The role of radicals in the phototransformations of β-ionone in the presence of electron donors in solution

Formation of radical anions of trans-β-ionone (I) has been detected during its photolysis in acetonitrile in the presence of the electron donors triphenylphosphine (TPP) and triphenylamine (TPA) by using chemically induced dynamic nuclear polarization (CIDNP). Transformation of radical anions of (E)-β-ionone (I) to neutral radicals by protonation has also been observed, as has the participation of these paramagnetic particles: radical anions and neutral free radicals in the processes of geometrical isomerization of trans- to cis-(Z)-β-ionone (III) in the presence of TPA. The isomerization of I occurs in the triplet excited state (3I→I+III), or in the free radical of I (IH↔IIIH). The triplet excited state of I results from back electron transfer in the triplet radical ion pair of radical ions of β-ionone and triphenylamine.