Design of Luminescent, Heteroleptic, Cyclometalated Pt^II and Pt^IV Complexes: Photophysics and Effects of the Cyclometalated Ligands

Abstract

Neutral pentafluorophenyl benzoquinolinyl Pt^II [Pt(bzq)(HC^N-κN)(C₆ F₅ )] (1 a-g) complexes, bearing nonmetalated N-heterocyclic HC^N ligands [HC^N=2,5-diphenyl-1,3,4-oxadiazole (Hoxd) a, 2-(2,4-difluorophenyl)pyridine (dfppy) b, 2-phenylbenzo[d]thiazole (pbt) c, 2-(4-bromophenyl)benzo[d]thiazole (Br-pbt) d, 2-phenylquinoline (pq) e, 2-thienylpyridine (thpy) f, 1-(2-pyridyl)pyrene (pypy) g], and heteroleptic bis(cyclometalated) Pt^IV fac-[Pt(bzq)(C^N)(C₆ F₅ )Cl] (2 b-g, bzq: benzo[h]quinolinyl) derivatives, generated by oxidation of 1 b-g with PhICl₂ , are reported. The oxidation reaction of 1 a evolved with formation of the bimetallic Pt^IV complex syn-[Pt(bzq)(C₆ F₅ )Cl(μ-OH)]₂ 3. The crystal structures of 1 a,d,f, 2 b,d,e and 3 were corroborated by X-ray crystallography. A comparative study of the absorption and photoluminescence properties of the two series of complexes Pt^II (1) and Pt^IV (2), supported by time-dependent DFT calculations (TD-DFT), is presented. The low-lying transitions (absorption and emission) of Pt^II complexes 1 a-e [solution and polystyrene (PS) films] were assigned to the IL/MLCT mixture located on the cyclometalated Pt(bzq) unit, with minor IL'/ML'CT/LL'CT contributions involving the non-metalated ligand. Complex 1 g, bearing the more delocalized pyridyl pyrene (Hpypy) as an ancillary ligand, shows dual ¹ ππ* and ³ ππ* (Hpypy) emission in fluid CH₂ Cl₂ and dual ³ IL/³ MLCT [Pt(bzq)] and [³ ππ*, Hpypy] phosphorescence at 77 K. Upon oxidation, Pt^IV complexes 2 b-f display (solution, PS) ligand-based phosphorescence that arises from the bzq in 2 b (³ LC) or from the second C^N ligand in 2 c-f (³ L'C) with some ³ LL'CT in 2 f. Despite metalation of the pyrenyl group, 2 g exhibits dual emission ¹ ππ*/³ ππ* located on the pypy chromophore.