Influence of energetics and electronic coupling on through-bond and through-space electron transfer within U-shaped donor-bridge-acceptor arrays

We present here a study of multistep electron transfer mechanisms within donor-bridge-acceptor arrays consisting of functionalized aromatic imide and diimide donors and acceptors arranged in rodlike linear structures and in U-shaped folded structures on xanthene scaffolds. Femtosecond and nanosecond transient absorption spectroscopy is used to explore the relative efficiency of through-bond and through-space electron transfer in these molecules. The magnitude of the electronic coupling between the oxidized donor and the reduced acceptor is probed specifically by direct measurements of the singlet-triplet splitting, 2J, within the radical ion pairs using magnetic field effects on the yield of triplet states resulting from radical ion pair recombination. These data are used to quantitatively assess the effects of both energetics and electronic coupling on the electron transfer mechanism. Through-space electron transfer is found to be a viable mechanism in the U-shaped structures when reduction of the acceptor that is folded back toward the donor is energetically more favorable than reduction of the acceptor directly bonded to the donor.