Synthesis, structures and luminescence properties of N^C^N-coordinated platinum(II) complexes based on an anthracene core: A red shift and a twist

The platinum (II) complex of the new N^C^N-coordinating ligand 1,8-bis(2-pyridyl)anthracene shows an extraordinary twist of the anthracene core, which may account for the lack of observable luminescence. In contrast, the Pt(II) complex of an N^C^N-binding benzodiquinoline ligand is a vividly red emitter, displaying vibrationally structured phosphorescence in solution. [Display omitted] 1,8-Bis(2-pyridyl)anthracene HL2 is shown to act as a tridentate, N^C^N-coordinating ligand to platinum(II), undergoing metallation at position 9 of the anthracene. The structures of Pt(N^C^N-L2)Cl and of HL2 have been determined by X-ray diffraction. The complex features two 6-membered chelate rings and an N-Pt-N bite angle of 178.16(7)°, close to the “ideal” value of 180°. The attainment of this angle necessitates not only a twisting of the pyridyl rings relative to the anthracene, but also a remarkable twisting distortion of the anthracene unit itself. The complex is isomeric with the previously reported Pt(d8qb)Cl {d8qb = 1,3-di(8-quinolyl)benzene}. A new derivative of the latter has been prepared, PtL3Cl, featuring methyl substituents in the central benzene ring ortho to the quinolines. Like PtL2Cl, its structure shows twisting of the coordinated heterocycles relative to the plane of the central aromatic ring. The Pt(II) complex PtL1Cl of a related N^C^N-coordinating ligand featuring extended conjugation, benzo[1,2-h: 5,4 = h’]diquinoline HL1, has also been prepared following a previously described procedure. PtL1Cl is brightly phosphorescent in deoxygenated solution at ambient temperature, in the red region of the spectrum. The impressive photoluminescence quantum yield of 0.30 and long emission lifetime of 38 μs are indicative of a rigid structure with little excited state distortion. In contrast, PtL2Cl shows no detectable emission, probably due to the distorted nature of the structure leading to rapid non-radiative decay of the excited state. This result contrasts with the weak red emission displayed by the isomeric Pt(d8qb)Cl and its dimethylated derivative PtL3Cl.