Evaluation of EPR monitoring of singlet oxygen production using the photosensitizer chloroboron subphthalocyanine

Singlet molecular oxygen produced in the presence of 2,2,6,6-tetramethyl-4-piperidone (TMPD) in 1,4-dioxan, by boron subphthalocyanine chloride as the sensitizer, was monitored by Electron Paramagnetic Resonance (EPR) via the oxidation of TMPD to the corresponding aminoxyl radical. This radical product is formed in low yields along the singlet oxygen pathway. A kinetic analysis of the process allows one to understand the reasons behind this, while affording more accurate quantitative insights into the mechanistic details. A crucial aspect of this reaction system, which explains the low aminoxyl radical yields along the singlet oxygen pathway, is the disappearance of the sensitizer triplet state in a reaction with TMPD, which has a rate constant of 1 x 10(9) M-1.s(-1).