Comparative study of the self-assembly behaviour of 3-chloro-4-hydroxy-phenylazo dyes

The complexity of intermolecular interactions and the difficulty to predict assembly behaviour solely based on chemical constitution was demonstrated by studying the self-assembly of three one-fold negatively charged 3-chloro-4-hydroxy-phenylazo dyes (Yellow, Blue and Red). Dye self-assembly was investigated using UV/vis- and NMR-spectroscopy, light- and small-angle neutron scattering. Significant differences between the three dyes were observed. While Yellow does not self-assemble, Red assembles into higher-order aggregates and Blue forms well-defined H-aggregate dimers with a dimerization constant of K D = (728 ± 8) L mol −1 . Differences between dyes were suggested to emerge from variations in the propensity to form π-π-interactions due to electrostatic repulsion, sterical constraints and hydrogen-bonding interactions. Three azo dyes with a congruent structural sub-unit significantly differ in their self-assembly in solution. Assembly structure was elucidated and reasons for differences discussed.