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

Understanding of polymorphism 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 "alpha" and "beta") of the material, 2,2'-(2,2'-thiophene-5,5'diyl)bis(5-butyl-SH-thieno[2,3-c]pyrrole-4,6)-dione (C4-NT3N), were determined. In the a phase, the molecules interact strongly by g-stacking, forming columns which are bonded via C-H-O and chalcogen bonds, and this packing is consistent with the elastic behavior observed with the crystals. Instead, the beta phase has the molecules aligned along their core forming layers. While the molecules interact strongly within the layers, they are practically unbound between the layers. The presence of slip planes in this form explains the plastic deformation induced by applying a force perpendicular to the (001). The thermal behavior and the enantiotropic relationship of the polymorphs are reported.