Isomers of organic semiconductors based on dithienothiophenes: the effect of sulphur atoms positions on the intermolecular interactions and field-effect performances

2,5-Distyryl-dithieno[2,3-b: 2',3'-d] thiophene (DEP-bt-DTT), an isomer of 2,5-distyryl-dithieno [2,3-b:3',2'-d]-thiophene (DEP-bb-DTT) and 2,5-distyryl-dithieno[3,2-b:2',3'-d] thiophene (DEP-tt-DTT), was synthesized. Organic field-effect transistors (OFETs) based on these three isomers were fabricated. The structure cell parameters and the formation of intermolecular interactions in their single crystals show regular change when the positions of sulphur atoms vary from top-bottom-top in DEP-tt-DTT to bottom-bottom-top in DEP-bt-DTT, then to bottom-bottom-bottom in DEP-bb-DTT. Combining the results of theoretical calculations and OFET performances, it reveals that: (1) the positions of sulphur atoms determine the contribution extent of sulphur atoms to the molecular conjugation and the formation of intermolecular interactions; (2) the existence of the intermolecular interactions, especially for S-pi, benefits for the charge transport; (3) the field-effect mobility (mu) increases with increasing of the sulphur atom contribution to the molecular conjugation.