Uniting metal clusters and liquid crystals easily
LCs are composed of molecules in a phase of matter intermediate between a crystalline solid and an isotropic liquid. Popular for their use in LC displays, these materials are slowly being replaced by organic light-emitting diodes that use less energy when it comes to flat low-power displays. Further improvements can be achieved by replacing the amorphous light-emitting organic compounds with stable inorganic phosphors (phosphorescent materials) that have LC properties. Liquid crystalline materials are easy to process and capable of spontaneous self-assembly over large areas. The first report of LCs containing transition metal clusters (clustomesogens) was published just a few years ago and the field has since witnessed a meteoric rise in attention. The EU-funded project LH-NAN-LC (Luminescent hybrid nanomaterial showing liquid crystalline properties) set out to introduce a bright red near-infrared (NIR) inorganic emitter into a LC matrix. Scientists chose the one nanometre-sized octahedral clusters based on molybdenum or rhenium as scaffolds due to their excellent luminescence. They are prepared by solid-state chemistry but, once solubilised and functionalised with suitable organic molecules, can yield hybrid nanomaterials with self-assembly capabilities. Clustomesogens are typically prepared by grafting mesogenic promoters on the surface of inorganic entities or by replacing inorganic counter-cations with functional organic ones bearing LC promoters. LH-NAN-LC developed an innovative and simple alternative that exploits host-guest complex formation and electrostatic interactions. Scientists demonstrated its utility in rational design of structure to achieve desired properties in a new class of liquid crystalline materials. Using a variety of advanced techniques, the team confirmed production of luminescent nematic or discotic (types of LCs) hybrid liquid crystalline materials. The nematic LC phase is the most important for applications, so the LH-NAN-LC hybrid materials have a bright future in systems requiring stable deep red or red-NIR emission. The knowledge-based design and synthesis promises to spur development of clustomesogens across the board for novel applications in optics, electronics, biology, energy and medicine.