All-Photochemical Rotation of Molecular Motors with a Phosphorus Stereoelement.Unidirectional molecular rotation based on alternating photochemical and thermal isomerizations of overcrowded alkenes is well established, but rotary cycles based purely on photochemical isomerizations are rare.  Furthermore, X-ray crystal structures of all four diastereomeric states of two of these new motors were obtained, which constitute the first achievements of crystallographic characterization of the full 360° rotational cycle of overcrowded-alkene-based molecular motors.  Finally, the axial phosphorus stereoelement in the phosphine motor can be thermally inverted, and this epimerization enables a "shortcut" of the traditional rotational cycle of these compounds.
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