Static and Dynamic Behavior of 2:1 Inclusion Complexes of Cyclodextrins and Charged Porphyrins in Aqueous Organic Media

Two heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin (TMe-β-CD) molecules strongly include the peripheral substituents at the 5- and 15-positions of the charged meso-tetrasubstituted porphyrins, PorSub₄ [TPPS₄ (Sub = p-C₆H₄-SO₃^-), TPPOC3PS (p-C₆H₄-O-(CH₂)₃-p-C₆H₄-SO₃^-), TCPP (Sub = p-C₆H₄-CO₂^-), and TPPOC3Py (p-C₆H₄-O-(CH₂)₃-Py⁺Br^-), where Py⁺ = N-alkylpyridinium] in aqueous ethylene glycol. The binding constants (K₁ and K₂) and the rate constants (k₁ and k₂) for formation of the 1:1 and 2:1 complexes of TMe-β-CD and PorSub₄ were determined. Both the binding constants and the rate constants for anionic TPPS₄, TCPP, and TPPOC3PS were much larger than those for cationic TPPOC3Py. The smaller k₁ and k₂ values for TPPOC3Py indicate a higher barrier for penetration of a cationic guest into the TMe-β-CD cavity. The methyl groups at the rims and the cavity wall of the host are positively polarized due to the inductive effect of the ethereal oxygens. The positively polarized rims and interior of the host cavity should prevent the penetration of the cationic substituent of TPPOC3Py into the TMe-β-CD cavity. The 2:1 TMe-β-CD−PorSub₄ complexes are extraordinary stable in aqueous solutions, even in the case of cationic TPPOC3Py. Formation of both 1:1 and 2:1 complexes is promoted by negative and large enthalpy changes, suggesting a strong van der Waals interaction as the main binding force.