T. Buranda et al., PHOTOINDUCED ELECTRON-TRANSFER IN PORPHYRIN QUINONE DONOR ACCEPTOR PAIRS - PH-DEPENDENT FREE-ION YIELD/, Journal of physical chemistry, 100(48), 1996, pp. 18868-18874
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.