Diketopyrrolopyrrole assembly into J‐aggregates

Realizing the potential of the diketopyrrolopyrrole (DPP) heterocyclic scaffold as a dye or semiconductor in the context of molecular electronics, organic photovoltaics, and supramolecular chemistry requires understanding fully the factors that drive its aggregation. To determine how solubilizing side chains, conjugation length, and H-bonding groups affect DPP assembly into J-aggregates, we performed variable temperature ultraviolet–visible titrations in toluene on a series of DPP derivatives. The structures of these clusters in solution are extrapolated from single-crystal x-ray data and computational modeling. Spectroscopic changes in response to variations in temperature and concentration were fit to an isodesmic assembly model to determine ΔH, ΔS, ΔGo, and K of association for each of the DPP derivatives. The resulting thermodynamic assembly parameters predict how aggregate size responds to perturbations, which can be used to design stimuli-responsive systems with DPP as a functional component. Finally, a new mathematical model is presented that demonstrates how these thermodynamic parameters can be used to estimate the average number of DPP monomers in a cluster in solution at a given temperature and concentration. This study provides guidelines for anticipating how changes to DPP molecular structure affect assembly, and more generally, they show that even the simplest supramolecular association process can only be understood as a complex matrix that considers simultaneously the synergistic interactions of molecular structure, solvent, and assembly pathway. Copyright © 2016 John Wiley & Sons, Ltd.