4-{2-[5-(3,5-Difluorophenyl)-2-methylthiophen-3-yl]-3,3,4,4,5,5-hexafluorocyclopent-1-en-1-yl}-1,5-dimethylpyrrole-2-carbonitrile

In the title compound, C23H14F8N2S, the dihedral angles between the pyrrole and thiophene groups and the almost planar C—C=C—C unit of the cyclopentene ring (r.m.s. deviation = 0.4193 Å) are 43.6 (5) and 50.1 (2)°, respectively. The distance of 3.612 (3) Å between the potentially reactive C atoms of the two heteroaryl substituents is short enough to enable a photocyclization reaction.

In the title compound, C 23 H 14 F 8 N 2 S, the dihedral angles between the pyrrole and thiophene groups and the almost planar C-C C-C unit of the cyclopentene ring (r.m.s. deviation = 0.4193 Å ) are 43.6 (5) and 50.1 (2) , respectively. The distance of 3.612 (3) Å between the potentially reactive C atoms of the two heteroaryl substituents is short enough to enable a photocyclization reaction.

Comment
The title compound when dissolved in hexane shows photochromism. Upon irradiation with 365 nm light, the colorless hexane solution turns blue rapidly. The blue compound displays an absorption maximum at 592 nm. 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 two absorption maximum at 253 nm and 294 nm. In a polymethylmethacrylate amorphous film, the title diarylethene also exhibits photochromism similar to that in hexane.
The solid product was purified by column chromatography on silica with petroleum ether as the eluent to give the title compound (0.55 g, 1.10 mmol) in 47.8% yield. Analysis calc. for C 23 H 14 F 8 N 2 S: C 54.98, H, 2.81%; fFound C 55.02, H 2.95%.

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 supplementary materials sup-3 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.