Direct and Sensitized Photoisomerization of 1,4-Diphenylbutadienes

Article abstract of DOI:10.1021/ja00215a027

Photoisomerization of the three stereoisomers of 1,4-diphenylbutadiene (DPB) in room-temperature cyclohexane solution was studied by direct excitation and by triplet sensitization.The photostationary mixtures in both singlet and triplet experiments contain only trans,trans and cis,trans isomers; no cis,cis isomer or any photoproducts were observed.The two cis isomers exhibit no measureable fluorescence or triplet formation. results are interpreted in terms of allylmethylene-like intermediates.Efficient internal conversion of singlet DPBs is inferred from low isomerization quantum yields by direct excitation.Triplet-sensitized isomerizations show a marked DPB concentration dependence of photostationary compositions and quantum yields.Analysis of these data reveals a quantum chain process involving excited triplet DPB as the chain-carrying species.Transient absorption measurements show that sensitized excitation of any DPB isomer leads directly to a fast equilibrium between triplet all-trans and trans-twisted forms.