How donor-bridge-acceptor energetics influence electron tunneling dynamics and their distance dependences

Long-range electron transfer may occur via two fundamentally different mechanisms depending on the combination of electron donor, acceptor, and the bridging medium between the two redox partners. Activating the so-called hopping mechanism requires matching the energy levels of the donor and the bridge. If electrons from the donor can thermodynamically access bridge-localized redox states, the bridge may be temporarily reduced before the electron is forwarded to the acceptor. As a result, electron transfer rates may demonstrate an extremely shallow dependence on distance. When transient reduction of the bridging medium is thermodynamically impossible, a tunneling mechanism that exponentially depends on distance becomes important for electron transport. Fifty years ago, superexchange theory had already predicted that electron transfer rates should be affected by donor-bridge-acceptor energetics even in the tunneling regime, in which the energy gap (Δε) is too large for electrons to hop from the donor onto the bridge. However, because electron tunneling rates depend on many parameters and the influence of donor-bridge energy gaps is difficult to distinguish from other influences, direct experimental support for the theoretical prediction has been difficult to find. Because of remarkable progress, particularly in the past couple of years, researchers have finally found direct evidence for the long-sought but elusive tunneling-energy gap effect. After a brief introduction to the theory of the tunneling mechanism, this Account discusses recent experimental results describing the importance of the tunneling-energy gap. Experimental studies in this area usually combine synthetic chemistry with electrochemical investigations and time-resolved (optical) spectroscopy. For example, we present a case study of hole tunneling through synthetic DNA hairpins, in which different donor-acceptor couples attached to the same hairpins resulted in tunneling rates with significantly different dependences on distance. Recent systematic studies of conjugated molecular bridges have demonstrated the same result: The distance decay constant (β), which describes the steepness of the exponential decrease of charge tunneling rates with increasing donor-acceptor distance, is not a property of the bridge alone; rather it is a sensitive function of the entire donor-bridge-acceptor (D-b-A) combination. In selected cases, researchers have found a quantitative relationship between the experimentally determined distance decay constant (β) and the magnitude of the tunneling-energy gap (Δε). The rates and efficiencies of charge transfer reactions occurring over long distances are of pivotal importance in light-to-chemical energy conversion and molecular electronics. Tunneling-energy gap effects play an intriguing role in the formation of long-lived charge-separated states after photoexcitation: The kinetic stabilization of these charge-separated states frequently exploits the inverted driving-force effect. Recent studies indicate that tunneling-energy gap effects can differentiate the distance dependences of energy-storing charge-separation reactions from those of energy-wasting charge-recombination processes. Thus, the exploitation of tunneling-energy gap effects may provide an additional way to obtain long-lived charge-separated states.