A series of organometallic complexes and the singlet oxygen sensitization properties of these complexes are provided. Complexes with acetylacetonate ligands give singlet oxygen quantum yields near unity, whether exciting the ligand-based state or the lowest energy excited state (MLCT+.sup.3LC). The singlet oxygen quenching rates for these .beta.-diketonate complexes are small, roughly three orders of magnitude slower than the corresponding phosphorescence quenching rate. Similar complexes were prepared with glycine or pyridine tethered to the Ir(III) center (i.e. (bsn).sub.2Ir(gly) and (bt).sub.2Ir(py)Cl, where gly=glycine, and py=pyridine). The glycine and pyridine derivatives give high singlet oxygen yields.