Photoinduced double charge accumulation in a molecular compound

Photochemistry involves elementary steps in which single electrons are transferred, but artificial photosynthesis requires multi-electron reactions. This discrepancy necessitates light-driven charge accumulation, which has so far proved very difficult to achieve without sacrificial redox reagents. Here we report a molecular donor–photosensitizer–acceptor compound in which light absorption leads to the reversible accumulation of two positive and two negative charges. The resulting photoproduct forms with an overall quantum yield of 37%, has a lifetime of more than 100 ns and stores 3.0 eV of energy. The use of a structurally well-defined molecular compound provides fundamental insights into how light-driven multi-electron transfer can generally be performed efficiently and sustainably, at irradiance levels orders of magnitude below those required in comparable systems. This represents a step towards more application-oriented research on solar fuels from fundamental studies of photoinduced (single) electron transfer. The photoinduced accumulation of redox equivalents is a challenging requirement for artificial photosynthesis. Now a molecule has been developed in which the sequential absorption of photons results in the temporary accumulation of two holes and two electrons. The employed strategy opens up new possibilities for charge accumulation under low photon densities.