Structural and spectroscopic characterization of E- and Z-isomers of azobenzene.

Monomers of azobenzene were isolated in argon matrices at 15 K and characterized by infrared spectroscopy and theoretical calculations. When the equilibrium vapors existing over the azobenzene crystals at room temperature were trapped in the matrix, only the thermodynamically most stable E-azobenzene was detected. In an attempt to convert E-azobenzene into the Z isomer, the matrix-isolated E-monomers were irradiated either by broad-band or narrow-band UV-visible light of different wavelengths, in the 600-200 nm range. However, no E-to-Z transformation was observed under these conditions. In an alternative experiment, E-azobenzene was irradiated by UV-visible broad-band light in the gas phase prior to trapping in a matrix. In this case, the E-to-Z photoisomerization occurred, and both E- and Z-azobenzene monomers were detected in the matrix sample. Subsequent irradiation of the matrix with narrow-band tunable visible or UV light (λ < 550 nm) resulted in back conversion of Z-azobenzene into the E-form. The observed photoinduced E-to-Z isomerizations allowed for the reliable vibrational characterization of both azobenzene isomers. The two-dimensional potential energy surfaces of Z- and E-azobenzene were explored as functions of the torsional movement of the two phenyl rings. They exhibit large flat areas around the minima, for both isomers, allowing for large-amplitude zero-point torsional vibrations. For the Z-form, these vibrations were found to be responsible for significant changes in the equilibrium NN bond length (up to 0.3 pm). This also allowed to explain the experimentally observed frequency smearing of the N=N stretching vibration in this isomer.