Nature of the Structural and Dynamical Disorder in Organic Cocrystals with a True Nanometric Size Channel-Like Architecture

The nature of the structural and dynamical disorder of the nanoporous organic cocrystal carbamazepine-tartaric acid designed by liquid-assisted grinding is investigated through complementary solid-state NMR, X-ray diffraction, and broadband dielectric spectroscopy experiments combined with molecular dynamics simulations. In this article, we especially highlight that the tartaric acid molecules present in the channel-like cocrystalline architecture show both translational and rotational dynamical disorder. Such a disorder seems only partial since tartaric acid molecules are strongly hydrogen-bonded to the carbamazepine molecules which form the channels, and they thus share with them some order. Tartaric acid species are organized as one-dimensional interrupted single files of molecules weakly hydrogen-bonded between them. Translational dynamics occurs by small hops of about 6-7 angstrom, consistent with the distance between first neighbors. At short times, it can be described as a single-file diffusion process, while at longer times, the classical diffusion (Fickian) is recovered. Random motions are explained by the presence of several short single files of molecules in the channel instead of just one single file. Rotational dynamics is interpreted as rotational jumps between preferred orientations. It gives rise to a change of the molecular dipole moments orientations, which are detected by dielectric relaxation spectroscopy. Freezing out of the rotational molecular mobility is detected in the temperature range [173-193] K concomitantly in the presence of a kink in the temperature evolution of the crystalline cell volume, which is usually associated with the glass transition phenomenon. It reveals a remarkable link between the molecular mobility of the tartaric acid molecules and the overall crystal anharmonicity. The present findings aim to demonstrate the interest of disordered channel-like cocrystals for investigation of dynamics in nanoconfinement environments.