UCLA

Molecular machines are molecular and supramolecular entities capable of producing quasi-mechanical movements. While many examples of functional artificial molecular machines have been studied in solution, crystalline aggregates of molecular machines have been explored infrequently because of the limited freedom of motion in closely packed solids. Although previous work on amphidynamic crystals has demonstrated that rotational dynamics in crystals could be achieved and engineered by proper structural design, the existing examples are limited to simple structures and small dynamic groups. As part of the efforts to prepare artificial molecular machines capable of performing useful work, the goals of my doctoral research were to design and synthesize crystalline molecular rotors with complex structures and explore their dynamics with solid-state NMR spectroscopy.

I have prepared two shape-persistent dendritic molecular rotors with molecular weight over 2000 Da by different synthetic strategies. The molecular rotor prepared by a convergent synthesis provided crystals with a low packing density, which allowed megahertz rotation of all aromatic groups in the rotor. The softening of local environments with temperature, resulting from the dynamics of virtually all components in the crystal, led to a new concept of crystal fluidity. The divergent strategy used in the synthesis of the other macromolecular rotor made it possible to incorporate large rotator groups in the structure. When a bulky triptycene group was used, the shape-persistent rotor was able to support its kilohertz rotation in a semicrystalline sample.

In order to realize faster triptycene rotations and to understand the structural elements required for effective solid-state gearing motions, I designed and prepared three pillared paddlewheel MOFs. I demonstrated that megahertz rotation of triptycene could be realized in a catenation-free framework, which offered a loose environment for the rotator. Since the rotator is in close contact only with the solvent molecules, its rotation is dictated solely by the hydrodynamic behavior of DMF and constitutes a diffusion-controlled process. As a result, the temperature dependence of observed rotation could shed light on the effective viscosity change of trapped solvents with temperature.