Dependence of the Triplet Potential of Retinal Homologues on the Chain Length: Resonance Raman Spectroscopy and Analysis of Triplet-Sensitized Isomerization

Article abstract of DOI:10.1021/j100373a030

The Raman spectra of triplet species produced from the all-trans, 7-cis and 9-cis isomers of β-ionylideneacetaldehyde (C15 aldehyde) and of β-ionylidenecrotonaldehyde (C17 aldehyde) were recorded.Each isomer of C15 aldehyde showed its own triplet Raman spectrum, while all the isomers of C17 aldehyde showed an identical triplet spectrum.The results were compared with those of isomeric retinal (C20 aldehyde) and retinylideneacetaldehyde (C22 aldehyde) obtained previously.Triplet-sensitized isomerization as well as direct photoisomerization of the all-trans isomer and the complete set of mono-cis isomers of C15, C17, C20, and C22 aldehydes were analyzed by HPLC.Mutual isomerization among the all-trans was seen for C17, C20, and C22 aldehydes.The quantum yield of triplet-sensitized isomerization for each isomer of the above aldehydes was determined. all the results are discussed in terms of triplet potentials with minima at cis and trans positions, the relative stability being dependent on the length of the polyene chain; the cis minima are as stable as the trans minimum for C15 aldehyde, while the cis minima are far less stable than the trans minimum for C17, C20, and C22 aldehydes.