Invisible but common motion in organic crystals: A pedal motion in stilbenes and azobenzenes

Some molecules that have a molecular skeleton similar to stilbenes and azobenzenes are known to show an orientational disorder in the crystals. Some of the disorders are known to be dynamic and mediated by a pedal motion in crystals. Dynamic processes of (E)-stilbene (1) and (E)-3,3',4,4'-tetramethylazobenzene (2) in the crystals were investigated by X-ray diffraction analyses. The dynamic disorder and the pedal motion were detected at higher temperature, even in the molecules that showed no traces of the disorder at room temperature. The results demonstrated that the pedal motion should always be taken into account, even if no disorder is detected. The reasons for the nonoccurrence of the disorder and for the prevalence of the pedal motion are also discussed.