Abstract
Mono- and di-alkylated 1,4-diketo-3,6-dithiophenylpyrrolo[3-4-c]pyrrole derivatives (TDPPs) have been synthesised and their solid state packing and absorption properties have been correlated. In this library of compounds the bulkier substituents distort the geometry of the chromophores and shift the lowest energy absorption band as a consequence of reduced π–π stacking and inter-chromophore overlap. Longitudinal displacement of the conjugated core is affected by donor–acceptor intermolecular interactions and twisting of the thiophene ring out of the plane of the DPP core, whereas lateral displacement was correlated to distortion of the NLactam–C(R) bond out of the plane of the DPP core. The di-substituted TDPP with hexyl units exhibit high molecular planarity, strong close packing of the conjugated core and significant red shift of the maximum of absorption in the solid, whereas the mono-substituted compounds with hexyl and ethyl acetate units are the least distorted of the series because of strong intermolecular hydrogen bonding that increases the molecular overlap and planarity of the chromophores. Therefore the family of mono-substituted TDPPs and more specifically the ones with ethyl acetate substituents show good potential for modulating the molecular geometry and optimizing the charge transport in materials for organic electronic applications.
Highlights
PaperScheme 1 Synthesis of TDPPs 1A–E and 2A–E.reasons, including synthetic accessibility, and high planarity which results in high π-delocalization and favourable π–π intermolecular interactions
We present the synthesis, spectroscopic properties and thorough solid state structural analysis of two new series of mono- and di-alkylated TDPP derivatives, with a focus on the differences in the solid state packing caused by the nature of the side-appended alkyl units
Mono-alkylated TDPPs have different dihedral angles of the planes of DPP moiety and the two thiophenyl groups ranging from 0.79° to 6.11° for the thiophene ring adjacent to the free lactam unit and from 2.59° to 19.8° for the thiophene ring interacting with the alkyl chain, demonstrating the influence of the substituent on the overall molecular conformation (Table 2, Fig. S8 and S9, Electronic supplementary information (ESI)†)
Summary
PaperScheme 1 Synthesis of TDPPs 1A–E and 2A–E.reasons, including synthetic accessibility, and high planarity (small dihedral angle between the DPP core and the thiophene rings) which results in high π-delocalization and favourable π–π intermolecular interactions. In the crystal packing, the TDPPs bearing two alkyl units (compound family 1) show slipped stacking in a classical edge-to-face herringbone fashion similar to the packing of previously described dihexylphenyl DPP and thiophene DPP 1B (Fig. S2, ESI†).[12] The mono-alkylated derivatives form strong hydrogen bonds between two molecules through the free amide functionalities and the dimers formed further adopt slipped packing as a result of π–π stacking.
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