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, S ₂, 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 C=C 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.