Superexchange pathways in charge transfer through a DNA π-stack

We studied contributions of different electronic states of intervening bases to the superexchange interaction between donor and acceptor in single- and double-strand DNA fragments with quantum chemistry calculations. Hole-transfer pathways through thymine bases entailed substantial destructive interference whereas the influence of high-lying electronic states of adenine turned out to be weak. Contributions of different superexchange pathways depended on the duplex conformation. For bridges of several base pairs, very many electronic states had to be taken into account to estimate properly the electronic coupling if the superexchange approach is broken down to the separate bridge units. From these results, we concluded that qualitative results on superexchange-mediated couplings in DNA fragments can be obtained with only a few low-lying virtual electronic states of the bridge (one or two states per nucleobase pair). However, reliable quantitative estimates of the coupling matrix element for hole transfer in DNA π-stacks require summing the contributions of tens of bridge states.