Aryl−Perfluoroaryl Interaction in Photochromic Diarylethene Crystals

Aryl-Perfluoroaryl Interaction in Photochromic Diarylethene Crystals Masakazu Morimoto, Seiya Kobatake, and Masahiro Irie* Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, Hakozaki 6-10-1, Higashi-ku, Fukuoka 812-8581, Japan Received May 11, 2003;

CRYSTAL GROWTH & DESIGN 2003 VOL. 3, NO. 5 847-854

Revised Manuscript Received July 2, 2003

ABSTRACT: A diarylethene derivative with two pentafluorophenyl groups, 1,2-bis(2-methyl-5-pentafluorophenyl3-thienyl)perfluorocyclopentene (1a), was synthesized in an attempt to prepare photochromic single crystals with well-controlled photochromic properties by utilizing intermolecular noncovalent aryl-perfluoroaryl interactions. Compound 1a formed stoichiometric cocrystals with aromatic hydrocarbons, benzene (Bz) and naphthalene (Np), based on aryl-perfluoroaryl interactions. In crystal 1a/Bz (1a:Bz ) 2:1), a linear chain structure was constructed, in which each benzene molecule is sandwiched between pentafluorophenyl rings of two 1a molecules. In crystal 1a/Np (1a:Np ) 1:1), a discrete sandwiched structure was formed, in which two 1a molecules sandwich two naphthalene molecules between its pentafluorophenyl groups. In the 1a, 1a/Bz, and 1a/Np crystals, the diarylethene molecules underwent photochromic reactions. Absorption properties of the photogenerated closed-ring isomers were different from each other depending on conformations of the diarylethene molecules packed in the crystals. Introduction Photochromism is referred to as a photoinduced reversible transformation between two isomers having different absorption spectra.1 Photochromic compounds have attracted much attention because of their potential applicability to photonics devices, such as rewritable optical memory media2 and optical switches.3 Although a number of photochromic compounds have been so far reported, compounds that show photochromic reactivity in a crystalline phase are rare. Typical examples of the crystalline photochromic compounds are paracyclophanes,4 triarylimidazole dimers,5 diphenylmaleonitrile,6 aziridines,7 2-(2′,4′-dinitrobenzyl)pyridine,8 Nsalicylideneanilines,9 and triazines.10 In most cases, however, photogenerated colored isomers in the crystals are thermally unstable, and the colored crystals return to the initial colorless ones in the dark. Recently, we have found that some of diarylethene derivatives underwent thermally stable and fatigue resistant photochromic reactions in the single-crystalline phase.11-23 Upon irradiation with ultraviolet light, colorless single crystals of diarylethenes turn yellow, red, blue, or green, depending on the molecular structure. These colors are due to photogenerated closed-ring isomers, which are thermally stable in the dark. Upon irradiation with visible light, the colored crystals are completely bleached and return to the initial colorless ones. Such coloration/decoloration cycles could be repeated many times (>104 cycles) by alternative irradiation with UV and visible light.13 The photochromic diarylethene crystals exhibit characteristic properties depending on the crystal structure. The photogenerated colored crystals exhibit dichroism under polarized light, which arises from the regular molecular alignment in the crystals.12a,13 Molecular structural changes following photocyclization/cycloreversion reactions trigger photoreversible nanoscale morphological changes of crystal* To whom correspondence should be addressed. Phone: +81-92642-3556. Fax: +81-92-642-3568. E-mail: [email protected].

