Laser-induced in vitro isomerization of urocanic acid in UVA region and the origin of excited triplet state

We demonstrate here unambiguously that (E)-urocanic acid, an epidermal and photobiologically important chromophore, undergoes isomerization to the (Z) isomer upon excitation with UVA (320–400 nm) light using monochromatic 355 and 340 nm laser radiation. Additionally, chemical evidence is presented that supports the isomerization coming from the excited singlet state, thus proving unequivocally that the previously observed triplet state is populated from the excited singlet manifold via intersystem crossing rather a weak S0→T1 transition.