Nitro-Functionalized Naphthobisthiadiazole as a Building Unit for Electron-Transporting π-Conjugated Polymers

A nitro group is one of the strongest electron-withdrawing substituents; however, nitro-functionalized electron-deficient building units remain largely unexplored in the design of organic semiconductors, particularly π-conjugated polymers. In this study, we designed and synthesized a nitro-functionalized naphthobisthiadiazole (NO2-NTz) as an electron-deficient building unit, along with a π-conjugated polymer consisting of NO2-NTz and dialkyl-substituted quaterthiophene moiety, named PNNT4T. Owing to the stronger electron-withdrawing ability of the nitro group compared to the cyano group, NO2-NTz exhibits greater electron-deficiency than the cyano-functionalized NTz (CN-NTz) we previously reported. Consequently, PNNT4T showed a significantly deeper lowest unoccupied molecular orbital (LUMO) energy level than its CN-NTz counterpart PCNT4T. Notably, PNNT4T formed a crystalline structure with a relatively short π–π stacking distance of 3.63 Å, likely due to the enhanced interchain dipole–dipole interaction, despite an unfavorable backbone twisting, as indicated by calculation. Organic field-effect transistors based on PNNT4T showed ambipolar behavior with electron mobilities of up to 0.17 cm2 V–1 s–1, and demonstrated more pronounced n-type behavior than those based on PCNT4T. These results highlight nitro functionalization as a powerful design strategy for creation of strongly electron-deficient building units toward high-performance electron-transporting organic semiconductors.