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

Abstract

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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.

Conformational isomers of stilbene on Si(100)

“…on opposite sides) conformation around the central bridge, for cis-or trans-stilbene, respectively. This isomerisation around the C=C-double bond can be induced by UV irradiation, as already shown in the gas phase and in solution and described theoretically [6][7][8][9]. The molecule is chemically stable against radiation over a broad wavelength range, and is in fact used as a laser-dye [10][11][12].…”

“…5. Following the hula-twist mechanism as the assumed isomerisation pathway, only the C-H-bonds on the C=C bridge are rotated out of the molecular plane while the actual cis-trans-rearrangement takes place within the molecular plane [9] and only afterwards the tilt within the cis-isomer is established. For such a mechanism, the molecule should not be significantly hindered by the substrate.…”

“…The ''one-bond flip'' corresponds to a rotation around the double bond excited by the UV photons and leads to a transition state conformation of the two phenyl rings perpendicular to each other. The ''hula-twist'' mechanism shown by Fuß and coworkers [9], being a low-energy pathway, is a simultaneous rotation around the central double-bond and one of the adjacent C-C-single bonds, keeping the phenyl groups parallel to each other at all times. Photoinduction of the isomerisation process requires UV light in the wavelength range of 250-320 nm, with isomerisation times in the gasphase and in solution of !…”

Light‐Driven Unidirectional Rotation in a Molecule: ROKS Simulation

“…The p system corresponds to that of binaphthyl ethene, which is similar to stilbene. [26,27] The spatial structure of the molecule is determined by two factors, namely, by the configuration of the double bond, that is, cis or trans isomers, and by the helicity of the molecule.…”

Predistortion amplified in the excited state

“…In this case the branching ratio at the last CI (implying the quantum yields) is the same for the forward and backward reactions. An example of this kind is the cis-trans isomerization of stilbenes: after substantial progress along the C¼C torsion the molecules turn to reach the last CI by one-sided pyramidalization [8,103] (which is slightly asymmetric [8] and thus opens the way to a Hula-twist type isomerization [104]). In fact, stilbenes have the same branching ratio at the S 1 /S 0 CI, whether the wave packet arrives there from the trans or cis isomer; without the sharp turn before the CI, this would probably not be so.…”