Abstract
The protonation of 2,3-dimethoxy-5-methyl-1,4-benzoquinone (UQ0) has been determined to improve the quantum efficiency of charge separation between a zinc meso-tetra(4-sulfonatophenyl)porphine (MP) donor and UQ0 (UQ0H+ upon protonation) acceptor system. The primary effect of protonation is to increase the driving force of the initial photoinitiated electron transfer (ET). Although this does not alter the diffusion-limited rate of forward ET, it may have a substantial effect on the nature of the ion pair formed by the initial ET and the subsequent competition between free ion separation and back electron transfer (BET). Transient absorption data show that although the rates of ET from photoexcited 3MP* to UQ0 or UQ0H+ are diffusion limited, there is a >3-fold increase in free ion product yields upon UQ0 protonation. The results are assessed in terms of current electron transfer theories. It is argued that the increase in product yield is a result of BET for the reactions of UQ0H+ being deeper in the normal Marcus region. The importance of these results for understanding trends in the rates of BET is discussed.
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