The role of conformational heterogeneity in the excited state dynamics of linked diketopyrrolopyrrole dimers.

Diketopyrrolopyrrole (DPP) derivatives have been proposed for both singlet fission and energy upconversion as they meet the energetic requirements and exhibit superior photostability compared to many other chromophores. In this study, both time-resolved electronic and IR spectroscopy have been applied to investigate excited state relaxation processes competing with fission in dimers of DPP derivatives with varying linker structures. A charge-separated (CS) state is shown to be an important intermediate with dynamics that are both solvent and linker dependent. The CS state is found for a subset of the total population of excited molecules and it is proposed that CS state formation requires suitably aligned dimers within a broader distribution of conformations available in solution. No long-lived triplet signatures indicative of singlet fission were detected, with the CS state likely acting as an alternative relaxation pathway for the excitation energy. This study provides insight into the role of molecular conformation in determining excited state relaxation pathways in DPP dimer systems.