Guest loading and multiple phases in single crystals of the van der Waals host p-tert-butylealix[4]arene

We have followed the loading of xenon into the low density form of a van der Waals solid host, p-tert-butylcalix[4]arene (tBC), with solid-state NMR and X-ray diffraction (XRD), techniques sensitive to local and long-range order, respectively. Even though there was little change in the unit cell parameters, C-13 and Xe-129 solid-state NMR spectra indicate that significant structural changes occur in local order even at low levels of loading. In particular, 129Xe double quantum experiments, which probe distance-dependent Xe-129-Xe-129 dipolar interactions, reveal that the closest Xe-Xe distances increase with Xe loading into the tBC host, suggesting that the tBC undergoes structural rearrangements as it absorbs Xe. Indeed, in light of the solid-state NMR results, a re-examination of partially loaded single crystals by XRD showed that up to a loading level (Xe/tBC ratio) of 0.25, the structure was closely related to that of the empty form with the typical calixarene bilayer structure; however, at higher loading (0.5), the structure is substantially different with a constricted zigzag channel for Xe. In the latter structure, alternate bilayers are distinct because of different molecular orientations and much enhanced thermal parameters. What is remarkable is that the changes described take place with the different structural motifs apparently coexisting in the same single crystal. These different structures have almost identical unit cell parameters; however, the structures are quite different, and the phase transitions are more easily followed with NMR spectroscopy than with diffraction. The fact that ordered domains are always present suggests that cooperative dynamics play an important role, with the experimental results giving snapshots of the loading process at different stages.