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Two diketopyrrolopyrrole-co-thieno[3,2-b]thiophene derivatives substituted with either branched ethylhexyl (TTDPP-EH) or linear hexyl side chains (TTDPP-C6) have been synthesized. The impact of the side chain architecture on the structure and optical properties has been evaluated. TTDPP molecules crystallize in triclinic unit cells observed in both single crystals and in thin films. The most striking difference between the two compounds is the packing of the molecules. For TTDPP-EH, pairs of molecules overlap only at their thienothiophene (TT) ring tips leading to a weak excitonic coupling of the J-type character. In contrast, TTDPP-C6 molecules stack in a 1D columnar structure with extended molecular overlapping. A transverse displacement of the molecules along their molecular axis allows a partial overlap of electron-rich TT and electron-poor DPP units. This leads to a stronger excitonic coupling with apparent coexistence of H- and J-like absorption features. Interestingly, both single crystals and oriented thin films change color with light polarization. This sensitivity to light polarization is related to the presence of two different excitonic couplings within TTDPP-C6.

cited By 15; International audience; A simple methodology is proposed to quantitatively discuss the quality of the 3D dispersion of grafted silica nanoparticles (NPs) mixed with free chains in nanocomposites. 3D observations of NPs are obtained by transmission electron tomography (TEMT). The NPs are individualized from the tomograms by segmentation and watershed processing. Equivalent dispersions of spherical objects are then generated. By computing 3D Voronoi tessellation from the spheres, the local environment is investigated for each NP through geometrical measurements. Two types of populations of NP can be extracted: either densely packed or isolated. As the length of the free chains decreases, when the length of the free chains are close to the ones of the grafted chains, this second population becomes predominant and the quality of the dispersion gradually increases. The width of the local volume fraction distribution surrounding each NP can be considered as a quantitative parameter indicative of the homogeneity of the microstructure. It becomes narrower as the quality of the dispersion increases. © 2014 American Chemical Society.