Crystalline molecular machines: encoding supramolecular dynamics into molecular structure

Abstract

Crystalline molecular machines represent an exciting new branch of crystal engineering and materials science with important implications to nanotechnology. Crystalline molecular machines are crystals built with molecules that are structurally programmed to respond collectively to mechanic, electric, magnetic, or photonic stimuli to fulfill specific functions. One of the main challenges in their construction derives from the picometric precision required for their mechanic operation within the close-packed, self-assembled environment of crystalline solids. In this article, we outline some of the general guidelines for their design and apply them for the construction of molecular crystals with units intended to emulate macroscopic gyroscopes and compasses. Recent advances in the preparation, crystallization, and dynamic characterization of these interesting systems offer a foothold to the possibilities and help highlight some avenues for future experimentation.

Fig. 1.

The blueprints contain all of the structural information necessary to build and assemble the components of a mechanical wristwatch. Each component must have the precise structural information to fit within the assembly so as to surrender its own degrees of freedom to the degrees of freedom of the collective.

Fig. 2.

Phase order–molecular motion diagram illustrating possible forms of condensed-phase matter. Crystalline molecular machines would require a combination of rigidity and mobility to warrant a high level of order and desirable dynamic processes.

Fig. 3.

Macroscopic gyroscopes and compasses retain their dynamic capabilities both as isolated units and as aggregates. A proposed molecular structure that may emulate their function consists of a rigid frame, or stator (in blue), and a rotator linked by a barrierless axle (in red).

Fig. 4.

Molecular gyroscopes with open topologies based on triptycyl (compounds 1 and 5) and triphenylmethyl (2–4) frames and phenylene rotators. Rates of “gyroscopic” rotation at the corresponding temperatures are shown below (arrows are not intended to suggest unidirectional rotation).

Fig. 5.

Space-filling models illustrating the packing relation between solvent molecules (light blue), the central rotator (red), and the frame provided by the triptycyl or trityl groups. (A) Three views of 1 illustrating the interdigitation experienced by neighboring molecules and meta-xylene. (B) Partial structure of the benzene-clathrate of 2 with a molecule in the front plane removed to illustrate the parallel-displaced benzene dimer. (C) Packing structure of the solvent-free structure of 2 showing the proximity between trityl groups and central rotators.

Fig. 6.

Suggested analogy between a gyroscope schematic and a filling model from the x-ray structure of the dodeca-tert-butyl compound 4 with a 3-Å solvent-accessible surface illustrating the steric shielding provided by the tert-butyl groups. Rotation of the central phenylene (in red) at 300 K occurs at a frequency higher than 100 MHz (39).

Fig. 7.

The synthesis of molecular gyroscope 5 can be accomplished by a highly convergent method from simple chemicals (41). Crystals grown from bromobenzene, removed from the center of a cage formed by four molecular gyroscopes, result in gyroscopic rotation with a barrier of only 4.3 kcal/mol.

Fig. 8.

Suggested analogy between a macroscopic compass and the space-filling models of compound 2 and the monofluoro derivative 2F. The isosteric structures form isomorphous crystals, which have very similar rotary dynamics in the solid state.