Ayako
Fujimoto
a,
Noriko
Fujinaga
a,
Ryo
Nishimura
a,
Eri
Hatano
a,
Luna
Kono
a,
Akira
Nagai
a,
Akiko
Sekine
b,
Yohei
Hattori
a,
Yuko
Kojima
c,
Nobuhiro
Yasuda
d,
Masakazu
Morimoto
e,
Satoshi
Yokojima
f,
Shinichiro
Nakamura
g,
Ben L.
Feringa
h and
Kingo
Uchida
*a
aDepartment of Materials Chemistry, Faculty of Science and Technology, Ryukoku University, Seta, Otsu, Shiga 520-2194, Japan. E-mail: uchida@rins.ryukoku.ac.jp; Fax: +81-77-543-7483; Tel: +81-77-543-7462
bDepartment of Chemistry, School of Science, Tokyo Institute of Technology, Ookayama 2-12-1, Meguro-ku, Tokyo 152-8551, Japan
cMaterials Characterization Laboratory, Mitsubishi Chemical Corporation 1000, Kamoshida-cho, Aoba-ku, Yokohama 227-8502, Japan
dJapan Synchrotron Radiation Research Institute, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan
eDepartment of Chemistry and Research Center for Smart Molecules, Rikkyo University, Nishi-Ikebukuro 3-34-1, Toshima-ku, Tokyo 171-8501, Japan
fTokyo University of Pharmacy and Life Science, Horino-uchi 1432-1, Hachioji, Tokyo 192-0392, Japan
gNakamura Laboratory, RIKEN Cluster for Science, Technology and Innovation Hub, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
hStratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands. E-mail: b.l.feringa@rug.nl; Fax: +31-50-363-4296
First published on 26th October 2020
We report a swinging motion of photochromic thin broad sword shaped crystals upon continuous irradiation with UV light. By contrast in thick crystals, photosalient phenomena were observed. The bending and swinging mechanisms are in fact due to molecular size changes as well as phase transitions. The first slight bending away from the light source is due to photocyclization-induced surface expansion, and the second dramatic bending toward UV incidence is due to single-crystal-to-single-crystal (SCSC) phase transition from the original phase I to phase IIUV. Upon visible light irradiation, the crystal returned to phase I. A similar SCSC phase transition with a similar volume decrease occurred by lowering the temperature (phase IIItemp). For both photoinduced and thermal SCSC phase transitions, the symmetry of the unit cell is lowered; in phase IIUV the twisting angle of disordered phenyl groups is different between two adjacent molecules, while in phase IIItemp, the population of the phenyl rotamer is different between adjacent molecules. In the case of phase IIUV, we found thickness dependent photosalient phenomena. The thin broad sword shaped crystals with a 3 μm thickness showed no photosalient phenomena, whereas photoinduced SCSC phase transition occurred. In contrast, large crystals of several tens of μm thickness showed photosalient phenomena on the irradiated surface where SCSC phase transition occurred. The results indicated that the accumulated strain, between isomerized and non-isomerized layers, gave rise to the photosalient phenomenon.
Reversible bending was observed not only for diarylethene derivatives but also for azobenzenes,16 anthracene,17 furylfulgides,18 and salicylideneanilines19 in their crystalline states, as well as liquid crystalline polymer films.20–22 Particularly the photo-induced crystal movements of diarylethene derivatives are well studied among photochromic crystalline systems because of the thermal stability of the closed-ring isomer, for which the X-ray analysis of the photo-irradiated intermediate in the crystal was carried out. For example, photoinduced helical twisting of the crystals23 and photoinduced fragmentation of the crystals were reported.24–27 Particularly photoinduced fragmentation and jumping of crystals were named the “photosalient effect” by Naumov.24 Very recently complex behaviour in bending phenomena including twisting upon UV light irradiation was reported. It was based on the combination of a photochromic reaction and a reversible single-crystal-to-single-crystal (SCSC) phase transition.28 Also, such photoinduced phenomena accompanied by phase transition of a salicylideneaniline derivative were reported.29
After the publication of our first paper concerning the bending phenomena of diarylethene crystals,30 we observed the swinging motions under constant irradiation with UV light on the single crystals of a diarylethene derivative 1oRR (Fig. 1a) with the thin broad sword shaped one prepared by sublimation. It underwent a dramatic concave bending toward the UV incidence followed by the reported backward bending under UV irradiation. The movements are summarized in Fig. 1b and ESI Movies 1 and 2.† Obviously the first backward swing motion is understood by photo-induced surface expansion. By contrast, the second is unexpected. We examined the cause of this drastic concave bending by preparing samples with different irradiation doses (in situ X-ray analysis), and observed a sudden change of diffraction spot numbers. This phase transition was studied in comparison with temperature-induced thermal phase transition. Finally, we reported the photosalient phenomena, since these phenomena are caused by phase transition depending on the thickness of the samples.
