Optical spectroscopy of ordered dyes in aqueous lyotropic nematic liquid crystals

Aqueous lyotropic nematic liquid crystals can be uniformly ordered by magnetic fields of ∼20 kG in flat glass capillaries up to ∼1 mm in path length. Once removed from the field the orientation decays very slowly, and optical spectra of the oriented samples can easily be recorded in conventional instruments. In this manner, the visible spectra of a substituted porphine (TPPS4) and the dye Congo Red were recorded in three nematic lyotropics which consist of rod-shaped micelles. The polarization of the absorption bands can easily be assigned. TPPS4 orients with its plane perpendicular to the optic axis of the lyotropic rods with a low order parameter, and the long axis of Congo Red also orients perpendicular to the optic axis with a considerably higher order parameter. In the nematic system (potassium laurate/decanol/D2O) addition of TPPS4 at a sufficiently high concentration stabilizes a disk-like nematic phase.