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40854-AC3
Singlet Oxygen Sensitization by Cyclometallated Iridium and Platinum Complexes

Mark E. Thompson, University of Southern California and Matthias Selke, California State University (Los Angeles)

<>Cyclometalated Iridium and Platinum Complexes as Singlet Oxygen Photosensitizers: Quenching Rates, Quantum Yields, Singlet Oxygen Quenching Rates and Correlation with Electronic Structures

<>A study was conducted on a series of organometallic iridium and platinum complexes that can serve as photosensitizers for singlet oxygen production.  Photophysical properties were measured for a series of cyclometalated platinum and iridium complexes that generate singlet oxygen upon irradiation.  The complexes have the formula (C^N)₂Ir(O^O) or (C^N)Pt(O^O) where C^N is a monoanionic cyclometalating ligand such as 2-(phenyl)pyridyl (ppy) and 2-(phenyl)quinolyl, and O^O is the ancillary ligand acetylacetonate (acac) or dipivolymethane (dpm).  Also examined were a series of (N^N)PtMe₂ complexes where N^N is a diimmine such as 2,2'-bipyridyl.  The quantum yields for singlet oxygen production and rates of singlet oxygen quenching by the complexes were measured at excitation wavelengths of 355 nm or 532 nm in benzene and deuterated methanol solution.  In general, the cyclometalated complexes are excellent photosensitizers for the production of singlet oxygen, while the (N^N)PtMe₂ complexes were ineffective at this reaction.  Quantum yields of singlet oxygen production range from 0.9–1.0 for the cyclometalated Pt complexes and 0.5–0.9 for Ir complexes.  Luminescence quenching of the Ir complexes occurs from a combination of electron and energy transfer processes as demonstrated by the rapid quenching rates (2.9 x 10⁹ – 2.3 x 10¹⁰ M^-1 s^-1, the latter value for (ppy)₂Ir(acac) being close to the diffusion controlled limit).  The concurrent high yields of singlet oxygen production for the Ir complexes indicate that formation of singlet oxygen can also occur from both energy and electron transfer pathways.  A possible mechanism for the formation of single oxygen by the latter route involves electron-hole recombination in a process similar to what occurs during chemiluminescence.  For Ir complexes with low emission energy, physical deactivation of the triplet excited state becomes competitive with energy transfer to ground state dioxygen.  The Pt complexes have slower quenching rates (near 1/9 the diffusion rate) and thus, only react by energy transfer.  However, little to no physical deactivation of the excited state occurs with the Pt complexes as the formation fractions of singlet oxygen are close to unity.  The rates of singlet oxygen quenching for the complexes are in the range of 2 x 10⁵ – 2 x 10⁷ M^-1 s^-1 for Ir complexes, 6 x 10⁶ – 2 x 10⁷ M^-1 s^-1 for Pt cyclometalated complexes, and up to 10⁹ M^-1 s^-1 for the (N^N)PtMe₂ derivatives.  Differences between the Ir and Pt cyclometalates in the efficiency of both forming and quenching singlet oxygen were rationalized using the results from density functional theory calculations on the ppy analogs.  The relatively high rates of the Pt complexes are believed to come about from the exposed coordination geometry in these planar species.   

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