Azobenzenes potential energy curves

Fig. 6. Left schematic potential energy curves explaining the absorption transients seen in the newly formed trans-absorption bands. The strain between the peptide part and the azobenzene chromophore delays the transition to the relaxed ground state and causes the blue-shift observed as a function of time. Right View of the two phases of peptide motion occurring after the initial isomerzation of the azobenzene chromophore.

The use of Walsh diagrams, based on one-electron molecular orbitals, shows that on n —> 7t excitation the azobenzene molecule is stretched, which is the beginning of inversion. All calculations and suggestions for an inversion mechanism agree that the potential energy curve for inversion has a relatively steep slope at the E- and the Z- geometries. This is corroborated by the experimental evidence of a continuous n tc absorption band in both isomers. In fact, a structured n band in an azo compound that can isomerize has never been observed. 

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