Two tridentate pyridinyl-hydrazone zinc(II) complexes as fluorophores for blue emitting layers

Two new complexes were obtained by reaction of zinc (II) acetate with 4-fluoro-N'-(pyridin-2-ylmethylene) benzohydrazide or with 4-(hexyloxy)-N'-(pyridin-2-ylmethylene)benzohydrazide ligands in pyridine. Both ligands have a pyridinyl-hydrazone moiety able to act as a mono-negative tridentate ligand toward zinc ion in 2:1 stoichiometric ratio, producing an octahedral environment. The derived complexes are poor emitters in diluted solution. Instead, they exhibit intense blue fluorescence in the solid state due to AIE (aggregation induced emission) effect. The crystalline complexes are bright yellow in natural light and blue under UV–visible light with remarkable Stokes Shifts. Their photoluminescence quantum yields are ranging 20–30%, making them very promising as fluorophore dopants for blue emissive layers. One ligand and its complex were characterized by single crystal X-ray diffraction analysis that revealed an almost planar conformation of two ligands coordinated to the metal and the presence of significant π-π stacking of molecules at about 3.4 Å.