ABSTRACT
The photochemical fixation of carbon dioxide has stimulated considerable interest in providing models of prebiotic photosynthesis. 1 Intensive efforts have been made during the last decade to mimic the natural process of photosynthesis, using reaction systems which included photosensitizers (such as porphyrin or bipyridine derivatives), electron transfer mediators (such as methyl viologen), and also sacrificial electron donors (such as tertiary amines). The need for sacrificial electron donors is due to the fact that the reduction products of carbon dioxide, such as formic acid, formaldehyde, and methanol, are themselves very effective reducing agents. Earlier work on such artificial photosynthesis was reviewed.2a Photochemical fixation of carbon dioxide is also of interest for the potential utilization of this atmospheric constituent on Mars, where it amounts to 95.3% of the lower atmosphere.2b
The primary steps in the reactions of carbon dioxide or of the bicarbonate or carbonate ions in solution were identified mainly by pulse radiolysis but also by flash photolysis, electron spin resonance, and electrochemical experiments. The radiolysis of water results in short-lived radicals, ions, and atoms, which may react with certain solutes to produce secondary radicals. If these secondary radicals have intense UV or visible absorption bands, then the rates of their formation and decay can be determined by kinetic spectrophotometry. Their interaction with other solutes may result in increases in the decay rates, from which the second-order rate constants of such interaction can be derived.3
