Potochemical E⇔Z isomerization of urocanic acid and methyl urocanate

Abstract

The E$\rightleftharpoons$Z photoisomerization of urocanic acid (UA) and methyl urocanate (MU) is studied. The isomerization of UA in aqueous solution was efficient ($\Phi_{E\to{Z}}=0.45,\;\Phi_{Z\to{E}}=0.36$), but surprisingly, that of MU in absolute ethanol was inefficient (0.11, 0.14 respectively). The triplet photosensitized isomerization of UA and MU is very effective and the low energy triplet quenchers fail to impede the isomerization. In addition, loss of MU is negligible in direct photolysis, but MU disappeared gradually in the photosensitized reaction. Thus other than isomerization occur in the triplet state or the sensitizer photoreacts with MU. From the results of sensitization and quenching studies, the triplet energy of MU is estimated to be 54.3 kcal/mol or less, and E$\rightleftharpoons$Z photoisomerization of MU and UA proceeds through a singlet excited state or via a very short-lived triplet state.