Ultra-Fast Molecular Rotors within Porous Organic Cages.

Using variable temperature 2 H static NMR spectra and 13 C spin-lattice relaxation times (T1 ), we show that two different porous organic cages with tubular architectures are ultra-fast molecular rotors. The central para-phenylene rings that frame the "windows" to the cage voids display very rapid rotational rates of the order of 1.2-8×106  Hz at 230 K with low activation energy barriers in the 12-18 kJ mol-1 range. These cages act as hosts to iodine guest molecules, which dramatically slows down the rotational rates of the phenylene groups (5-10×104  Hz at 230 K), demonstrating potential use in applications that require molecular capture and release.