DOI: 10.1002/chem.200700560 Rational Tuning of Melting Entropies for Designing Luminescent Lanthanide-Containing Thermotropic Liquid Crystals at Room Temperature Aude Escande, [a] Laure GuØnØe, [a] Homayoun Nozary, [a] GØrald Bernardinelli, [b] FrØdØric Gumy, [c] Annina Aebischer, [c] Jean-Claude G. Bünzli, [c] Bertrand Donnio, [d] Daniel Guillon, [d] and Claude Piguet* [a] Introduction Up to the turn of the 20th century, thermotropic lanthanido- mesogens, that is, lanthanide-containing liquid crystals, re- mained a curiosity, despite potential attractive technological applications for improving brightness, view-angle and color quality in flat liquid-crystal displays. [1] Pioneering research efforts indeed focussed on two well-identified classes of complexes that incorporated trivalent lanthanides, Ln III , and exhibited thermotropic liquid crystalline properties. 1) Neu- tral disk-like sandwich complexes [LnACHTUNGTRENNUNG(Li-2H) 2 ](i = 1–5, as shown) [2] or triple-decker complexes [Ce 2 (Porphyrin) 3 ] [3] have been shown to stack on top of each other to produce hexagonal columnar mesophases (Col h ) in the 20–250 8C range, whereas; 2) lanthanide size-dependent lamellar or- ganizations (usually smectic A, SmA) have been established to be common for the rod-like complexes [LnACHTUNGTRENNUNG(Li-H) 3 X 3 ] (i = 6–8;X = NO 3 ,H 25 C 12 OSO 3 , Cl, b-diketonate). [4] The in- tense scientific activity dedicated to this topic has been thor- oughly reviewed in 2002 by Binnemans and Gçrller-Wal- rand, [1] who have significantly contributed to the rapid evo- lution of thermotropic lanthanidomesogens during the past four years with: 1) the preparation of the first d–f heterome- Abstract: The connection of twelve pe- ripheral and divergent dodecyloxy chains to a central tridentate aromatic binding unit provides the dodecacate- nar ligand L11, for which room-temper- ature mesomorphism is detected. An enthalpically unbalanced large melting entropy (DS L11 m = 226Jmol 1 K 1 ) results from the programmed microsegrega- tion induced in the crystalline phase, a phenomenon which is maintained in the associated lanthanide complexes [LnACHTUNGTRENNUNG(L11)ACHTUNGTRENNUNG(NO 3 ) 3 ] and [LnACHTUNGTRENNUNG(L11)- ACHTUNGTRENNUNG(CF 3 CO 2 ) 3 ] 2 . Low-temperature melting processes (43  T m 25 8C) produc- ing room-temperature hexagonal col- umnar liquid-crystalline phases thus result for these novel lanthanidomeso- gens. A combined photophysical (high- resolution emission spectroscopy), ther- modynamic (differential scanning calo- rimetry, DSC) and structural (small- angle X-ray diffraction, SA-XRD) in- vestigation of the melting process shows minor structural changes occur- ring between the crystal (Cr) and the hexagonal columnar mesophase (Col h ) in the complexes, which allows the as- signment of the existence of these un- usual first-order phase transitions to the negligible mixing entropy produced by the two viscous phases at low tem- perature. Extension of the concept of chemical tuning of melting entropies for the global design of room-tempera- ture metallomesogens and liquid crys- tals is discussed. Keywords: entropy · lanthanides · liquid crystals · metallomesogens · phase transitions [a] A. Escande, Dr. L. GuØnØe, Dr. H. Nozary, Prof.Dr. C. Piguet Department of Inorganic, Analytical and Applied Chemistry University of Geneva 30 quai E. Ansermet, 1211 Geneva 4 (Switzerland) Fax:(+ 41)22-379-6830 E-mail: Claude.Piguet@chiam.unige.ch [b] Dr. G. Bernardinelli Laboratory of X-ray Crystallography, University of Geneva 24 quai E. Ansermet, 1211 Geneva 4 (Switzerland) [c] F. Gumy, Dr. A. Aebischer, Prof.Dr. J.-C.G. Bünzli Laboratory of Lanthanide Supramolecular Chemistry École Polytechnique FØdØrale de Lausanne BCH 1402 1015 Lausanne (Switzerland) [d] Dr. B. Donnio, Dr. D. Guillon Institut de Physique et Chimie des MatØriaux de Strasbourg-IPCMS CNRS-ULP UMR 7504, 23 rue du Loess B.P. 43 67034 Strasbourg Cedex 2 (France) Supporting information for this article is available on the WWW under http://www.chemeurj.org/ or from the author and includes the experimental section, as well as the associated analytical data. Addi- tional tables (Tables S1–S6) and figures (Figure S1–S14) for structural and spectroscopic analyses and crystallographic data for compound 18. # 2007 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim Chem. Eur. J. 2007, 13,8696–8713 8696