LIQUID-CRYSTALLINE J-AGGREGATES FORMED BY AQUEOUS IONIC CYANINE DYES

NMR spectroscopy (H-2 and H-1), polarized-light optical microscopy, an d synchrotron X-ray diffraction have been used to determine the liquid -crystal structures (mesophases) formed at room temperature by four an ionic cyanine dyes in water. A sharp, intense, bathochromically-shifte d absorption band appeared in the UV-visible absorption spectra on inc reasing dye concentration for all the dyes, demonstrating that so-call ed J-aggregates are formed. The onset of J-aggregate formation is prac tically concomitant with the occurrence of mesophases. In some instanc es, these mesophases form. at much less than 0.1% w/w. X-ray diffracti on shows that three of the dyes form layer (lamellar) phases while one forms columnar nematic and hexagonal phases, These structures are con sistent with the textures observed by optical microscopy. Unusually, t he columns have a multimolecular cross section rather than being unimo lecular. We propose a novel ''hollow pipe'' structure for these aggreg ates. The liquid-crystal type appears to be largely dependent on the p recise molecular structure of the dye, presumably due to the short-ran ge intermolecular interactions (electrostatic, steric, and van der Waa ls). Previously postulated stacking geometries for low-dimensional J-a ggregates are compared to the liquid-crystal structures. Since the J-a ggregates form a separate liquid-crystalline phase they consist of tho usands of molecules or more, rather than the small aggregation numbers deduced by mass action considerations.