X-ray Diffraction Analysis of Nonequilibrium States in Crystals: Observation of an Unstable Conformer in Flash-Cooled Crystals

Some molecules with a molecular skeleton similar to that of stilbenes and azobenzenes show
orientational disorder in the crystals. If the disorder is dynamic, a conformational interconversion takes
place through a pedal motion. In this study X-ray diffraction analyses of (E)-stilbene (1) and azobenzene
(2) were carried out at various temperatures. We succeeded in observing thermodynamic nonequilibrium
states that were generated by fast freezing of the conformational interconversion at low temperatures. The
populations of the two conformers in crystals of 1 at 90 K varied with the cooling rate. Flash cooling of a
crystal of 2 from room temperature to 90 K made it possible to observe the minor unstable conformer that
does not exist in the equilibrium state at the same temperature.