Pinacyanol chloride forms mesoscopic H- and J-aggregates in aqueous solution--a spectroscopic and cryo-transmission...

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 ∼511 nm. The aggregates possess a tubular architecture with a single-layer wall thickness of ∼2.5 nm and an outer diameter of ∼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 ∼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.