Cryogenic Deuteron Solid-state NMR Spectroscopy of Ultra-low Barrier Rotors for "Inertial" Crystalline Molecular Machines Assembled via Metal-organic Frameworks
EMSL Project ID
45292
Abstract
Condensed-phase artificial molecular machines are an extremely promising platform for the development of stimuli-responsive bulk materials. By choosing well-characterized molecular "parts" with specific functionalities, artificial molecular machines may be synthesized, assembled and self-organized in a predetermined manner, amenable to perform sophisticated functions. Our group has synthesized crystalline assemblies of machine components based upon molecular analogues of macroscopic gyroscopes and compasses, and established structure/function relationships to develop strategic engineering methods for specific types of dynamic behavior. One major objective is to develop crystalline molecular machines with barriers to rotation that are less than the residual thermal energy at room temperature (~0.6 kcal/mol), in order to engineer diffusional, or "inertial", molecular machines. In these systems, very small energy perturbations arising from interactions of dipole-functionalized rotators (or "compasses") would lead to dynamic, interacting domains that could be interfaced with an external field to modulate the dynamics in a directional manner. In order to achieve such low barrier rotation in solids, our previous structure/function relationships have established that free-volume and a low intrinsic (gas phase) electronic barrier to rotation are essential. Knowing that metal-organic frameworks (MOFs) can maintain permanent porosity, it was expected that their low crystal densities could impart the free-volume needed to facilitate diffusional rotation of engineered molecular machines when constructed with ligands possessing negligible intrinsic rotational barriers. A bicyclo[2.2.2]octane 1,4-dicarboxylic acid (BODCA) ligand was synthesized based on the previous use of this rotator core in crystalline molecular machines. The ligand was treated with zinc acetate dihydrate to obtain the BODCA MOF as a polycrystalline powder. The resulting powder x-ray diffraction pattern was analyzed via Monte Carlo, refinement, and ab initio charge-flipping methods to establish a cubic MOF structure. Preliminary proton wideline solid-state NMR spin-lattice relaxation (T1) measurements from 25 K - 210 K have established that the rotators possess a rotational barrier of only 0.54 kcal/mol above 100 K. Below 100 K, the data suggests that additional factors may contribute to the T1 relaxation, as T1 becomes decoupled from the cooling power. Eyring analysis of the T1 shows a large, negative transition state entropy of -131 J/K*mol that may be related to additional vibrational modes that occur in MOFs, of which many have associated negative thermal expansion coefficients due to such vibrational modes. Solid-state deuteron quadrupolar-echo NMR spectra on an isotopically-enriched BODCA MOF have been acquired to 150 K, however the spectra at all temperatures are consistent with diffusional rotation. In this study, deuteron solid-state NMR spectra will be acquired at cryogenic temperatures in the range of 10 K - 150 K to reach motion in the intermediate and slow exchange limits to further characterize this system as an "inertial" solid-state molecular machine, and gain insight into the dynamics and energetic potential. This study will provide fundamental information regarding the dynamics of ultra-low barrier anisotropic rotation in molecular solids for the development of molecular machines, and shed light on how the unique nanoporous architectures of MOFs influence dynamics and ultimately the bulk properties of these materials.
Project Details
Project type
Exploratory Research
Start Date
2011-10-07
End Date
2012-10-07
Status
Closed
Released Data Link
Team
Principal Investigator
Related Publications
Vogelsberg CS, FJ Uribe-Romo, AS Lipton, S Yang, KN Houk, SE Brown, and MA Garcia-Garibay. 2017. "Ultrafast rotation in an amphidynamic crystalline metal organic framework." Proceedings of the National Academy of Sciences of the United States of America 114(52):13613-13618. doi:10.1073/pnas.1708817115