FAN Cong-Bin CUI Shi-Qiang LIU Gang

(Jiangxi Key Laboratory of Organic Chemistry,Jiangxi Science & Technology Normal University, Nanchang 330013, China)

INTRODUCTION

Photochromic compounds have been vastly investigated due to their promising applications in molecular switches for the development of various photoresponsive systems[1-3]. Among different families of known organic photochromes, dithienylethenes are of particular interest as they can undergo fatigueresistant six-p-electron electrocyclization and cycloreversion between two stable isomers[2]. The difference in either their electronic or geometric structures between the open- and closed-ring isomers has been exploited to successively modulate a broad range of properties and functions[2,4-10].

However, the reports focusing on the hexatriene backbone of photochromic perfluorocyclopentene systems is composed of five-membered heterocyclic rings or the combination of five-membered aryl ring and vinyl group. To the best of our knowledge, photochromic hybrid diarylethene derivatives bearing both five- and six-membered moieties have rarely been reported[11-12].

For broad understanding the photochromism of diarylethene derivatives with various aryl groups, we report a new class of photochromic diarylethenes by replacing a five-membered heterocyclic ring with a six-membered aryl ring to investigate their novel characteristics. In this article, we present our results on the synthesis and crystal of 1-(2-methyl-5-formyl-3-thienyl)-2-(2-methoxylphenyl) perfluorocyclopentene.

2 EXPERIMENTAL

2.1 Materials and instruments

All chemicals were purchased from Aldrichor Acros chemical Co. and used without any further purification.

The elemental analyses were measured with a PE CHN 2400. The melting point determination was performed on a WRS-1B melting point apparatus.NMR spectra were recorded on a Bruker AV400(400 MHz) spectrometer with CDCl3as the solvent and tetramethylsilane as an internal standard. Infrared spectra (IR) were recorded on a Bruker Vertex-70 spectrometer. The absorption spectra were measured using an Agilent 8453 UV/vis spectrometer. Photo-irradiation was carried out using an SHG-200 UV lamp, CX-21 ultraviolet fluorescence analysis cabinet and a BMH-250 Visible lamp. The required wavelength was isolated by the use of appropriate filters. The PMMA films were prepared by dissolving 10 mg of diarylethene sample and 100 mg of polymethylmethacrylate (PMMA) in chloroform (1 mL) with the aid of ultrasound, then the homogeneous solution was spin-coated on a glass substrate (20 mm × 10 mm × 1mm) at 1500 rpm.

2.2 Synthesis of 1-(2-methyl-5-formyl-3-thienyl)-2-(2-methoxylphenyl)-perfluorocyclopentene (1o)

1,3,3,4,4,5,5-Heptafluoro-2-(methoxyphenyl)cyclo pentene[12](a, 1.5 g, 5.0 mmol) reacted with 4-bromo-5-methyl-2-(1,3-dioxolane)thiophene[13](b, 1.25 g, 5.0 mmol) in the presence of n-butyllithium (2.5 M in hexane, 2.0 mL, 5.0 mmol) at 195 K under a nitrogen atmosphere (Scheme 1). After an hour, the reaction was quenched by the addition of water. The solid product was purified by column chromatography on silica using petroleum ether as the eluant to give the compound {1-(2-methoxyphenyl), 2-[2-methyl-5-(1,3-dioxolane)-3-thienyl]}-3,3,4,4,5,5-hexafluorocyclopentene (c) (1.01 g, 2.25 mmol, yield 45%). This compound (1.01 g, 2.25 mmol) and p-toluenesulfonic acid (0.3 g) were dissolved in the mixture of water (30 mL) and acetone (90 mL); 2 mL pyridine was added into the mixture and the reaction mixture was refluxed for 24 h (Scheme 1).After the reaction was complete, the mixture was washed sequentially by aqueous NaHCO3and water.The organic layer was dried over anhydrous Na2SO4,filtrated, and evaporated. The crude product was purified by column chromatography on SiO2using ethyl acetate and petroleum ether mixture (v/v = 1/6)as the eluent to give 0.85 g of compound 1o in 84%yield. Analysis calculated for C20H16F6O3S: C, 53.33;H, 3.58%. Found: C, 53.42; H, 3.67%. MS: m/e 406.05. m.p. 67–68 ℃.1H NMR(CDCl3, 400 MHz,TMS): δ 2.01 (s, 3H, -CH3), 3.51 (s, 3H, -OCH3),6.83, 6.85 (d, 1H, J = 8.0 Hz, phenyl-H), 7.01 (t, 1H,J = 8.0 Hz, phenyl-H), 7.35–7.42 (m, 2H, phenyl-H),7.69 (s, 1H, thienyl-H), 9.83 (s, 1H, -CHO).13CNMR (100 MHz, CDCl3, TMS): δ 15.02, 55.19,111.52, 116.56, 121.02, 127.51, 132.32, 137.02,141.00, 150.69, 156.77, 182.23; IR(KBr): v (C-H)2852 cm-1, (C=O)1668 cm-1, (C=C) 1618 cm-1, 1496 cm-1, (C-F) 1401 cm-1(o-substituted benzene) 759 cm-1.

