Network Superstructure of Pseudoisocyanine J-Aggregates in Aqueous Sodium Chloride Solution Revealed by Cryo-Transmission Electron Microscopy

Cryo-transmission electron microscopy (cryo-TEM) has been applied to characterize J -aggregates formed by the dye 1,1‘-diethyl-2,2‘-cyanine chloride (pseudoisocyanine chloride) in 200 mM sodium chloride solutions as a function of the dye concentration within the range of (2.5−6.1) × 10 -4 mol/L. Electron micrographs reveal the formation of a network superstructure consisting of isolated fibers and complex fiber bundles. Upon dilution, more of the isolated threadlike J -aggregates become visible, which are characterized by a diameter of 2.3 nm, lengths of several hundreds of nanometers and a high stiffness. Such J -aggregates already appear at room temperature at a concentration as low as 2.5 × 10 -4 mol/L. Ends of the threadlike J -aggregates can be found, but they are extremely rare, indicating a large end-cap energy. Because of their morphological appearance, the J -aggregates resemble ordinary polymers rather than “equilibrium polymers”. Concentration-dependent growth of J -aggregates cannot be proved. H -aggregates of mesoscopic size are not found.