Cyano substituted benzothiadiazole: a novel acceptor inducing n-type behaviour in conjugated polymersElectronic supplementary information (ESI) available. CCDC 1020253. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4tc02008a

We report the synthesis of the novel acceptor, 4,7-di(thiophen-2-yl)-5,6-dicyano-2,1,3-benzothiadiazole (DTDCNBT) and compare its properties to those of the previously reported 4,7-di(thiophen-2-yl)-5,6-difluoro-2,1,3-benzothiadiazole (DTDFBT). Co-polymers of both monomers with the donor monomers indacenodithiophene (IDT) and dithienogermole (DTG) were prepared and investigated. The DTDCNBT unit was found to be a much stronger electron acceptor than DTDFBT. The electron affinity of the cyanated polymers was increased by up to ∼0.4 eV, resulting in red-shifted absorptions and reduced optical band gaps. In field effect transistors it was found that replacing the fluorine substituents of the polymers with cyano groups changed the charge transport from unipolar p-type to unipolar n-type. The displacement of fluoride by cyanide offers a simple route to strongly electron accepting cyano substituted benzothiadiazole monomers. Copolymerisation with electron rich donors leads to low band gap polymers which exhibit electron transporting behaviour.