Photosynthesis and porphyrin excited state redox reactions

TLDR: This comprehensive study into the irreversible oxidation and reduction properties of the triplet state of zinc uroporphyrin (ZnUP) found the similarities in the redox and spectral properties between ZnUP and chlorophyll excited states rendered this study relevant.

Abstract: The necessity of unraveling the complex reactions of photosynthesis inspired this comprehensive study into the irreversible oxidation and reduction properties of the triplet state of zinc uroporphyrin (ZnUP). The similarities in the redox and spectral properties between ZnUP and chlorophyll excited states rendered this study relevant. In addition, the eight ionized acetic and propionic acid groups on the periphery of the uroporphyrin molecule permitted the determination of a distance of electron transfer from the triplet state t o various acceptors. The importance of these findings t o porphyrin photochemistry and photosynthesis will be presented in this report. The absorption spectra and decay kinetics of some free base and metalloporphyrin triplet states have been That the ground state of metalloporphyrins undergo successive one-electron aromatic ring oxidations6-'0 and reductions1 '-' is well documented. Similarly, one-electron electrochemical oxidations of tetraphenylporphin,' and successive one-electron electrochemical reductions of various free-base porphyrins have been reported.' 6 i 1 Photochemical reductions of uroporphyrin' 8 -2 and zinc porphin2 have been demonstrated, although the reactive state was not conclusively identified. The one-electron reduction of ground state zinc etioporphyrinZ3 and the one-electron oxidations of the triplet state of zinc t e t r a p h e n y l p ~ r p h y r i n ~ ~ by chemical means have also been reported. PhotoreductionSv2 5 , 2 6 and p h o t o ~ x i d a t i o n ~ > ~ 6 9 2 7 of chlorophyll and chlorophyll-related2 8 30 compounds have also been studied, and reaction from the triplet state was demonstrated in several cases.26-28g30 A quantitative mechanistic study, however, has not been reported and will be presented for triplet state ZnUP in this report.