Symmetry control of radiative decay in linear polyenes: Low barriers for isomerization in the [S.sub.1] state of hexadecaheptaene

The controversy related to experiments implying that the [S.sub.1] states of longer polyenes have local energy minima, corresponding to a range of conformations and isomers separated by relatively low (2-4 kcal) barriers is reevaluated. Low barriers to isomerization and conformational change might account for the complex kinetics of [S.sub.2] -->[S.sub.1] nonradiative decay in carotenoids.