Bisubstituted-biquinoline Cu(I) complexes: synthesis, mesomorphism and photophysical studies in solution and condensed states

New ionic Cu(I) coordination complexes with 4,4′-bisubstituted-2,2′-biquinolines showing low temperature lamello-columnar and columnar hexagonal thermotropic mesomorphism, depending on the substituents, are synthesized and characterized. All complexes show good stability both in solution and in condensed states, confirming the usefulness of biquinoline ligands in stabilizing Cu(I) species. Single crystal X-ray diffraction studies on a model compound have confirmed the ionic nature of the new species, with the cations in a tetracoordination star-like geometry containing the Cu(I) center bis-chelated by the two biquinolinate ligands. The crystal packing has shown a strong tendency towards the formation of columns of molecules interacting via p–p stacking interactions. The mesomorphic behavior was investigated via polarized optical microscopy (POM), differential scanning calorimetry (DSC) and powder X-ray diffraction (PXRD) at variable temperature. Moreover, photophysical studies have shown that although the emission is quenched in solution due to deactivation paths derived from flattening distortions, in condensed states the star-like geometry is stabilized by intermolecular interactions and emission is observed in the red region. A correlation between the new ionic Cu(I) complexes and their Ag(I) analogues previously reported is also undertaken.