Platinum(II) Complexes of Nonsymmetrical NCN-Coordinating Ligands: Unimolecular and Excimeric Luminescence Properties and Comparison with Symmetrical Analogues

[Image: see text] A series of seven new platinum(II) complexes PtL(n)Cl have been prepared, where L(n) is an NCN-coordinating ligand comprising a benzene ring 1,3-disubstituted with two different azaheterocycles. In PtL(1–5)Cl, one heterocycle is a simple pyridine ring, while the other is an isoquinoline, a quinoline, a pyrimidine (L(1), L(2), L(3)), or a p-CF(3)- or p-OMe-substituted pyridine (L(4) and L(5)). PtL(6)Cl incorporates both a p-CF(3) and a p-OMe-substituted pyridine. The synthesis of the requisite proligands HL(n) is achieved using Pd-catalyzed cross-coupling methodology. The molecular structures of six of the Pt(II) complexes have been determined by X-ray diffraction. All the complexes are brightly luminescent in deoxygenated solution at room temperature. The absorption and emission properties are compared with those of the corresponding symmetrical complexes featuring two identical heterocycles, PtL(nsym)Cl, and of the parent Pt(dpyb)Cl containing two unsubstituted pyridines [dpybH = 1,3-di(2-pyridyl)benzene]. While the absorption spectra of the nonsymmetrical complexes show features of both PtL(nsym)Cl and Pt(dpyb)Cl, the emission generally resembles that of whichever of the corresponding symmetrical complexes has the lower-energy emission. PtL(1)Cl differs in that—at room temperature but not at 77 K—it displays emission bands that can be attributed to excited states involving both the pyridine and the isoquinoline rings, despite the latter being unequivocally lower in energy. This unusual behavior is attributed to thermally activated repopulation of the former excited state from the latter, facilitated by the very long-lived nature of the isoquinoline-based excited state. At elevated concentrations, all the complexes show an additional red-shifted emission band attributable to excimers. For PtL(1)Cl, the excimer strikingly dominates the emission spectra at all but the lowest concentrations (<10(–5) M). Trends in the energies of the excimers and their propensity to form are compared with those of the symmetrical analogues.