Mixed-Valent Metallomesogens
Transition-metal containing liquid crystals (TMLC's) or metallomesogens combine the physical characteristics of metal coordination complexes with those of liquid crystalline organic molecules. Diruthenium carboxylates are good candidates for the develop-ment of metallomesogens, due to ruthenium's high polarizability and the ease with which the carbox-ylates can be replaced by long-chain saturated or unsaturated fatty acids showing mesogenic properties.
Cukiernik et al. studied the reduced Ru2II,II(μ-O2CR)4 and, to a lesser extent, the mixed-valent species [Ru2II,III(μ-O2CR)4]X (R = a saturated or unsaturated hydrocarbon chain; X = carboxylate, Cl-, or dodecylsulphate). In 1998, we prepared the air-stable mixed-valent metallomesogens containing the unsaturated palmitoleic and oleic acids as the bridging carboxylates. The complexes are polymeric with four intradimer and one interdimer carboxylate bridge, {[Ru2II,III(μ -O2CR)4](μ'-O2CR)}n, where R = -C15H31, -C17H35. Using differential scanning calorimetry (DSC), we saw solid to liquid crystal transitions (Tc's) at 125 C for both compounds, lower those seen for the saturated chain analogues. When the materials were cooled, a hexagonal discotic mesophase was observed using polarizing optical microscopy and X-ray powder diffraction. Remarkably, the materials maintain their mesophase down to room temperature, where reversion back to the solid phase takes about 3 days.
We are currently studying more systematically the dependence of the Tc and mesophase temperature range on the degree and position of saturation (or rigidity). This will be done by looking at systems incorporating fatty acids with 2 or more double bonds located near the center and outer ends of the chain, as well as by adding side-chains to the acids at varying distances from the dimetal core. Increased unsaturation near the middle of the chains appears to lead to less efficient packing and a lowering of the enthalpy of the transition and Tc. This effect has been seen for our systems, where the Tc drops from about 140o for the saturated palmitic and stearic analogues to 125o for the unsaturated palmitoleic and oleic derivatives, and also by Maldivi in some reduced Ru2II,II(μ -O2CR)4 complexes. An extensive study of the effects of varying the degree and position of unsaturation has not been undertaken.
The environment directly next to the diruthenium core plays a key role in how the molecules are ordered in the liquid crystal phase, although the exact nature of this effect is hard to predict. A second strategy we are employing is to use modified metallocene carboxylates as the bridging groups. The modifications will involve introducing a long hydrocarbon chain (saturated or unsaturated) between either, a) the carboxyl group and the cyclopentadienyl (Cp) ring or, b) onto the Cp ring that does not contain the carboxylate group. Conventional exchange reactions will be employed similar to those mentioned in 1 and 2 above, although the use of more non-polar solvents may be necessary. We hope to generate a liquid crystal that contains a mixture of metals, as well as the possibility of multiple mixed-valency. We are aware of only a few examples in the literature of mixed-metal metallomesogens and no examples of multiply mixed-valent liquid crystals. These features will introduce novel electronic and magnetic properties not seen in the simpler metallomesogens.