A new class of cyclornetalated tetradentate platinum complexes of the type Pt[(NC)-C-boolean AND-O-LL'] was synthesized and characterized. (NC)-C-boolean AND is a cYclmnetalating ligand such as phenyl-pyrazole (ppz), phenyl-methylimidazole (pmi), or phenyl-pyridine (ppy) and LL' is an ancillary ligand such as phenoxyl-pyridine (Popy) The complexes in this series are highly luminescent, emitting blue to green light in solution with quantum efficiencies ranging from 0.39 to 0.64 and luminescent lifetimes from 2 to 9 mu s. When doped in a poly( methyl nnethacrylate) (PMMA) thin film, measured quantum efficiencies increase to 0.81-0.97 with lifetimes ranging from 4.5-10.4 mu s. One notable example, the metal complex PtOO3, emits green light with a luminescent quantum efficiency approaching 100% and achieves approximately 100% electron-to-photon conversion efficiency in device settings.