Proton transport is ubiquitous in biology — for example, through the hydrogen-bonded networks of protein channels. While some synthetic proton shuttles have been realized, such systems have rarely been used as a switching mechanism in functional materials. Now, Osamu Sato and colleagues at Kyushu University and the Institute for Materials Science in Japan have designed supramolecular organic frameworks (SOFs) that contract and expand at the macroscale on intramolecular proton shuttling.
The researchers first prepared a SOF from an azodipyridine and a tetra-carboxylic acid. These two groups are hydrogen-bonded to one another to form a rhombic grid and act as the basis of a rack-and-pinion (gear) cascade. On a temperature change, the pyridine groups facilitate an intra-carboxyl proton shuttle, resulting in rotation of the azodipyridine and a correlated translation of the tetra-acid. In other words, rotary motion is converted to linear motion. Density functional theory calculations suggested that ionization of the carboxyl groups kinetically aids the azodipyridine pedalling motion. This thermally-induced process involves a single crystal to single crystal phase transition. Propagation of the motion through the network causes a macroscopic expansion and contraction of the crystal of around 1.82 mm (123–333 K).