A density functional theory study on the thermal and photochemical isomerization mechanism of 4,4′-azobenzene disulfonate

The isomerization pathways of 4,4′-azobenzene disulfonate in the S0 and T1 states have been studied by using density functional theory (DFT) with B3LYP method at the levels of 6-31G(d,p) and 6-311++G(d,p), respectively. There are two isomerization pathways in the S0 state. One is the inversion of one CNN angle combined with certain degree of rotation around the CN bond, and it is worthy to notice that the potential energy profile includes three sequential transition states. The other pathway is the rotation of CNNC dihedral angle involved inversion of one CNN angle, while the two isomers are connected through the only one transition state. Calculation indicates that the molecular structures at the highest points on the potential energy profiles of two pathways are identical, and the energy barriers are the same, 20.52 kcal/mol. In the T1 state, there exists the rotation pathway (rotation of CNNC dihedral angle) and its energy barrier is 4.11 kcal/mol. In the excited states (T1, S1, T2, and S2), the potential energy profiles of the vertical excitation are obtained by time dependent density functional theory (TD-DFT) at the B3LYP/6-311++G(d,p) level. The photoexcitation at 342 nm results in the reactant molecule populated in the S2 state, but isomerization does not occur directly on the S2 state due to the high energy barrier. It could undergo a rapid relaxation to the minimum of S1 state, and then the isomerization occurs using the inversion or rotation pathway. The results show that the rapid energy redistribution among the various vibrations renders the concentration of such amounts of energy in the inversion coordinate very improbable, while there are two possible photoisomerization pathways by the rotation pathway. The isomerization can easily occur through the S0/S1 conical intersection and the S0–T1–S0 crossing to reach the product. The primary isomerization pathways for 4,4′-azobenzene disulfonate can go through the inversion and rotation forms in the S0 state and the rotation mechanism in the excited state.