Structure of J-aggregates of pseudoisocyanine dye in aqueous solution

Cryo-transmission electron microscopy (cryo-TEM), absorption spectroscopy w ith polarized light, and polarized-light optical microscopy have been used to characterize the J-aggregates formed by the dye 1,1'-diethyl-2,2'-cyanin e chloride (pseudoisocyanine chloride) in aqueous solution. Cryo-TEM visual izes for the first time directly the rodlike morphology of the J-aggregates . A rod diameter of 2.3 +/- 0.2 nm is estimated. Absorption spectroscopy sh ows that J-aggregation is a strong function of dye concentration and starts in dilute solution before the viscosity increases and mesophases appear. F or a 12.5 x 10(-3) mol solution, the length of the J-aggregates is on the o rder of 350 nm, which corresponds to aggregation numbers of approximate to 3000. Optical microscope textures reveal columnar nematic and hexagonal pha ses at higher dye concentrations. Structural alternatives for the molecular packing within the J-aggregate based on the estimated rod geometry are dis cussed. A quasi-two-dimensional superstructure is proposed which could bett er explain the optical properties of the J-aggregates than previous models.