fig. 2.4).

2.2 4-Aminoazobenzene

Plate-like microcrystals of trans-4-aminoazobenzene trans-1b were also obtained by sublimation and condensation. The top surface was identified as the (1 0 −1) face by XRD, and the longitudinal direction was assigned to b-axis based on comparisons with plate-like bulk crystals. Figure 2.6a shows a piece of a plate-like microcrystal (200 × 25 × 1.2 mm³) in which the lower portion was adhered to the glass surface and the upper portion was free. When the (1 0 −1) surface was irradiated at 365 nm (40 mW/cm²) from the left side, the crystal quickly bent away from the light, reaching a displacement angle of 34° after 0.5 s (Fig. 2.6b). Stopping the illumination resulted in the gradual return of the crystal to its initial linear shape after 4 min (Fig. 2.6c–f).

Fig. 2.6

Photomechanical bending of the trans-1b microcrystal: (A) (a) before irradiation, (b) after UV irradiation from the left side for 0.5 s, and no irradiation for (c) 10 s, (d) 60 s, (e) 120 s, (f) 240 s; (B) (g) before irradiation, (h) after UV irradiation from the left side for 0.5 s, and after visible light (530 nm) irradiation from the left side for (i) 2 s, (j) 14 s, (k) 30 s, (l) 60 s. The scale bar is 100 mm (from [29] with permission. © 2012 Elsevier)

The microcrystal bending speed and displacement angle increased in proportion to the UV light intensity (Fig. 2.7). When the microcrystals were irradiated with 2.5 mW/cm², 8 s was required to reach the maximum displacement angle of 27°. When 50 mW/cm² was used, 2 s was required to reach the maximum displacement angle of 41°.

Fig. 2.7

Dependence of (a) UV irradiation time and (b) UV light intensity on the displacement angle of a plate-like microcrystal (200 × 25 × 1.2 mm³) of trans-1b

After 30 s of UV irradiation at 365 nm, the absorption spectrum (l _max = 375 nm) of trans-1b in benzene [35] changed to that of cis-1b (photostationary state) with two bands at l _max = 344 and 442 nm, corresponding to (π, π*) and (n, π*) excitations, respectively (Fig. 2.8a). When irradiation was stopped for 10 min, the initial spectrum of trans-1b was recovered, demonstrating thermal cis-to-trans isomerization. Visible light irradiation at 530 nm led to a faster recovery (1 min) of the initial trans-1b spectrum. Thus, 4-aminoazobenzene 1b in benzene underwent reversible photoisomerization between the trans and cis isomers via alternate irradiation with UV (365 nm) and visible (530 nm) light. In contrast, the absorption spectrum of trans-1b microcrystals did not change after UV irradiation at 365 nm for 30 s (Fig. 2.8b).

Fig. 2.8

(a) Absorption spectra of trans-1b (0.05 M) in benzene before (yellow) and after UV irradiation at 365 nm for 30 s (red). Stopping irradiation for 10 min recovered the initial spectrum in benzene (orange). (b) Absorption spectra of the trans-1b microcrystals before (yellow) and after UV irradiation for 30 s (red)

However, the changes in trans-1b microcrystals upon UV irradiation were detectable by XRD (Fig. 2.9). The 2 0 −2 peak intensity decreased to 87% of the original value after UV irradiation (40 mW/cm²) for 60 s because of the trans-to-cis photoisomerization and deterioration of crystallinity. However, no shift in the diffraction angle 2q or new peaks were observed in the XRD profile, even after continued irradiation. This suggests that the cis-1b molecules did not form a new crystalline phase. The XRD peak intensity almost recovered 15 min after irradiation was stopped due to cis-to-trans thermal isomerization (Fig. 2.9a). In contrast, irradiation with visible light at 530 nm (10 mW/cm²) recovered the initial intensity after 4 min due to cis-to-trans photoisomerization induced by (n, π*) excitation (Fig. 2.9b). As discussed above, the crystal returned to its initial, linear shape 4 min after irradiation was extinguished (Fig. 2.6f). In contrast, visible light irradiation recovered the initial straight crystal in 1 min (Fig. 2.6l), revealing that both bending and straightening of 1b microcrystals could be controlled by irradiation with UV and visible light, respectively.

Fig. 2.9

Dependence of irradiation time on the XRD 2 0 −2 peak intensity of trans-1b microcrystals. (a) Blue: UV light was applied for 60 s and then stopped. (b) Green: UV light was applied for 60 s and then visible light was applied at 530 nm (from [29] with permission. © 2012 Elsevier)

3 Photomechanical Bending of Salicylideneaniline Crystals

Salicylideneanilines are typical photochromic compounds in the crystalline state [36–38]. Their photochromicity is based on the formation of colored species by photoinduced proton transfer. This section describes plate-like microcrystals of N-3,5-di-tert-butylsalicylidene-3-nitroaniline (2) in their enol form that exhibit reversible bending upon alternate irradiation with UV and visible light (Fig. 2.10). The bending mechanism was determined by changes in crystal structure before and after photoirradiation.

