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 ( K dim ≈ 77 000 dm 3 mol -1 at I = 0.01) and that it occurs more readily at higher ionic strength, where trimerization also occurs ( K dim ≈ 580 000 dm 3 mol -1 and K trim ≈ 2700 dm 3 mol -1 at I = 0.1); the driving force is enthalpic rather than entropic (Δ H dim ≈ −53 kJ mol -1 and Δ S dim ≈ −90 J K -1 mol -1 at I = 0.01). Dimerization occurs much less readily in cellophane ( K dim ≈ 42 dm 3 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.