1-(2-Chlorophenyl)-2-(2-methyl-5-phenyl-3-thienyl)-3,3,4,4,5,5-hexafluorocyclopent-1-ene: a new photochromic diarylethene

The title compound, C22H13ClF6S, is a hybrid diarylethene derivative with one 3-thienyl substituent, and a Cl-substituted six-membered aryl unit bonded to the double bond of a hexafluorocyclopentene ring. In the crystal structure, the molecule adopts a photo-active antiparallel conformation that can undergo effective photocyclization reactions. The distance between the two reactive C atoms is 3.848 (3) Å. The dihedral angles between the least-squares cyclopentene plane and those of the adjacent thiophene and chlorophenyl rings are 49.39 (8) and 59.88 (8)°, respectively. The F atoms are disordered over two positions, with site occupancy factors of 0.6 and 0.4.


Comment
Organic photochromic materials have attracted much attention, because of their potential application to optical memory media and optical switches. (Dürr & Bouas-Laurent, 1990;Tian & Yang, 2004). Among all organic photochromic compounds, diarylethenes with heterocyclic aryl groups are the most promising candidates for those application, mainly due to the excellent thermal stability of the respective isomers, notable fatigue resistance, and high reactivity in the solid state (Irie, 2000). The backbone of all photochromic perfluorocyclopentene systems are composed of five-membered heterocyclic rings (Zheng et al., 2007) or the combination of a five-membered aryl ring and a vinyl group. we decided to investigate if replacing the five-membered heterocyclic ring in the diarylethene with a six-membered aryl ring would induce novel characteristics. This paper presents the synthesis and crystal structure of the title compound a six-membered aryl ring group bearing a Cl atom.
The thienyl and the 2-chlorophenyl rings are in cis-position with respect to the C7=C11 double bond ( Such conformation is crucial for the compound to exhibit photochromic and photoinduced properties (Woodward & Hoffmann, 1970). The intramolecular distance between the two reactive C atoms (C1-C13) is 3.848 (3) Å. This distance indicates that the crystal can be expected to undergo photochromism to form compound (Ib) ( Fig. 2), because photochromic reactivity usually appears when distance between the reactive C atoms is less than 4.2 Å (Ramamurthy & Venkatesan, 1987;Kobatake et al., 2004). Crystal of (Ib) shows photochromism in accordance with the expected ring closure to form (Ib).
Upon irradiation with 313 nm light, the colorless single-crystal of (Ia) turned red quickly. When the red crystal was dissolved in hexane, the solution also showed a red color, with an absorption maximum at 523 nm, consistent with the presence of the closed-ring isomer (Ib). Upon irradiation with visible light with wavelength greater than 510 nm, the red crystal can return to its initial colorless state, and the absorption spectrum of the hexane solution containing the colorless crystal is the same as that of solution of the open-ring form, (Ia), with the absorption maximum at 273 nm.

Refinement
All H atoms attached to C were fixed geometrically and treated as riding with C-H = 0.96 Å (methyl) or 0.93 Å (aromatic) with U iso (H) = 1.2U eq (aromatic) or U iso (H) = 1.5U eq (methyl).
The F atoms attached to the cyclopentene ring are disordered over two positions. The occupancy factors of the two positions were refined using an overall isotropic thermal parameter and by restraining the sum of the occupancy to remain equal to 1.0. The ratio between the two occupancies was found to be 0.6/0.4. The C-F distances were restrained using SADI (SHELXL-97) instructions and similar Uij restraints as well as rigid bond restraints were used in the final refinement cycles.   1-(2-Chlorophenyl)-2-(2-methyl-5-phenyl-3-thienyl)-3,3,4,4,5,5-hexafluorocyclopent-1-ene

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.