line surfaces, and the morphological changes reflect the molecular packing structure in the crystal.19 Photocyclization and photocycloreversion quantum yields also strongly depend on the molecular conformations in the crystals.20,22,23 Photochromic diarylethene crystals are able to find applications in optical memory media,18,24 optical switches,25 color displays,21 and photodriven nano-actuators.19 For such applications, the photochromic reactivity and other properties should be tuned precisely. In the field of crystal engineering, the essential tools to design and fabricate desired structures are intermolecular noncovalent bonds.26 Systems based on hydrogen bonds27 and/or coordinate bonds28 have been widely developed. In addition, considerable attention is focused on developing systems based on noncovalent interactions between aromatic units.29 Aryl-aryl interactions have been widely employed in supramolecular chemistry and played a critical role to the crystal structures of organic molecules.30 Aryl-perfluoroaryl interaction is a special case, which was reported for the first time by Patrick and Prosser in 1960. They showed the formation of a 1:1 solid complex composed of benzene and hexafluorobenzene.31 Pure benzene adopts a crystal structure with T-shaped edge-to-face orientations, while a mixture of the two liquids gives a 1:1 binary cocrystal in which benzene and hexafluorobenzene molecules stack alternatively in a face-to-face mode.32 This stacking arrangement is thought to result not from chargetransfer interactions but from van der Waals forces and favorable electrostatic interactions between the molecules.33 Aryl-perfluoroaryl interactions, via which complementary molecular pairs assemble spontaneously to produce regular arrangements in crystals, have been used in the crystal engineering, especially binary cocrystals (i.e., crystals with lattices containing complementary pairs of organic molecules).34 In this paper, we report on the first trial to control the crystal structure of photochromic diarylethenes by the use of aryl-perfluoroaryl interactions. A novel

10.1021/cg034076t CCC: $25.00 © 2003 American Chemical Society Published on Web 08/13/2003

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Table 1. Crystal Data and Structure Refinement Details for 1a, 1a/Bz, and 1a/Np formula formula weight temperature (K) crystal system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Fcalcd. (g cm-3) µ (mm-1) F(000) θ range (deg) reflections collected independent reflections no. of restraints/parameters goodness-of-fit on F2 R1 (I > 2σ(I)) wR2 (all data) largest diff. peak/hole (e Å-3)

1a

1a/Bz

1a/Np

C27H8F16S2 700.45 123(2) orthorhombic Pbca 12.157(3) 18.073(4) 22.720(5) 90 90 90 4992.0(18) 8 1.864 0.353 2768 1.79-27.49 30095 5493 0/408 1.031 0.0352 0.0880 0.403/-0.337

(C27H8F16S2)1(C6H6)0.5 739.51 123(2) triclinic P1 h 8.565(3) 12.886(4) 12.896(4) 91.569(6) 101.802(5) 100.209(6) 1368.1(8) 2 1.795 0.327 734 1.61-27.53 8743 5518 0/435 1.025 0.0479 0.1379 0.391/-0.435

(C27H8F16S2)1(C10H8)1 828.62 123(2) monoclinic C2/c 29.337(6) 8.0513(17) 31.583(6) 90 117.188(4) 90 6636(2) 8 1.659 0.280 3312 1.45-27.52 19862 7105 11/523 1.009 0.0540 0.1518 0.388/-0.555

Figure 1. Absorption spectral changes of 1 in hexane (1.8 × 10-5 M): 1a (- - - -), 1b (ss), and 1 in the photostationary state (- - - - ).

diarylethene 1a, having two pentafluorophenyl groups at the 5-positions of thienyl rings, was designed and synthesized (Scheme 1). Compound 1a is expected to form binary cocrystals with various aromatic hydrocarbons via complementary π-π interactions. Crystal structures and photochromic properties of a single crystal of 1a and binary cocrystals composed of 1a and simple aromatic hydrocarbons, such as benzene and naphthalene, have been examined. Scheme 1.