Fig. 2.10

Photochromic reaction of salicylideneaniline 2

3.1 Bending Motion

Plate-like microcrystals of enol-2 were obtained on a silanized glass plate by sublimation and condensation at approximately 10 °C below the melting point (132 °C). The top surface of the plate-like microcrystals was identified as the (0 0 1) face based on consistencies with existing crystallographic data [39], and its longitudinal direction was assigned to the a-axis based on comparisons with plate-like bulk crystals, which exhibit a (0 0 1) face along the a-axis. Figure 2.11a shows the frontal (0 0 1) face of a plate-like enol-2 crystal (73 × 4.5 × 1.1 mm³) with one end fixed to an adjacent crystal. When the (0 0 1) face was irradiated from the right at 365 nm (40 mW/cm², Fig. 2.11b), the crystal bent away from the light to reach a maximum tip displacement angle of 45° after 5 s (Fig. 2.11c). Subsequent illumination with a halogen lamp equipped with a color filter (530 nm, 10 mW/cm²) returned the bend to its initial straight shape after 10 s (Fig. 2.11b).

Fig. 2.11

A plate-like enol-2 microcrystal is shown (a) at the frontal (0 0 1) face (b) before and (c) after UV irradiation from the right side. The scale bars are 10 mm. (d) The repeatability of the reversible bending can be observed over as many as 200 cycles (from [30] with permission. © 2011 Royal Society of Chemistry)

A 2-s pulse of UV irradiation, with simultaneous and continuous illumination with visible light (>390 nm) from the above halogen lamp, bent the crystal to a displacement angle of 37°. When the UV source was blocked, the crystal returned to its initial straight form after 4.6 s. This reversible bending was observed over 200 cycles of alternating UV irradiation (2 s on, 5 s off) under continuous illumination with visible light (Fig. 2.11d). The bending motion was accompanied by a color change from pale yellow to reddish orange due to the formation of trans-keto-2 (Fig. 2.12a). The reddish-orange crystal also returned to the initial pale-yellow color due to photochemical fading to the enol-2 isomer (Fig. 2.12b).

Fig. 2.12

Absorption spectra of enol-2 microcrystals upon (a) UV irradiation at 365 nm, and then (b) visible light irradiation at 530 nm, or (c) in the dark. The lifetimes estimated from the changes of absorbance at 480 nm are (a) 2.6 s, (b) 63 s, and (c) 42 days

The bending motion of the microcrystals depended on the intensity of UV and visible light. When a plate-like microcrystal (50 × 6.3 × 2.1 mm³, Fig. 2.13a) was irradiated at 365 nm at 5–40 mW/cm² for 30 s, the tip displacement angle increased from 9° to 25°, respectively, in proportion to the UV intensity (Fig. 2.13b, c). In the absence of UV radiation, the crystals straightened to their initial form after 500 to 1,400 s. The lifetime of thermal fading from the trans-keto-2 to the enol-2 isomer is very long (42 days, k = 2.8 × 10⁻⁷/s at 25 °C, Fig. 2.12c). This is due to stabilization of the trans-keto-2 form by intermolecular hydrogen bonding and dimer formation between the N–H group of the trans-keto imine and the oxygen atom of the nitro group [40]. However, the lifetime was dramatically shortened to 63 s upon illumination with 530 nm light at 10 mW/cm² due to photochemical fading (Fig. 2.12b). The time required to recover the initial straight form was also dramatically shortened from 1,400 to 43 s with increasing visible light intensity up to 10 mW/cm² (Fig. 2.13d, e). This recovery time is comparable to the 63 s lifetime observed with 530 nm light when the intensity was increased to 10 mW/cm².

Fig. 2.13

(a) A microcrystal (50 × 6.3 × 2.1 mm³) is shown before and after irradiation with UV (365 nm) and visible light (530 nm). The scale bar is 10 mm. (b) The bending motion is dependent on the UV irradiation time. (c) Tip displacement angle is shown as a function of UV intensity. (d) The straightening motion is dependent on the visible irradiation time, and (e) the time to recover the initial straightness is shown as a function of visible light intensity (from [30] with permission. © 2011 Royal Society of Chemistry)

The degree of tip displacement with plate-like microcrystals increased with increasing aspect ratio (length/thickness). For example, maximum displacement angles of 25 °C, 52 °C, and 70 °C were observed with microcrystals having aspect ratios of 25 (50 × 6.3 × 2.1 mm³), 71 (120 × 7.1 × 1.7 mm³), and 132 (159 × 6.8 × 1.2 mm³), respectively.

