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

Abstract

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.

Photophysical Properties of Luminescent Zinc(II)‒Pyridinyloxadiazole Complexes and their Glassy Self‐Assembly Networks

“…A similar assembly of parallel sheets of polymeric chains was found in tridentate meta ‐pyridinylhydrazone derivatives. [10a] In those cases, the assembly of crystals into packed polymeric structures led to an enhancement of the PL emission of meta ‐substituted complexes.…”

“…Moreover, the value measured for [Zn(L m ) 2 py 2 ] is among the highest reported in the literature for solid‐state zinc complexes and comparable to a zinc meta derivative reported in previous works. [10a], , , This value is definitely suitable for a lighting device.…”

“…The results of calculations on all the complexes as mononuclear species in pyridine showed that the HOMO and LUMO orbitals lie on the ligand (see Figure ) and that the pyridines are not involved, in contrast to previously synthesized analogous complexes. [10a] The zinc is not directly involved in electronic transitions, as usually happens for d 10 metals. The predicted absorption maxima are in good agreement with the experimental values (Table ).…”

“…In the last years, our research group has focused interest on N,O ligands apt to coordinate transition metals as stable structures with specific properties . Zinc chelate complexes have attracted our interest for their structural features and optical properties.…”

“…As expected, zinc can drive various coordination modes. [10a], The employment of the ligands in pyridine produced crystalline complexes with the general formula [Zn(L i ) 2 py 2 ], the coordination sphere being completed by the nitrogen of two solvent molecules. The same reaction performed in the absence of pyridine produced disordered coordination networks with the same [Zn(L i ) 2 ] building block.…”