Photochromism of 1

took place by photoirradiation. The colored isomer was isolated by HPLC (see Experimental Procedures) and assigned to 1b by 1H NMR and mass spectrometry. Compound 1b has an absorption maximum at 566 nm ( ) 13 000 M-1 cm-1) in the visible region. Compound 1b was thermally stable in the dark. Upon irradiation with visible light (λ > 450 nm), the bluish purple solution of 1b was completely bleached, and the absorption spectrum returned back to that of 1a. The coloration/decoloration cycles could be repeated many times by alternative irradiation with UV and visible light. The conversion ratio from 1a to 1b by irradiation with 289 nm light was 91%, as determined from the absorption spectra shown in Figure 1. The photocyclization quantum yield of 1a was determined to be 0.47 (by irradiation with 289 nm light), which is slightly smaller than that of 1,2-bis(2-methyl5-phenyl-3-thienyl)perfluorocyclopentene (2a) (0.59) shown in Scheme 2.14 On the other hand, the photocycloreversion quantum yield of 1b was 0.022 (by irradiation with 566 nm light), which is larger than that of 2b (0.013).14 Electron accepting character of the pentafluorophenyl groups increased the cycloreversion quantum yield.35 Introduction of the fluorine atoms in the phenyl rings influenced the photochromic reactivity. Scheme 2.

Photochromism of 2

Results and Discussion Photochromism of 1 in Hexane. Figure 1 shows the absorption spectral change of 1 in hexane. A hexane solution of the open-ring isomer 1a was colorless. Compound 1a has absorption maxima at 266 nm ( ) 34 000 M-1 cm-1) and 289 nm ( ) 34 000 M-1 cm-1) as shown in Figure 1. Upon irradiation with UV light, the colorless solution turned bluish purple. The color change suggests that photocyclization reactions from 1a to 1b

Single Crystal of 1a. Recrystallization of 1a from hexane gave a colorless plate with a hexagonal surface. The X-ray crystallographic data are listed in Table 1. The crystal has an orthorhombic crystal system, and the space group is Pbca. One diarylethene molecule is crystallographically independent, and eight molecules are included in the unit cell. Figure 2 shows the molecular structure of 1a in the crystal. The dia-

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Figure 4. Polarized absorption spectra of photogenerated blue crystal 1a (a) and polar plots of absorbance at 630 nm (b). 0° showed the highest intensity angle.

Figure 2. ORTEP drawings of crystal 1a. The ellipsoids were drawn at 50% probability level.

Figure 3. Molecular packing diagram of crystal 1a. Blue dashed lines showed H‚‚‚F contacts in C-H‚‚‚F interactions.

rylethene has a photoreactive antiparallel conformation,36 and the distance between reactive carbon atoms, C1 and C10, is 3.49 Å, which is short enough for the photocyclization reaction to take place in the crystalline phase.22 Torsion angles between thiophene and pentafluorophenyl rings are -23.4° (C3-C4-C16-C17) and -16.1° (C12-C13-C22-C23). Figure 3 shows a molecular packing diagram of crystal 1a. In the crystal, the intermolecular C-H‚‚‚F interaction is clearly noticed.37 Fluorine atom F6 of the hexafluorocyclopentene ring serves as an acceptor for hydrogen atom H3 of the thiophene ring of the adjacent molecule. The H‚‚‚F distance and the bond angle for C3-H3‚‚‚F6 are 2.55 Å and 163.0°, respectively. Any type of π-π interactions is not discerned in this crystal. The photochromic behavior of crystal 1a was examined. Upon irradiation with 370 nm light, the colorless crystal turned blue. The blue color is due to the photogenerated closed-ring isomer 1b in the single crystal. Upon irradiation with visible light (λ > 450 nm),

