n-Type Conjugated Polymers Based on Double B←N Bridged Bipyridine Unit

ConspectusBoth p-type conjugated polymers and n-type conjugated polymers are required for organic optoelectronic devices, such as organic solar cells (OSCs), organic field-effect transistors (OFETs), organic thermoelectrics (OTEs), etc. The development of n-type conjugated polymers lags far behind that of the p-type counterparts in view of material diversity and optoelectronic device performance. This is mainly due to the lack of strong electron-withdrawing building blocks, which are always based on the imide unit. Double boron-nitrogen coordination bond (B←N) bridged bipyridine (BNBP), which was first developed in 2016, is an alternative kind of electron-withdrawing building block based on a B←N unit. BNBP itself possesses a planar and fixed configuration, low-lying electronic energy levels, strong fluorescence, and facile functionalization. A family of BNBP-based conjugated polymers has been developed. They show excellent and tunable optoelectronic properties, such as high electron mobility, low-lying and tunable lowest unoccupied molecular orbital (LUMO) energy level (ELUMO), medium bandgap, narrow absorption spectra in the visible range, high fluorescence quantum efficiency, etc. With rational molecular design of BNBP-based conjugated polymers, they have been widely used in organic optoelectronic devices with high performance, including OSCs, OFETs, and OTEs, indoor photovoltaics (IPVs), organic light-emitting diodes (OLEDs), organic photodetectors (OPDs), etc. Therefore, BNBP-based conjugated polymers have become an important class of optoelectronic materials. In this Account, we summarize the research progress on BNBP-based conjugated polymers.At first, we discuss BNBP itself, including its molecular design, synthesis, chemistry, and optoelectronic properties. Then we introduce the optoelectronic properties of BNBP-based conjugated polymers, including their light absorption property, fluorescence, electron mobility, and frontier electronic energy levels. We have systematically elucidated the relationship between the chemical structures, optoelectronic properties, and optoelectronic device performance of BNBP-based n-type conjugated polymers. The unique property of BNBP-based conjugated polymers is the high electron mobility in the amorphous state. Other noteworthy properties of these polymers are the medium bandgap and absorption spectra in the visible range. Next, we discuss the applications of BNBP-based conjugated polymers in OSCs, IPVs, OFETs, OTEs, OPDs, and OLEDs. The excellent optoelectronic device performance is noteworthy, such as power conversion efficiency (PCE) of 10% in OSCs, PCE of 26% in IPVs, electron mobility of 0.3 cm2 V-1 s-1 in OFETs, power factor of 25 μW m-1 K-2 in OTEs, specific detectivity of 1.79 × 1013 cm Hz1/2 W-1 in OPDs. Finally, we propose that great attention should be paid to the deep understanding of the electronic structures of BNBP itself and BNBP-based conjugated polymers as well as the new applications of BNBP-based conjugated polymers.

Pyrazinacene conjugated polymers: a breakthrough in synthesis and unraveling the conjugation continuum

Pyrazinacenes are next generation N-heteroacenes and represent a novel class of stable n-type materials capable of accepting more than one electron and displaying intriguing features, including prototropism, halochromism, and redox chromism. Astonishingly, despite a century since their discovery, there have been no reports on the conjugated polymers of pyrazinacenes due to unknown substrate scope and lack of pyrazinacene monomers that are conducive to condensation polymerization. Breaking through these challenges, in this work, we report the synthesis of previously undiscovered and highly coveted conjugated polymers of pyrazinacenes. In order to understand the intricacies of conjugation extension within the acene and along the polymer backbone, a series of electronically diverse four pyrazinacene conjugated polymers were synthesized. Polymers synthesis required optimizing a few synthetic steps along the 12-step synthetic pathway. The generated pyrazinacene monomers are not amenable to the popular condensation polymerizations involving Pd or Cu catalysts. Gratifyingly, Pd and Cu free dehydrohalogenation polymerization of the monomer with HgCl2 resulted in high molecular weight organometallic conjugated pyrazinacene polymers within a few minutes at room temperature. The dual role played by the Hg(ii) during the polymerization, combined with the self-coupling of the RHgCl (intermediate), is at the core of successful polymerization. Notably, the self-coupling of intermediates challenges the strict stoichiometric balance typically required for step-growth polymerization and offers a novel synthetic strategy to generate high molecular weight conjugated polymers even with imbalanced monomer stoichiometries. A combination of electrochemical studies and DFT-B3LYP simulations indicated that the presence of the reduced pyrazine ring promotes interacene π-conjugation through the metal center, in contrast to completely oxidized tetrazaazaanthracene. The extension of conjugation results in ca. 2 eV lower reduction potential for polymers compared to the monomer, placing the LUMO energy levels of these polymers on par with some of the best-known n-type polymers. Also, the presence of NH protons in the pyrazinacene polymers show ionochromism and red-shift UV-vis absorption maximum by ca. 100 nm. This work not only shows a way to realize highly desirable and elusive pyrazinacene conjugated polymers but also paves the way for a library of n-type conjugated polymers that can undergo multi-electron reduction.