Is it common for charge recombination to be faster than charge separation?

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Abstract
Attaining long‐lived charge‐transfer (CT) states is of the utmost importance for energy science, photocatalysis, and materials engineering. When charge separation (CS) is slower than consequent charge recombination (CR), formation of a CT state is not apparent, yet the CT process provides parallel pathways for deactivation of electronically excited systems. The nuclear, or Franck‐Condon (FC), contributions to the CT kinetics, as implemented by various formalisms based on the Marcus transition‐state theory, provide an excellent platform for designing systems that produce long‐lived CT states. Such approaches, however, tend to underestimate the complexity of alternative parameters that govern CT kinetics. Here we show a comparative analysis of two systems that have quite similar FC CT characteristics but manifest distinctly different CT kinetics. A decrease in the donor‐acceptor electronic coupling during the charge‐separation step provides an alternative route for slowing down undesired charge recombination. These examples suggest that, while infrequently reported and discussed, cases where CR is faster than CS are not necessarily rare occurrences.
Funding Information
  • National Science Foundation (CHE 1800602, CHE 1465284, CBET 0923408)
  • Division of Chemical, Bioengineering, Environmental and Transport Systems (CBET 0923408)
  • Narodowe Centrum Badań i Rozwoju (PL‐TWIII/17/2016)
  • Division of Chemistry (CHE 1465284, CHE 1800602)

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