Solvent‐Dependent Singlet Fission in Diketopyrrolopyrrole Dimers: A Mediating Charge Transfer versus a Trapping Symmetry‐Breaking Charge Separation

Abstract Three diketopyrrolopyrrole (DPP) dimers, linked via different dithienylphenylene spacers, ortho‐DPP (o‐DPP), meta‐DPP (m‐DPP), and para‐DPP (p‐DPP), are synthesized, characterized, and probed in light of intramolecular singlet fission (i‐SF). Importantly, the corresponding DPP reference (DPP‐Ref) singlet and triplet excited state energies of 2.22 and 1.04 eV, respectively, suggest that i‐SF is thermodynamically feasible. The investigations focus on the impact of the relative positioning of the DPPs, and give compelling evidence that solvent polarity and/or spatial overlap govern i‐SF dynamics and efficiencies. Polar solvents make the involvement of an intermediate charge transfer (CT) state possible, followed by the population of ¹(T₁T₁) and subsequently (T₁ + T₁), while spatial overlap drives the mutual interactions between the DPPs. In o‐DPP, the correct balance between polar solvents and spatial overlap leads to the highest triplet quantum yield (TQY) of 40%. Notable is the superimposition of CT and triplet excited states, preventing an accurate TQY determination. For m‐DPP, poorer spatial overlap correlates with weaker CT character and manifests in a TQY of 11%. Strong CT character acts as a trap and prevents i‐SF, as found with p‐DPP. The DPP separation is decisive, enabling a symmetry‐breaking charge‐separated state rather than CT formation, shutting down the formation ¹(T₁T₁).