The three investigated azobenzenes differ in the substituents in para positions with respect to the carbons bound to the azo (N --N) group: AZO-1 has the electron-donating hydroxy group ( -OH) on one of the phenyl rings, whereas AZO-2 has an electron-donating amino group ( -NH 2 ) in the same position; AZO-3 contains both the electron-donating amino group on one ring and the electron-withdrawing nitro group ( -NO 2 ) on the other ring. AZO-1 and AZO-2 are sometimes referred to as electron-donating azobenzenes; AZO-3 is a prototypical push-pull system (electron donor-acceptor azobenzene).