Noboru Ando

Assignment of electronic bands of some benzidines by dichroism

Abstract The dichroic spectra in the stretched polyvinyl alcohol (PVA) sheet of benzidine, 3,3′-dichlorobenzidine, o -tolidine, and dianisidine were analyzed in the region above 220 mμ. The first electronic band appearing at around 310 mμ in the neutral state corresponds to the first band of biphenyl (ca. 250 mμ), and its transition moment is directed along the long axis of the molecule. At the shorter wavelengths of the first band a weak absorption (ca. 270 mμ) is hidden, the transition moment of which is directed along the short axis of the molecule and is attributed to the 1 L b transition of benzene. But in the case of dianisidine, the corresponding band appears at about 290 mμ as a shoulder. The intense band below 220 mμ, of which only the tail was observable, has the same polarization as that of the hidden band and is assigned to the 1 B b transition of benzene. All samples in the acid state have the first band with the longitudinal polarization at about 250 mμ and the second band with the latitudinal below 220 mμ. Besides, dianisidine in the acid state has another band at 292 mμ with the polarization slanting against the molecular axis. This may have originated from the 1 L b transition of benzene which has become intensified and has changed its polarization due to the perturbation of the methoxy groups.