Highly planar cross-conjugated alternating polymers with multiple conformational locks: synthesis, characterization and their field-effect properties

Abstract

It is meaningful to explore new design principles for organic semiconductors. Herein, we have developed two cross-conjugated alternating polymers based on 1,2-di(thiophen-2-yl)ethane-1,2-dione (DTO), namely PDTO-C1 and PDTO-C3, and investigated their charge transport properties by fabricating field-effect transistors devices. Single crystal X-ray crystallography shows that non-covalent S⋯O and C–H⋯O interactions exist inside the DTO units. These non-covalent interactions in combination with the C–H⋯O interactions of the thiophene-flanked dithienothiophene units, acting as conformational locks, are beneficial for acquiring the planar backbone conformation. PDTO-C1 and PDTO-C3 possess broad absorption spectra and HOMO and LUMO energy levels of ca. −5.50 and −3.6 eV, respectively. The highest mobility of 0.54 cm² V⁻¹ s⁻¹ was achieved in the PDTO-C3-based transistor devices, whereas PDTO-C1 affords a mobility of 0.22 cm² V⁻¹ s⁻¹. Further thin film microstructure investigations indicate that both polymers can form highly-ordered lamellar packing with small π–π stacking distances down to 3.50 Å. These results demonstrate that the incorporation of cross-conjugation may be used as an additional design tactic for organic semiconductors to alter their optoelectronic properties.