Effects of Thiophene-Fused Isomer on High-Layered Crystallinity in -Extended and Alkylated Organic Semiconductors

Here, we systematically investigated the effects of the thiophene-fused isomer and the end-cap substitution on high-layered crystallinity, film formability, and field-effect transistor characteristics in pi-extended and alkylated organic semiconductors (OSCs). We developed four kinds of unsymmetric rod-like OSCs based on syn-/anti-isomers of benzothieno-[6,5-b]-/benzothieno-[5,6-b]-benzothieno-[3,2-b]-thiophene (BTBTT) with phenyl/alkyl substitutions with different alkyl chain lengths: synCn and antiCn (n = 6, 10). The layered molecular packing motifs of the compounds are distinct from the thiophene orientation of isomeric pi-cores but are unaffected by the alkyl chain length. The synCn forms a bilayer-type layered herringbone (b-LHB) packing composed of head-to-head arrangement of unidirectionally aligned molecular layers showing high-layered crystallinity and high carrier mobility over 10 cm2 V-1 s-1. By contrast, the antiCn forms an antiparallel alkyl-interdigitated herringbone (aai-HB) structure in which the respective pi-core layer is composed of alternating antiparallel alignment of pi-cores and the alkyl chains are interdigitated with each other between the adjacent pi-core layers. The latter shows relatively poor crystalline-film formability and moderate carrier mobility. Dispersion-corrected density functional theory calculations of intermolecular interaction energy reveal that the overall shape of the rigid pi-core components is crucial for achieving unidirectionally aligned and closely packed 2D pi-core layers and that the flexible end-cap substituents strengthen and balance the layered crystallinity. The findings will be crucial for designing and developing the highly layered crystalline and high-performance OSCs.