Recent Progress of Conjugated Polymers Containing Quinoidal Structure

In recent years, conjugated polymers have attracted extensive attention from both industry and academia due to their excellent solution processability and good mechanical properties. The incorporation of quinoidal unit, featured with good pi-electron delocalization, rigidity structure and low-lying LUMO energy level, into conjugated backbone is a promising approach to developing high-performance conjugated polymers. However, how to introduce such structure into polymer backbone remains a challenge. In this feature article, we summarized the recent progress of conjugated polymers containing quinoidal structure. According to the structures of quinoidal units, we discussed the design and synthesis of quinoidal monomers. Particularly, we synthesized a series of oxindole- and indandione-terminated quinoidal compounds. The terminal aromatic rings in these quinoidal compounds provide an opportunity to feasibly adjust the properties of conjugated polymers. Furthermore, compared with the reported quinoids, oxindole- and indandione-terminated quinoidal compounds tend to exhibit lower LUMO energy levels, making them promising for the construction of n-type semiconductors. The applications of conjugated polymers containing quinoidal structure in different optoelectronic devices also have been discussed. Such polymers have been widely used in organic thin-film transistors (OTFTs), organic solar cells (OSCs), and organic thermoelectric (OTE) devices. For example, electron mobility of above 1 cm(2)center dot V-1 center dot s(-1) in OTFTs has been obtained for the polymers; air stable n-type OTE devices with power factor above 4.24 mu W center dot m(-1)center dot K-2 can be fabricated based on these polymers. Finally, the current problems and future development directions in the research process of quinoid based polymers are also discussed, hoping to provide useful information for the development of high-performance polymer semiconductor materials. The major issues for this field include: (1) development of new configuration-locking methods for the synthesis of isomer-free quinoid monomers; (2) development of new quinoidal monomers for the synthesis of conjugated polymers with LUMO energy levels below - 4.4 eV; (3) synthesis of conjugated polymers using quinoid units for NIR-II photothermal therapy. [GRAPHICS] .