Different photochemical behavior of bis(biphenyl)ethylenes and ethenes in solution and in the solid-state: Structurally controlled Z/E-photoisomerization in the solid-state

Several bis(biphenyl)ethylenes and ethenes were prepared and their photoisomerization both in solution and in the solid-state was investigated. While all ethylenes and ethenes gave the solutions containing both E- and Z-isomers in the manner of the mutual photoisomerization, the ethylenes having at least one hydrogen atom on the sp2 carbons underwent unidirectional photoisomerization from the Z-isomers to the E-isomers in the solid-state even at low temperature. On the other hand, both the E- and Z-ethenes having no hydrogen atom on the double bonds did not change when irradiated in the solid-state even at room temperature. These different photochemical behaviors can be explained by the Hula-twist (HT) mechanism in such confined environments.