The luminescence properties of a set of metallo-AHP derivatives, M[Me-AHP] (M = Mn, Pd, Pt), were examined in toluene. All solutions were thoroughly deoxygenated with ultra-pure nitrogen (99.99999% purity). For all three compounds, we failed to register any emission in the red to NIR spectral range (up to 1020 nm), leading all us to conclude that the compounds are not emissive or exhibit such weak emission (F << 0.1%) that precludes more thorough photophysical studies. (c) Singlet oxygen sensitization. Singlet oxygen quantum yields were determined as previously described. The assay relies on the singlet oxygen acceptor 9,10-dimethylanthracene, whose characteristic UV absorption vanishes upon reaction with 1 O2. Methylene blue, with F( 1 O2) = 0.48, was used as the standard. Figure depicts the bleaching of 9,10dimethylanthracene at 358 nm in presence of M[Me-AHP] (M = Mn, Pd, Pt), with the corresponding absorption spectra included as Supporting Information. It should be noted that both 9,10-dimethylanthracene and the AHP photosensitizer absorb at 358nm, where both 9,10demethylanthracene and the photosensitizer absorb light; however, since no degradation of the photosensitizer was observed throughout the experiment (Figures ), the spectral changes can be attributed solely to the bleaching of 9,10-dimethylanthracene. As can be seen, both the Pd(II) and Pt(II) complexes generate singlet oxygen, whereas no bleaching of dimethylanthracene was observed for Mn[Me-AHP], i.e., the F( 1 O2) is essentially 0. The Pd(II) complex was found to be a very efficient photosensitizer with F( 1 O2) of = 0.84 ± 0.02 (average of 3 assays) in 9:1 v/v ethanol:tetrahydrofuran, which compares favorably with the best singlet oxygen quantum yields measured for 5d metallocorroles. On the other hand, the F( 1 O2)