Aggregation of squaraine dyes: Structure–property relationships and solvent effects

In characterizing the UV–visible absorption properties of a series of seven anilino class squaraine dyes in dimethyl sulfoxide (DMSO) – water mixtures, spectral features characteristic of aggregation were observed. These included hypsochromic and hypochromic shifts of the absorption maximum, relative to the nonassociated state of these dyes. Previously aggregation of this class of squaraine dyes had only been reported in studies of Langmuir–Blodgett (LB) films prepared with squaraine dyes. In the present study two distinctly different and novel solution dye aggregates, designated as type A and type B, were identified on the basis of their characteristic absorption properties. The type A aggregate was characterized by an absorption maximum that was hypsochromically and hypochromically shifted, in comparison with that of the nonassociated dyes, and had a secondary absorption occurring at approximately the same wavelength at which nonassociated squaraine dyes absorb. These spectral features are believed to be the result of a "face-to-face" alignment of molecules in the aggregate. The type B aggregate featured a broad absorption typically from 525 to 700 nm. Based on similarities between the spectral features of type B aggregates and LB films of surfactant squaraines, the alignment of molecules in the solution aggregate was believed to be such that the electron-deficient squarylium moieties interacted with the electron-rich dialkylamino phenyl moieties. Certain squaraine dyes that were investigated as part of this study were found to form both type A and type B aggregates, depending upon the composition of the DMSO–water medium. Typically, the type A aggregate formed preferentially in media of intermediate DMSO content (50–70% DMSO) and the type B aggregate in more water-rich media (<20% DMSO). In the intervening region it was possible to monitor spectroscopically a dynamic conversion from the type B aggregate to the type A aggregate. An examination of structure–property relationships indicated that dyes with less hydrophobic N-alkyl substituents formed only type A aggregates. Squaraine dyes with more hydrophobic N-alkyl substituents were found to form type B aggregates in water and type A aggregates in some DMSO–water mixtures. A model of squaraine aggregation was proposed in which the type A squaraine aggregate was "thermodynamically" preferred while the type B aggregate was "kinetically" preferred. The stability of the type B aggregate was proposed to be enhanced by increasing hydrophobicity of the N-alkyl substituents and decreased by increasing amounts of DMSO in the solvent system. Keywords: squaraine dyes, aggregation, crystallochromy, solvent effects.