Garner, KL; Parkes, LF; Piper, JD; Williams, HF; ,
Platinum(II) complexes of the form [PtLnCl](+) are reported, containing the N boolean AND N boolean AND N-coordinating ligands 2,6-di-(8-quinolyl)pyridine (L), 2,6-di(8-quinolyl)-4-methoxypyridine (L-2), or 2,6-di(7-aza-indolyl)-pyridine (L). Metathesis of the chloride co-ligand in [(PtLCl)-Cl-1](+) can be accomplished under mild conditions, as exemplified by the formation of the complexes [(PtLOMe)-O-1](+) and [PtL1(C C-tfp)](+), in which L-1 remains bound as a terdentate ligand {HC C-tfp = 3,5-bis(trifluoromethyl)-phenylacetylenel. An N boolean AND C boolean AND N-coordinated, cyclometalated analogue of [(PtLCl)-Cl-1](+) has also been prepared, namely, (PtLCl)-Cl-4 where HL4 is 1,3-di(8-quinolyl)benzene. The common feature among the six new complexes described here is that they contain 6-membered chelate rings, rather than the usual 5-membered rings that form when more common N boolean AND N boolean AND N ligands, such as 2,2':6',2 ''-terpyridine (tpy), bind to Pt(II). All the quinolyl-based complexes are phosphorescent in solution at room temperature, with quantum yields up to 4%. This contrasts with the well-established lack of emission from [Pt(tpy)Cl](+) under these conditions. Density functional theory calculations suggest that the improvement may stem, at least in part, from the relief of ring strain associated with the larger chelate ring size, leading to a more optimal bite angle at the metal, close to 1800, and hence to a stronger ligand field. Consideration of the luminescence parameters, including data at 77 K, together with absorption and electrochemical data and the results of TD-DFT calculations, suggests that the lowest-lying singlet states have metal-to-ligand charge-transfer (MLCT) character, but that the triplet state from which emission occurs has more predominant ligand-centered character. The azaindolyl complex [(PtLCl)-Cl-3](+) is not emissive at room temperature, apparently owing to a particularly small radiative rate constant. The cyclometalated complex (PtLCl)-Cl-4 emits at lower energy than [(PtLCl)-Cl-1](+) but, in this case, the luminescence quantum yield is inferior to related complexes with 5-membered chelate rings.