PHOTOINDUCED ELECTRON-TRANSFER IN PORPHYRIN QUINONE DONOR ACCEPTOR PAIRS - PH-DEPENDENT FREE-ION YIELD/

The protonation of 2,3-dimethoxy-5-methyl-1,4-benzoquinone (UQ(0)) has been determined to improve the quantum efficiency of charge separatio n between a zinc meso-tetra(4-sulfonatophenyl)porphine (MP) donor and UQ(0) (UQ(0)H(+) upon protonation) acceptor system. The primary effect of protonation is to increase the driving force of the initial photoi nitiated electron transfer (ET). Although this does not alter the diff usion-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 (B ET). Transient absorption data show that although the rates of ET from photoexcited (3)MP to UQ(0) or UQ(0)H(+) are diffusion limited, ther e is a >3-fold increase in free ion product yields upon UQ(0) protonat ion. The results assessed in terms of current electron transfer theori es. It is argued that the increase in product yield is a result of BET for the reactions of UQ(0)H(+) being deeper in the normal Marcus regi on. The importance of these results for understanding trends in the ra tes of BET is discussed.