Scheme 1. Synthetic route for the title compound 1o

2.3 Structure determination and refinement

A colorless crystal of the title compound having approximate dimensions of 0.47mm × 0.37mm ×0.31mm was mounted on the top of a glass fiber. X-ray diffraction data were collected on a Bruker Smart-1000 CCD diffractometer equipped with a graphite-monochromatized MoKa radiation(0.71073 ?) by using an ω/φ scan mode in the range of 2.6≤θ≤25.5o(–18≤h≤18, –10≤k≤10, –15≤l≤15) at 291(2) K. A total of 12839 reflections were collected with 3308 unique ones (Rint= 0.058), of which 2584 with (I ＞ 2σ(I)) were observed. The structure was solved by direct methods and refined on F2by full-matrix least-squares procedure with SHELXS-97 and SHELXL-97[14]. All non-hydrogen atoms were determined from difference Fourier maps and anisotropically. All hydrogen atoms were determined from difference Fourier maps. The final refinement converged to R = 0.0579 and wR =0.1566 (w = 1/[σ2(Fo2) + (0.0719P)2+ 1.4916P],where P = (Fo2+ 2Fc2)/3), (Δ/σ)max= 0.000, S =1.071, Δρ(max)= 0.421 and Δρ(min)= –0.364 e/?3.Selected interatomic distances and bond angles are given in Table 1. All H atoms attached to C were fixed geometrically and treated as riding with C–H =0.96 ? (methyl) or 0.93 ? (aromatic) with Uiso(H) =1.2Ueq(aromatic) or Uiso(H) = 1.5Ueq(methyl). The F atoms attached to C atoms in the cyclopentene ring are disordered over two positions. The occupancy factors of the two positions were refined using an overall isotropic thermal parameter by restraining the sum of the occupancy to keep 1.0. The ratio between the two occupancies was found to be 0.5657/0.4343. The C–F distances were restrained using SADI (SHELXL-97) instructions and similar Uijrestraints as well as rigid bond restraints were used in the final refinement of cycles.

3 RESULTS AND DISCUSSION

The selected bond lengths and bond angles together with the torsion angles are listed in Table 1.The molecular structure of the title compound is shown in Fig. 1. The crystal adopts an antiparallel conformation that can undergo effective photocyclization reactions by a conrotatory mechanism. The packing diagram of the title compound in a unit cell is shown in Fig. 2. Neighboring molecules are antiparallel and crisscross to each other in the cell.The center-to-center distances between the phenyl rings of neighboring molecules are 4.071 ? (see left of Fig. 2), showing weak π-π stacking interactions.Intermolecular hydrogen bonds C–H··F and C–H··O connect compounds 1o with each other(Table 2). These molecular interactions together with hydrogen bonding may strengthen the stability of the framework structure.

Fig. 1. ORTEP drawing of the title crystals showing 30% probability displacement ellipsoids

Fig. 2. Fragment of the crystal packing of the crystal of compound 1o

Table 1. Selected Bond Lengths (?), Bond Angles (°) and Torsion Angles (o) for Compound 1o

Table 2. Hydrogen Bond Lengths (?) and Bond Angles (o)

The thiophene and benzene rings are linked by the C(8)=C(12) bond (1.348(4) ?, see Table 1). The methyl and methoxyl rings are located on the opposite sides of the double bond and are directed trans relative to the thiophene and benzene planes,as reflected in the torsion angles 127.49ofor C(11)–C(12)–C(14)–C(13) and –122.08ofor C(9)–C(8)–C(6)–C(1). The dihedral angle between the main central cyclopentene ring and adjacent thiophene, benzene rings are 46.7o for S(1)/C(13)– C(16)and 48.2o for C(1)–C(6), respectively. The conformation leads to a C(1)–C(13) separation of 3.518 ?. This distance is short enough, theoretically, for a ring-closure reaction to take place in the crystalline phase to generate an isomeric compound. It is known that the photochromic activity in similar compounds is usually present when this distance is less than 4.2 ?[15-18]. Therefore, the title crystal can undergo photochromism in the crystalline phase.However, the fact is contrary to the above analysis.The title compound can not show any photochromism in the crystalline phase. This is verified by the fact that irradiating single crystals of 1o with UV light for 12 h results in no observable change in the color of the crystal. When the title crystal was dissolved in hexane, the solutions turned colorless and their absorption spectra were the same as those of the open-ring isomer 1o. Thus, the important rule described above should restrict its applied condition:it is inapplicable to diarylethene systems bearing a six-membered aryl ring reported in this paper at least.

The title compound when dissolved in hexane shows photochromism (Scheme 2). Upon irradiation with 297 nm light, the colorless hexane solution turns blue rapidly. The blue compound displays an absorption maximum at 588 nm (Fig. 3). Upon irradiation with visible light with wavelength longer than 510 nm, the blue hexane solution reverts to its initial colorless state; a colorless hexane solution of the title compound has an absorption maximum at 257 nm. In a PMMA amorphous film, the title diarylethene also exhibits photochromism similar to that in hexane.

Fig. 3. Absorption spectral changes of the title compound in hexane solution (about 2.0 × 10-5 mol/L)

Scheme 2. Photochromic interconversion of the diarylethene 1

4 CONCLUSION

The title compound has been synthesized and characterized, and its crystal structure was determined by X-ray single-crystal diffraction. The thiophene and benzene rings are linked by the C(8)=C(12) bond of the central cyclopentene ring.The methyl and methoxyl rings are located on opposite sides of the double bond (C(8)=C(12)) and are directed trans relative to the thiophene and benzene planes, as reflected in the torsion angles 127.49oand –122.08o, respectively. The dihedral angle between the main central cyclopentene ring and adjacent thiophene, benzene rings are 46.7 and 48.2o, respectively.

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