Photochemical mechanisms of radiationless deactivation processes in urocanic acid

Urocanic acid is a UV filter found in human skin that protects the skin from UV damage but has also been linked to the onset of skin cancer and to photoimmunosuppression. We report on ab initio investigations of two rotameric forms of each of the two tautomers of neutral (E)- and (Z)-urocanic acid. We have computed the vertical singlet excitation energies of eight isomers and have explored the singlet excited-state reaction paths of several photochemical processes for radiationless excited-state deactivation: the E/Z photoisomerization, an electron-driven proton transfer for an intramolecularly hydrogen-bonded Z isomer, as well as the hydrogen-atom detachment process and the ring-puckering process involving the NH group inherent to the imidazole moiety. We have optimized the S₁/S₀ conical intersections for each of these processes and located additional ππ/nπ* conical intersections. Because of the reversed energetic order of the nπ* and ππ* states in the N3H and N1H tautomers, an energy window exists where the N3H tautomers can be excited to the nπ* state, from which only the photoisomerization process is accessible, while the N1H tautomers can be excited to the ππ* state, from which several deexcitation processes compete from the onset of the absorption. These results explain the unusual dependence of the quantum yield for E→Z photoisomerization on the excitation wavelength. The present work provides novel insight into the complex photochemistry of this biomolecule and paves the way for future computational studies of the photoinduced excited-state dynamics of urocanic acid.