A series of four zinc porphyrin-spacer-benzoquinone molecules were studied in which the spacer is either spiro [4.4] nonane or trans-decalin. The benzoquinone is attached to the porphyrin at two fixed distances each possessing two fixed orientations of the porphyrin relative to the quinone. The rate constants for photoinduced electron transfer from the lowest excited singlet state of the porphyrin to the quinone to form the Zn porphyrin+-quinone- ion pair and the subsequent dark charge recombination reaction were measured as a function of solvent polarity. The observed orientation dependent differences in rate constants for these two reactions can be attributed to orientation dependent changes in electronic coupling alone, because the Franck-Condon factors for electron transfer are similar for each molecule. The rate constant data suggest that the donor-acceptor orientation effects observed are due to variations in the sum of the direct, through-space interaction between the donor and acceptor and the indirect, through-solvent term. The rate constant data and AM1 molecular orbital calculations support the idea that the indirect interaction of the donor with the acceptor through the covalent bonds of the spacer is approximately constant for the spacers employed in this study.
ASJC Scopus subject areas
- Colloid and Surface Chemistry