the color was completely bleached, and the crystal returned to the initial colorless one. To confirm that the photochromic reactions take place in the crystal lattice, dichroism of the colored crystal was measured under linearly polarized light.12a,13 Figure 4a shows the polarized absorption spectra of the colored crystal. The absorption maximum in the visible region appears at 630 nm. The absorption intensity decreased by rotating the crystal as much as 90° under polarized light. Figure 4b shows the polar plots of the absorbance at 630 nm. The absorption anisotropy gave the order parameter ((A0° - A90°)/(A0° + 2A90°)) of 0.61. The relatively large order parameter indicates that 1a underwent the photochromic reactions in the crystal lattice.12a,13 Quantum yields of the photochromic reactions in crystal 1a were determined according to the method described in the literature.20 The photocyclization quantum yield was determined to be 0.97 (by irradiation with 370 nm light), which is close to unity. This indicates that all photons absorbed in the crystal are used for the cyclization reaction. The high efficiency is ascribed to that all molecules in the crystal are fixed in a photoreactive antiparallel conformation.20,22,23,36 The photocycloreversion quantum yield was determined to be 0.044 (by irradiation with 618 nm light), which is twice as large as that in hexane. The large cycloreversion quantum yield is ascribed to the constrained conformation of the photogenerated closed-ring isomers in the crystal lattice, which is different from the most stable conformation of the closed-ring isomers.14,20 Cocrystals of 1a and Aromatic Compounds. Compound 1a having two pentafluorophenyl groups is anticipated to form stoichiometric cocrystals with aromatic hydrocarbons by aryl-perfluoroaryl interaction. Cocrystals with different molecules may change crystal structures and photochromic properties of the diarylethene. We tried to prepare cocrystals composed of 1a and aromatic hydrocarbons, such as benzene and naphthalene, and examined molecular conformations, molecular packing structures, and photochromic performance of the crystals.

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Figure 5. ORTEP drawings of cocrystal 1a/Bz. The ellipsoids were drawn at 50% probability level.

Compound 1a was dissolved into a mixed solvent of benzene/hexane (volume ratio 1:1), and the solution was recrystallized by slow evaporation of the solvent. Colorless crystals with a needlelike shape were obtained. The X-ray crystallographic data are listed in Table 1. The unit cell is triclinic P1 h , which is different from that of crystal 1a. One diarylethene molecule and half of the benzene molecule are crystallographically independent, and the unit cell includes two diarylethene molecules and one benzene molecule. The crystal is composed of

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1a and benzene in the molar ratio of 2:1. Figure 5 shows ORTEP drawings of 1a and benzene in the crystal. The diarylethene has a photoreactive antiparallel conformation,36 and the distance between reactive carbon atoms, C1 and C10, is 3.48 Å, which is short enough for the photocyclization reaction to take place in the crystalline phase.22 Torsion angles between the thiophene and pentafluorophenyl rings are 41.5° (C3-C4-C16-C17) and 39.8° (C12-C13-C22-C23). In crystal 1a/Bz, a linear chain structure, which is composed of 1a and benzene in the molar ratio of 2:1, is noticed as shown in Figure 6a. The repeating unit is a dimeric structure, in which one benzene molecule is sandwiched between pentafluorophenyl rings of two 1a molecules and an inversion center lies in the center of the benzene molecule. Figure 6b,c shows enlarged views of the stacking of the benzene molecule and the pentafluorophenyl rings. The aromatic rings are stacked with another phenyl ring nearly in a parallel-displaced manner, not in T-shaped one that is found in crystalline benzene.32a The two top-and-bottom pentafluorophenyl rings are parallel to each other, but the benzene molecule in the middle is slightly tilted. The distance between centers in the benzene and pentafluorophenyl rings is 4.38 Å. The shortest intermolecular contact between carbon atoms is 3.88 Å for C22-C30, which is slightly longer than the sum of van der Waals radii (3.4 Å for C(sp2)-C(sp2)). The aryl-perfluoroaryl interaction between benzene molecules and pentafluorophenyl groups is not strong enough. In addition to the above π-π stacking interactions, C-H‚‚‚F interaction is noticed between benzene molecules and hexafluorocyclopentene rings of 1a.37 The H‚‚‚F distance and the bond angle for C28-H28‚‚‚F6 are 2.65 Å and 156.3°, respectively. The dimeric unit is stabilized by both arylperfluoroaryl and C-H‚‚‚F interactions. The repeating units composed of two diarylethene molecules and one benzene molecule are linked with the adjacent ones via face-to-face π-π stacking of pentafluorophenyl rings to form the linear chain as shown in Figure 6a. Figure 6d,e shows enlarged views exhibiting

Figure 6. Linear chain structure in cocrystal 1a/Bz (a) and enlarged top and side views of stacking of the benzene molecule and pentafluorophenyl rings (b and c) and two pentafluorophenyl groups (d and e). Red and blue dashed lines showed C‚‚‚C contacts in π-π interaction and H‚‚‚F contacts in C-H‚‚‚F interaction, respectively.

