Solubility of thioindigo dopants in a smectic liquid crystal host evaluated by H-2 NMR spectroscopy

The solubility of two partially deuterated thioindigo dopants in a smectic liquid crystal host was evaluated by variable temperature H-2 NMR spectrosc opy and polarized microscopy. H-2 NMR spectra showed that the dopant (+/-)- 6,6'-bis(2-octyloxy)-5,5'-dinitrothioindigo-d(6) forms a homogeneous soluti on with the smectic phases of the liquid crystal host (+/-)-4-(4-methylhexy loxy)phenyl 4-decyloxybenzoate (PhB) up to its saturation point of 3 mol %. These results are consistent with polarized microscopy observations of the dopant crystallizing out of solution upon reaching a concentration of 3 mo l %. On the other hand, H-2 NMR spectra of (+/-)-5,5'-dichloro-6,6'-bis(2-o ctyloxy)thioindigo-d(6) dissolved in PhB showed evidence of a partitioning of the solution between smectic and isotropic microdomains, which increases with increasing dopant concentration-from 1.2 to 9.1 mol %. To a large ext ent, this smectic/isotropic microphase separation could not be detected by polarized microscopy. These results suggest that H-2 NMR spectroscopy can p rovide a more accurate determination of the occurrence and extent of microp hase separation in doped liquid crystal samples.