Large Assembly Formation via a Two-Step Process in a Chromonic Liquid Crystal

IR-806 is a near-infrared cyanine dye that forms assemblies in aqueous solutions which in turn orientationally order into a liquid crystal phase at concentrations as low as 0.5 wt %. Unlike many chromonic liquid crystals, the absorption spectrum of IR-806 changes dramatically with concentration, showing an isodesmic assembly process at lower :concentrations followed by a second process at higher concentration that is not isodesmic. The lower concentration assembly process is characterized by a free energy change per molecule of about 9 k(B)T, not unlike other chromonic systems. However, X-ray scattering measurements suggest that the assemblies that form during the higher concentration process are much larger than what is observed for many chromonic liquid crystals. Although there is a transitional region between the liquid crystal and isotropic phases of 10-15 degrees C, unlike most chromonic liquid crystals, no biphasic region is observed using polarizing microscopy.