The binding of bis-amine ligands to a series of cyclic zinc porphyrin dimers has been characterized by NMR and electronic spectroscopy. The cavity within the flexible dimers is kept closed by π-π interactions between the porphyrin and aromatic bridging groups but can be opened by ligands whose binding is strong enough to overcome this energetic barrier. Analysis of the binding of these bifunctional and the corresponding monofunctional ligands to the porphyrin dimers and corresponding monomeric porphyrins allowed definition of the parameters ΔΔG, ΔΔH, and ΔΔS. These are specifically associated with the energetic costs of conformational switching and yield information about the strength of the π-π interactions that hold the cavity closed. It is estimated that the porphyrin-pyromellitimide π-π interaction has an energy of 28-56 kJ mol-1 (7-13 kcal mol-1), while the porphyrin-biphenyl interaction is ca. 22 kJ mol-1 (5 kcal mol-1). A bis-pyridyl porphyrin ligand which has additional recognition sites binds within the cavity with no thermodynamic barrier. It is shown that similar values of ΔΔG can mask radically different binding modes: the values ΔΔH and ΔΔS are more informative, allowing the dissection of the various factors that contribute to or inhibit ligand binding within a cavity. © 1990, American Chemical Society. All rights reserved.
