Molecular Design Principles for Achieving High-Efficiency Light-Induced Charge Separation at the Nanometer Scale.

The light-induced separation of charges, fundamental to natural photosynthesis, is key to converting solar energy into chemical energy in artificial systems. One challenge is that charges tend to spontaneously recombine in a downhill, energy-wasting process. This recombination can be slowed by increasing the distance between charges, but doing so also reduces the efficiency of the initial charge-separating step. We investigated how the quantum efficiency of energy-storing charge separation and the rate of energy-wasting charge recombination vary over distances from 22 to 44 Å in three structurally well-defined molecular donor-photosensitizer-acceptor compounds. Our key finding is that separation efficiency can remain high, around 60%, even at distances of up to 44 Å when redox relays are used instead of passive molecular bridges. At the same time, undesirable charge recombination remains slow, occurring on a time scale approaching the low-millisecond range. These results support the widely accepted view that multistep hopping is more effective than single-step electron tunneling. However, while previous studies have strongly focused on kinetics and charge-transfer rates, the question of how the efficiency of light-induced charge separation in donor-photosensitizer-acceptor systems is affected by the transition from tunneling to hopping remains underexplored. The newly gained insight from focusing on quantum efficiencies rather than charge-separation rates is critically important, as light-independent charge recombination events often go undetected when only rates of light-induced steps are in focus. Overall, our findings provide valuable design principles for creating artificial photosynthetic systems with high light-to-chemical energy conversion efficiency, offering important insights for the broader fields of solar energy conversion and artificial photosynthesis.