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Optical Anisotropy of Nematic Compounds

1974; Volume: 28; Issue: 1-2 Linguagem: Inglês

10.1080/15421407408083165

0026-8941

H. S. Subramhanyam, C. S. Prabha, D. Krishnamurti,

Photochemistry and Electron Transfer Studies

Abstract The calculation of the principal polarizabilities of nematic compounds with reference to the two approaches using either the Vuks formulae or the Neugebauer's relations is discussed. The polarizabilities and internal field constants have been calculated from the birefringence data in the nematic phase of seven compounds viz., (i) p-azoxyanisole, (ii) p-azoxyphene-tole, (iii) anisaldazine, (iv) p-methoxybenzylidene p-butylaniline, (v) 4-n-butyl-4 ′-ethoxy-α-chlorostilbene, (vi) 4-n-octyl-4′-etnoxy-α-chlorostilbene and (vii) mixture 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 polahzability and the polarizability α‖ parallel to the long axis of the molecules 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 respective bond polarizabilities have values which correspond to their double bond polarizabilities and that the ratio of βt (the bond polarizability transverse to the bond) to βt (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 π bonds in conjugated systems. Also, it emerges that the values of α‖ 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 α‖ from the effective polarizabilities αe and αo of the nematic phase. The reported diamagnetic ansiotropies in the nematic phase of the above compounds are found to be consistent with their optical anisotropy (αe-αo) calculated from Neugebauer's relations. Order parameters are also calculated for anisaldazine and MBBA. The paper also discusses the inherent limitations with regard to obtaining accurate values of the order parameter from birefringence data.