Cross-Linking of Poly(arylenebutadiynylene)s and Its Effect on Charge Carrier Mobilities in Thin-Film Transistors

Thermal or photochemical 1,4-coupling reactions of butadiyne units can be employed to cross-link polymers. Poly­(arylenebutadiynylene)­s are solution-processable organic semiconducting polymers which are used in organic electronic devices. However, effects of the topochemical cross-linking reactions of the main chain backbone on the electronic device performances have not been reported before. Intermolecular π–π stacking of the main chain backbones was expected to assist the alignment of the butadiyne units, and this would omit the necessity of introducing conventional aligning groups such as hydrogen-bonding amide units. Two different poly­(arylenebutadiynylene)­s, namely, crystalline poly­(thieno­[3,2-b]­thiophen-2,5-diylbutadiynelene) (pDETT) and rather amorphous poly­(thiophen-2,5-diylbutadiynelene) (pDET), were prepared, and their topochemical cross-linking was comprehensively studied. Grazing-incident wide-angle X-ray scattering (GIWAXS) measurements and density functional theory calculations of pDETT demonstrated the required butadiyne alignment suitable for the thermal topochemical reaction. As the newly formed covalent bonds were almost perpendicular to the polymer backbone plane, the resulting twist of the polymer backbone disrupted the extent of π-conjugation, which should have negatively affected the charge transport properties. However, the very tight molecular packings of pDETT inhibited the formation of excess cross-links. Consequently, the partially cross-linked pDETT film showed an improved charge carrier mobility in organic field-effect transistors as compared to the precursor polymer film. On the other hand, amorphous pDET produced cross-links at the randomly π–π stacked regions by both thermal and UV-induced topochemical reactions, and the resulting films were completely deficient of charge transport.