Optical Anisotropy of Nematic Compounds

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Abstract
The calcn. of the principal polarizabilities of nematic compds. with ref. to the 2 approaches using either the Vuks formulae or the Neugebauer's relations is discussed. The polarizabilities and internal field consts. were calcd. from the birefringence data in the nematic phase of 7 compds. viz., (1) p-azoxyanisole, (2) p-azoxyphenetole, (3) anisaldazine, (4) p-methoxybenzylidene-p-butylaniline, (5) 4-n-butyl-4'-ethoxy-α-chlorostilbene, (6) 4-n-octyl-4'-ethoxy-α-chlorostilbene and (7) mixt. of the isomers of 4-methoxy-4'-butylazoxybenzene. The results support the validity of the Neugebauer's relations rather than the Vuks formulae. The isotropic polarizability and the polarizability parallel to the long axis of the mols. are accounted for on the basis that, in the case of the conjugated bonds immediately on either side and in between the benzene rings, the resp. bond polarizabilities have values which correspond to their double bond polarizabilities and that the ratio of the bond polarizability transverse to the bond to the bond polarizability parallel to the bond is zero. The justification for the assumptions is to be sought for in the great mobility of the electron clouds of � bonds in conjugated systems. Also, it emerges that the values of the parallel polarizability given by the Vuks formulae are too large to be accounted for in terms of bond polarizability data. An extrapolation procedure is also discussed for obtaining the parallel polarizability from the effective polarizabilities αe and α0 of the nematic phase. The reported diamagnetic anisotropies in the nematic phase of the above compds. are consistent with their optical anisotropy calcd. from Neugebauer's relations. Order parameters are also calcd. for anisaldazine and p-methoxybenzylidene-p-butylaniline. The inherent limitations with regard to obtaining accurate values of the order parameter from birefringence data are discussed. [on SciFinder(R)

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