Ab initio study on the structures and properties of trans-p-coumaric acid in low-lying electronic states

The five lowest-lying electronic states of trans-p-coumaric acid have been studied by using the complete active space self-consistent field and multi-reference configuration interaction methods. The adiabatic excitation energies from the ground state to the S1, T1, S2, and T2 states have been determined in the present study. On the basis of the optimized structures, the calculated energies, and analysis of molecular orbitals, the S1 and S2 states were assigned to 1nπ* and 1ππ* in nature, respectively, which are different from the assignment in previous work. The T1 and T2 states, which have not been investigated before, were characterized as 3ππ* localized in the CC double bond and the aromatic ring regions, respectively. Photoisomerization dynamics of trans-p-CA should be re-investigated, due to controversy over the character of the excited electronic states.