Molecule spins in its crystal

Envision a crystalline material and you might think of atoms holding static positions within a regular molecular lattice. A new dendrimeric molecular rotor represents another extreme: In crystalline form, the molecule’s aliphatic framework stays stationary, but its 25 phenyl rings rapidly rotate (J. Am. Chem. Soc. 2016, DOI: 10.1021/jacs.6b01398). A team led by Xing Jiang, Miguel A. Garcia-Garibay, and Kendall N. Houk of UCLA constructed the molecule (shown), which has a phenylene core connected on each end to a triphenylpropynyl group. Each of those six phenyl rings in turn connects to another triphenylpropynyl group. The molecule consequently has three types of phenyl rings: one core phenylene, six branch phenylenes, and 18 peripheral phenyls. X-ray, NMR, and molecular dynamics data show that crystals of the compound are loosely packed and each of the three kinds of phenyl rings may rotate independently of the others. Within a particular set of phenyl rings, however, the motion may be coordinated. In a set of three branch phenylenes, for example, one ring may rotate while the other two oscillate, or all three rings may rotate synchronously. The molecule could find use in molecular machines.