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

► Two sulfonate-substituted amphiphilic cyanines form mixed J-aggregates in water. ► Absorption spectrum is not a superposition of pure component spectra. ► Cryo-TEM reveals 13 nm thin bilayered nanotubes with helical ultrastructure. ► The spatial structure of aggregates is determined by 3D-reconstruction techniques.