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

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, P1̄, 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 π acceptors producing a slight increase of the energy of the 1ππ* and 3ππ*
levels centered on the terdentate binding unit. Intermolecular π-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 ColR
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, P1̄, 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 3ππ* prevents Eu(5D0) → 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.