Solution-Processable Quinoidal Dithioalkylterthiophene-Based Small Molecules Pseudo-Pentathienoacenes via an Intramolecular S···S Lock for High-Performance n-Type Organic Field-Effect Transistors.

A new organic small-molecule family comprising tetracyanoquinodimethane-substituted quinoidal dithioalky(SR)terthiophenes ( DSTQ s) ( DSTQ-6 ( 1 ); SR = SC ₆ H ₁₃ , DSTQ-10 ( 2 ); SR = SC ₁₀ H ₂₁ , DSTQ-14 ( 3 ); SR = SC ₁₀ H ₂₁ ) was synthesized and contrasted with a nonthioalkylated analogue ( DRTQ-14 ( 4 ); R = C ₁₄ H ₂₉ ). The physical, electrochemical, and electrical properties of these new compounds are thoroughly investigated. Optimized geometries obtained from density functional theory calculations and single-crystal X-ray diffraction reveal the planarity of the SR-containing DSTQ core. DSTQ s pack in a slipped π-π stacked two-dimensional arrangement, with a short intermolecular stacking distance of 3.55 Å and short intermolecular S···N contacts of 3.56 Å. Thin-film morphological analysis by grazing incident X-ray diffraction reveals that all DSTQ molecules are packed in an edge-on fashion on the substrate. The favorable molecular packing, the high core planarity, and very low lowest unoccupied molecular orbital (LUMO) energy level (-4.2 eV) suggest that DSTQ s could be electron-transporting semiconductors. Organic field-effect transistors based on solution-sheared DSTQ-14 exhibit the highest electron mobility of 0.77 cm ² V ^-1 s ^-1 with good ambient stability, which is the highest value reported to date for such a solution process terthiophene-based small molecular semiconductor. These results demonstrate that the device performance of solution-sheared DSTQ s can be improved by side chain engineering.