Complexation by α- and β-cyclodextrin C(6) linked homo- and hetero-dimers of Brilliant Yellow tetraanion: A study of host-guest size relationships

Complexation by α- and β-cyclodextrin (αCD and βCD) homo- and heterodimers linked at C(6) by a urea linker (N,N′-bis(6^A-deoxy-α-cyclodextrin-6^A-yl) urea, N-(6^A-deoxy-α-cyclodextrin-6^A-yl)-N′- (6^A-deoxy-β-cyclodextrin-6^A-yl)urea and N,N′-bis(6^A-deoxy-β-cyclodextrin-6^A-yl) urea1-3) of the tetraanion of the dye Brilliant Yellow (4) has been studied. In aqueous solution at 298.2 K, pH 10.0 (borate) and I = 0.10 mol dm^-3 (NaClO₄) the spectrophotometrically determined complexation constants K = (1.40 ± 0.08) × 10⁴, (9.05 ± 0.16) × 10⁴ and (3.92 ± 0.06) × 10⁴ dm³ mol^-1, for the complexes 1.4, 2.4 and 3.4, respectively, that compare with K = (1.05 ± 0.08) × 10⁴ and (2.20 ± 0.05) × 10³ dm³ mol^-1 for the αCD.4 and βCD.4 complexes, respectively. Thus, the complexation of 4 in 1.4 shows little cooperativity consistent with the annulus of each αCD component of 1 being too small to pass over the phenylsulfonate component of 4. It is probable that two complexes are formed when 2 complexes 4: one in which 4 is complexed by the αCD component alone and which has a similar stability to 1.4 and a second complex where 4 is complexed by both the αCD and βCD components of 2 to form a complex 8.6 and 41 times more stable than αCD.4 and βCD.4, respectively. The cooperativity between the two βCD components of 3 causes 3.4 to be 18 times more stable than βCD.4. These conclusions are supported by ¹H NMR spectroscopic studies.