Rotationally-hindered furyl fulgides

Fulgides are a representative class of photochromic organic molecules which exhibit several interesting properties for diverse applications in fields such as data storage or high-resolution spectroscopy. The crystal structures of three furyl fulgides with different steric constraints were determined and for two of the compounds both the E and Z isomer structures were defined. The compounds are 3-[(E)-1,3-dimethyl-4,5,6,7-tetrahydro-2-benzofuran-4-ylidene]-4-isopropylidenetetrahydrofuran-2,5-dione, C17H18O4, (I-E), 3-[(E)-1,3-dimethyl-5,6,7,8-tetrahydro-4H-cyclohepta[c]furan-4-ylidene]-4-isopropylidenetetrahydrofuran-2,5-dione, C18H20O4, (II-E), and the Z isomer, (II-Z), and 3-isopropylidene-4-[(E)-1-(5-methoxy-2-methyl-1-benzofuran-3-yl)ethylidene]tetrahydrofuran-2,5-dione, C19H18O5, (III-E), with two molecules in the asymmetric unit, and the Z isomer, (III-Z). The structures of the E and Z isomers show only little differences in the bond lengths and angles inside the hexatriene unit. Because of the strained geometry there are deviations in the torsion angles. Furthermore, small differences in the distances between the bond-forming C atoms in the electrocyclization process give no explanation for the unequal photochromic behaviour.