Cyclometalated Iridium and Platinum Complexes as Singlet Oxygen Photosensitizers: Quantum Yields, Quenching Rates and Correlation with Electronic Structures Billy Hernandez,† Ruomei Gao,† Pasha L. Hunt,† Matthias Selke,*,† Peter I. Djurovich,‡ Drew Murphy,‡ Mark E. Thompson,*,‡ †Department of Chemistry and Biochemistry, California State University at Los Angeles, Los Angeles, CA 90032, and ‡Department of Chemistry, University of Southern California, Los Angeles, CA 90089 The photophysical properties are described for a series of cyclometalated platinum and iridium complexes that can serve as photosensitizers for singlet oxygen (1O2). 1O2 quenching Cyclometalated complexes form 1O2 from the triplet excited state. Yields of 1O2 formation are unity for Pt derivatives. Energy transfer occurs through contact with the cyclometalated ligand. (ppy)2Ir(acac) (ppy)Pt(acac) 1O2 formation Triplet spin density Cyclometalated complexes are poor quenchers of 1O2, whereas (diimmine)PtMe2 complexes are efficient at quenching 1O2. Energy transfer is believed to occur by direct contact with the metal center. HOMO