Triplet-Sensitized and Thermal Isomerization of All-Trans, 7-Cis, 9-cis, 13-Cis, and 15-Cis Isomers of β-Carotene: Configurational dependence of the Quantum Yield of Isomerization via the T1 State

Article abstract of DOI:10.1021/j100172a016

The products of triplet-sensitized photoisomerization (excitation at 337 nm of the sensitizer, anthracene) and thermal isomerization of β-carotene in n-hexane, starting from the all-trans, 7-cis, 9-cis, 13-cis, and 15-cis isomers, were analyzed by HPLC.Direct photoisomerization (excitation at 488 and 337 nm) was also examined for comparison.Three different isomerization patterns were found in both triplet-sensitized and thermal isomerization: pattern A, cis to trans isomerization around each cis bond; pattern B, trans to cis isomerization in the central part of the conjugated chain; and pattern C, cis to another cis isomerization.In the T₁ state, the pattern A isomerization was predominant even for the peripheral-cis (7-cis and 9-cis) isomers and its efficiency was extremly high for the central-cis (13-cis and 15-cis) isomers.In the S₀ state, the pattern B isomerization, instead, was predominant for the peripheral-cis isomers, and the pattern A isomerization was predominant only for the central-cis isomers.The quantum yields of triplet-sensitized isomerization (decrease of the starting isomer per triplet species generated) were determined to be as follows: all-trans, 0.04; 7-cis, 0.12; 9-cis, 0.15; 13-cis, 0.87; and 15-cis, 0.98.In direct photoisomerization, the quantum yield of isomerization at 488-nnm (337 nm) excitation was 4 (3) orders of magnitude lower than the above values, the relative values among the isomers being similar to the above.Further, the overall isomerization patterns of direct photoexcitation were similar to those of triplet-sensitized isomerization, supporting the idea that isomerization takes place via the T₁ state even in the case of direct photoexcitation.Carbon-carbon ? bond orders of model polyenes in the T₁ and S₀ states were calculated by using the Pariser-Parr-Pople CI theorie; bond lengths were optimized by using a bond order-bond length relationship.Isomerization characteristics in the T₁ and S₀ states observed were discussed based on the results of the calculations.