Architecting layered molecular packing in substituted benzobisbenzothiophene (BBBT) semiconductor crystals

The construction and control of 2D layered packing motifs with pi-extended fused-ring molecules is of crucial importance for developing organic electronic materials and devices. Herein, we demonstrate that, when adequately substituted, two kinds of layered packing motifs are obtainable for benzo[1,2-b:4,5-b']bis[b]benzothiophene (BBBT), which itself does not show layered crystallinity. We synthesized BBBT derivatives substituted with a combination of alkyl chains and a phenyl ring in a symmetric/asymmetric manner, 2,8-didecyl-BBBT (diC10-BBBT) and 2-decyl-8-phenyl-BBBT (Ph-BBBT-C10). We found that diC10-BBBT forms a layered pi-stack (L pi S) structure mainly composed of slipped parallel stacks, while Ph-BBBT-C10 forms a typical layered herringbone (LHB) packing structure chiefly composed of T-shaped contacts. This feature is associated with the non-layered packing motif in BBBT: typical pi-stack and herringbone structures, both of whose polymorphs show a large slip along the molecular long axis. Calculations of intermolecular interaction energies between neighbouring molecules in the crystals reveal that the interchain interactions suppress the long-axis slip, leading to the formation of the L pi S and the LHB, respectively. Both diC10-BBBT and Ph-BBBT-C10 form uniform (ultra)thin films originating from the layered crystallinity, and exhibit good transistor characteristics with a hole mobility of about 1 cm(2) V-1 s(-1). We discuss how the substituent modifications are useful as crystal engineering to explore the potential of pi-extended molecules for electronic applications.