To clarify the details of the mechanism in this complicated bending phenomenon, we conducted XRD of the single crystal using synchrotron radiation (SPring-8).31 Block crystals whose crystalline structure is the same as that of thin broad sword shaped crystals of 1oRR were prepared by recrystallization from MeOH to monitor the structural changes upon UV irradiation (Tables 1 and S1, and Fig. S1†). Upon UV irradiation of the block crystal for 8 min, all the axes expanded about 0.3 to 0.8%. At this stage, closed-ring isomer 1cRR generated by UV light irradiation was present at about 8% in the single crystal. After irradiation for an additional 1 minute, the number of diffraction spots suddenly increased approximately twice (Fig. 2 and S2†). Then, upon visible light irradiation, the number of diffraction spots returned to the initial value. This phenomenon is attributed to a photoinduced single-crystal-to-single-crystal (SCSC) phase transition. Herein, there are two possibilities for inducing phase transition.
1oRR (initial) | 1oRR-UV (after 313 nm light irradiation for 8 min) | 1oRR-UV (after 313 nm light irradiation for 9 min) | 1oRR-Vis. (after white LED light irradiation for 3 min) | |
---|---|---|---|---|
a The ratios of the unit cell change due to photoirradiation in comparison to initial structure 1oRR are shown by percentage. b In order to compare the cell size before and after the phase transition, the half-length of the diagonal length between the c and a axes (c′ axis) was compared. c The β′-angle is obtained as an angle formed by the a-axis and the diagonal line of the a- and c-axes. The angle was compared with the β-angle before phase transition. The details are described in Fig. S3. | ||||
T/K | 173 (2) | 173 (2) | 173 (2) | 173 (2) |
Crystal system | Monoclinic | Monoclinic | Monoclinic | Monoclinic |
Space group | P21 | P21 | P21 | P21 |
a/Å | 12.3769 (5) | 12.457 (3) + 0.65% | 12.131 (11) − 2.0% | 12.851 (7) + 3.8% |
b/Å | 14.6220 (6) | 14.737 (3) + 0.79% | 14.308 (12) − 2.1% | 14.275 (7) − 2.4% |
c/Å | 19.3798 (8) | 19.461 (4) + 0.41% | 36.28 (3) | 19.163 (10) − 1.1% |
(c′/2 = 18.96–2.2%)b | ||||
α/° | 90 | 90 | 90 | 90 |
β/° | 106.751 (7) | 107.079 (8) + 0.31% | 91.678 (7) | 105.447 (7) − 1.2% |
(β′ = 106.97 + 0.21%)c | ||||
γ/° | 90 | 90 | 90 | 90 |
V/Å3 | 3358.4 (3) | 3415.1 (13) | 6294 (9) | 3388 (3) |
Volume for a molecule (Å3) | 839.6 | 853.8 + 1.6% | 786.8–6.3% | 847 + 0.88% |
Z | 4 | 4 | 8 | 4 |
R 1 (I > 2σ(I)) | 0.0516 | 0.0597 | 0.0777 | — |
wR2 (I > 2σ(I)) | 0.1397 | 0.1591 | 0.2186 | — |
R 1 (all data) | 0.0561 | 0.0798 | 0.1721 | — |
wR2 (all data) | 0.1447 | 0.1726 | 0.2743 | — |
CCDC | 1938355 | 1938356 | 1938357 | — |
(i) The photogenerated closed-ring isomer has a rigid six-membered-ring in its molecular structure,32 and its original unit cell is different from the open-ring isomer, and therefore the photogenerated closed-ring isomers themselves induce a strain to the surrounding unit cells of 1oRR, which works as a kind of pressure to induce the phase transition.28,29
(ii) During the first bending by expansion of the photo-irradiated side, the opposite side of the crystal suffers a compression to induce phase transition. However, the SCSC phase transition was also observed for rod-shaped crystals without bending. Therefore, the SCSC transition is attributed to the former mechanism of (i).
Analysis of the diffraction patterns showed that the c axis length expanded from 19.461 (4) to 36.28 (3) Å, the β angle contracted from 107.079 (8) to 91.678 (7)° and the number of the molecules in the unit cell increased from 4 to 8 after the SCSC phase transition. The transition induced the dramatic concave bending as shown in Fig. 1b.
