Aromatic π-Stacking in Solution as Revealed through the Aggregation of Phenylacetylene Macrocycles

Αggregation of phenylacetylene macrocycles (PAMs) in solution
has been studied by 1H NMR spectroscopy
and vapor pressure osmometry. The association constant for
dimerization, Kassoc, has been determined by
curve
fitting the concentration dependence of 1H NMR chemical
shifts to a model for monomer−dimer equilibrium.
The
reliability of the NMR-determined aggregation constants and aggregate
size have been independently verified by
vapor pressure osmometry measurements. Thermodynamic parameters
for association have been obtained from
van't Hoff analyses which show the aggregation to be favored
enthalpically. The aggregation of PAMs bearing
various endo- and exo-annular functional groups
and PAMs of different geometry and ring size has been
studied.
The type of pendant functional groups and the manner in which
these groups are arranged on the macrocycle is
shown to strongly influence self-association. PAMs substituted
with electron withdrawing groups (e.g., esters) are
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 group 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 macrocycles, similarly substituted open-chain
oligomers and a nonplanar macrocycle show
much weaker association. These findings are discussed in the
context of face-to-face π−π interactions between
aromatic rings. Consideration has also been given to π−π
interactions between aromatic and ethynyl groups and
between a pair of acetylenes, but these are concluded to be less
significant based on an analysis of data from the
Cambridge Structural Database.