Charge-transfer character in a covalent diketopyrrolopyrrole dimer: implications for singlet fission

Diketopyrrolopyrrole (DPP) is a strongly absorbing, photostable chromophore that can undergo singlet fission (SF), a photophysical process that promises to significantly enhance solar‐cell performance. In the solid state, DPP packs in a herringbone arrangement that maximizes intermolecular donor–acceptor interactions, suggesting that charge‐transfer (CT) states play a role in DPP SF. In order to characterize intermolecular DPP CT states in molecular assemblies, we have synthesized a covalent DPP dimer bridged by a xanthene linker, which places two thiophene‐substituted DPPs (TDPPs) in a cofacial arrangement that mimics chromophore π–π stacking in the thin film. After photoexcitation in polar solvents, symmetry‐breaking charge separation forms the fully charge separated TDPP+.–TDPP−. ion‐pair state. In nonpolar solvents, charge separation is incomplete leading to the TDPPδ+–TDPPδ− CT state, which is in pseudoequilibrium with the relaxed S1S0 state observed by transient absorption and emission spectroscopy. This study highlights the importance of intramolecular coupling as well as the importance of entropy to promoting SF in chromophore dimers for which SF is endo‐ or isoergic