Dynamics in Flexible Pillar[]arenes Probed by Solid-State NMR.

Pillar[]arenes are supramolecular assemblies that can perform a range of technologically important molecular separations which are enabled by their molecular flexibility. Here, we probe dynamical behavior by performing a range of variable-temperature solid-state NMR experiments on microcrystalline perethylated pillar[]arene ( = 5, 6) and the corresponding three pillar[6]arene xylene adducts in the 100-350 K range. This was achieved either by measuring site-selective motional averaged C H heteronuclear dipolar couplings and subsequently accessing order parameters or by determining H and C spin-lattice relaxation times and extracting correlation times based on dipolar and/or chemical shift anisotropy relaxation mechanisms. We demonstrate fast motional regimes at room temperature and highlight a significant difference in dynamics between the core of the pillar[]arenes, the protruding flexible ethoxy groups, and the adsorbed xylene guest. Additionally, unexpected and sizable C H heteronuclear dipolar couplings for a quaternary carbon were observed for -xylene adsorbed in pillar[6]arene only, indicating a strong host-guest interaction and establishing the -xylene location inside the host, confirming structural refinements.