Electron-Transporting Properties and Ultrasmall Band Gap of BODIPY-Based Conjugated Polymers: Teach an Old Dog New Tricks

4,4′-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) is a well-known functional dye. BODIPY-based conjugated polymers have been intensively studied, and they always exhibit moderate hole-transporting properties with a band gap larger than 1.0 eV. Herein, we report a conjugated polymer based on an alternating BODIPY unit and vinylene unit with excellent electron-transporting properties and ultrasmall band gap. The α-linking mode of the BODIPY unit and the vinylene insertion lead to well-delocalized lowest unoccupied molecular orbital (LUMO) and much improved effective conjugation of the polymer backbone. As a result, the polymer exhibits a LUMO energy level of as low as −4.17 eV, a decent electron mobility of 0.07 cm V s, an electrical conductivity of 5.16 S cm after n-doping, an ultrasmall band gap of 0.80 eV, and an onset absorption wavelength of 1545 nm. To the best of our knowledge, for conjugated polymers based on nonfused BODIPY reported so far, this LUMO energy level is the lowest, this electron mobility is the highest, and this band gap is the smallest. Moreover, this is the first report of n-doping of BODIPY-based conjugated polymers. As BODIPY has been intensively studied for more than 50 years, we teach an old dog new tricks in this work. Our results indicate new optoelectronic properties of BODIPY-based polymers and will open up many new applications of BODIPY-based polymers.