Article pubs.acs.org/crystal
Polymorphic Crystallization and Thermodynamic Phase Transition between the Polymorphs of a Photochromic Diarylethene Chika Iwaihara, Daichi Kitagawa, and Seiya Kobatake* Department of Applied Chemistry, Graduate School of Engineering, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan S Supporting Information *
ABSTRACT: We synthesized a photochromic diarylethene, 1 , 2 - b i s ( 2 - m e t h y l - 5 - ( 4 - h e x y l o x y p h e ny l ) - 3 - t h i e n y l ) perfluorocyclopentene (1a), and found that it has two types of crystals, a needle-like α-crystal and a plate-like β-crystal. From X-ray crystallographic analysis, the space groups of the α- and β-crystals were determined to be P1̅ and C2/c, respectively. The molecular conformation and packing of two crystal forms are quite different. The α- and β-crystals can be obtained individually by recrystallization from acetone at different temperatures. The solvent-mediated phase transition from the α-crystal to the β-crystal was found to occur in the acetone solution at room temperature. Moreover, the thermodynamic phase transition from α-form to β-form was found to take place above 88 °C, as confirmed by differential scanning calorimetry measurement, optical microscopic observation under crossed Nicols, and powder X-ray diffraction measurement. The phase transition from α-form to β-form was also observed at 78 °C by photochromic reaction of 1a, and the phase transition proceeded from the UV irradiated part to the nonirradiated part.
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state.40,41 The research on the polymorphism of diarylethene derivatives also has been reported. When 1,2-bis(2-methyl-5-(4methoxyphenyl)-3-thienyl)perfluorocyclopentene is crystallized out from n-hexane, four types of crystals can be obtained. The photocycloreversion quantum yields of the closed-ring isomer in these four crystals are different from each other, which is ascribed to a difference in the conformation of the closed-ring isomers in the crystals.8 A platelet crystal of 1,2-bis(2-methyl-6-nitro-1benzothiophen-3-yl)perfluorocyclopentene changes to a needlelike crystal via crystal melting. The phase transition takes place at a much lower temperature in the presence of the photogenerated closed-ring isomers.17 1,2-Bis(5-phenyl-2-propyl-3-thienyl)perfluorocyclopentene has two polymorphic forms, of which one undergoes a thermodynamic phase transition to the other without melting the crystal. The motion of the molecules in the crystal during the phase transition was revealed by single-crystal X-ray crystallographic analysis.42 Recently, we have found that 1,2-bis(2-methyl-5-(4-hexyloxyphenyl)-3-thienyl)perfluorocyclopentene (1a) has two polymorphs, α- and β-crystals, when crystallized out from acetone. Here, we report on the polymorphism, photochromism, solventmediated phase transition, and thermodynamic phase transition of diarylethene 1a. Furthermore, the photochromic reactionassisted phase transition of the crystal is also described.
INTRODUCTION Crystal polymorphism has attracted much attention because of its importance in various applications in crystallographic chemistry and medication.1 5-Methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile, known as ROY, is one of the most famous compounds that show polymorphic crystallization. ROY has at least seven polymorphs with solved structures by single-crystal X-ray crystallographic analysis.2,3 There are various different properties between polymorphs, such as crystal density, melting point, refractive indices, dielectric constants, conductivity, magnetic properties, absorption, fluorescence, and photochromism.4−9 In crystal polymorphism, the phase transition between the polymorphs takes place due to the Gibbs free energy difference between the two polymorphic forms. To exceed the activation energy of the phase transition between polymorphs, the external stimuli such as heat, light, and mechanical force are required as a trigger. In other words, the solid state properties can be controlled by the external stimuli. Therefore, the polymorphic phase transition is suitable for application to new functional switching devices, such as fluorescence switching, electronic property switching, and so on.10,11 Among various photochromic compounds, photochromic diarylethene derivatives are the most promising compounds for optical memory,12 optical switches,13 optoelectrical devices,13 photoresponsive surfaces,14−24 light-starting sensors,25−30 and photomechanical devices31−39 because of their thermal stability of both isomers, fatigue-resistant property, high sensitivity, rapid response, and the capability of photochromism in the solid © XXXX American Chemical Society
Received: February 8, 2015 Revised: March 5, 2015
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DOI: 10.1021/acs.cgd.5b00196 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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1.9 (m, 4H), 1.94 (s, 3H), 3.97 (t, J = 6.6 Hz, 2H), 6.90 (d, J = 8.7 Hz, 2H), 7.15 (s, 1H), 7.45 (d, J = 8.7 Hz, 2H). FAB HRMS (m/z) [M]+ = 720.2535, Calcd for C39H42F6O2S2: 720.2530.
