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

Abstract

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 “α” and “β”) 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 α phase, the molecules interact strongly by π-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 β 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.

Rubbing induced reversible fluorescence switching in thiophene-based organic semiconductor films by mechanical amorphisation

“…It is worth noting that the recrystallization which starts at 90 1C in thin films, leads to the formation of form b, and not a, even if the latter form should be stable up to 205 1C. 23 Eventually, XRD measurements show that the rubbing-heating process induces a preferential orientation with the plane (100) for the b phase parallel to the substrate.…”

“…Samples prepared by drop casting exhibit the typical XRD pattern, with the Bragg peaks coming from two different a and b crystal phases, indicating the coexistence of both a and b polymorphs. 23,25 Both polymorphs are oriented with the long axis almost perpendicular to the substrate surface, i.e. (001) and (100) planes for a and b respectively.…”

“…20 Compound 1 is a multifunctional organic semiconductor used in optoelectronics 20 and in time-temperature integrator devices 5,21,22 capable of forming two polymorphs, discernible by the fluorescence colour: form a, that exhibits yellow/orange fluorescence and form b, that displays a green/yellow fluorescence. ‡ 22,23 The a and b polymorphs of 1 have different mechanical properties. 23 The b polymorph is prone to irreversible plastic deformation; in particular the application of a mechanical force on (100) plane causes slippage of adjacent p-stacked layers, giving rise to an irreversible plastic deformation of the crystals.…”

“…‡ 22,23 The a and b polymorphs of 1 have different mechanical properties. 23 The b polymorph is prone to irreversible plastic deformation; in particular the application of a mechanical force on (100) plane causes slippage of adjacent p-stacked layers, giving rise to an irreversible plastic deformation of the crystals. When the mechanical force is applied to different faces the crystals become very fragile.…”

Control of polymorphism in thiophene derivatives by sublimation-aided nanostructuring

“…31 1 is a well-characterized thermochromic material used in organic optoelectronics 31 and in time-temperature integrator devices, 26,32 capable of forming two types of polymorphs, discernible by the fluorescence colour. 9,33 As sublimating compound 2 we used p-dichlorobenzene, a commercial substance that sublimes under ambient conditions, and whose effectiveness in ASB-SANS processes has already been demonstrated. [27][28][29][30] The two substances were chosen for their ability to achieve separate precipitation from the solution, due to their different solubility under the specific experimental conditions.…”