Photochromism of novel [1]benzothien-2-yl fulgides

Novel [1]benzothien-2-yl fulgides were synthesized and their structures established based on analysis by 1H, 13C NMR and IR spectroscopy, mass-spectrometry and X-ray diffraction. All the compounds exist in the ring-opened Z-isomeric forms. Under UV-irradiation fulgides with no substituent at the methylidene bridge undergo thermally or photochemically reversible Z/E-photoisomerization, but do not convert to the cyclic isomeric forms. Under the same conditions, 3-(1-(3-methyl[1]benzothien-2-yl)ethylidene)-4-(1-methylethylidene)-dihydrofuran-2,5-dione with a methyl group at the methylidene bridge rearranges into the thermally stable colored 1,3-cyclohexadiene ring-closed isomer which possesses improved spectral characteristics as compared with 2-thienyl and [1]benzothien-3-yl analogues. Irradiation with visible light results in the fast backward isomerization into the initial colorless hexatriene ring-opened form.