Singlet Oxygen Quantum Yields. Nanosecond time-resolved luminescence spectroscopy was used to determine the singlet oxygen quantum yields for 6MP and Aza following UVA excitation. Briefly, a GCR-150-30 Nd:YAG laser (Spectra Physics, 355 nm excitation wavelength, 7 ns pulse width) was used as the excitation source. Singlet oxygen phosphorescent decay traces were collected at 1270 nm using a modified Fluorolog-3 spectrometer (HORIBA, Jobin Yvon), with a near-IR sensitive photomultiplier tube (H10330A-45, Hamamatsu) as detector. The decay traces were stored on a digital oscilloscope (TDS 360, Tektronics). Solutions of 6MP, Aza, and the phenalenone standard were prepared in Tris-buffered D2O and acetonitrile at an optical density of 0.3 at 355 nm in 1 cm path length quartz cuvettes. The O2-saturated solutions were bubbled with ultrapure oxygen gas for at least 20 min prior to testing.