Mixed-ligand Pt(II) and Pd(II) complexes on the basis of cyclometalated 2-phenylpyridine and polypyridyl ligands

1H NMR spectroscopy, electronic absorption and emission spectroscopy, and cyclic voltammetry were used for comparative study of the complexes [M(ppy)(dpz)]ClO4, [M(ppy)ddpq]ClO4, and [(M(ppy)2(μ-tpbq)](ClO4)2 (M = Pt(II), Pd(II); ppy− = deprotonated 2-phenylpyridine; dpz = 2,3-dipirydylpyrazine, ddpq = 6,7-dimethyl-2,3-dipyridylquinoxaline, tpbq = 2,2′,3,3′-tetrapyridyl-6,6′-biquinoline). The complexes feature trans arrangement of the pyridine moieties of the cyclometalated ppy and polypyridyl (N∧N) ligands. Steric interactions orient the noncoordinated pyridine moiety of the (N∧N) ligands orthogonally to the coordination plane of the complexes. One-electron reduction potentials and parameters of the absorption and luminescence spectra of the complexes were determined. It was shown in terms of the localized molecular orbital model that the LUMOs of the [M(ppy)dpz]+ complexes are primarily localized on the {M(ppy)} metal complex fragment. The presence of low-energy vacant π* orbitals in ddpq and tpbq results in that the LUMOs in the [M(ppy)ddpq] and [(M(ppy)2μ-tpbq]2+ complexes are localized on the polypyridyl (N∧N) ligand, which is responsible for highly efficient photoexcitation energy transfers [M(ppy)→ddpq]+ and [M(ppy) → (μ-tpbq)←M(ppy)]2+.