Photochemistry of azobenzene

When the azobenzene molecule is illuminated, one of the rings rotates (figure 4 ) to avoid steric repulsion if its π cloud is placed opposite the other aromatic ring. This arrangement of aromatic rings is also observed in the proton nuclear magnetic resonance (1H NMR) spectrum. NMR signals from the cis isomer appear with a stronger field than NMR signals corresponding to the trans isomer, due to the anisotropic effect of the π cloud of the aromatic ring. The UV-visible absorption spectrum of azobenzene shows two absorption band features corresponding to the π → π* and n → π* electronic transitions