Synthesis and properties of water-soluble squaraine oligomers containing pendant propanesulfonate moieties

Abstract

Polycondensation of squaric acid with a water-soluble pyrrole derivative containing an N-propanesulfonate pendant group gave a water-soluble A-B type oligomer. Similarly, several water-soluble random oligomers were prepared by the polycondensation of pyrrole, sodium 3-(pyrrol-1-yl)propanesulfonate, and squaric acid. Molecular weight analysis of the new oligomers showed a low degree of polymerization (M_n ~ 1800-2000) due to the premature precipitation of the condensation products in the reaction medium. Comparison of the IR and ¹H NMR spectral data of the new oligomers with those of a model squaraine dye, 9, revealed the presence of zwitterionic squaraine repeat units. The negative solvatochromic property of the polycondensation products and the model compound 9 was analogous to that of several reported squaraine dyes, indicating their structural similarities. Comparison of the solvatochromic properties of the polycondensation products with that of the model dye 9 revealed the oligomeric nature of the polycondensation products. All new oligomers showed enhanced conductivities without external doping when compared to an analogous material without pendant propanesulfonate groups. For example, the conductivity of the condensation product of squaric acid with pyrrole was 1.1 × 10^-6 S/cm, whereas that of the condensation product of squaric acid and sodium 3-(pyrrol-1-yl)propanesulfonate was 3.2 × 10^-5 S/cm. The enhanced conductivities of the new squaraine oligomers could be due to enhanced conjugation or due to intramolecular doping by the pendant propanesulfonate groups.