Crystal phases before and after photoinduced SCSC phase transition are named phases I and IIUV, respectively. Viewing the crystal structure along the b axis, the shape of the unit cell of phase I is a parallelogram (red line in Fig. 3a), while that of phase IIUV is nearly a rectangle (blue line in Fig. 3b). Comparing β angles, the angles before and after photoinduced SCSC phase transition were 106.751 (7) and 91.678 (7)°, respectively. Comparing the overlapping diagram of the unit cells of phases I and IIUV (Fig. 3c), the overall unit cell contracted from phase I to phase IIUV as explained below. Although both crystal systems are monoclinic, the reduced cell of phase IIUV is different from the double of phase I; the β angle of phase IIUV is 91.678 (7)° which is near 90° relative to the β angle of phase I. Therefore, the c axis and the β angle before and after the phase transition cannot be directly compared. We took a new c′ axis and β′ angle for a new unit cell of phase IIUV as shown in Fig. S3.† Thus, we could discuss the change of c′/2 and β′ which correspond to c and β in phase I. In the new unit cell of phase IIUV, all axes contracted about 2% and the volume for one molecule decreased about 6.3% compared to that in phase I (Table 1). This shrinkage corresponded to the dramatic concave bending of thin crystals in Fig. 1b. By X-ray single crystal structure analysis of phase IIUV, it was found that closed-ring isomer 1cRR generated by UV light irradiation was present at about 10% in the blocky single crystal.
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Fig. 3 Crystal structures viewed along the b axis (a) before and (b) after photoinduced SCSC phase transition and (c) overlapping of them. |
The mechanism of the photoinduced bending process can be interpreted by these changes of the unit cell. First, the crystal bended away from the light source because all axes expanded by the ring closure reaction near the surface. Then, as the ratio of the closed-ring isomer reached about 10% where most of the closed-ring isomers are concentrated near the surface, a phase transition occurred from phase I to phase IIUV at the surrounding unit cells of 1oRR. Now it became clear that the second stage of bending toward the light source occurred, because the volume of the cell decreased rapidly by about 6.3%. Upon visible light irradiation for 3 min, the blue crystal turned to the initial colourless crystal, and along with it the number of diffraction spots decreased. Although we could not obtain the precise crystal structure after visible light irradiation, because the crystal became brittle by the salient phenomenon as we will discuss later, we could determine a unit cell with a c axis of 12.851 (7) Å, β angle of 105.447 (7)°, and Z = 4 (Table 1). The photoinduced swing of the crystals upon alternate irradiation with UV and visible light was reversible. After repeating the photoinduced swing, the number of diffraction spots gradually decreased from the initial number, indicating the breaking of the crystal structure.
Comparing the crystal structures before and after photoinduced SCSC phase transition, the length of the c′ axis of phase IIUV was almost doubled relative to the c axis of phase I. The number of molecules in a unit cell was also doubled from 4 to 8. In both crystals, the phenyl groups circled in Fig. 4a and b were disordered. Two phenyl group structures in red circles (shown in blue and magenta in Fig. 4d) were rotated about 80°. On the other hand, two phenyl group structures in a blue circle (shown in orange and pale blue in Fig. 4c) were slightly deviated and overlapping. In phase IIUV, the structure of the terminal phenyl group at the disordered moiety is different in two adjacent molecules, and thus the symmetry is lowered. Therefore, the cell could not be regarded as the same cell as phase I and, as a result, the c axis and the number of molecules in a unit cell doubled. The initial population ratios of the conformations of phenyl rings shown in dark blue and magenta coloured ones before UV irradiation at 173 K are 79 and 21%, respectively, from X-ray analysis (Fig. 4). After UV irradiation for 9 min, the ratio became 76 and 24%, respectively; besides the populations of disordered phenyl rings of orange and pale blue coloured ones are 57 and 43%, respectively (Fig. 4). Similar rotation of phenyl rings was also observed during the cooling process with shrinkage of the unit cell in the next section.