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RESULTS AND DISCUSSION Photochromism in Solution. Figure 1 shows the absorption spectral change of 1a in n-hexane upon UV light
EXPERIMENTAL SECTION
General. Solvents used were spectroscopic grade and purified by distillation before use. 1H NMR (300 MHz) spectra were recorded on a Bruker AV-300N spectrometer with tetramethylsilane as the internal standard. Mass spectra were obtained on a JEOL JMS-700/700S mass spectrometer. UV−vis absorption spectra were measured with a JASCO V-560 absorption spectrometer. Photoirradiation in solution was conducted using a 200 W mercury−xenon lamp (Moritex MUV-202) as a light source. Monochromatic light was obtained by passing the light through a monochromator (Jobin Yvon H10 UV) and glass filters. Polymorphic crystals were observed using a Keyence VHX-500 digital microscope. Solvent-mediated phase transition of crystals was observed using a Leica MZ7.5 stereomicroscope, equipped with a video camera system. Thermodynamic phase transition of crystals was observed using a Nikon ECLIPSE E600POL polarizing optical microscope, equipped with a Mettler-Toledo FP82HT hot stage and FP90 central processor. Differential scanning calorimetry (DSC) was run using a Seiko DSC6200. Polarized absorption spectra were measured using the polarizing optical microscope connected with a Hamamatsu PMA-11 photodetector. Powder X-ray diffraction profiles were recorded on a Rigaku RINT-2100 diffractometer employing Cu Kα radiation (λ = 1.54184 Å). Single crystal X-ray crystallographic analysis was carried out using a Rigaku RAXIS RAPID imaging plate diffractometer with Mo Kα radiation (λ = 0.71073 Å) monochromated by graphite. The crystal structures were solved by a direct method using SIR92 and refined by the full-matrix least-squares method on F2 with anisotropic displacement parameters for non-hydrogen atoms using SHELXL-97. UV irradiation to the crystal was carried out using a Keyence UV-LED UV-400 (365 nm light) or a super high pressure mercury lamp (100 W; UV-1A filter (365 nm light excitation)) attached to the polarizing optical microscope. Visible light irradiation was carried out using a halogen lamp (100 W). Materials. Diarylethene 1a was synthesized as follows. 3-Bromo-2-methyl-5-(4-hexyloxyphenyl)thiophene. To 50 mL of dry THF solution containing 3,5-dibromo-2-methylthiophene (2.9 g, 11 mmol) was added 8.5 mL of 1.6 M n-BuLi hexane solution (14 mmol) at −78 °C under argon atmosphere, and the solution was stirred for 1 h at that temperature. Tri-n-butylborate (3.7 mL, 14 mmol) was slowly added to the reaction mixture at −78 °C, and the mixture was stirred for 1 h at that temperature. The reaction mixture was quenched with water. To the mixture were added 20 wt % Na2CO3(aq) (20 mL), 1-iodo-4hexyloxybenzene (2.9 g, 9.5 mmol), and Pd(PPh3)4 (0.30 g, 0.26 mmol). After the mixture was refluxed for 24 h, it was neutralized with HCl(aq) and extracted with ether. The organic layer was dried over MgSO4, filtrated, and concentrated. The residue was purified by silica gel column chromatography using n-hexane as the eluent to give 2.6 g of the product in 67% yield based on 3,5-dibromo-2-methylthiophene: 1H NMR (300 MHz, CDCl3): δ 0.91 (t, J = 7.0 Hz, 3H), 1.3−1.5 (m, 6H), 1.7−1.9 (m, 2H), 2.40 (s, 3H), 3.97 (t, J = 6.5 Hz, 2H), 6.88 (d, J = 8.8 Hz, 2H), 6.97 (s, 1H), 7.41 (d, J = 8.8 Hz, 2H). 