Solution-Phase Dimerization of an Oblong Shape-Persistent Macrocycle

A new
shape-persistent macrocycle comprising two 2,3-triphenylene
moieties bridged by m-phenylene ethynylenes has been
synthesized. UV–vis and fluorescence spectroscopies indicate
limited interaction between the two triphenylene units. The compound,
which has a pronounced oblong shape (the core measures approximately
2.2 × 1.0 nm), aggregates in CDCl3 and toluene-d8 to give stacked dimers, as indicated by the 1H NMR signals corresponding to protons on or near the core,
which shift upfield with increasing concentration. These changes in
NMR shielding were modeled using DFT calculations on candidate dimer
geometries. The best match to the experimental data was obtained for
a dimer consisting of arene–arene stacking by 3.6 Å (on
average) with a displacement along the molecular long axis of 3.5–4.0
Å. This displacement is larger than can be explained by the electronic
effects of aromatic stacking interactions. Instead, the minimization
of steric interactions between the side chains appears to control
the dimer geometry, with the alkoxy groups of one molecule sliding
into registry with the gaps along the periphery of the other. Such
lateral displacement (as opposed to rotation) is a direct consequence
of the extended oval shape of the compound.