Material’s rotors spin freely and quickly

Researchers have designed a crystalline material with tethered but freely spinning molecular groups that rotate incredibly quickly for a solid-state material. The material is the best example yet of an “amphidynamic material,” a crystalline solid with both relatively static and rapidly moving molecular units. Miguel García-Garibay of the University of California, Los Angeles, and coworkers synthesized the material by combining a Zn₄O-based metal-organic framework with spinning bicyclo[2.2.2]octane-1,4-dicarboxylate units that have nearly zero spatial or electronic barriers to rotation (Proc. Natl. Acad. Sci. USA 2017, DOI: 10.1073/pnas.1708817115). The bicyclooctane units spin at up to 50 billion rotations per second, only one order of magnitude slower than the maximum possible rotation rate for unhindered gas-phase molecular rotors in vacuum. At the same time, the Zn₄O framework undergoes normal low-frequency vibrations. In previous amphidynamic crystals, rotors had larger internal steric or electronic barriers that caused them to rotate in discrete steps. The new material’s low barriers enable the bicyclooctane groups to rotate continuously instead, and the rotors also switch direction many times a second. Amphidynamic materials like this new one have potential implications for the design of smart materials and molecular machines, the researchers say.