Charge Separation and Recombination Pathways in Diblock DNA Hairpins

Achieving high-yielding photoinduced charge separation through the π-stacked bases of DNA is a critical requirement for realizing numerous DNA-based technologies. In the current work, we combine two strategies for achieving high-yield charge separation. First, a chromophore with a high driving force for charge injection, naphthalenediimide (NDI), is used because it generates hot carriers that enhance charge-transfer rates. Second, a diblock DNA sequence is used with two or three adenines followed by a series of guanines to implement an energy landscape that accelerates charge separation while retarding charge recombination. The photoinduced dynamics of these NDI diblock oligomers with and without a terminal hole acceptor are probed by femtosecond transient absorption spectroscopy. The measured rate constants for various charge separation and recombination processes are interpreted within the context of a full kinetic model of these systems. We find that the A₂ and A₃ oligomers achieve similar charge separation yields (as high as 20-25%) for a given length, yet the critical recombination process that determines these yields occurs at different distances from the NDI chromophore and on different time scales. This type of analysis could be used to predict charge separation efficiencies in candidate DNA structures.