Irradiation Hula twist

The photoisomerization of all-s-trans-all-trans 1,3,5,7-octatetraene at 4.3 K illustrates the need for a new mechanism to explain the observed behavior [150]. Upon irradiation, all-s-trans-all-trans 1,3,5,7-octatetraene at 4.3 K undergoes conformational change from all-s-trans to 2-s-cis. Based on NEER principle (NonEquilibrium of Excited state Rotamers), that holds good in solution, the above transformation is not expected. NEER postulate and one bond flip mechanism allow only trans to cis conversion rotations of single bonds are prevented as the bond order between the original C C bonds increases in the excited state. However, the above simple photochemical reaction is explainable based on a hula-twist process. The free volume available for the all-s-trans-all-trans 1,3,5,7-octatetraene in the //-octane matrix at 4.3 K is very small and under such conditions, the only volume conserving process that this molecule can undergo is hula-twist at carbon-2. 

Reactions of 1,4-Diketones. - The hula-twist mechanism is suggested as the only possible mechanism for the ,Z-isomerism of tru s-1,2-dibenzoylethene in the crystalline phase. The irradiation in sunlight of the yellow bi-indenylidenedione (152) brings about a colour change to red. The red compound has been shown to be the biradical (153). The red colour can be dispersed thermally when the radical (153) reverts to the parent (152). The photoinduced electron transfer within (154) has been studied. ... 

Liu and co-workers have shown that irradiation of the 2,2 -dimethyl-stilbene isomers in glassy 2-methylpentane brings about isomerization by a hula-twist mechanism. A hula-twist mechanism to afford the trans isomer is also involved in the isomerization of a complex (1 1) of c/.y-3,3 -bis(di-phenylhydroxymethyl)stilbene and acetone. " The conformational dynamics... 

Reviews of the hula-twist photoisomerization of dienes have been published. New calculations dealing with the isomerism of -cw-butadiene have been reported.Molecular dynamic simulations of the photochemical behaviour of butadiene have shown that cis-trans isomerism occurs. The regioselectivity observed in the photochemical isomerism of trans,trans-l-fluorohexa-2,4-diene is proposed as due to electrostatic control. Direct irradiation of the fluorinated dienes (102) results in preferential conversion to the Z,3E isomers (103). DCA-sensitized irradiation of the butatrienes (104) in various alcohols afford the adducts (105). The simpler alcohols give moderate yields of product, while -butanol affords only a trace of the adduct. ... 

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