Exploratory study of the quenching of photosensitizers by initiators of free radical "living" polymerization

Several compounds of the type PhCR¹R²-TEMPO (where TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxide) have been prepared and examined as potential photosensitized initiators for "living" free radical polymerizations. For example, these compounds are excellent quenchers of triplet xanthone (rate constants in the range (8.0–13) × 10⁸ M⁻¹ s⁻¹) and result in the formation of benzylic and TEMPO radicals. The importance of excited state energy and nature of the excited triplet state have also been explored. We also tested pyrene as a possible singlet sensitizer. While pyrene fluorescence is efficiently quenched, the process does not lead to cleavage of the C—O bond. A model is proposed to account for this anomaly in the photosensitized decomposition of these TEMPO adducts. Keywords: TEMPO, laser flash photolysis, time resolved emission spectroscopy, sensitizers, quenching, photoinitiator.