Hula-twist cis–trans isomerization: The role of internal forces and the origin of regioselectivity

In photochemical Hula-twist isomerization of conjugated polyenes, a double bond and an adjacent single bond twist concertedly by 180°. It is here considered to be driven by passing through the last conical intersection (CI), between potentials of the dark covalent S1 (2A) and the ground S0 (1A) states. If several such CIs are available, that is, if isomerization of different bonds is possible, regioselectivity can be caused by a sterically induced pre-twist in S0, so that the pre-twisted group isomerizes: this deformation is amplified in the Franck–Condon region of the spectroscopic (1B) state; on entering the 2A surface with this geometry, the nearest S1/S0 CI will be chosen, if the reaction is ultrafast. If the reaction is slower, that is, if there is a barrier before each CI, a local pre-twist reduces one of them and thus also selects the site of isomerization. Another pre-twist can be caused in the initially excited (ionic) 1B state by electrostatic effects, thus also giving rise to regioselectivity. The explanations only consider potentials of the molecule, in the majority of cases with no external forces. Also other observations are summarized that support the idea that Hula twist can work without influence of the environment. A variant of Hula twist is also proposed and used, in which the torsion of the two adjacent bonds is not disrotatory as usual but conrotatory.

► Hula twist isomerization is often considered to be due to environmental constraints.
► But it is the normal path for nonpolar polyenes also in free molecules.
► It is caused by internal forces (slopes through the last conical intersection).
► This assumption explains many observations, including regioselectivity.