Polarisation directions of the near ultraviolet band systems of disubstituted benzene derivatives

Abstract

The induced circular dichroism (i.c.d.) spectra of homo- and hetero-disubstituted benzene derivatives and of the monocyclic diazines in a cholesteric liquid crystal solution have been measured for radiation propagated along the helix axis of the plane texture. The polarisation direction of the ¹L_a and ¹L_b benzenoid transitions and, in the case of the diazines, of the n→¹π* transition are derived from the i.c.d. spectra through the order matrix elements which express the statistical orientation of the solute molecular axes in the mesophase solution. It is found that the ¹L_b moment near 380 nm is polarised in the direction of the longer in-plane axis in 1,2- and 1,3-homodisubstituted benzenes but is rotated through π/2 to the shorter in-plane axis for the corresponding heterodisubstituted derivatives, C₆H₄XY, with one substituent meta-directing and the other ortho–para-directing, which satisfy the induced spectroscopic moment condition, m_x[graphic omitted] –m_y. The ¹L_a moment near 230 nm similarly changes polarisation from the shorter in-plane axis in 1,2- or 1.3-C₆H₄X₂ to the longer in the corresponding C₆H₄XY with m_X [graphic omitted] –m_y. The changes in polarisation direction and other results support a vector addition model for the resultant transition moment in benzene derivatives based upon the assignment of the ¹L_b and ¹L_a bands to the transitions ¹A_1g→¹B_2u and ¹A_1g→¹B_1u, respectively, in the parent hydrocarbon. The i.c.d. spectra confirm that the n→¹π* transition of the diazines is polarised perpendicular to the molecular plane.