1,2-Bis[5-(4-cyanophenyl)-2-methyl-3-thienyl]-3,3,4,4,5,5-hexafluorocyclopent-1-ene: a photochromic diarylethene compound

The molecules of the title compound, C29H16F6N2S2, a photochromic dithienylethene with 4-cyanophenyl substituents, adopt an antiparallel arrangement that is reponsible for photoactivity. The molecule lies on a twofold rotation axis. The dihedral angle between the nearly planar cyclopentenyl and heteroaryl rings is 142.5 (3)°, and that between the heteroaryl and benzene rings is 22.4 (3)°. The distance between the heteroaryl rings of adjacent molecules is 3.601 (2) Å, indicating a π–π interaction.


Comment
Dithienylethenes are the most promising organic photochromic compounds for photoelectronic devices (Irie, 2000).Dithienylethenes bearing terminal phenyl groups are of special interest, because phenyl group can be substituted by many electron-donating or electron-withdrawing groups that influence the properties (Pu, Yang,Xu et al.,2005). In order to investigate the substituent effect at the para-position on the photochemical properties, we have now synthesized the title compound, (Ia), and its structure is presented in this paper.
To the best of our knowledge,this is the first symmetrical dithienylethene compound with phenyl groups bearing para substituent. The molecular structure of (Ia) is shown in Fig. 1 and selected geometric parameters are given in Table 1.
The molecule contains two thiophene rings substituted by two para-cyanophenyl rings in a photoactive antiparallel conformation. In the cyclopent-1-ene ring, the C12=C12A bond is clearly a double bond, and the other bonds in the ring are clearly single bonds(see Table 1). The two thiophene rings are linked by the C12=C12A double bond.The two methyl groups are located on different sides of the double bond and are thus trans with respect to the double bond. Such a configuration is crucial for the compound to exhibit photochromic and photoinduced properties (Woodward & Hoffmann, 1970). The dihedral angles between the least-squares plane of the atoms of the central cyclopent-1-ene ring and the adjacent thiophene rings are 142.5 (3)°, and those between thiophene rings and the adjacent benzene rings are both 22.4 (3)°.The distance between the two reactive C atoms (C4_C4A) is 3.601 (2) Å. This distance indicates that the crystal can undergo photochromism in the crystalline phase because the photochromic reactivity of crystals depends on the distance between the reactive C atoms being less than 4.2 Å (Kobatake et al., 2004).
Upon irradiation with 313 nm light, colorless single crystals of (Ia) turned to blue rapidly, and the blue color remained stable in the dark. When the blue crystals were dissolved in hexane, the solution also remained blue.The absorption maximum of this solution is observed at a wavelength of 596 nm, consistent with the presence of the closed-ring isomer, (Ib).This result suggests that the title compound undergoes a photochromic reaction to produce the closed-ring molecule of (Ib) in the single-crystal phase. We have not, so far, been able to determine the crystal structure of (Ib). Furthermore, upon irradiation with wavelengths longer than 450 nm, the blue crystal changes back to colorless, and the absorption spectrum of a hexane solution of the colorless crystals is the same as that of a solution of the open-ring form, (Ia), with the absorption maximum at 314 nm.

Special details
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 > σ(F 2 ) is used only for calculating Rfactors(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.