Notably, upon UV irradiation, a plate-like crystal (120 × 7.1 × 1.7 mm³) was able to flip a silica gel particle that was approximately 50-fold heavier than the microcrystal itself, suggesting that the mechanical force generated in these crystals is relatively large. Most of the plate-like crystals prepared by sublimation adhered to the glass surface and were unable to bend freely. UV irradiation over the silanized glass plate, however, bent most of the crystals away from the light source, stripping the stuck crystals from the glass surface. In contrast, crystals prepared on bare glass plates did not bend, indicating that silanization decreased the degree of interaction between the crystals and the glass.

3.2 Mechanism of Crystal Bending

Crystallographic analyses of a thin enol-2 microcrystal (200 × 30 × 20 mm³) were performed at 123 K before and after successive UV irradiation at room temperature. Here, the thinness of the crystal allows for complete light penetration. A nearly constant unit cell size was attained after 1 h (Fig. 2.14). The resulting crystal exhibited a disordered structure with contributions from both the enol-2 and the trans-­keto-2 isomers at a ratio of 0.905(2):0.095(2). Figure 2.15a shows the ORTEP drawing of this structure. This is identical to structures obtained following irradiation at 360 nm [40] and following two-photon excitation at 730 nm [39].

Fig. 2.14

(a) Changes in the unit cell parameters and (b) the relative changes in unit cell lengths of enol-2 crystal upon UV irradiation

Fig. 2.15

(a) ORTEP drawings show the disordered structures of the enol-2 (black) and trans-­keto-2 (red) isomers after irradiation at 365 nm at the 25% probability level. Hydrogen atoms, except H(1) and H(10), are omitted for clarity. (b) Molecular arrangements on the (0 0 1) face

In the trans-keto-2 molecule, the oxygen atom O(1′) and the nitrogen atom N(1′) are trans with respect to the C(1′)–C(7′) bond. The bond lengths of C(2′)–O(1′), C(1′)–C(7′), and C(7′)–N(1′) were 1.305(9), 1.415(9), and 1.349(9) Å, respectively, which differed significantly from those (1.355(4), 1.441(5), and 1.273(4) Å) of the enol-2 molecule. The C(8′)–N(1′)–C(7′)–C(1′) torsion angle of the trans-keto-2 was −172.4(18)°, and the corresponding angle of the enol-2 isomer was 177.2(3)°. The phototransformation likely occurs through the motion of a pair of benzene rings, analogous to the bicycle pedal motion of stilbenes and azobenzenes, without requiring major structural changes [41–43].

The length of the a-axis in the unit cell increased by 0.42% from 6.0913(2) to 6.1171(4) Å (Figs. 2.14 and 2.15b). Hence, photoisomerization elongates the crystal length along the a-axis near the (0 0 1) crystal surface. In contrast, since no photoisomerization occurs in the absence of light, the unit cell dimensions remain constant at the non-illuminated surface, resulting in bending of the microcrystal away from the light source. When UV or visible (530 nm) irradiation is stopped, the reverse isomerization from trans-keto-2 to enol-2 brings the crystal back to its initial straight conformation. These molecular-level shape changes therefore result in bending of the bulk crystal.

The (0 0 1) surface of the enol-2 microcrystal was smooth before irradiation, as observed by AFM (Fig. 2.16a). Small dents with a depth of 1.5 nm (0.3% of the total crystal thickness) appeared after UV irradiation for 40 s (Fig. 2.16b). The overall surface morphology, however, remained constant. Subsequent irradiation with visible light for 180 s did not alter the surface morphology (Fig. 2.16c). These results suggest minimal molecular movement, such as a pedaling motion, during photoisomerization. The minimal movement affords excellent repeatability and reversibility over as many as 200 cycles (Fig. 2.11d).

Fig. 2.16

AFM images show the (0 0 1) top surface of an enol-2 microcrystal (a) before and after irradiation with (b) UV and (c) visible light

4 Photomechanical Bending of Furylfulgide Crystals

Fulgides constitute a class of photochromic compounds that undergo reversibly electrocyclic ring-closure and ring-opening reactions [44]. The furylfulgide (E)-2-[1-(2,5-dimethyl-3-furyl)ethylidene]-3-isopropylidenesuccinic anhydride (E3) in its E-form exhibits photochromism in the crystalline state (Fig. 2.17) [45–48]. This section describes plate-like microcrystals of E3 with reversible bending upon alternate irradiation with UV and visible light. The bending mechanism is based on changes in crystal structure before and after photoirradiation.

Fig. 2.17

Photochromic reaction of furylfulgide 3

Microcrystals of E1 were prepared by sublimation and condensation of crystalline powders in a small platinum pan covered with a glass plate. Plate-like microcrystals grew on the surface of the glass plate after heating to approximately 20 °C below the melting point (126 °C) and holding for several hours. X-ray diffractograms of the microcrystals contained two peaks, assigned to the 1 0 1 and 2 0 2 reflections based on consistencies with existing crystallographic data [45, 46, 48].

