Cis–trans isomerization mechanism of 4-aminoazobenzene in the S0 and S1 states: A CASSCF and DFT study

The isomerization mechanism of the ground-state and the first single excited-state (S0 and S1) for 4-aminoazobenzene (4-AAB) was studied on the basis of the DFT-B3LYP and CASSCF calculations. The results indicate that the thermal cis–trans (E ↔ Z) isomerization of 4-AAB mainly concerns the inversion mechanism, while the photoisomerization on the S1 state involves the rotational pathway that is characterized by the S1/S0 conical intersection with a twisting structure. By means of the investigation on the geometrical parameters, it was found that the relaxation modes, which determine the decay dynamics of the excited states, are different for the E- and Z-isomers upon excited in the S1 state. Moreover, the S1 potential energy surface of 4-AAB is found to be similar to that of parent azobenzene. During the decay process of excited E-isomer through the rotation of the NN bond, however, the activation energy barrier of 4-AAB is lower than that of azobenzene, which suggests that the corresponding decay lifetime of S1 excited E-4-AAB is smaller than that of E-azobenzene.