Designing Zn(II) complexes as a support of bifunctional liquid crystal and luminescent materials

Four series of 1-(pyridyl)-3-(alkyloxyphenyl)propane-1,3-dione type ligands, named L_2N1, L_2N2, L_4N1 and L_4N2, have been prepared and their thermal behaviour studied. These compounds have been classified on the basis of the position of the pyridinic nitrogen atom (2- or 4-position, 2N or 4N) and the number of alkyl chains at the phenyl substituent (one or two). In each family the alkyl chains are increased from 12 to 18 carbon atoms. In contrast with the absence of mesomorphism of the monocatenar ligands L_2N1 and L_4N1, the dicatenar ones L_2N2 and L_4N2 have been found to be liquid crystals exhibiting lamellar columnar mesophases (Col_L). Those were established through the columnar ordering of disc-like molecules, which is achieved by the approach of the complementary shape between two hemidiscotic molecules. The crystalline structures of selected prototype compounds have been solved. The dicatenar ligands exhibit a dimeric structure with a disc-like shape, which is in agreement with the above mentioned Col_L mesophase. By contrast, monomeric compounds having a hockey-stick shape are found for the monocatenar species, which were not liquid crystals. Reactions of those diketone-based ligands with Zn(II) dichloride afforded to four new families of Zn(II) compounds. Two of them, containing the monocatenar ligands, are liquid crystal materials, showing smectic A (SmA) mesophases and so, indicating that mesomophic behaviour is induced by complexation with the metal. However, the Zn(II) complexes carrying dicatenar ligands were not liquid crystal materials. Most of the Zn(II) complexes also exhibit green/blue emission at room temperature in solution. Additionally, the ionochromic behaviour of selected ligands towards Zn²⁺ and Cu²⁺ ions has also been proved. The results of both the mesomorphic and photoluminescence behaviours are established and compared as a function of the nitrogen position on the pyridine substituent as well as the number of pheripherical chains on the diketone ligand.