CHEMISTRY OF PYRROLE PIGMENTS .29. ANAEROBIC PHOTOCHEMISTRY OF BILE-PIGMENTS - MECHANISM OF THE PHOTOISOMERIZATION AT EXOCYCLIC DOUBLE-BONDS

Studies of heavy atom effects as well as sensitization and quenching experiments with several model compounds suggested that in pyrromethenones and biladienes-ac the photoisomerization at exocyclic double bonds proceeds predominantly via the singlet path on direct irradiation. The energy of the triplet state of pyrromethenones and arylmethylenepyrrolinones was estimated by sensitization experiments to be in the region from 140 to 160 kJ/mol. On sensitization the reaction follows the triplet path. With bilatrienes-abc the situation is complicated by the fact that in homogenous phase there is only unidirectional isomerization but there is evidence that the direct process proceeds also via the singlet path. These experiments are supported by calculations using the Formosinho theory of radiationless deexcitation. © 1979 Springer-Verlag.