1,2-Bis(2-methyl-5-(4-hexyloxyphenyl)-3-thienyl)perfluorocyclopentene (1a). To 100 mL of dry THF solution containing 3-bromo-2-methyl-5-(4-hexyloxyphenyl)thiophene (1.6 g, 4.5 mmol) was added 5.2 mL of 1.0 M sec-BuLi hexane solution (5.2 mmol) at −78 °C under argon atmosphere, and the solution was stirred for 1.5 h at that temperature. Octafluorocyclopentene (0.3 mL, 2.3 mmol, Nippon Zeon) was slowly added to the reaction mixture at −78 °C, and the mixture was stirred for 4 h at that temperature. The reaction was stopped by the addition of water. The product was extracted with ether. The organic layer was dried over MgSO4, filtrated, and concentrated. The residue was purified by silica gel column chromatography (hexane/dichloromethane = 9:1) and by recrystallization from acetone to give 920 mg of the product in 56% yield: 1H NMR (300 MHz, CDCl3): δ 0.91 (t, J = 6.9 Hz, 6H), 1.3−1.5 (m, 12H), 1.7−
Figure 1. (a) Absorption spectra of 1a (solid black line), 1b (solid blue line), and the solution in the photostationary state upon irradiation at 313 nm (broken blue line) in n-hexane, and (b) a plot of the photocyclization conversion against irradiation time.
irradiation. Diarylethene 1a has an absorption maximum at 295 nm. Upon irradiation with UV light, the colorless solution of 1a turned blue, in which a visible absorption maximum was observed at 582 nm. This absorption spectral change is ascribed to the photoisomerization from the colorless open-ring isomer to the colored closed-ring isomer. The blue color disappeared by irradiation with visible light (>500 nm), and the absorption spectrum returned to that of 1a. The photoisomerization conversion from 1a to 1b was determined to be 98% in nhexane upon irradiation with 313 nm light. The open-ring isomer 1a and the closed-ring isomer 1b were stable at room temperature. The photocyclization and cycloreversion quantum yields were determined to be 0.54 (upon irradiation with 313 nm light) and 0.0039 (upon irradiation with 582 nm light), respectively, whereas the ε values are 47 500 M−1 cm−1 at 295 nm for 1a and 22 200 M−1 cm−1 at 582 nm for 1b in n-hexane. These values are similar to those of 1,2-bis(2-methyl-5-(4methoxyphenyl)-3-thienyl)perfluorocyclopentene.8 Polymorphism and Photochromism of Crystal 1a. When diarylethene 1a was recrystallized from acetone, two types of crystals, needle-like 1a-α crystals and plate-like 1a-β crystals, were obtained as shown in Figure 2. X-ray crystallographic analysis of these crystals was carried out to confirm the polymorphic forms. The crystallographic data are summarized in Table 1. The crystal systems and space groups of 1a-α and 1a-β are triclinic P1̅ and monoclinic C2/c, respectively. The crystal of 1a-α has two molecules in the unit cell and one molecule in the B
DOI: 10.1021/acs.cgd.5b00196 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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The hexyl groups in diarylethene of 1a-α exist mainly in all-trans conformation, while those of 1a-β contain a gauche form. The hexyl groups in both α- and β-crystals include disordered structures. These results indicate that α- and β-crystals are evidently polymorphic forms. In the α- and β-crystals, all 1a molecules exist in antiparallel conformations. Figure 3 shows the shape and molecular packing diagrams of α- and β-crystals, determined by X-ray crystallographic analysis. The distances between the reactive carbons in the α- and βcrystals were 3.583(4) and 3.601(5) Å, respectively, which are sufficiently short for photocyclization to take place in the crystalline phase.43 Upon UV light irradiation, the colorless single crystals of α- and β-forms turned blue. The colored crystals of α- and β-forms had absorption maxima at 584 and 605 nm, respectively. The blue-colored crystals returned to the colorless crystals upon irradiation with visible light as shown in Figure 4.
