Comparison of the photoinduced electron transfer reaction in a rigid cyclophane and its corresponding bimolecular donor/acceptor complex
The back electron transfer following photoexcitation of the bimolecular charge-transfer complex between 1,4-dimethoxybenzene (DMB) and 7,7-dicyanobenzoquinone methide (DCBM) is compared to the dynamics observed for the corresponding spatially constrained cyclophanes. In a recent letter (J. Phys. Chem. 1999, 703, 2740), we reported that the back electron transfer dynamics for two structurally related cyclophanes of DMB and DCBM were identical and that these data provided direct evidence for the through-bond mechanism of electron transfer in this bridged organic donor/acceptor systems. The study of the noncovalent bimolecular system herein enables us to experimentally determine the through-space electron transfer rate for this charge transfer pair. We find that the rate of back electron transfer is faster in the cyclophane than in the bimolecular complex. This difference in reaction rates can be accounted for quantitatively using the Bixon-Jortner equation for the electron transfer reaction rate, if the difference in driving force for the two systems is taken to be the energy difference between the absorption maxima of the corresponding charge transfer bands. Using the same approach for determining the relative driving force for the two cyclophane structures studied, we find that that the experimental data requires a common reaction distance, despite the fact that the center-to-center distance between the donor and acceptor molecules is increased by 25%. These data provide convincing evidence that the reaction process in the cyclophanes occurs by a through-bond mechanism. The origin of the different driving forces among the two cyclophanes and the noncovalent bimolecular complex is attributed to conformational changes in the donor and acceptor moieties that result from the constraints imposed by the alkyl spacers. © 1999 American Chemical Society.
Pullen, SH; Studer-Martinez, SL; Edington, MD; Harris, AL; Long, A; Baldwin, SW; Staab, HA; Simon, JD
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