Dual Character of Excited Radical Anions in Aromatic Diimide Bis(radical anion)s: Donor or Acceptor?

Abstract

Intramolecular electron transfer (ET) processes in the excited aromatic diimide bis(radical anion)s (ADI•–*–ADI′•–) were systematically investigated by applying femtosecond laser flash photolysis to bis(radical anion)s of naphthalenediimide (NDI) and perylenediimide (PDI), including NDI•––m-NDI•–, NDI•––p-NDI•–, PDI•––m-PDI•–, and NDI•––m-PDI•– (m and p indicate the substitution positions). The excitation of NDI•––m-NDI•– and NDI•––p-NDI•– initiated disproportionation reactions generating NDI and NDI2– with different ET rate constants. For the first time, the dual characteristics of ADI•–* were confirmed upon selective excitation of NDI•––m-PDI•–: NDI•–* was unambiguously demonstrated to function as an electron donor in NDI•–*–m-PDI•–, whereas PDI•–* acted as an electron acceptor in NDI•––m-PDI•–* because of the energetically preferable production of NDI–m-PDI2–. The relationship between the ET rate constants and driving forces in ADI•–*–ADI′•– can be reasonably analyzed by using the Marcus theory. The current findings provide a new viewpoint regarding the bipolaron-generating nature of ADI•–*–ADI′•– and facilitate simulating various types of photocarrier migration in the densely charged regions of homo- and heterogeneous n-type semiconductor materials upon irradiation.