On the aggregation behavior of pseudoisocyanine chloride in aqueous solution as probed by UV/vis spectroscopy and static light scattering

In this work. the aggregation behavior of the cyanine dye, pseudoisocyanine chloride, in aqueous solutions both in the absence and presence of added e lectrolyte (0.01 M NaCl) was studied by a combination of UV/ vis spectrosco py and static light scattering (SLS). In the case of pure aqueous dyestuff solutions, an apparent aggregation number of N-app similar to 3 was found. A correction of the apparent molecular weights, M-w,M-app, for the amount o f the still present residual monomers gave only slightly larger values (N s imilar to 4). However, it could be shown that, in the presence of 0.01 M Na Cl at low dyestuff concentration (0.75 x 10(-3) mol L-1), the absorption sp ectrum shows the typical features of what is commonly believed to represent a dimer spectrum, whereas the corresponding SLS data unequivocally prove t he existence of larger aggregates, consisting of at least 30 molecules. At higher dyestuff concentrations (similar to2 x 10-3 mol L-1) and in the pres ence of salt, apparent aggregation numbers of similar to 1000 (uncorrected) and similar to 2000 (corrected for cm) were obtained. A particle form fact or analysis of the SLS data by means of the Koyama approximation revealed t ypical wormlike aggregate structures for the higher concentration solutions . Additionally, the linear mass densities could be obtained at large scatte ring vectors and from those the number of molecules per unit length. They r ange from 1 to 2 nm(-1) (uncorrected) and 2 to 6 nm(-1) (corrected for cm) and are consistent with a bundle formation of single aggregated strands. Th is is in agreement with recent cryo-transmission electron microscopy result s obtained for the same compound.