Aryl-Perfluoroaryl Interactions

Figure 7. ORTEP drawings of cocrystal 1a/Np. The ellipsoids were drawn at 50% probability level. The perfluorocyclopentene ring was disordered in the ratio of 92:8. Only the major structure was illustrated for clarity.

the π-π stacking. The two pentafluorophenyl rings are parallel to each other but partially overlapped. The distance between the two parallel phenyl rings is 3.34 Å. The shortest intermolecular contact between carbon atoms is 3.35 Å for C19-C21, which is shorter than the sum of van der Waals radii. A mixture of 1a and naphthalene (molar ratio 1:1) was dissolved into hexane, and the solvent was slowly evaporated. Although crystal 1a was formed in the early stage, colorless needle crystals were also obtained from the same batch after several days. The crystallographic data of the needle crystal are listed in Table 1. The crystal has a unit cell of monoclinic C2/c. One diarylethene molecule and one naphthalene molecule are crystallographically independent. The unit cell includes eight diarylethene molecules and eight naphthalene molecules. The crystal is composed of both 1a and naphthalene in the molar ratio of 1:1. ORTEP drawings of 1a and naphthalene are shown in Figure 7. Compound 1a has a photoreactive antiparallel conformation,36 and the distance between reactive carbon atoms, C1 and C10, is 3.59 Å, which is short enough for the photocyclization reaction to take place in the crystalline phase.22 Torsion angles between thiophene and pentafluorophenyl rings are -21.7° (C3-C4-C16-C17) and -32.8° (C12-C13-C22-C23). In crystal 1a/Np, a discrete sandwiched structure composed of 1a and naphthalene molecules is noted as shown in Figure 8a. Two diarylethene molecules sandwich two naphthalene molecules between its pentafluo-

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rophenyl groups via an aryl-perfluoroaryl interaction. The sandwiched structure has the symmetry of an inversion center. The four contact parts of pentafluorophenyl groups and naphthalene molecules are noticed. Parts A and B shown in Figure 8a are not equivalent to each other. Figure 8b-e shows enlarged views of contact parts A and B. The pentafluorophenyl rings and the naphthalene molecules overlap each other, but they are not completely parallel. The distances between a center of the pentafluorophenyl ring and a plane of the naphthalene molecule are 3.41 Å in part A and 3.43 Å in part B. The intermolecular close-contact distances are 3.29 Å (C18-C33) and 3.29 Å (C19-C28) in part A and 3.36 Å (C25-C33) and 3.31 Å (C26-C34) in part B, respectively. Such close contacts are ascribed to arylperfluoroaryl π-π interactions between the pentafluorophenyl rings and the naphthalene molecules. Photochromism of the Cocrystals. Cocrystals 1a/ Bz and 1a/Np also underwent photochromic reactions. Upon irradiation with 370 nm light, colorless crystal 1a/ Bz turned reddish purple. The absorption spectrum of the colored crystal is shown in Figure 9b. Absorption maximum in the visible region appeared at 555 nm. On the other hand, colorless crystal 1a/Np turned blue upon irradiation with 370 nm light. Absorption maximum appeared at 620 nm as shown in Figure 9c. The absorption maximum of 1a/Np shifts as much as 65 nm in comparison with the maximum of 1a/Bz. The photogenerated colors of crystals 1a/Bz and 1a/Np were thermally stable in the dark and completely bleached by irradiation with visible light (λ > 450 nm).38 To confirm the origin of the spectral shift, the photogenerated colored crystals were dissolved in hexane, and the absorption maxima were compared. Both solutions gave the maxima at 566 nm. This indicates that the colors in both crystals are due to the closed-ring isomer 1b. Pure single crystal 1a gave the absorption maximum at 630 nm upon UV irradiation as shown in Figure 9a. The color is considered to reflect the π-conjugation length of the photogenerated closed-ring isomer 1b in the crystals.23 As can be seen from the molecular structures shown in Figures 2 and 7, pentafluorophenyl rings in crystals 1a and 1a/Np are almost parallel to molecular planes. Therefore, it is supposed that the photogenrated closed-ring isomers in crystals 1a and 1a/ Np have high planarity and the π-conjugation extends all over the molecule. In crystal 1a/Bz, on the other hand, planes of pentafluorophenyl rings largely tilt to a molecular plane as shown in Figure 5. This means that the pentafluorophenyl rings hardly enter the π-conjugation of the central part of the closed-ring isomer. The closed-ring isomers generated in crystals 1a and 1a/Np, in which π-electrons are delocalized over the whole molecule, have the absorption at longer wavelength than that in crystal 1a/Bz, in which the π-conjugation is limited.23 Conclusion A novel diarylethene with two pentafluorophenyl groups was prepared, and the photochromic performance was examined in solution, in the pure single crystal, and in the cocrystals with benzene or naphthalene. The diarylethene underwent thermally irreversible and photochemically reversible photochromic reactions

