Electronic and vibrational linear and circular dichroism of nematic and cholesteric systems

Abstract

The vibrational and electronic linear dichroism of oriented nematic p-n-butyl-N-(p-methoxybenzylidene)aniline (MBBA) and of the azoxy analogue (MBAOB) are reported together with the corresponding circular dichroism of cholesteric plane textures formed by the addition of 2 mole % of (–) menthol to MBBA or of (+)-carvone to MBAOB. The induced c.d. absorptions of perylene and tungsten hexacarbonyl in cholesteric solvents have been measured. The c.d. absorption of cholesteric systems is shown to arise from two primary sources; first the differential radiation field strength of left and right circularly polarised light in circularly birefrigent media, and secondly the linear dichroism of oriented anisotropic solute or solvent molecules. The first effect alone gives rise to the c.d. absorption of isotropic or randomly-oriented solute species in a cholesteric solvent.