The aggregation behaviour of the cationic pinacyanol chloride in aqueous
solution is investigated using absorption and linear dichroism spectroscopies,
optical microscopy and cryogenic transmission electron microscopy (cryo-TEM).
The investigations are focused on solutions in a concentration range from 50
μM up to 1 mM. At a concentration of 0.7 mM H-aggregates are detected that are
characterized by a broad absorption band centred at [similar]511 nm. The
aggregates possess a tubular architecture with a single-layer wall thickness
of [similar]2.5 nm and an outer diameter of [similar]6.5 nm. Linear dichroism
spectroscopy indicates that the molecules are packed with their long axis
parallel to the tube axis. These H-aggregates are not stable, but transform
into J-aggregates on the time scale of weeks. The kinetics of J-aggregation
depends on the dye concentration and the route of sample preparation, but can
also be enhanced by shear stress. J-aggregates possess a split absorption
spectrum composed of two longitudinally polarized J-bands and one H-band that
is polarized perpendicular to the aggregate axis. The J-aggregates are
[similar]9 nm wide and several micrometer long fibrils consisting of stacked
pairs of ribbons with a dumbbell-shaped density cross-section. Upon aging
these ribbons laterally stack face-to-face to form tape-like aggregates
