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 >10(4) M-1), but they enter as such only in the larger CB[8]. In terms of orientation, H-1 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], H-1 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 H-1 NMR shielding calculations) in the determination of the stereochemistry of those complexes.