Mono-, Di-, and Polymeric Pyridinoylhydrazone Zn^II Complexes: Structure and Photoluminescent Properties

A series of zinc complexes with tridentate pyridinoylhydrazone ligands bearing two different end groups and coordinated pyridine molecules were synthesized and characterized. Crystal structures with mono-, di-, and polymeric arrangements were obtained, depending on the position of the pyridinoyl nitrogen atom. Direct involvement of the pyridinoyl moiety in the coordination to the metal was observed when the nitrogen was in the ortho or meta position. The meta derivatives led to 1D polymers with the same coordination environment, regardless of the bulkiness of the end group. In this case, unprecedented solid-state photoluminescence quantum yields were recorded. In the ortho ligands, the further bidentate N,N site involving the pyridinoyl nitrogen atom was able to coordinate a further zinc acetate unit. Molecular and periodic calculations were performed within density functional theory (DFT) to rationalize the photophysical behavior. Excellent agreement between the theoretical and experimental absorptions was observed. The HOMOs and LUMOs of the complexes were investigated both in the solid state and in solution.