Brightly Luminescent Platinum Complexes of (NC-boolean AND N)-C-boolean AND Ligands Forming Six-Membered Chelate Rings: Offsetting Deleterious Ring Size Effects Using Site-Selective Benzannulation

Brightly emissive platinum(II) complexes (lambda(emission,max) = 607-612 nm) of the type (LPtCl)-L-R are reported, where L-R is a cyclometalated (NC-boolean AND N)-C-boolean AND-coordinating ligand derived rom 1,3-di(2-trifluoromethyl-4-phenanthridinyl)benzene ((LH)-L-CF3) or 1,3-di(2-tert-butyl-4-phenanthridinyl)benzene ((LH)-L-tBu). Metathesis of the chlorido ligand can be achieved under mild conditions, enabling isolation of ionic compounds with the formula [(LPtL)-L-CF3']PF6 where L' = pyridine or (4-dimethylamino)-pyridine (DMAP), as well as the charge-neutral species (LPt)-L-tBu-(C C-C6H4-tBu) (C C-C6H4-tBu = 4-tert-butylphenylacetylido). Compared with (NNN)-N-boolean AND-N-boolean AND-ligated Pt(II) complexes that form 5-membered chelates, these compounds all contain 6-membered rings. Expanding the chelate ring size from 5 to 6 has been previously demonstrated to enhance emission in some (NNN)-N-boolean AND-N-boolean AND-coordinated Pt(II) species-for example, in complexes of 2,6-di(8-quinolinyl)pyridine vs those of 2,2':6',2 ''-terpyridine-but in related (NC-boolean AND N)-C-boolean AND-coordinated species, luminescence quantum yields are significantly lower for the 6-membered chelate ring complexes. Here, we demonstrate that site-selective benzannulation of the quinolinyl side-arms can offset the deleterious effect of changing the chelate ring-size and boost photophysical properties such as the quantum yield. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations suggest that benzannulation counterintuitively destabilizes the emissive triplet states compared to the smaller pi-system, with the "imine-bridged biphenyl" form of the phenanthridinyl arm helping to buffer against larger molecular distortions, enhancing photoluminescence quantum yields up to 0.09 +/- 0.02. The spontaneous formation under aerated conditions of a Pt(IV) derivative ((LPtCl3)-L-CF3) is also reported, together with its molecular structure in the solid state.