Mixtures of achiral amphiphilic cyanine dyes form helical tubular J-aggregates

Abstract

The amphiphilic 3,3´-bis(2-sulfoethyl) and 3,3´-bis(2-sulfopropyl) substituted 5,5´,6,6´-tetrachlorobenzimidacarbocyanine dyes (C8S2 and C8S3) self-aggregate in aqueous solution to form mixed J-aggregates. The individual dyes form planar (C8S2) or tubular (C8S3) aggregates, while the mixed solution forms tubular aggregates that are morphologically identical to those obtained from the pure C8S3. The optical spectra however differ significantly indicating a different structure on the molecular length scale. The structure and morphology is independent of the mixing ratio if the content of C8S2 is above 10%. The structure of the mixed C8S3/C8S2 aggregates is investigated in detail by means of cryogenic transmission electron microscopy (cryo-TEM) in combination with a 3D-image reconstruction technique, which yields a structure with a spatial resolution of ∼2 nm. A comparison with the 3D structure of the tubular aggregates of C8S3 could not resolve any differences. Any structural difference that could describe the spectroscopic difference of the two types of tubular aggregates must be below this resolution limit.