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

Intramolecular electron transfer (ET) processes in the excited aromatic diimide bis­(radical anion)­s (ADI<sup>•–</sup>*–ADI′<sup>•–</sup>) were systematically investigated by applying femtosecond laser flash photolysis to bis­(radical anion)­s of naphthalenediimide (NDI) and perylenediimide (PDI), including NDI<sup>•–</sup>–<i>m</i>-NDI<sup>•–</sup>, NDI<sup>•–</sup>–<i>p</i>-NDI<sup>•–</sup>, PDI<sup>•–</sup>–<i>m</i>-PDI<sup>•–</sup>, and NDI<sup>•–</sup>–<i>m</i>-PDI<sup>•–</sup> (<i>m</i> and <i>p</i> indicate the substitution positions). The excitation of NDI<sup>•–</sup>–<i>m</i>-NDI<sup>•–</sup> and NDI<sup>•–</sup>–<i>p</i>-NDI<sup>•–</sup> initiated disproportionation reactions generating NDI and NDI<sup>2–</sup> with different ET rate constants. For the first time, the dual characteristics of ADI<sup>•–</sup>* were confirmed upon selective excitation of NDI<sup>•–</sup>–<i>m</i>-PDI<sup>•–</sup>: NDI<sup>•–</sup>* was unambiguously demonstrated to function as an electron donor in NDI<sup>•–</sup>*–<i>m</i>-PDI<sup>•–</sup>, whereas PDI<sup>•–</sup>* acted as an electron acceptor in NDI<sup>•–</sup>–<i>m</i>-PDI<sup>•–</sup>* because of the energetically preferable production of NDI–<i>m</i>-PDI<sup>2–</sup>. The relationship between the ET rate constants and driving forces in ADI<sup>•–</sup>*–ADI′<sup>•–</sup> can be reasonably analyzed by using the Marcus theory. The current findings provide a new viewpoint regarding the bipolaron-generating nature of ADI<sup>•–</sup>*–ADI′<sup>•–</sup> and facilitate simulating various types of photocarrier migration in the densely charged regions of homo- and heterogeneous n-type semiconductor materials upon irradiation.