Blue luminescent zinc(II) complexes with polypyridylamine ligands: crystal structures and luminescence properties

Abstract

A series of blue luminescent zinc(II) complexes, [Zn(dpa)X₂] [dpa = di-2-pyridylamine, X = OAc 1a, Cl 1b, CN 1c or 4-MeC₆H₄S 1d], [Zn(dpa)₂][CF₃SO₃]₂ 2, [Zn(tpda)X₂] [tpda = 2,6-bis(2-pyridylamino)pyridine, X = OAc 3a or Cl 3b] and [Zn(tpda)(CF₃SO₃)₂] 4 has been prepared. Their molecular structures, except complex 4, have been established by X-ray crystallography. In the crystal lattice of 2 the [Zn(dpa)₂]²⁺ cations and CF₃SO₃^– anions are disposed in pairs via intermolecular hydrogen bonds [N(2)· · ·O(2) 2.865(4) Å]. The crystal packing of [Zn(dpa)(OAc)₂] 1a revealed that two adjacent molecules associate in pairs through intermolecular hydrogen bonding [N(2)· · ·O(4′) 2.816(3) Å]. However, the crystal lattice of its tpda derivative 3a shows that the molecules are linked by extensive intermolecular hydrogen bonding between the amino groups and the acetate ligands [N(2)· · ·O(2′) 2.805(3) and N(4)· · ·O(4′) 2.860(3) Å] resulting in an interlocking hydrogen bond network. Polymeric one-dimensional tapes are generated through extended π–π stacking interactions between the molecules of [Zn(dpa)(4-MeC₆H₄S)₂] 1d, and the thiolate groups are aligned in an all-anti conformation along the metal chain. In the case of [Zn(dpa)(CN)₂] 1c, co-operative intermolecular hydrogen bonds and aromatic π–π interactions in its solid state lead to a supramolecular two-dimensional sheet. All the zinc(II) complexes display high energy intraligand ¹(π–π*) fluorescence in degassed MeOH at 298 K, and intraligand ³(π–π*) phosphorescence in a glassy solution (MeOH–EtOH 1∶2 at 77 K). An emission band observed for 1c (418 nm) and 1d (481 nm) in their solid state emission spectra is ascribed to excimeric emission due to aromatic π–π interactions.