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Luminescent liquid crystalline materials based on metal clusters

Final Report Summary - MESOCLUST (Luminescent liquid crystalline materials based on metal clusters)

The MESOCLUST project focused on the design of new hybrid materials based on octahedral metallic cluster cores that display liquid crystalline properties as well as bright luminescence in the red-NIR region. Liquid crystals (LC) are self-assembling molecular materials that have found various applications ranging from the manufacturing of LCD to different molecular sensors and detectors, optical switches, spatial light modulator, etc., based on their unique features to retain a high degree of mobility besides long or short-range ordering in the fluid phase that give rise to anisotropy-related optical, electrical and magnetic properties. Introduction of octahedral metallic cluster complexes, added unique valuable properties to the liquid crystalline phase (such as magnetic, electronic and most important photoluminescence - quantum yields of such nanometer sized octahedral clusters can reach up to 100%) besides an interesting structural role when compare to purely organic liquid crystals, and represent a new promising class of materials that are able to preserve almost entirely unaffected the cluster based properties. The affordable costs and preparation methods as well as thermal stability, the ease of handling and processing made them very appealing materials to be used in different applications based on emission properties (LED, etc.).

Combining such nanoobjects with liquid-crystalline ordering was a challenging task and we used different strategies based on covalent binding or ionic self-assembly (ISA) to induce LC properties in such systems. The molybdenum, tungsten and rhenium based octahedral metallic clusters (M6) incorporated in LC matrix display broad emission in the near-red range (600 - 900 nm) upon UV-visible excitation, emission lifetimes in the microsecond range and high luminescence quantum yield, up to 100%. The main strategy to promote LC behaviour to the new hybrid materials based on metallic clusters was to employ carefully designed promesogenic organic ligands that can give rise to controlled liquid crystalline properties both regarding the phase type and phase transition temperatures. By using neutral ligands, such is the case of phosphine oxide or pyridone derivatives, we were able to vary the number, as well as the position, of mesogenic groups and this, indeed, was a great step further towards smart functionalization of octahedral metallic clusters. Moreover, this allowed a deeper understanding of the requirements to control the supramolecular LC organization and how LC properties can be adjusted according to desired application of such hybrid nanomaterials. In this way we successfully demonstrated that utilizing various neutral ligands as coordinating entities to the cluster core on one hand, or coupling with different organic promesogenic cations on the other hand, did not affect its emission properties while these derivatives could be functionalized with various mesogenic groups and, moreover, by LC incorporation these properties are preserved unaffected over the entire temperature range up to isotropic state. For instance, nematic phases – the simplest and the widely used LC phase in the construction of LC devices – were evidenced for molybdenum and tungsten clusters close to ambient temperature.

The results obtained during the realisation of the MESOCLUST project afforded a strong correlation to be made between the chemical structure of newly prepared hybrid materials and liquid crystalline behaviour. Additionally, several suitable materials have been identified (with low melting points, wide mesomorphic ranges as well as stability and mesophase accessibility at temperatures near to room temperature) and these were used in order to evaluate the electro-optical and photophysical properties.