The rise of digital photography has brought the major photography companies to attenuate the degree of research and development of new dyes for classic color photography. A genuine concern for the loss of the established knowledge base prompted to study a lyotropic chromonic liquid crystal that was intensively studied by the photography industry R&D, namely, Sunset Yellow (SY) or Yellow #6. SY is a dianionic azo dye which forms an approx. 20 unit aggregate and a lyotropic nematic LC when solvated in water (1,2). Our concentration gradient cells reveal a columnar phase in adition to the previously reported nematic phase, which are both enantio-thermotropic.

Lyotrpic and isotrpic system of SY in D2O were prepared in order to study them by NMR. The 2H-NMR spectrum of the nematic phase shows a high dipolar coupling of the D2O signal, which is indicative of a very high order parameter. In order to get more insight about the nematic aggregates, we studied a solution that is just shy of the concentration needed for a nematic phase by NMR. Diffusion NMR measurements on a series of concentrations of the same ionic strength show that below the I-N phase transition concentration, SY forms only a dimer (i.e, in the most concentrated isotropic solution the size of SY is twice as large as the most dilute sample). It is interesting to note that without normalizing the ionic strength, the size of the aggregate in the concentrated solution was 22 times bigger compared to the size of SY in the dilute solution.

NOESY correlations are compatible with three possible π stacking conformations that all exist in equilibrium. The barrier for aryl rotation is lower than the threshold for detection by dynamic NMR methods, implying that in the isotropic solution the dimer is most likely a solvent separated ionic sandwich.

(1) V. R. Horowitz, L. A. Janowitz, A. L. Modic, P. J. Heiney, and P. J. Collings, 2005, Phys. Rew. E 72, 041710 (2) R. Meyer - private communication