Spotlight on the ligand: luminescent cyclometalated Pt(IV) complexes containing a fluorenyl moiety

The synthesis, electrochemistry and photophysical properties of a family of Pt(IV) complexes with cyclo-metalated 2-(9,9-dimethylfluoren-2-yl)pyridine (flpy) are reported. Homoleptic and heteroleptic tris-cyclometalated complexes with a meridional configuration, mer-[Pt(C<^>N)(2)(flpy)]OTf, with C<^>N = flpy or cyclometalated 2-phenylpyridine (ppy), were prepared by reacting the bis-cyclometalated precursors [Pt(C<^>N)(2)Cl-2] with flpyH in the presence of two equivalents of AgOTf. The corresponding facial isomers were obtained by photoisomerization. The Pt(IV) complexes with flpy display intense absorptions in the near-visible region and yellow phosphorescence in fluid solutions at 298 K, with quantum yields in the range 0.06-0.28 and lifetimes of hundreds of microseconds. The emissions arise from essentially (LC)-L-3 (flpy) states in all cases, with little metal-orbital contribution. However, computational calculations and experimental data demonstrate that subtle variations in the contribution of metal orbitals to the emitting state have a profound impact on quantum yields, while nonradiative deactivation through the thermal population of deactivating LMCT states does not have a significant influence.