5-n-Butyl-4-[2-(2-ethyl-1-benzothiophen-3-yl)-3,3,4,4,5,5-hexafluorocyclopent-1-en-1-yl]thiophene-2-carbaldehyde

The title compound, C24H20F6OS2, exhibiting photochromic behaviour, has thienyl and benzothienyl substituents attached to the double-bond C atoms of the envelope-shaped cyclopentene ring. The mean planes of aromatic systems form dihedral angles of 43.0 (1) (thienyl) and 73.8 (1)° (benzothienyl) with the mean plane of the C—C=C—C portion of the cyclopentene ring. This conformation avoids steric hindrance between the n-butyl and ethyl substituents. The formyl substituent of the thienyl group, as well as the ethyl substituent of the benzothienyl group, are disordered [occupancies of 0.788 (17):0.212 (17) and 0.64 (5):0.36 (5), respectively].

The title compound, C 24 H 20 F 6 OS 2 , exhibiting photochromic behaviour, has thienyl and benzothienyl substituents attached to the double-bond C atoms of the envelope-shaped cyclopentene ring. The mean planes of aromatic systems form dihedral angles of 43.0 (1) (thienyl) and 73.8 (1) (benzothienyl) with the mean plane of the C-C C-C portion of the cyclopentene ring. This conformation avoids steric hindrance between the n-butyl and ethyl substituents. The formyl substituent of the thienyl group, as well as the ethyl substituent of the benzothienyl group, are disordered [occupancies of 0.788 (17)

Comment
The title compound has thienyl and benzothienyl substituents on the double-bond C atoms of the envelope-shaped cyclopentene ring. The planes of aromatic systems form dihedral angles 43.0 (1)° (thienyl) and 73.8 (1)° (benzothienyl) with the mean plane of the C4-C11=C15-C16 portion of the cyclpentene ring. Such conformation allows to avoid steric hindrance between the n-butyl and ethyl substituents (Fig. 1). The terminal atoms of formyl substituent on the thienyl group as well as the ethyl substituent on the benzothienyl group are disordered over two positions. The intramolecular distance between C3 and C20 is 3.989 (8) Å. This distance indicates that the compound may undergo photochromism in crystalline phase to form the closed ring isomer, as photochromic reactivity was shown to occur when the distance between the potentially reactive C atoms is less than 4.2 Å (Ramamurthy & Venkatesan, 1987;Kobatake & Irie, 2004).
Indeed, crystals of the title compound show photochromism: upon irradiation with 365 nm light, the colourless crystals rapidly turn blue, and the blue crystals turn colourless again upon irradiation with visible light (>510 nm). When dissolved in hexane, the blue compound displays an absorption maximum at 581 nm; the solution of colourless compound has absorption maximum at 257 nm.
Experimental 2.0 mL (5 mmol) of n-butyllithium was added under stirring in nitrogen atmosphere to 30 mL of THF solution containing 1.46 g (5.0 mmol) of 4-bromo-5-n-butyl-2-(1,3-dioxolane)-thiophene (Zheng et al., 2007) at 195 K. 40 min later, 10 mL of THF solution containing 1.77 g (5.0 mmol) of 1-(2-ethyl-1-benzothien-3-yl)heptafluorocyclopentene (Pu et al., 2008) was added to the reaction mixture and stirring under nitrogen atmosphere at 195 K was continued for two more hours. The reaction mixture was extracted with diethyl ether and evaporated in vacuum. Then the obtained compound was hydrolyzed by p-toluenesulfonic acid (0.4 g) in mixture of water (30 ml) and acetone (90 ml). Pyridine (2 ml) was added, and the solution was refluxed for 24 hours and then washed with aqueous sodium bicarbonate. The mixed compound was extracted with diethyl ether and evaporated in vacuum. The crude product was purified by column chromatography on silica, with ethyl acetate and petroleum ether (v/v 1/6) as the eluent, to give 1.58 g (3.15 mmol, 63% yield) of the title compound. Elemental analysis: calc. for C 24 H 20 F 6 OS 2 : C 57.36, H, 4.01%. Found C 57.22, H 3.90%.
supplementary materials sup-2 The formyl oxygen atom is also disordered over two positions; the C19-O1 and C19-O1' distances were restrained to be equal within 0.01 Å. The occupancies of the O1 and O1' atoms refined to a 0.64 (1):0.36 ratio.
The anisotropic displacement parameters of the disordered atoms were restrained to be nearly isotropic. Fig. 1. Molecular structure of the title compound and the atom-labelling scheme; thermal displacement ellipsoids are drawn at the 30% probability level. Minor components of the disorder are shown with the dashed bonds; H atoms are omitted.