![]() | ||
Fig. 5 Crystal structures (a) before and (b) after thermal SCSC phase transition viewed along the a axis. (c) Enlarged view showing phenyl groups of (b) circled in a blue line. (d) Enlarged view showing phenyl groups of (a) and (b) circled in a red line. (e) Temperature dependence of the population of the disordered phenyl groups. For increasing the temperature from 20 K to 173 K, the heating rate was 10 K min−1. For the measurements of the crystal structure, the temperature was maintained for 20 min during the measurements. Dark blue and magenta lines correspond to the populations of dark blue and magenta coloured phenyl rings in (c), while pale blue and orange lines correspond to those of pale blue and orange coloured phenyl rings in (d). When the measurement temperature was below 110 K, the populations of the disordered phenyl groups between two adjacent molecules were different. Thus, symmetry is lowered, adjacent molecules were considered to be in different cells, the length of the c axis is doubled, and the number of molecules in the unit cell doubled from 4 to 8 (CCDC numbers: 1938358–1938363†). |
As shown in Fig. 5b for phase IIItemp, both disordered terminal phenyl rings circled in red and blue circles were rotated approximately 80° due to the strain accompanied by the SCSC phase transition. Although we showed here a similar SCSC transition by UV irradiation and temperature control, there is a difference in detail; one of the disordered terminal phenyl rings overlaps (Fig. 4d). Commonly, in the photoinduced phase transition, the increase of the ratio of 1cRR in the surrounding 1oRR lattice induced the strain which is the driving force of phase transition. Similarly, the contraction of the cell volume triggers the thermal phase transition.
To evaluate the percentage of the disordered phenyl rings, a crystal was analysed as a function of the temperature from 20 to 173 K with a heating speed at 10 K min−1 and its structures at each temperature were analysed (Fig. 5e). When the crystal of 1oRR was cooled at 20 K, the percentages of phenyl rings shown in orange and pale blue (Fig. 5e) rotated approximately 80° were 95% and 5%, and those of disordered phenyl rings shown in blue and magenta were 5 and 95%, respectively. At 90 K, the population of the phenyl rings in blue and magenta gradually became closer, and finally reached the same population (around 50%).
On the other hand, the phenyl rings in orange and pale blue made up 80 and 20% of the population in the same range of the temperature, respectively. On increasing the temperature from 90 to 173 K above thermal SCSC phase transition, the percentage of the phenyl ring in blue and magenta became 80 and 20%, respectively. In addition, the SCSC phase transition showed hysteresis between the cooling and heating process. But, when the crystal was cooled and heated within the temperature range of phase I, the hysteresis was not observed (Fig. S7†). In the thermal phase transition case, the ratio of the terminal phenyl group at the disordered moiety is different between two adjacent molecules therefore the symmetry is lowered. Thus, the length of the c axis is doubled. To confirm whether the ratio of the disordered phenyl ring changes after a long time, we kept the crystal at 90 K for three hours. The ratio remained almost the same (Fig. S8†). The transformation of the single crystal of 1oRR did not depend on the retention time, but only on the measurement temperature. The reversible and continuous phase transition was also observed by alternate cooling and heating. Therefore, we consider that the thermal phase transition of 1oRR is a martensitic-like transformation.33–35
As the temperature decreased, the volume of a molecule gradually decreased due to thermal contraction. As shown in Table 2, comparing 20 K with 173 K, all axes contracted (see Fig. S3† for the comparison method), the volume for a molecule decreased by 19.4 Å3 (−2.3%), the density increased by 0.031 g cm−3 (+2.4%), and the average distance of the intermolecular hydrogen bond decreased by 0.01 Å (−0.3%). To examine the contribution of the volume contraction by thermal SCSC phase transition, we performed the X-ray analysis of an orthorhombic polymorph crystal prepared by the vapour diffusion method of hexane vapor to THF solution with changing the temperature. The orthorhombic broad sword shaped crystal has a similar crystal structure to the monoclinic one but never gives rise to thermal SCSC phase transition. The contraction ratio of the orthorhombic crystal was almost the same as that of the monoclinic crystal of 1oRR. We obtained the results which support these arguments (Table S2 and Fig. S9†). Therefore, it is considered that the volume contraction of 1oRR was due to cooling to a low temperature.
Temp. (K) | Volume of a unit cell (Å3) | Volume of a molecule (Å3) | Density (g cm−3) | Intermolecular hydrogen bonding distance (av.) (Å) |
---|---|---|---|---|
173 | 3354.8 | 839 | 1.312 | 2.864 |
20 | 6554.2 | 819–2.3% | 1.343 + 2.4% | 2.854 − 0.3% |
For further consideration of the photoinduced bending of the crystal depending on the chirality of the molecular structure, we measured the single crystal of 1oSS which has opposite chirality to 1oRR. At 173 K, the crystal structure of 1oSS was isomorphic to 1oRR. When the crystal of 1oSS was cooled to 80 K, thermal SCSC phase transition occurred similarly to that in 1oRR.
