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The Photochemical cis–trans Isomerization of Free Stilbene Molecules Follows a Hula-Twist Pathway


  • This work was supported by the Deutsche Forschungsgemeinschaft. We are indebted to René Lapouyade for a sample of 2 and to Jörg Schroeder for helpful discussions.


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A motion breaking with tradition: The cis–trans isomerization of nonpolar conjugated π systems does not proceed simply by the 180° twist of a C[DOUBLE BOND]C bond (“one-bond flip”, OBF), but by the rotation of a central CH group out of the plane (“hula twist”, HT). The hula-twist motion does not require any external constraint but is driven by an internal force: the slope on the way through a conical intersection.