Figure 2.18a shows the hexagonal face of a plate-like E3 microcrystal (115 × 60 × 2 mm³) with the left end fixed to an adjacent crystal and the remaining portion free. The top surface of the plate-like microcrystal was identified as the (1 0 1) face with its longitudinal direction along the b-axis, based on comparisons with bulk crystals having a hexagonal surface, which exhibit a (1 0 1) face along the b-axis. When the (1 0 1) face of the microcrystal was irradiated from the diagonal underside at 365 nm (10 mW/cm²) for 1 s, the crystal curled from the right upper corner toward the light, reaching a maximum twisted curl after 2 s with a color change from pale yellow to red due to the formation of the closed C3 isomer (Fig. 2.18a–c).

Fig. 2.18

The (1 0 1) face of a plate-like E3 microcrystal before (a) and after UV irradiation for (b) 1 s and (c) 2 s. The scale bar is 50 mm

When the (1 0 1) face of a narrow, plate-like microcrystal (109 × 6 × 2 mm³) was irradiated at 365 nm from the lower side, the crystal bent toward the light, reaching a maximum tip displacement angle of 9° after 2 s (Fig. 2.19a, b). Subsequent illumination with a halogen lamp equipped with a filter (>390 nm, 10 mW/cm²) returned the crystal to its initial straight shape after 30 s (Fig. 2.19a). This reversible bending was observed over 200 cycles of alternating irradiation with UV (2 s) and visible light (30 s) (Fig. 2.19c). The bending motion was accompanied by a color change from pale yellow to red (l _max = 512 nm) due to the formation of the closed C3 isomer in the crystals. The red crystal also returned to the initial pale-yellow color due to the photochemical ring-opening reaction, generating the E3 isomer.

Fig. 2.19

Bending of a narrow, plate-like crystal of E3 (a) before and (b) after UV irradiation from the lower side. Scale bar is 20 mm. (c) Reversible bending was repeatable over as many as 200 cycles (from [31] with permission. © 2011 Chemical Society of Japan)

The bending effect was ascribed to a gradient in the extent of electrocyclic ring closure as a function of light penetration, such that shrinkage of the irradiated crystal surface along the b-axis resulted in a bent macrostructure. In the E3 crystal, the E3 molecules exhibit a torsional conformation with a dihedral angle of 38.3° between the succinic anhydride portion and the furan ring (Fig. 2.20a) and are arranged at the (1 0 1) face in a twofold screw along the b-axis [45, 46]. Upon UV irradiation, the torsional E3 molecules underwent electrocyclic ring closure to C3 molecules at the (1 0 1) crystal surface. The crystalline C3 isomer is nearly planar with a dihedral angle of 13.8° between the succinic anhydride portion and the furan rings (Fig. 2.20b) [47]. Therefore, the ring-closure photoisomerization from the torsional E3 isomer to the nearly planar C3 isomer shrank the length of the b-axis of the unit cell. By contrast, because photoisomerization does not occur in the absence of light, the unit cell dimensions remained constant at the non-illuminated surface, causing the microcrystals to bend toward the light source.

Fig. 2.20

Molecular structures of (a) the E3 isomer and (b) the C3 isomer in crystals of E3 and C3, respectively. (c) ORTEP drawings show the disordered structures of the E3 (black) and the C3 (red) isomers after irradiation at 742 nm at the 25% probability level. (d) Molecular arrangement on the (1 0 1) face. Hydrogen atoms are omitted for clarity

Crystallographic analysis of E3 after two-photon excitation at 742 nm by pulsed laser light provides more direct evidence for the mechanism of photomechanical bending [48]. The resulting crystal exhibited a disordered structure with contributions from both the E3 and the C3 isomers at a ratio of 0.945:0.055 (Fig. 2.20c). The length of the b-axis of the unit cell decreased 0.13% from 9.6203(5) to 7.6101(8) Å after two-photon excitation (Fig. 2.20d), bending the plate-like microcrystal toward the light source.

5 Conclusions

Several typical photochromic crystals composed of azobenzene, salicylideneaniline, and furylfulgide exhibit mechanical bending upon photoirradiation. The bend motion is reversible and repeatable by tuning the illuminating wavelength. The mechanism of bending in salicylideneaniline and furylfulgide crystals was elucidated by X-ray crystallographic analyses before and after photoirradiation. Molecular-level structural changes resulted in macroscale mechanical motion of the bulk crystals. Synthetic molecular machines have attracted a great deal of interest from the perspective of basic research and offer a wide range of potential applications. Further studies are necessary for the development of molecular machinery.