PHOTOCHEMICAL STABILITY OF VARIOUS PORPHYRINS IN SOLUTION AND AS THIN-FILM ELECTRODES

The stabilities of various porphin analogue compounds such as unsubstituted and substituted phthalocyanines, tetraphenylporphyrin, naphthalocyanine and tetraazaporphyrin have been investigated. The compounds were dissolved either in organic solvents or water. Electronic spectra in the visible region were recorded after the samples were kept either under inert gas or air in the dark and under illumination. The rate of decay of absorption is explained by a decomposition reaction which is of first-order kinetics with respect to the porphyrin concentration. Illumination, and the presence of oxygen, enhance the rate of decomposition. In general, an increase of the size of the ligand and an increase of the electron donating ability of substituents further enhance the rate of decomposition. Central metal ions with open shell arrangements of electrons favour the photodecomposition of the molecules. Thin solid films of phthalocyanines obtained by vapour deposition were investigated for their stability in contact with either air or an aqueous electrolyte. The influence of a cathodic bias potential was studied in the dark and under illumination. When compared to the dissolved molecules the crystalline solids turned out to be much more stable although, decomposition could be detected in a long-term photoelectrochemical experiment. Decomposition products were determined and appropriate mechanisms are discussed.