Monometallic mechanism evidence

While a limited amount of experimental evidence does lend support to the bimetallic concept, major objections were voiced by Ziegler, who was of the opinion that like dimeric aluminum alkyls the Ti-Al complex is not likely to be the effective catalyst agent. Other more recent work also favors the second and simpler alternative, the monometallic mechanism. 

While a limited amount of experimental evidence does lend support to the bimetallic concept, majority opinion favors the second and simpler alternative, the monometallic mechanism. 

The above model for monometallic mechanism, though now widely accepted, is still occasionally questioned. Some evidence, for instance, has been presented over the years to support a bimetallic mechanism. It was shown that elimination of the oiganometallic portion of the complex... 

Electron transfer effects in porphyrins are of biological relevance for a number of reasons, not least in the understanding of mechanisms involved in photodynamic therapy. Studies reported include fluorescence quenching of porphyrins by oxidants such as p-benzoquinone , photoinduced electron transfer of porphyrin-acceptor molecules in solid state °, and ps experiments on quinone substituted monometallic porphyrin dimer which show evidence for super-exchange mediated electron transfer in photosynthetic systems . 

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