The trans-urocanic acid, a UV chromophore in the epidermis of human skin, was found to exhibit a wavelength dependent isomerization property. The isomerization quantum yield to cis-urocanic is greatest when being excited to the S-1 state, whereas exciting the molecule to the S-2 state causes almost no isomerization. The comparative photochemical behavior of the trans-urocanic on the S-1 and S-2 states continues to be the subject of intense research effort. This study is concerned with the unique photo-behavior of this interesting molecule on the S-2 state. Combining the on-the-fly surface hopping dynamics simulations and static electronic structure calculations, three decay channels were observed following excitation to the S-2 state. An overwhelming majority of the molecules decay to the S-1 state through a planar or pucker characterized minimum energy conical intersection (MECI), and then decay to the ground state along a relaxation coordinate driven by a pucker deformation of the ring. A very small fraction of molecules decay to the S-1 state by a MECI characterized by a twisting motion around the CC double bond, which continues to drive the molecule to deactivate to the ground state. The latter channel is related with the photoisomerization process, whereas the former one will only generate the original trans-form products. The present work provides a novel S-2 state decay mechanism of this molecule, which offers useful information to explain the wavelength dependent isomerization behavior. Published by AIP Publishing.
