Functional polymer materials containing heavy group-14 elements focusing on germanium and tin

In this review, the synthesis, functions, and applications of the polymers containing germanium and tin, which are heavy group 14 elements, in their polymer frameworks are summarized. Germanium and tin can form similar skeletal structures with their homologues carbon and silicon, whereas the polymers containing germanium and tin show unique properties derived from their large atomic radii and weak binding energies. For example, polygermane and polystannane exhibited light absorption in the UV-visible region and conductivity because of the σ-conjugation through the polymer main-chain constructed by σ-bonds between heavy elements. The σ-conjugation was affected by the conformational change of the polymer main-chain, and thermochromic properties can be induced. Furthermore, the weak bonds were able to be cleaved homolytically upon photoirradiation, and radicals were subsequently generated. By incorporating hypervalent heavy elements into the π-conjugated system, it was possible to modulate the electronic structures of the π-conjugated system through σ*-π* conjugation with highly coordinated elements. Finally, applications for organic solar cells, organic light-emitting materials, and chemical sensors have been achieved. Herein, representative synthetic methods and unique properties for creating smart materials with germanium and tin will be explained.