Rotating solid-state crystals could outperform liquid crystals for optoelectronic applications, claim US scientists.

John Price and colleagues from the University of Colorado, Boulder, have investigated metal organic framework (MOF) crystals that can act as molecular rotors.

Rotating or twisting molecules using an applied electric field is the principle behind liquid crystals, an important class of optoelectronic materials. However, solid-state crystals - such as MOF crystals - could lead to more robust materials for use in the data communications industry.

MOF crystals are metal clusters linked by organic groups to form 1-, 2- and 3-D frameworks. Price's team used an existing MOF containing octahedrally coordinated zinc oxide clusters and incorporated polar functional groups into the organic linker parts of the framework. They demonstrated that in the presence of an electric field, the effect on the polar group causes the molecule to rotate. Rotation was not observed for an analogous MOF with no polar functional groups.

MOFs have better structural predictability than we currently have with other solid-state crystals which makes them particularly appealing,' says Erick Winston, a member of Price's group. However, more work is needed as the barrier to rotation of 7.3 kcal mol^-1 for this particular polar MOF means that it can not compete with liquid crystals.

The Colorado group plan to keep searching for an MOF structure which can compete with liquid crystals. 'Dealing with these challenges does not appear to be a far off goal,' says Winston, 'particularly if the community becomes more interested in molecular rotation in MOF crystals.'

Miguel Garcia-Garibay, an expert in MOFs from the University of California, Los Angeles, US, says that the work of the Price group represents 'an important step forward' in the application of crystals as molecular rotors.

Ruth Doherty