Conformations and threshold rotational mechanisms of the (Z)-1,2- diarylethene moiety: A structural correlation study

The crystal structures of substituted (Z)-1,2-diarylethenes and (Z)-1,2- diarylcyclopropenes, -cyclobutenes, -cyclopentenes, and -cyclohexenes that were reported in the literature were retrieved from the Cambridge Structural Database, and the dihedral angles of the aryl rings with the double bond were plotted in a conformational map. Analysis of the data by the Structural Correlation method together with molecular mechanics calculations suggest that the conformation and the threshold rotational mechanism of the (Z)-1,2- diarylvinyl fragment strongly depend on its steric environment. In 1,2- diarylcyclopropenes the aryl rings are nearly coplanar with the double bond, whereas in 1,2-diarylcycloalkenes with larger rings a propeller conformation is adopted. The threshold rotational mechanism is the one-ring flip for (Z)- 1,2-diphenylethene and 1,2-diphenylcyclobutene, and is the two-ring flip for 1,2-diarylcyclopentene and 1,2-diarylcyclohexene. The calculated rotational barriers of the aryl rings in the threshold mechanism for all systems were very low (0.2-2.4 kcal mol^-1). The different conformations and rotational behaviors are dictated by an interplay of conjugation and steric effects.