Polarized Luminescence from Aligned Samples of Nematogenic Lanthanide Complexes

Abstract

Liquid-crystalline lanthanide complexes (lanthanidomesogens) are a class of molecular materials that have intensively been studied during the last decade. [1] Whereas the first examples of lanthanidomesogens were phthalocyanine [2] or Schiff base complexes, [3] progressively new types of lanthanide-containing liquid crystals have been discovered. These include alkanoates, [4] b-diketonates, [5] b-enaminoketones, [6] imidazo[4,5-f]-1,10-phenanthrolines, [7] 2,6-bis(benzimidazol-2yl)pyridines, [8] crown ethers, [9] polyoxometalate surfactant complexes, [10] and complexes of lanthanide salts with non-ionic surfactants. [11] The rationale for incorporating lanthanide ions into liquid crystals is that this approach allows to add unique magnetic and spectroscopic properties to the anisotropy-related properties of liquid crystals. [1a] For example, it is possible to obtain liquid crystals that can be more easily aligned in an external magnetic field [3c,12] or luminescent liquid crystals. [13] The coordination geometries of the metal ion in f-block metallomesogens differ from those of the d-block metallomesogens, in the sense that the coordination number of the lanthanides is high (typically eight or nine). It is a challenge to obtain liquid-crystalline lanthanide complexes, because it is difficult to make the bulky coordination sphere of the lanthanide ion compatible with the structural requirements for the existence of liquid-crystal phases (rod-like or