Study of cis-trans isomerization mechanism of 3,3'-azobenzene disulphonate in the lowest singlet and triplet electronic states by density functional theory

The cis-trans isomerization pathways of 3,3'-azobenzene disulphonate in the S-0 and T-1 states are studied by DFT method at the B3LYP/6-31G(d,p) level. In the S-0 state, the cis-trans isomerization concerns the complex pathway that is characterized by the inversion of one NNC angle combined with rotation around the NC bond, and the three sequential transition states are also found on the potential energy profile. Therefore, the cis-trans isomerization of 3,3'-azobenzene disulphonate can be understood in terms of a pathway involving successive rotation, inversion, and rotation processes. The energy barrier of the S-0 state is 22.79 kcal mol(-1). In the T-1 state, the isomerization mainly concerns the rotational pathway around the NN double bond, and the two isomers are connected through only one transition state. The isomerization of the T-1 state is related to a lower energy barrier, 5.02 kcal mol(-1), but requires a change in spin-multiplicity.