Abstract:

Photoisomerization pathways for 3,3'-azobenzene sulfonate (3,3'-AbS) in the S0 and T1 states were studied by using density functional theory (DFT) at the B3LYP/6-311++G(d,p) level. In the S0 state, there are two isomerization pathways: the inversion of one NNC angle combined with the rotation around the NC bond and the rotation of the CNNC dihedral angle. The energy barriers of the two pathways are 94.2 and 124.3 kJ·mol-1, respectively. It is worthy to notice that there exist second-order transition states on the combination pathways of inversion and rotation. In the T1 state, only the rotation pathway exists and its energy barrier is 21.1 kJ·mol-1. To investigate the photoisomerization pathway, the potential energy profiles of the vertical excitation for the excited states (T1, S1, T2, and S2) were calculated by time dependent density functional theory (TD-DFT) at the B3LYP/6-311++G(d,p) level along the two pathways. A photoexcitation at 330 nm results in the reactant molecule populating the S2 state and then undergoing a rapid relaxation to the minimum of the S1 state. Two possible isomerization pathways exist through the rotation pathway as follows: (1) the isomerization can easily occur through the S0/S1 conical intersection (CI) and descend to the S0 state; (2) a relaxation to the T1 state from the S1 state may occur and the reaction can take place via the S0-T1-S0 path. Calculation results show that the primary isomerization pathways for 3,3'-AbS are a combination pathway of inversion and rotation in the S0 state and the rotation pathway when it is excited.

Key words: Density fuctional theory, 3,3’-azobenzene sulphonate, Photoisomerization mechanism, Excited state, Potential energy profile