Effect of Chalcogen Atom on the Properties of Naphthobischalcogenadiazole-Based π-Conjugated Polymers

We here discuss the effect of chalcogen atom on the properties of π-conjugated polymers based on electron deficient fused π-electron systems, naphtho[1,2-c:5,6-c']bis[1,2,5]oxadiazole (NOz), naphtho[1,2-c:5,6-c']bis[1,2,5]thiadiazole (NTz), and naphtho[1,2-c:5,6-c']bis[1,2,5]selenadiazole (NSz). We synthesize new donor-acceptor polymers having NOz and NSz combined with a quaterthiophene donor unit, namely, PNOz4T and PNSz4T, respectively. Physicochemical properties, structures, and charge transport properties of PNOz4T and PNSz4T in comparison with the sulfur analogue (PNTz4T) are discussed. As both NOz and NSz have a higher electron deficient nature than NTz, PNOz4T and PNSz4T are found to possess deeper frontier orbital energy levels. In particular, PNOz4T have a sufficiently deep lowest unoccupied molecular orbital (LUMO) energy level which allows for ambient ambipolar behavior in transistor devices. Interestingly, whereas PNSz4T is mostly amorphous in the thin film, PNOz4T forms a highly crystalline structure that is similar to PNTz4T. This is probably due to the difference of the atomic size, where the large selenium may cause a larger steric repulsion between NSz and the adjacenet thiophene ring, resulting in a larger defect in the polymer backbone than the others. As a result, whereas PNSz4T exhibited p-channel behavior with modest hole mobilities of the order of 10^-2 cm² V^-1 s^-1, PNOz4T demonstrated p-channel behavior with high hole mobilities of ∼1 cm² V^-1 s^-1 and ambipolar behavior with balanced hole and electron mobilities of ∼0.5 and ∼0.3 cm² V^-1 s^-1 depending on the surfactant used for the devices. These results show clear correlations between the chalcogen atom and the properties of the naphthobischalcogenadiazole-based polymers. Overall, in addition to NTz, NOz is found to be another fascinating acceptor unit for π-conjugated polymers.