ENERGETICS OF PYROCHLOROPHYLL-SENSITIZED PHOTOREDUCTION OF NITRO COMPOUNDS

Comparison of the quantum yields of pyrochlorophyll-sensitized photoreduction of substituted nitrobenzenes by hydrazobenzene in ethanol-pyridine solution with the polarographic quarter-wave reduction potentials of the nitro compounds leads to the conclusion that an electron can be transferred from triplet-excited pyrochlorophyll to the nitro compound without input of additional activation energy, if the potential of the nitro compound is greater than -0.80 V (sce) in aqueous ethanol-pyridine. The liquid-junction potential between water and the aqueous ethanol-pyridine mixture used for polarography is estimated to be 0.05 ± 0.02 V from the polarography of complexes of cadmium with pyridine, 2,2′-bipyridyl, and o-phenanthroline. With the correction, it is estimated that triplet-excited chlorophyll can reduce oxidants exothermically if their oxidation-reduction potentials are greater than -0.60 ± 0.035 V (nhe) in water, a value compatible with the reported reducing properties of illuminated chloroplasts. Pyrochlorophyll also sensitizes the reduction of low-potential nitro compounds by ascorbic acid, by a mechanism involving reduction of triplet-excited pyrochlorophyll by ascorbic acid.