Design of Luminescent Heteroleptic Cyclometalated PtII and PtIV Complexes: Photophysics and Effects of the Cyclometalated Ligands.

Pentafluorophenyl benzoquinolinyl PtII [Pt(bzq)(HC^N-κN)(C6F5)] (1a-g) complexes, bearing non-metalated N-heterocyclic HC^N ligands (HC^N = Hoxd a, dfppy b, pbt c, Br-pbt d, pq e, thpy f, pypy g), and heteroleptic bis(cyclometalated) PtIV fac-[Pt(bzq)(C^N)(C6F5)Cl] (2b-g) derivatives, generated by oxidation of 1b-g with PhICl2, are reported. The oxidation reaction of 1a evolves with formation of the bimetallic PtIV complex syn-[Pt(bzq)(C6F5)Cl(µ-OH)]2 3. The crystal structures of 1a,d,f, 2b,d,e and 3 have been corroborated by X-ray crystallography. A comparative study of the absorption and photoluminescence properties of the two series of complexes PtII (1) and PtIV (2), supported by calculations, is presented. The low-lying transitions (absorption, emission) of PtII complexes 1a-e (solution and polystyrene PS films) were associated to admixture IL/MLCT located on the cyclometalated Pt(bzq) unit, with minor IL'/ML'CT/LL'CT contributions involving the non-metalated ligand. 1g, bearing the more delocalized pyridyl pyrene (Hpypy) as ancillary ligand, shows dual 1ππ* and 3ππ* (Hpypy) emission in fluid CH2Cl2 and dual 3IL/3MLCT Pt(bzq)] and [3ππ*, Hpypy] phosphorescence at 77 K. Upon oxidation, PtIV complexes 2b-f display (solution, PS) ligand-based phosphorescence, which arises from the bzq in 2b (3LC) or from the second C^N ligand in 2c-f (3L'C) with some 3LL'CT in 2f. Despite metalation of the pyrenyl group, 2g exhibits dual emission 1ππ*/3ππ* located on the pypy chromophore.