Adapting semiconducting polymer doping techniques to create new types of click postfunctionalization

After the historical development from the insoluble polyacetylene film to soluble and processible aromatic polymers, donor-acceptor-type aromatic polymers have recently emerged as a new class of semiconducting polymers. The polymer energy levels and band gaps can be tuned by the appropriate selection of the donor and acceptor moieties, and some of these polymers showed good optoelectronic or photovoltaic performances. The conventional synthetic method for achieving donor-acceptor-type aromatic polymers is based on the metal-catalyzed polycondensation between donor-type monomers and acceptor-type co-monomers. In this tutorial review, a new methodology for introducing donor-acceptor chromophores into semiconducting polymers is described. The donor-acceptor structures are constructed in the main chains and side chains of semiconducting polymers using a polymer reaction based on high-yielding addition reactions between the electron-rich alkynes and strong acceptor molecules, such as tetracyanoethylene (TCNE) and 7,7,8,8-tetracyanoquinodimethane (TCNQ). Considering the p-type doping features of TCNE and TCNQ, the experimental procedure is the same as the conventional doping technique for semiconducting polymers. However, the resulting donor-acceptor type polymers are chemically stable due to the absence of unstable unpaired electrons (polarons). The donor-acceptor alternating polymers were achieved in one step from the precursor poly(aryleneethynylene)s and poly(arylenebutadiynylene)s. When the side chain alkynes were post-functionalized, the polymer energy levels were controlled by the species and amount of the employed acceptor molecules. These atom-economic acceptor additions satisfy most of the requirements of the "click chemistry'' concept. In contrast to the conventional click chemistry reactions, the reactions between electron-rich alkynes and acceptor molecules provide a wide variety of polymers with promising optoelectronic applications.