Tuning crystallochromism in diketopyrrolopyrrole-co-thieno[3,2-b]thiophene derivatives by the architecture of their alkyl side chains

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.