Fig. 3. CASPT2 potential energies of azobenzene (“azo”) and diazocine (“diazo”) in their S0, T1 and S1 states as a function of the CNNC dihedral angle. Vertical transitions (S0 → T1) are indicated with arrows (filled for diazocine). Energies of azobenzene and diazocine are relative to their most stable ground state (S0) isomers (E azobenzene and Z diazocine). Note that the CNNC torsion in Z azobenzene is ∼12° because of the non-planarity caused by steric hindrance of the two phenyl rings. The CNNC torsion in E diazocine deviates strongly from 180° (147°) due to the large strain in the trans configured eight-membered diazocine ring. Energy scans, therefore, are restricted to the range of twist angles between the corresponding ground state minima (E and Z).
