Structure of J-Aggregates of Pseudoisocyanine Dye in Aqueous Solution

Abstract

Cryo-transmission electron microscopy (cryo-TEM), absorption spectroscopy with polarized light, and polarized-light optical microscopy have been used to characterize the J-aggregates formed by the dye 1,1‘-diethyl-2,2‘-cyanine chloride (pseudoisocyanine chloride) in aqueous solution. Cryo-TEM visualizes 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 shows that J-aggregation is a strong function of dye concentration and starts in dilute solution before the viscosity increases and mesophases appear. For a 12.5 × 10-3 mol solution, the length of the J-aggregates is on the order of 350 nm, which corresponds to aggregation numbers of ≈3000. Optical microscope textures reveal columnar nematic and hexagonal phases at higher dye concentrations. Structural alternatives for the molecular packing within the J-aggregate based on the estimated rod geometry are discussed. A quasi-two-dimensional superstructure is proposed which could better explain the optical properties of the J-aggregates than previous models.