Polyene isomerization, excited state reactions

The idea that the various more or less planar conformers of a conjugated trlene—or of a polyene in general—upon n - n excitation do not interconvert during their short (singlet) excited-state lifetime arose in the early days of the study of the photoisomerization reactions in the vitamin D field (14, 63). Its introduction served the purpose of rationalizing the values of the quantum yields of isomerization of prevltamln D and tachysterol into their primary photoproducts. 

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