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

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 (T 1 T 1 ) and subsequently (T 1 + T 1 ), 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 (T 1 T 1 ).