Structure-Mechanical Relationships in Polymorphs of Organic Semiconductor (C4-NT3N)

Understanding of polymoprhism of organic semiconducting materials is the key to structural control of their electrical and mechanical properties. Motivated by the ambipolar n-type charge transport and electroluminescence of thienopyrrolyldione end-capped oligothiophenes, here we studied the propensity of one representative to crystallize as different polymorphs which display distinctly different mechanically properties. The crystal structures of the two polymorphs (denoted ‘orange’ and ‘yellow’) of the material, 2,2'-(2,2'-thiophene-5,5'-diyl)bis(5-butyl-5H-thieno[2,3-c]pyrrole-4,6)-dione (C4-NT3N), were determined. In the orange phase, the molecules interact strongly by -stacking, forming columns which are bonded via C—H---O and chalcogen bonds, and this packing is consistent with the elastic behaviour observed with the crystals. Instead, the yellow phase has the molecules aligned along their core forming layers. Whilst the molecules interact strongly within the layers, they are practically unbound betw...