Jean-Claude Bunzli - Aromatic bent-core liquid crystals: An opportunity for introducing terdentate binding units into mesophases

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  Nozary,H., Piguet,C., Rivera,J.P., Tissot,P., Morgantini,P.Y.,   Weber,J., Bernardinelli,G., Bunzli,J.C.G., Deschenaux,R.,   Donnio,B., Guillon,D. Chemistry of Materials  2002 14 (3) 1075-1090

  Abstract: Lipophilic linear semirigid side arms   containing two or three successive phenyl rings separated by   carboxylate spacers have been connected to the 5 or 6 positions   of bent aromatic terdentate 2,6-bis(benzimidazol-2-yl)pyridine   binding units to give extended V-shaped (L11) and I-shaped   receptors (L12, L12b, and L13). The carboxylate spacers limit the   flexibility of the side arms and provide crossed arrangements of   the successive aromatic rings in the crystal structure of L12b   (C63H61N5O10; triclinic, P (1) over bar, Z = 2) in agreement with   semiempirical calculations performed on optimized gas-phase   geometries. Moreover, the carboxylate spacers in L11-L13 prevent   efficient electronic delocalization between the connected   aromatic rings and act as weak pi acceptors producing a slight   increase of the energy of the (1)pipi* and (3)pipi* levels   centered on the terdentate binding unit. Intermolecular   pi-stacking interactions observed in the crystal of L12b are   invoked to rationalize (i) the peculiar excimer emission of L11   in the solid state and (ii) the rich and varied calamitic   (I-shaped L12, L12b, and L13) and columnar (V-shaped L11)   mesomorphism observed at high temperature. The Col(R) mesophase   detected for L11 demonstrates that V-shaped bent terdentate   binding units are compatible with liquid-crystalline behavior.   Complexation of L11 with lanthanide(III) produces I-shaped   complexes [Ln(L11)(NO3)(3)] (Ln = La, Eu, Gd, Tb, and Lu)   possessing a large axial anisometry as found in the crystal   structure of [Lu(L11)(CF3CO2)(3)(H2O)] (LuC81H87N5O17F9;   triclinic, P (1) over bar, Z = 2), which exists in the solid   state as H-bonded dimers. No mesomorphism is detected for the   complexes as a result of the large perpendicular expansion   brought by the metallic coordination site, but the high energy of   the ligand-centered (3)pipi* prevents Eu(D-5(0)) --> L11 back   transfer in the Eu(III) complex, which thus exhibits sizable red   luminescence at room temperature, a crucial point for the design   of luminescent materials

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