Steric Effects in Azo Compounds. The Electric Dipole Moments and the Absorption Spectra of Azobenzene Derivatives

The electric dipole moments of trans-4-N,N-dimethylaminoazobenzene and its derivatives, and the absorption spectra of azobenzene derivatives and their conjugate acids have been measured. The dipole moment and the intensity of the π–π* band are clearly reduced by introduction of methyl groups into ortho positions to azo group. The twisting angle of the benzene rings with respect to the azo group has been estimated for 2,2′-disubstituted azobenzenes and their conjugate acids by means of the Braude equation. From the spectral intensity and transition energy, it was concluded that the conjugate acids are generally more susceptible than the bases to the steric effect.