Cyclometalated Spirobifluorene Imidazolylidene Platinum(II) Complexes with Predominant (LC)-L-3 Emissive Character and High Photoluminescence Quantum Yields

Two novel bidentate C<^>C*(spiro) cyclometalated platinum(II) complexes comprising a spiro-conjugated bifluorene ligand and different beta-diketonate auxiliary ligands are synthesized and characterized. Their preparation employs a robust and elaborate synthetic protocol commencing with an N-heterocyclic carbene precursor. Structural characterization by means of NMR techniques and solid-state structures validate the proposed and herein presented molecular scaffolds. Photophysical studies, including laser flash photolysis methods, reveal an almost exclusively ligand-centered triplet state, governed by the C<^>C*(spiro)-NHC ligand. The high triplet energies and the long triplet lifetimes in the order of 30 mu s in solution make the complexes good candidates for light-emitting diode-driven photocatalysis, as initial energy transfer experiments reveal. In-depth time-dependent density functional theory investigations are in excellent accordance with our spectroscopic findings. The title compounds are highly emissive in the bluish-green color region with quantum yields of up to 87% in solid-state measurements.