Solution-processable small molecule semiconductors based on pyrene-fused bisimidazole and influence of alkyl side-chain on the charge transport

Abstract

To explore the potential of pyrene-fused biimidazole as a building block for soluble small molecule semiconductors, we designed and synthesized PBI-L-Na and PBI-B-Na. Imidazole rings provided substitution positions for the solubilising groups at the K-region, however, DFT calculations revealed that the repulsive steric hindrance from the neighboring hydrogen atoms on pyrene forced alkyl side chains into a twisted conformation. Thus, side chains on this type of molecule exerted a prominent influence on the molecular packing and affected their optoelectronic properties intensively in the solid state. PBI-L-Na exhibited a more ordered packing of the large conjugated plane with a π–π stacking distance of 0.36 nm and showed a hole mobility up to 0.12 cm² V⁻¹ s⁻¹. Bulkier branched chains provided better solubility but impeded molecular packing of PBI-B-Na, thus giving a hole mobility of 4.6 × 10⁻³ cm² V⁻¹ s⁻¹.