Solid-State Absorption, Luminescence, and Singlet Fission of Furanyl-Substituted Diketopyrrolopyrroles with Different π-Stacking Arrangements

Abstract

Small modifications of the diketopyrrolopyrrole (DPP) molecular structure induced remarkable changes in its spectral and photophysical behavior. Using furan (F) heterosubstitution instead of thiophene (T) substituent resulted in a small blue shift and decreased Huang-Rhys factor of the absorption spectra in solution, irrespectively to N,N'-alkyls. Branching of alkyl side chains by formal 2-ethylation of n-hexyl substituent (C6 to EH) switched the slipped-stack arrangement, irrespectively on the heteroatoms. Consequent changes in steady-state absorption spectra of thin films were interpreted using time dependent density functional theory calculations, carried out on model dimers. Solid-state luminescence is weak and partially dependent on an excitation wavelength. Singlet fission was observed by femtosecond transient absorption spectroscopy, with considerably different yields for variously pi-stacked FDPP-EH (30 %) and FDPP-C6 (160 %). The shape of triplet-triplet absorption spectra was also influenced by various pi-stacking. The results are discussed in terms of different mixing of both Frenkel and charge transfer states in model dimers and different excitonic and electronic coupling in both types of pi-stacks, visualized by natural transition orbitals. Furanyl-substituted diketopyrrolopyrroles (DPP) with different branching of the alkyl side chains were studied. Both formed two distinct types of pi-stacking leading to H-aggregates with considerably different energetics in terms of mixed lowest Frenkel and charge transfer (CT) states. The combined effect of the molecular structure and solid-state arrangement lead to fast and efficient CT mediated singlet fission, with one of the highest efficiencies (160 %) ever reported within the DPP family.image