CHLOROPHYLL‐PHOTOSENSITIZED OXIDATION OF HYDROQUINONE AND p‐PHENYLENEDIAMINE DERIVATIVES

ESR and photovoltaic studies on light‐induced one‐electron transfer between chlorophyll a and electron donors in the absence of oxygen show (1) the possible conversion of photo‐reduced chlorophyll a and p‐benzosemiquinone ion radicals to their non‐ionic radicals in methanol solutions of low pH, (2) the production of ESR absorption of tetrachloro‐p‐benzosemi‐quinone even in benzene, enhanced by the addition of triethylamine or methanol, and (3) the transfer of one electron from tetramethyl‐p‐phenylenediamine to either excited chlorophyll a or pheophytin a in methanol at pH above 3.6 but not to pheophytin a at pH below 1 0 where its radical cation appears to accept an electron from excited pheophytin a . Bacteteriochloro‐phyll is also shown to be capable of photooxidizing hydroquinones and tetramethyl‐p‐phenyl‐enediamine. The presence of oxygen enhances chlorophyll a ‐photosensitized oxidation of hydroquinone and tetrachloro‐hydroquinone by one‐electron transfer to oxygen and of trimethylhydro‐quinone probably by two‐electron trnasfer to oxygen. A free radical from excited chlorophyll a‐oxygen interaction is formed in these reactions, but rapidly quenched in the case of trimethyl‐hydroquinone. This, kind of free radical is not formed in pheophytin a . Tetramethyl‐ p ‐phenyl‐enediamine readily undergoes chlorophyll a‐photosensitized oxidation by oxygen in any pH region.