J-aggregates and liquid crystal structures of cyanine dyes

Abstract

Low-angle X-ray diffraction and NMR measurements have been carried out on liquid crystals formed by four cyanine dyes dissolved in water. Three of the dyes form layer (smectic/lamellar-type) phases, while the fourth forms columnar nematic/hexagonal phases. The NMR measurements of water (²H₂O) quadrupole splittings give information on the phase equilibria, while X-ray diffraction has been employed to determine the long-range structure. Both the columnar and the layer phases can form at solute concentrations as low as 0.1 wt.% of dye. The dye aggregates are significantly more rigid than surfactant micelles. They self-associate via σ–π interactions between adjacent molecules rather than the hydrophobic effect. Using NMR, the layer phases were found to exist to higher temperatures in the presence of added electrolytes, while 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 dye mesophases resemble amphiphilic ones. It appears that a simple aggregation model originally developed for different surfactant micelle shapes can be applied to describe dye aggregates. One dye shows unusual NMR behaviour, with time-dependent and overlapping spectra. This indicates the coexistence of two mesophases in a manner which contradicts the phase rule. Possible explanations of this phenomenon are discussed.