Cyano substituted benzothiadiazole: a novel acceptor inducing n-type behaviour in conjugated polymers

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.