Photobleaching studies on azabenzoporphyrins and related systems: a comparison of the photobleaching of the zinc(II) complexes of the tetrabenzoporphyrin, 5-azadibenzo[b,g]porphyrin and phthalocyanine systems

The kinetics of photobleaching of zinc(II) tetra-t-butyltetrabenzoporphyrin 1, zinc(II) 13,17-diethyl-12,18-dimethyl-5-azadibenzo[b,g]porphyrin 2 and zinc(II) tetra-t-butylphthalocyanine 3 are studied. In methanol containing ca 0.1% pyridine, complex 1 undergoes photoaggregation, 2 is relatively photostable, while 3 undergoes a smooth true photobleaching process with loss of intensity across the visible region. Where appropriate, apparent first-order rate constants for the photodiminution of the α band in the spectra are reported, and the effects of solvent variation and of additives are measured. It has not been possible to provide an overall rationalization of the solvent effects, but the effects of additives support the view that the photo-oxidative processes involve singlet oxygen. Experiments with potassium superoxide do not support superoxide involvement. Preparative experiments in methanol have been carried out with 1 and 3. 1 leads to a large recovery of the starting material, indicating that the photoaggregation does not involve the formation of new covalent bonds. 3 gives rise to at least five products (besides the starting material). Two of these, isolated in low yields (<5%), are identified as a fragmentation (depolymerization) product, 4-t-butylphthalonitrile, and the expected oxidative product, 4-t-butylphthalimide. The results are discussed and conclusions drawn.