As. Shetty et al., AROMATIC PI-STACKING IN SOLUTION AS REVEALED THROUGH THE AGGREGATION OF PHENYLACETYLENE MACROCYCLES, Journal of the American Chemical Society, 118(5), 1996, pp. 1019-1027
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.