Synthesis of covalently linked diketopyrrolopyrrole oligomers and their singlet fission properties†

Singlet fission (SF) is a down-conversion process that could convert a high-energy singlet exciton into two low-energy triplet excitons. To date, there are very few SF materials with high efficiencies, high stability and relatively high triplet state energy simultaneously. Diketopyrrolopyrroles (DPPs) have been found to achieve highly efficient intermolecular SF; however, they have not yet achieved efficient intramolecular SF (iSF). Herein, a series of covalently linked DPP oligomers (E(T₁) = ∼1.1 eV) were synthesized, and their iSF properties were investigated via time-resolved ultrafast fluorescence/absorption spectroscopy. The absorption spectra of oligomers changed significantly compared to their corresponding monomers, suggesting the presence of strong inter-chromophore electronic coupling in these oligomers. Meanwhile, the fluorescence quantum yield of these oligomers also dropped a lot, especially in polar solvents. Transient absorption spectra revealed that these oligomers could conduct iSF in polar solvents, but not in non-polar solvents. This, together with the large dropped fluorescence quantum yield in polar solvents, suggests that iSF in these oligomers was mediated by the charge-transfer state. More importantly, the SF rate and efficiency increase gradually from dimer (∼3.71 ns and ∼45.3%) to trimer (∼1.62 ns and ∼66.8%) and then to tetramer (∼1.08 ns and ∼85.3%). This may be caused by the larger diffusion space for the separation of the triplet state provided by the increasing number of DPP units in trimers and tetramers. The 85% iSF yield of tetramers is the highest recorded for DPP derivatives in solution. This work opens a new avenue for the design of novel efficient iSF materials based on DPP derivatives.