J-AGGREGATES AND LIQUID-CRYSTAL STRUCTURES OF CYANINE DYES

Low-angle X-ray diffraction and NMR measurements have been carried out on liquid crystals formed by four cyanine dyes dissolved in water. Th ree of the dyes form layer (smectic/lamellar-type) phases, while the f ourth forms columnar nematic/hexagonal phases. The NMR measurements of water ((H2O)-H-2) quadrupole splittings give information on the phase equilibria, while X-ray diffraction has been employed to determine th e long-range structure. Both the columnar and the layer phases can for m at solute concentrations as low as 0.1 wt.% of dye. The dye aggregat es are significantly more rigid than surfactant micelles. They self-as sociate via sigma-pi interactions between adjacent molecules rather th an the hydrophobic effect. Using NMR, the layer phases were found to e xist to higher temperatures in the presence of added electrolytes, whi le the columnar phases are known to be unaltered or destabilised. The two-phase solution/mesophase co-existence regions are much larger for the layer phases than for the columnar phases. In these respects the d ye mesophases resemble amphiphilic ones. It appears that a simple aggr egation model originally developed for different surfactant micelle sh apes can be applied to describe dye aggregates. One dye shows unusual NMR behaviour, with time-dependent and overlapping spectra. This indic ates the coexistence of two mesophases in a manner which contradicts t he phase rule. Possible explanations of this phenomenon are discussed.