Liquid Crystalline Metallocyclophanes
Metallocyclophanes are flat, dimeric structures formed by the self-assembly of oligopyridine/pyrazine or oligopyridine/quinoxaline ligands with kinetically labile metal ions, such as copper(I), silver(I) or cobalt(II). Both copper(I) and silver(I) have tetrahedral binding domains, and this is well accommodated by the bidentate binding domains of compound 1; see Equation 1. One quinoxaline N atom remains uncomplexed in the metallocyclophane.
Molecules having both
lipophillic and polar domains can form
liquid crystals. In 2008, we showed that oligopyridine/quinoxaline
containing long-chain polyethers will, when combined with Ag(I) salts,
liquid crystalline, disilver(I) metallocyclophanes. The
dicopper(I) metallocyclophanes are low-melting, non-liquid crystalline
complexes. The structure of a typical dicopper(I) metallocyclophane
from a long-chain ligand 2n is shown
Liquid crystalline Ag(I) complexes are of interest for electronic display technologies. The complexes of Cu(I) with oligopyridine-type ligands, on the other hand, are luminescent, and have been identified as candidate materials in electroluminescent displays and organic photovoltaic semiconductors.
The challenge, which we have
identified, is therefore to
synthesize an improved generation of ligands. These will form liquid
crystalline dicopper(I) metallocyclophanes. For example, ligands
molecule 3n will feature phenyl groups as spacers
between the polar metal coordination sites and the lipophillic alkyl
chains. This feature is
well-known to favor liquid crystallinity. The synthesis of 3n
is well underway.