Aromatic Bent-Core Liquid Crystals: an Opportunity for Introducing Terdentate Binding Units into Mesophases

Lipophilic linear semirigid side arms contg. two or three successive Ph rings sepd. by carboxylate spacers were connected to the 5 or 6 positions of bent arom. 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 arom. rings in the crystal structure of L12b (C63H61N5O10; triclinic, space group P?1, Z = 2) in agreement with semiempirical calcns. performed on optimized gas-phase geometries. also, the carboxylate spacers in L11-L13 prevent efficient electronic delocalization between the connected arom. rings and act as weak ? acceptors producing a slight increase of the energy of the 1??* and 3??* levels centered on the terdentate binding unit. Intermol. ?-stacking interactions obsd. 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 obsd. at high temp. The ColR mesophase detected for L11 demonstrates that V-shaped bent terdentate binding units are compatible with liq.-cryst. 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, space group P?1, 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??* prevents Eu(5D0) ? L11 back transfer in the Eu(III) complex, which thus exhibits sizable red luminescence at room temp., a crucial point for the design of luminescent materials. [on SciFinder(R)]