Photochromism of Rotation-Hindered Furylfulgides Influenced by Steric Modifications

The syntheses of a bicyclic furylfulgide 14 and a (benzofuryl)fulgide 15 with increased steric constraints are described. Their photochromic behaviors were analyzed by means of UV/Vis spectroscopic measurements, X-ray crystallography, and NMR experiments, and the results were compared to those of the furyl(methyl) fulgide 12 and the furyl(isopropyl)fulgide 13. Compounds 13E and 14E exhibit large quantum yields of 0.57 and 0.53 for the coloration reaction (E) -> (C) compared with 12E and 15E (0.23 and 0.17). After irradiation with 350 nm light, 13E and 14E are transformed into the closed (C) forms almost quantitatively, whereas 12E and 15E result in a photostationary state with mixtures of the (E), (Z), and (C) forms. The crystal structures obtained for 13E, 14E, and 15E show that the fulgides adopt cyclizable helical (P)-E-alpha conformations with no significant differences in atomic distances in the hexatriene unit. 2D- and temperature-dependent NMR experiments showed that the enantio-and diastereotopomerization processes were suppressed in a fulgide for the first time. Compound 14E populates only the E-alpha conformational state. In contrast, 13E and 15E both exist in the cyclizable E-alpha and the non-cyclizable E-beta conformations in solution. Due to the annulated benzene ring, 15E exhibits a higher thermodynamic barrier than 13E, so the belly roll process was reduced for 15, but the (E) -> (Z) isomerization could not be suppressed. The structural modification of 14 successfully suppressed the (E) -> (Z) isomerization as well as the belly roll process. The way in which the isomerization reaction is suppressed by steric hindrance could not be fully elucidated by using these methods.