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Figure 8. Sandwiched structure in cocrystal 1a/Np (a) and enlarged top and side views of stacking of naphthalene molecules and pentafluorophenyl groups (b and c: part A, d and e: part B). Red dashed lines showed C‚‚‚C contacts in aryl-perfluoroaryl interaction.

Figure 9. Absorption spectra of photogenerated closed-ring isomers in crystal 1a (a), cocrystal 1a/Bz (b), and cocrystal 1a/Np (c).

not only in the pure single crystal of 1a but also in the cocrystals composed of 1a and the aromatic hydrocarbons. The structures of the diarylethene in the cocrystals were controlled by aryl-perfluoroaryl interactions. The absorption maxima of the photogenerated closedring isomers were different among the crystals and dependent on the molecular conformations of the diarylethene in the crystals. Experimental Procedures General. Solvents used were spectroscopic grade and purified by distillation before use. 1H NMR spectra were recorded on a Varian Gemini 200 spectrometer (200 MHz). Tetramethylsilane was used as an internal standard. Mass spectra were taken with a Shimadzu GCMS-QP5050A gas chromatograph-mass spectrometer. Absorption spectra in solution were measured with a Hitachi U-3410 absorption spectrophotometer. Photoirradiation was carried out using an Ushio 500 W super high-pressure mercury lamp or an Ushio 500 W xenon lamp as the light sources. Monochromic light was obtained by passing the light through a monochromator (Ritsu MV-10N). Absorption spectra in a single-crystalline

phase were measured using a Leica DMLP polarizing microscope connected with a Hamamatsu PMA-11 photodetector. The polarizer and analyzer were set in parallel to each other. Photoirradiation was carried out using a 75 W xenon lamp. The wavelength of the light was selected by passing the light through a band-pass filter. 3-Bromo-2-methyl-5-pentafluorophenylthiophene (3). To a dry THF solution (300 mL) containing 3,5-dibromo-2methylthiophene (46 g, 0.18 mol) was added 15% n-BuLi hexane solution (115 mL, 0.19 mol) at -78 °C under argon atmosphere, and the reaction mixture was stirred for 2 h at the low temperature. Tri-n-butylborate (73 mL, 0.31 mol) was slowly added to the reaction mixture at -78 °C, and the mixture was stirred for 2 h at the temperature. A small amount of water was added to the mixture. To the reaction mixture were added 20 wt % Na2CO3aq (370 mL), pentafluoroiodobenzene (53 g, 0.18 mol), and Pd(PPh3)4 (4.0 g, 3.5 mmol). The mixture was refluxed for 12 h at 75 °C. The mixture was neutralized with HCl and then was extracted with diethyl ether. The organic layer was dried over MgSO4, filtrated, and concentrated. The residue was purified by silica gel column chromatography using hexane as the eluent and recrystallization from methanol to give 3 (11 g, 18%) as crystals. mp ) 91.8-93.5 °C; 1H NMR (200 MHz, CDCl3, 25 °C, TMS) δ 2.46 (s, 3H, CH3), 7.34 (s, 1H, thienyl proton); MS m/z 342, 344 (M+). Anal. Calcd for C11H4F5BrS: C, 38.51; H, 1.18. Found: C, 38.29; H, 1.22. 1,2-Bis(2-methyl-5-pentafluorophenyl-3-thienyl)perfluorocyclopentene (1a). To a dry THF solution (80 mL) containing 3 (3.3 g, 9.6 mmol) was added 15% n-BuLi hexane solution (6.2 mL, 10 mmol) at -78 °C under argon atmosphere, and the reaction mixture was stirred for 2 h at the low temperature. Octafluorocyclopentene (0.64 mL, 4.8 mmol; Nippon Zeon) was slowly added to the reaction mixture at -78 °C, and the mixture was stirred for 12 h at that temperature. The reaction was stopped by the addition of water. The product was extracted with diethyl ether. The organic layer was dried over MgSO4, filtrated, and concentrated. The residue was purified by column chromatography on silica gel using hexane as the eluent and by recrystallization from methanol to give 1a (0.94 g, 28%) as colorless crystals. mp ) 115.0-115.7 °C;