Figure 2. Photographs of (a) α- and (b) β-crystals prepared by recrystallization from acetone.
Table 1. X-ray Crystallographic Data of 1a-α and 1a-β 1a-α empirical formula formula weight temperature crystal system space group unit cell dimensions
C39H42F6O2S2 720.85 303(2) triclinic P1̅ a = 6.441(3) Å b = 16.712(8) Å c = 18.87(1) Å α = 67.26(4)° β = 86.07(4)° γ = 81.85(3)° 1854.2(15) Å3 2 1.291 0.20 × 0.20 × 0.60 0.860 R1 = 0.0518, wR2 = 0.1291 R1 = 0.1237, wR2 = 0.1558 1043244
1a-β C39H42F6O2S2 720.85 303(2) monoclinic C2/c a = 35.72(2) Å b = 9.681(7) Å c = 10.770(7) Å β = 94.95(5)° 3710(4) Å3 4 1.290 0.20 × 0.30 × 0.60 0.842 R1 = 0.0524, wR2 = 0.1328 R1 = 0.1281, wR2 = 0.1589 1043245
Figure 4. Color changes of (a) 1a-α and (b) 1a-β upon alternating irradiation with UV and visible light.
asymmetric unit, while the crystal of 1a-β has four molecules in the unit cell and a half of the molecule in the asymmetric unit.
The face indices of α- and β-crystals were determined as shown in Figure 3. In the α-crystal, the long edge of the crystal is parallel to
volume Z density g/cm3 crystal size mm3 goodness-of-fit on F2 final R [I > 2σ(I)] R (all data) CCDC number
Figure 3. Shape and molecular packing diagrams of (a) α- and (b) β-crystals. C
DOI: 10.1021/acs.cgd.5b00196 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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solubility curves of α- and β-crystals. It was clarified that the solubility of α-crystal is always larger than that of β-crystal. Generally, it is well-known that the polymorphic crystallization relates the process of crystal nucleation, which is affected by solvent, evaporation rate of a solvent, recrystallization temperature, and so on.1 To know the crystallization conditions of αand β-crystals, we focused on the recrystallization temperature. The saturated acetone solution of 1a in various concentrations was cooled at a certain temperature and allowed to stand until crystals precipitated. From the shape of precipitated crystals observed by optical microscopy, we determined the crystal form of α- or β-crystals. The experiments were performed using a sealed vessel to prevent evaporation of the solvent. The experimental results are shown in Figure 5b. For example, when the saturated solution of 1a with a concentration of 60 g L−1 at 40 °C was cooled down at −3 °C, α-crystals crystallized out. On the other hand, when it was cooled down at 3 °C under the same conditions, β-crystals crystallized out. As a result, the αand β-crystals can be obtained from the solutions of all concentrations when the cooling temperature is below and above 3 °C, respectively. This result indicates that the crystallization condition of α- and β-crystals depends on the cooling temperature and does not depend on the concentration of 1a. Moreover, we found that the needle-like α-crystals undergo a transformation to plate-like β-crystals in acetone at room temperature. This phenomenon is well-known as the solventmediated phase transition.44 Figure 6 shows the optical microscopic photographs of the solvent-mediated phase transition from α-crystal to β-crystal at room temperature. When the needle-like α-crystals were added to the saturated solution of 1a in acetone at room temperature, the α-crystals were gradually dissolved, and the plate-like β-crystals crystallized out and grew up. Finally, there were only β-crystals in the solution. It took for 8 min to finish the solvent-mediated phase transition at 22 °C. The solvent-mediated phase transition can also be observed even at 0 °C, as shown in Figure S3. In this case, it took for 5 days to complete the solvent-mediated phase transition. This result indicates that the speed of the solventmediated phase transition depends on the temperature because the driving force of the solvent-mediated phase transition is ascribed to the difference in the solubility in the polymorphs. In
