Polyenes triplet energies

The cis-trans isomerization of various olefins in the triplet state was reviewed. The mode of cis-trans isomerization is correlated to the triplet energy of the aryl substituent (ArH), since only the planar triplet energy decreases with decreasing t of ArH. In addition, the energy minima as well as the stable conformations in the triplet state were discussed based on the triplet lifetime, the efficiency of isomerization, and the quenching experiments. Several examples of quantum chain process in olefins and polyenes were also described. 

Little, if any reliable spectroscopic information is available regarding the T, energies of polyenes or carotenoids with more than six conjugated double bonds. As a result, it has been necessary to rely on indirect measurements and/or extrapolations to estimate the T energies of carotenoids employed in photosynthetic systems. One such approach has been outlined by Bensasson et al. (1976, 1993) and discussed by Frank and Cogdell (1993). A plot of the reciprocal of the triplet energies (from Table 2) as a... 

The spectroscopic catalog of polyene T, triplet energies (Table 2) has expanded very little since the... 

Fig. 2. Potential energy curves for C=C bond twisting in polyene triplets ethylene A), 3.4 bond and 1.2 bond in trans—irons hexatriene B and D, respectively), irons butadiene (C)...

---