Aggregation of pseudoisocyanine iodide in cellulose acetate films: Structural characterization by FTIR

Cellulose acetate films dyed with pseudoisocyanine iodide (1,1'-diethyl-2,2 '-cyanine iodide) have been produced by spin coating and their structures c haracterized by FTIR spectroscopy. The aggregation of the cyanine during th e spinning process was induced by addition of KI to the precursor solution, and the formation of J-aggregates was observed by W-vis spectroscopy. The detailed analysis of the O-H stretching mode of cellulose acetate allowed u s to understand the types of hydrogen bonds existing in the pure matrix fil ms, in films containing just cyanine monomers and J-aggregates as well. It has been shown that: cyanine monomers, even in a large concentration, have a small influence on the cellulose acetate structure, by favoring the repla cement of some intermolecular by intramolecular hydrogen bonds. On the cont rary, the presence of cyanine J-aggregates remarkably modifies the arrangem ent of the polymer chains, inducing an extensive formation of intermolecula r hydrogen bonds in the C-2, C-3, and/or C-6 positions of the glucopyranose rings. These intermolecular bonds do not involve the carbonyl groups, as t he C=O stretching mode is not affected. This effect has been interpreted in terms of a higher degree of packing of the cellulose acetate amorphous pha se, due to the presence of J-aggregates. The infrared spectra of the cyanin e, in the wavenumber windows where the matrix does not absorb, have shown t hat the pseudoisocyanine molecule keeps the all-trans conformation upon agg regation. The modifications in the quinolines out of plane C-H deformations have indicated that the J-aggregates are formed by overlap of the phenyl r ings of neighbor cyanines, leaving the heterocyclic moieties unperturbed.