the a axis. The faces of the α-crystal can be distinguished by measuring the absorption anisotropy of the blue color at 584 nm because the direction of the absorption anisotropy viewed on the (0 0 1) and (0 1̅ 0) faces is different, as shown in Figure S1, Supporting Information. In the β-crystal, the lozenge face with the corner angles of 96° and 84° corresponds to the (1 0 0) face. The absorption anisotropy of the blue color at 605 nm viewed on the (1 0 0) face is shown in Figure S2, Supporting Information. Polymorphic Crystallization of 1a. First, we examined the solubility of α- and β-crystals in acetone. The solubility was determined based on the absorbance measurement of the saturated solution of 1a at each temperature. Figure 5a shows the
Figure 5. (a, b) Solubility diagram (solubility curve: 1a-α (- - -) and 1a-β ()) and (b) recrystallization condition of 1a in acetone (cooling temperature: 1a-α (square) and 1a-β (triangle)). When the saturated solution of 1a in a sealed tube was cooled down at a temperature shown by a square or triangle, the 1a-α or 1a-β crystal crystallized out.
Figure 6. (a−f) Photographs in the solvent-mediated phase transition from 1a-α to 1a-β in acetone at 22 °C. Each picture was taken every 1 min. The broken circles indicate the generation of the β-crystal. D
DOI: 10.1021/acs.cgd.5b00196 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 8 shows the optical microscopic photographs observed under normal view and crossed Nicols when the α-crystal was heated up to 100 °C at a rate of 0.1 °C min−1. When the α-crystal was heated up, the transparent crystal turned slightly opaque while maintaining the crystal shape. Under crossed Nicols, the crystal was bright from the beginning to the end of the phase transition, which means that the crystallinity was maintained during the phase transition. The phase transition took place at 85−90 °C. The thermodynamic phase transition from the α-form to the β-form is also confirmed by powder X-ray diffraction measurement. Figure 9 shows the profiles after heating at 30 and 90 °C
other words, the large difference of the solubility in polymorphs causes the fast solvent-mediated phase transition. From these experiments, it was clarified that the α-crystal can kinetically crystallize out according to the Ostwald rule of stages, which means that the crystal is first obtained from the metastable α-form. However, when the recrystallization was performed at high temperature, the β-crystal can be preferentially obtained because the solvent-mediated phase transition takes place quickly. Thermodynamic Phase Transition between the Polymorphs. During the course of study on the thermodynamic stability of the polymorphic crystals, the thermodynamic phase transition was found. Figure 7 shows the DSC traces of α- and β-
Figure 7. DSC curves of α- and β-crystals at a heating rate of 5 °C min−1. Figure 9. X-ray powder diffraction patterns measured at 30 °C after heating of 1a-α crystal at (a) 30 and (c) 90 °C. The calculated patterns of (b) 1a-α and (d) 1a-β were obtained from single crystal X-ray crystallographic analysis.
crystals. When the powder crystal of 1a-α was heated at a rate of 5 °C min−1, the crystal exhibited a small endothermic and exothermic behavior due to a phase transition accompanying the crystal melting and growth at around 100 °C and a large endothermic behavior due to the crystal melting at 118 °C. The β-crystal showed only a large endothermic behavior corresponding to the crystal melting at 118 °C. When the heating rate was lowered, the endothermic melting peak intensity decreased, as shown in Figure S4. This indicates that the α-crystal melted at a rate of 5 °C min−1, but the α-crystal did not melt at a rate of
