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.