Pathways for Photoinduced Charge Separation in DNA Hairpins

By means of correlated quantum-chemical calculations, we explore the chain-length dependence of the electronic coupling for photoinduced charge separation in DNA hairpins associated to conjugated linkers. Pathways for charge transfer from the linker chromophore to a guanine site located at a well-defined distance along the DNA strand are identified. Importantly, these involve not only the frontier molecular orbitals of the interacting donor, bridge, and acceptor units, but also deeper lying orbitals possessing both the appropriate energy and the symmetry to overlap significantly. The relative efficiency of these channels is found to be sensitive to the chemical structure of the linker, leading to falloff parameters for the charge-transfer rates ranging from ∼0.4 to ∼1.2 Å^-1.