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 characterized by FTIR spectroscopy. The aggregation of the cyanine during the 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 us to understand the types of hydrogen bonds existing in the pure matrix films, 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 replacement of some intermolecular by intramolecular hydrogen bonds. On the contrary, the presence of cyanine J-aggregates remarkably modifies the arrangement of the polymer chains, inducing an extensive formation of intermolecular 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 the 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 phase, due to the presence of J-aggregates. The infrared spectra of the cyanine, in the wavenumber windows where the matrix does not absorb, have shown that the pseudoisocyanine molecule keeps the all-trans conformation upon aggregation. The modifications in the quinolines out of plane C-H deformations have indicated that the J-aggregates are formed by overlap of the phenyl rings of neighbor cyanines, leaving the heterocyclic moieties unperturbed.