PHOTOCHEMISTRY OF DIENONES-VIII; THE LOW TEMPERATURE PHOTOCHEMISTRY OF (Z)-β-IONONE AND ITS PHOTO-ISOMERS

Article abstract of DOI:10.1016/S0040-4020(01)92097-6

The isomerizations of (E)-β-ionone 1, and of mixtures of the isomeric pyran 2 and (Z)-β-ionone 3 in CD3OD as solvent on direct irradiation with λ 254 nm and on triplet photo-sensitization have been studied at temperatures <= -50 deg C, where the thermal isomerization between 2 and 3 is fully inhibited.The direct irradiation of 1 at -60 deg C leads to 3 and (Z)-retro-γ-ionone 4 as primary products; 3 is subsequently rapidly photo-converted into mainly 2.Evidence is presented that 4 is also a primary photoproduct from both 2 and 3.The quantum yield ratio Φ_2<*>4:Φ_3<*>4=0.50.On starting with either 1 or mixtures of 2 and 3 the same photo-stationary equilibrium ratio of 1-3 is eventually obtained, viz 1:2:3=17:72:11. 4 is photostable relative to 1-3.The perdeuterobenzophenone triplet photo-sensitization with λ 366 nm at -50 deg C of 1 leads to 3 as the sole primary product, which isomer on triplet sensitization yields both 1 and 2.The triplet sensitized conversion is much faster for (Z)- than (E)-β-ionone.On starting with either 1 or mixtures of 2 and 3, eventually the same photo-stationary state is obtained, viz 1:2:3=39:46:15. (Z)-retro-γ-ionone 4 is not formed in the triplet sensitized irradiations of 1, 2 and 3 and in the direct irradiation it apparently results from the singlet excited state of the three substrates.The UV spectrum of the (unstable) (Z)-β-ionone 3 has been indirectly determined; its absorption occurs at lower wavelength and is of lower intensity than that of the (E)-isomer 1.