Here let us now discuss the detailed mechanism of photosalient phenomena. Speaking first from the conclusion, most probably the phenomena are closely related to the photoinduced phase transition. The macroscopic experimental results are shown in Fig. 6. Our arguments are as follows.
First, we considered the reason why the crack appears on the surface of the block crystal of 1oRR. Naturally, this is because the cyclization reaction occurred mainly on the surface of the crystal. Then, we tried to estimate the thickness of the photosalient depth on the surface of the crystal by observing the cross section of the crystal with SEM. The thickness from the surface was estimated to be 1.0–2.5 μm, which corresponds to the thickness of the thin broad sword shaped crystals. As a matter of fact, upon UV irradiation on such a thin broad sword shaped crystal, the strain due to the SCSC phase transition is mostly released by the bending of the crystal. On the other hand, in a block crystal, obviously the SCSC phase transition occurs only at the surface of the crystal. Therefore, the strain must be generated at the interface between the transition-occurred layer and unchanged deeper layer. This naturally induces cracking and peeling on the crystalline surface. In other words, the photoinduced phase transitions have a bending and swinging effect on the thin broad sword shaped crystal, whereas it gives the photosalient phenomena on a bulk crystal.
To verify this hypothesis, we carried out additional experiments by using the polymorph of 1oRR. We prepared the crystals of 1oRR by the vapor diffusion method of hexane vapor to THF solution (Fig. 6f–h and S9†), which is an orthorhombic broad shaped crystal without showing thermal SCSC phase transition as previously described. To our surprise, no salient phenomena were observed in this crystal in which the content of the photogenerated 1cRR was almost 9%. The obtained orthorhombic crystal did not show SCSC phase transition by cooling to 20 K (Table S3†). The results show neither a temperature phase change nor photosalient phenomena on the orthorhombic crystal. (In fact, upon 313 nm light irradiation, the crystal shows only slight bending toward the incident light.) We can conclude that the photoinduced SCSC phase transition is critical for the photosalient phenomena to occur.
We also succeeded in elucidating the relationship between the photosalient phenomenon and the crystal shape. The thin broad sword shaped crystals with a 3 μm thickness have never peeled because photoinduced SCSC phase transition occurred in the whole crystal. On the other hand, in large crystals with a thickness of several tens of μm, we found that SCSC phase transition occurred only on the irradiated surface and gave rise to the photosalient phenomenon when the crystal was not able to withstand the distortion.
UV hand lamp SPECTROLINE Model EB-280C/J (λ = 313 nm, 8 W) was used for UV irradiation of the thin and block crystals, and a white LED (SHODENSHA, AC100 ∼240 V, max power consumption 17 VA, 5 W) was used as a visible light source. A Nikon OPTIPHOT2-POL equipped with a Shimadzu Moticam 2000 2.0 M Pixel USB2.0 was used for monitoring the crystal shape changes. SEM images were recorded on a Keyence VE-8800. DSC curves were recorded on a TA Instruments DSC 2920 by Japan thermal consulting Co., Ltd.
A beamline BL40XU: Si(111) channel cut monochromator was used and the wavelength and the size of the X-ray beam were 0.78229 Å and 75 × 75 μm (square), respectively. The diffraction data were collected by the oscillation method using an EIGER detector at 173 K. The data were corrected for absorption effects by a multi-scan method with ABSCOR.43
A beamline BL02B1: Si(311) double crystal monochromator was used and the wavelengths were 0.4146 Å, 0.4246 Å, and 0.42677 Å. The sizes of X-ray beams were 127 (H) × 120 (V) μm, 127 (H) × 176 (V) μm and 123 (H) × 128 (V) μm. The diffractometer was equipped with a PILATUS3 X CdTe 1M detector (DECTRIS Ltd, Baden, Switzerland) from 20 to 173 K.
The structures were determined by the direct method and refined by the full-matrix least-squares method using the SHELX-2014/7 program. The positions of all hydrogen atoms were calculated geometrically and refined by the riding model.
The conversions from 1oRR to the closed-ring isomer 1cRR in the crystals upon UV irradiation were determined by disorder analysis of crystallography. Upon UV irradiation, the superposition of 1oRR and 1cRR appeared. Then we determined the conversion by the disorder of sulphur atoms of thiophene rings of 1oRR and 1cRR.44
Footnote |
† Electronic supplementary information (ESI) available. CCDC 1938354–1938363, 1938366, 1961657, and 1965755–1965757. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0sc05388k |
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