Aryl-Perfluoroaryl Interactions 1

H NMR (200 MHz, CDCl3, 25 °C, TMS) δ 2.05 (s, 6H, CH3), 7.45 (s, 2H, thienyl proton); MS m/z 700 (M+). Anal. Calcd for C27H8F16S2: C, 46.30; H, 1.15. Found: C, 46.59; H, 1.20. Closed-Ring Isomer of 1a (1b). Compound 1b was isolated by passing a photostationary solution containing 1a and 1b through a HPLC (Hitachi L-7100 pump system equipped with Hitachi L-7400 detector, silica gel column (Kanto, MightySil Si 60), hexane/ethyl acetate (96:4) as the eluent). Retention times for 1a and 1b were 14 and 12 min, respectively. 1H NMR (200 MHz, CDCl3, 25 °C, TMS) δ 2.23 (s, 6H, CH3), 6.67 (s, 2H, olefinic proton); MS m/z 700 (M+). General Procedure of X-ray Crystallographic Analysis. X-ray crystallographic analysis was performed using a Bruker SMART1000 CCD-based diffractometer (60 kV, 30 mA) with MoKR radiation. The crystals were cooled at 123 K by cryostat (Rigaku GN2). The data were collected as a series of ω-scan frames, each with a width of 0.3°/frame. The crystalto-detector distance was 5.118 cm. Crystal decay was monitored by repeating the 50 initial frames at the end data collection and analyzing the duplicate reflections. Data reduction was performed using SAINT software, which corrects for Lorentz and polarization effects and decay. The cell constants were determined by the global refinement. The structures were solved by direct methods using SHELXS-8639 and refined by full least-squares on F2 using SHELXL-97.40 The positions of all hydrogen atoms were calculated geometrically and refined by the riding model. Quantum Yields of Photochromic Reactions. Photocyclizaion and photocycloreversion quantum yields in hexane were determined by comparing the reaction yields of 1 in hexane against furylfulgide in hexane or toluene.41 Photocyclizaion and photocycloreversion quantum yields in crystal 1a were determined according to the method already reported.20

Acknowledgment. This work was partly supported by Grant-in-Aids for Scientific Research on Priority Areas (15033252 and 12131211), Scientific Research (S) (15105006), and the 21st century COE program “Functional Innovation of Molecular Informatics” from the Ministry of Education, Culture, Science, Sports, and Technology of Japan. We thank Nippon Zeon Co., Ltd. for their supply of octafluorocyclopentene. Supporting Information Available: Polarized absorption spectra of photogenerated colored crystals 1a/Bz and 1a/Np and X-ray crystallographic information files (CIF) for 1a, 1a/ Bz, and 1a/Np. This material is available free of charge via the Internet at http://pubs.acs.org.

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(37) (38)

(39) (40)

(41)

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