Rational Tuning of Melting Entropies for Designing Luminescent Lanthanide-Containing Thermotropic Liquid Crystals at Room Temperature

The connection of twelve peripheral and divergent dodecyloxy chains to a central tridentate aromatic binding unit provides the dodecacatenar ligand L11, for which room- temperature mesomorphism is detected. An enthalpically unbalanced large melting entropy ($\Delta S_m^{L11} =226 Jmol^{-1}1K^{-1}$) results from the programmed microsegregation induced in the crystalline phase, a phenomenon which is maintained in the associated lanthanide complexes [$Ln(L11)(NO_3)_3$] and $[Ln(L11)- (CF_3CO_2)_3]_2$. Low-temperature melting processes ($-43 \leq T_m \leq -25^{\circ}C$) producing room-temperature hexagonal columnar liquid-crystalline phases thus result for these novel lanthanidomesogens. A combined photophysical (highresolution emission spectroscopy), thermodynamic (differential scanning calorimetry, DSC) and structural (smallangle X-ray diffraction, SA-XRD) investigation of the melting process shows minor structural changes occurring between the crystal (Cr) and the hexagonal columnar mesophase ($Col_h$) in the complexes, which allows the assignment of the existence of these unusual first-order phase transitions to the negligible mixing entropy produced by the two viscous phases at low temperature. Extension of the concept of chemical tuning of melting entropies for the global design of room-temperature metallomesogens and liquid crystals is discussed.