Making Charge Recombination Spin-Forbidden for Efficient Generation and Excitation of Perylene Diimide Radical Anion...

The excited radical anion of perylene diimide (PDI) dyes is broadly used in thermodynamically challenging synthetic reactions via a consecutive photoinduced electron transfer (ConPET) mechanism. This approach relies on two sequential single-photon absorptions, in which the first triggers photoinduced electron transfer (PET) while the second populates the doublet excited state of the ensuing radical anion. Herein, we employ pump-pump-probe spectroscopy to investigate all of the elementary steps of this mechanism. We show that radical anion formation upon direct quenching of photogenerated ¹PDI^* is an inefficient strategy due to ultrafast charge recombination of the generated geminate ion pair (GIP). Alternatively, we demonstrate that ³PDI^* can be populated via triplet charge recombination with a halogenated electron donor and, from there, it can be quenched with a second donor, generating a triplet GIP and rendering charge recombination spin-forbidden to efficiently accumulate PDI^• - in solution. This anion can then be selectively excited by a second laser pump pulse, potentially triggering a catalytic reaction.