2,2′-Biquinolines as test pilots for tuning the colour emission of luminescent mesomorphic silver(i) complexes

The synthesis and characterization of a series of 2,2'-biquinolines differently substituted in the 4,4'-position and their corresponding silver(I) derivatives obtained through reaction with silver triflate in a 1 : 1 stoichiometric ratio are reported. In order to perform a systematic investigation on the role played by the substituents on the coordination to the silver(I) centre, structural studies through single crystal X-ray diffraction have been performed on two Ag(I) model complexes. Unlike their analogous 2,2'-bipyridine ligands, the biquinolines have been found to behave only as chelated ligands towards the silver(I) ion, irrespective of the substituents. The coordination sphere of the Ag(I) is filled by a solvent molecule and, depending on the presence and nature of the substituents on the organic ligand, by an oxygen atom coming from a coordinated triflate or from a carboxylic group of a symmetrically related molecule, giving rise to neutral or ionic species. For the highest Ag(I) triflate homologues the presence of long and flexible peripheral tails makes it possible to achieve liquid crystalline properties with columnar organization whose high order is due to the large and rigid core. Moreover, the metal coordination induces in all the Ag(I) species interesting emission properties both in solution and condensed states, giving rise to blue or green emitters, depending on the nature of the substituents on the biquinoline units.