Bent Tridentate Receptors in Calamitic Mesophases with Predetermined Photophysical Properties:  New Luminescent Lanthanide-Containing Materials

A new synthetic strategy has been developed to introduce bent and rigid tridentate 2,6-bis(benzimidazol-2‘-yl)pyridine cores into rodlike ligands L^11-17. The crystal structure of the nonmesogenic ligand L¹³ (C₃₉H₃₇N₅O₄, triclinic, <i>P</i>1̄, <i>Z</i> = 2) shows the expected <i>trans−trans</i> conformation of the tridentate binding unit, which provides a linear arrangement of the semirigid aromatic sidearms. The crystal structure of the related mesogenic ligand L¹⁶ (C₆₁H₈₁N₅O4, triclinic, <i>P</i>1̄, <i>Z</i> = 2) demonstrates the fully extended conformation adopted by the lipophilic side chains, leading to a slightly helically twisted I-shaped molecule. A rich and varied mesomorphism results which can be combined with the simultaneous tuning of electronic and photophysical properties via a judicious choice of the spacers between the rigid central core and the semirigid lipophilic sidearms. Ligands L^13,14 react with Ln(NO₃)₃·xH₂O to give quantitatively and selectively the neutral 1:1 complexes [Ln(L^<i>i</i>)(NO₃)₃] (Ln = La to Lu), which are stable in the solid state at room temperature but partially dissociate in acetonitrile to give the cationic species [Ln(L^<i>i</i>)(NO₃)₂]+. The crystal structure of [Lu(L¹³)(NO₃)₃]·3CH₃CN (30, LuC₄₅H₄₆N₁₁O₁₃, monoclinic, C2/<i>c</i>, Z = 8) reveals a U-shaped arrangement of the ligand strand arising from the <i>cis−cis</i> conformation of the coordinated tridentate binding unit. This drastic geometric change strongly affects the thermal behavior and the photophysical and electronic properties of the lipophilic complexes [Ln(L¹⁴)(NO₃)₃]. Particular attention has been focused on structure−properties relationships, which can be modulated by the size of the lanthanide metal ions.