The case of medium-dependent dual mechanisms for photoisomerization: one-bond-flip and hula-twist

Abstract

This paper critically reviews examples in the literature of photochemical cis-trans isomerization paying particular attention to the medium effect and accompanied conformational changes. A case is made that the Hula-Twist mechanism, postulated in 1985 as a photochemical reaction pathway for a polyene chromophore imbedded in a protein binding cavity such as those of rhodopsin and bacteriorhodopsin, is also a dominant reaction pathway for a diene, or a longer polyene confined in a rigid (relative to isomerization rate) medium. The conventional one-bond-flip process is the preferred reaction pathway in a fluid medium. While defining experiments are proposed, this dual mechanistic approach successfully accounts for all examples in the literature on photoisomerization reactions whether involving conformational changes or not.

Figure 1

Cis-trans isomerization of a segment of a polyene. (Upper) The Hula-Twist (HT) process showing the translocation of one C-H unit and the sliding sideways movement of the remaining polyene chain. (Lower) The conventional one-bond-flip (OBF) process showing the turning over of one-half of the molecule.