Spectroscopic studies of the intermolecular interactions of a bis-azo dye, Direct Blue 1, on di- and trimerization in aqueous solution and in cellulose

The intermolecular interactions of the bis-azo dye Direct Blue 1 (Chicago Sky Blue 6B) have been studied as a function of concentration in aqueous solution and in cellophane using UV−visible absorption, NMR, and resonance Raman spectroscopy. UV−visible spectroscopy indicates that dimerization occurs in aqueous solution (Kdim ≈ 77 000 dm3 mol-1 at I = 0.01) and that it occurs more readily at higher ionic strength, where trimerization also occurs (Kdim ≈ 580 000 dm3 mol-1 and Ktrim ≈ 2700 dm3 mol-1 at I = 0.1); the driving force is enthalpic rather than entropic (ΔHdim ≈ −53 kJ mol-1 and ΔSdim ≈ −90 J K-1 mol-1 at I = 0.01). Dimerization occurs much less readily in cellophane (Kdim ≈ 42 dm3 mol-1) than in aqueous solution, indicating that strong dye−cellulose interactions compete effectively with dye−dye interactions. NMR spectroscopy indicates that Direct Blue 1 molecules interact by π-stacking at the central biphenyl group, while resonance Raman spectroscopy indicates that the internal structure and bonding of the monomers is essentially retained on stacking. The UV−visible spectra are consistent with this interpretation, and the application of exciton theory indicates that stacking results in angles between adjacent molecules which are different in the dimer (θ ≈ 84°) and trimer (θ ≈ 58°); they are attributed to geometries which minimize the repulsion between charged naphthylsulfonate groups.