Orientation of Pyrylium Guests in Cucurbituril Hosts

According to recent reports, supramolecular complexes
of the pyrylium
cation with cucurbit­[x]­urils (CB­[x], x = 7, 8) show promising photoluminescence suitable
for electroluminescent devices. In turn, photoluminescence seems to
be related to the stereochemistry of the complexes; however, that
has been controversial. Here, we report that in H2O, 2,6-disubsituted-4-phenyl
pyryliums (Pylm) form dimers quantitatively (equilibrium
constants >104 M–1), but they enter
as
such only in the larger CB­[8]. In terms of orientation, 1H NMR shows that Me-Pylm, Ph-Pylm, and t-Bu-Pylm insert
their 4-phenyl groups in either the CB­[7] or CB­[8] cavity. The orientation of iPr-Pylm in the iPr-Pylm@CB­[7] complex is similar. Experimental conclusions are supported by DFT
calculations using the M062X functional and the 6-31G­(d) basis set.
In the case of (iPr-Pylm)2@CB­[8], 1H NMR of
both the guest and the host indicates that both guests might enter CB­[8] from the same side with their iPr groups
in the cavity, but DFT calculations leave room for ambiguity. In addition
to the size and hydrophobicity of the 2,6-substituents of the guests,
as well as the size and flexibility of the hosts, theory reveals the
importance of explicit solvation (H2O) and finite temperature
effects (particularly for 1H NMR shielding calculations)
in the determination of the stereochemistry of those complexes.