Terminal Cyano-Functionalized Fused Bithiophene Imide Dimer-Based n-Type Small Molecular Semiconductors: Synthesis, Structure-Property Correlations, and Thermoelectric Performances.

n-Doped small molecular organic thermoelectric materials (OTMs) hold advantages of high Seebeck coefficient and better performance reproducibility over their polymeric analogues; however, high-performance n-type small molecular OTMs are severely lacking. We report here a class of small molecular OTMs based on terminal cyanation of a bithiophene imide-based ladder-type heteroarene BTI2. It was found that the cyanation could effectively lower the lowest unoccupied molecular orbital (LUMO) level from -2.90 eV (BTI2) to -4.14 eV (BTI2-4CN) and thus lead to significantly improved n-doping efficiency. Additionally, terminal cyano-functionalization can maintain the close packing and efficient intermolecular charge transfer between these cyanated molecules, thus yielding high electron mobilities of up to 0.40 cm ² V ^-1 s ^-1 . Benefiting from its low LUMO-enabled efficient n-doping and high electron mobility, an encouraging n-type electrical conductivity of 0.43 S cm ^-1 and power factor (PF) of 6.34 μW m ^-1 K ^-2 were achieved for tetracyanated BTI2-4CN, significantly outperforming those of its noncynated BTI2 (<10 ^-7 S cm ^-1 , PF undetectable) and dicyanated BTI2-2CN (0.24 S cm ^-1 , 1.78 μW m ^-1 K ^-2 ). These results suggest the great potential of the terminal cyanation strategy of ladder-type heteroarenes for developing high-performance small molecular OTMs.