3-(4-{3,3,4,4,5,5-Hexafluoro-2-[5-(3-methoxyphenyl)-2-methyl-3-thienyl]cyclopent-1-enyl}-5-methyl-2-thienyl)benzonitrile

The title compound, C29H19F6NOS2, is a new unsymmetrical photochromic diarylethene derivative with different meta-phenyl substituents. The distance between the two reactive (i.e. can be irradiated to form a new chemical bond) C atoms is 3.501 (4) Å; the dihedral angles between the mean plane of the main central cyclopentene ring and the thiophene rings are 47.7 (5) and 45.1 (2)°, and those between the thiophene rings and the adjacent benzene rings are 29.4 (2) and 28.4 (3)°. The three C atoms and the F atoms of hexafuorocyclopentene ring are disordered over two positions, with site-occupancy factors of 0.751 (4) and 0.249 (4).

The title compound, C 29 H 19 F 6 NOS 2 , is a new unsymmetrical photochromic diarylethene derivative with different metaphenyl substituents. The distance between the two reactive (i.e. can be irradiated to form a new chemical bond) C atoms is 3.501 (4) Å ; the dihedral angles between the mean plane of the main central cyclopentene ring and the thiophene rings are 47.7 (5) and 45.1 (2) , and those between the thiophene rings and the adjacent benzene rings are 29.4 (2) and 28.4 (3) . The three C atoms and the F atoms of hexafuorocyclopentene ring are disordered over two positions, with site-occupancy factors of 0.751 (4) and 0.249 (4).

Comment
Photochromic diarylethene is one of the most promising candidates for photoelectronic applications, such as optical storage (Irie, 2000;Pu et al., 2008), photoswitches (Irie et al., 2001;Pu et al., 2007) (4) Å]. The title compound shows a photoactive antiparallel conformation (Fig. 1). The two independent planar thiophene ring systems have essentially identical geometries, and the dihedral angles between the main central cyclopent-1-ene ring and those of the two thiophene rings, S1/C6-C9 and S2/C18-C21, are 47.7 (5) and 45.1 (2)°, respectively. The dihedral angle between the thiophene and adjacent benzene ring is 29.4 (2)° for the C11-C16 benzene ring and 28.4 (3)° for the C23-C28 benzene ring. The two thiophene groups are linked by the C1=C5 double bond, with both of them attached to the ethylene group via the 2-position. The distance between the two reactive C atoms (C6···C18) is 3.501 (4) Å, which is short enough, theoretically, for the ring-closure reaction to take place in the crystalline phase (Ramamurthy & Venkatesan, 1987). Crystals of the title compound (1a) show photochromism in accordance with the expected ring closure, to form (1b) (Fig. 2); upon irradiation with 313 nm light, the colorless crystals turn blue rapidly. The blue compound when dissolved in hexane displays an absorption maximum at 583 nm. Upon irradiation with visible light with a wavelength greater than 450 nm, the blue crystals again turn initial colorless; a hexane solution has an absorption maximum at 294 nm.

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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )