AROMATIC PI-STACKING IN SOLUTION AS REVEALED THROUGH THE AGGREGATION OF PHENYLACETYLENE MACROCYCLES

Aggregation of phenylacetylene macrocycles (PAMs) in solution has been studied by H-1 NMR spectroscopy and vapor pressure osmometry. The ass ociation constant for dimerization, K-assoc, has been determined by cu rve fitting the concentration dependence of H-1 NMR chemical shifts to a model for monomer-dimer equilibrium. The reliability of the NMR-det ermined aggregation constants and aggregate size have been independent ly verified by vapor pressure osmometry measurements. Thermodynamic pa rameters for association have been obtained from van't Hoff analyses w hich show the aggregation to be favored enthalpically. The aggregation of PAMs bearing various endo- and exo-annular functional groups and P AMs of different geometry and ring size has been studied. The type of pendant functional groups and the manner in which these groups are arr anged on the macrocycle is shown to strongly influence self-associatio n. PAMs substituted with electron withdrawing groups (e.g., esters) ar e more strongly associated than those bearing electron donating groups (e.g., alkyl ethers) or macrocycles bearing a combination of the two substituents. The type of alkyl substituent on the ester or ether grou p is less important as long as the substituent is not branched and is exo-annular. Endo-annular alkyl ethers as well as branched exo-annular alkyl esters severely disrupt aggregation. Rigidity of the macrocycle also influences self-association. In contrast to hexameric macrocycle s, similarly substituted open-chain oligomers and a nonplanar macrocyc le show much weaker association. These findings are discussed in the c ontext of face-to-face pi-pi interactions between aromatic rings. Cons ideration has also been given to pi-pi interactions between aromatic a nd ethynyl groups and between a pair of acetylenes, but these are conc luded to be less significant based on an analysis of data from the Cam bridge Structural Database.