Synthesis and characterization of cyclic organogermanium element-block materials germa[N]pericyclynes composed of ethynylene-germylenes

Synthesis of cyclic germylene-ethynylene materials, germa N pericyclynes, and their characterization are described in this account. With the growing attention and importance of "element blocks" for functional materials, skipped polyynes are of particular interest among polyyne materials. Although they consist of non conjugated alkyne units interrupted by linker atoms, those are expected to enhance the through-bond and through-space interactions. Thus, use of group 14 elements such as germanium would be a preferable sp3 unit due to the hypercoordination abilities of heavy atoms and strong covalent bonds with the sp-carbon atom units. Moreover, germa N-pericyclynes of cyclic skipped polyynes are expected to fix their conformation and reinforce the orbital interaction by ring strain promoted reduced vertex angles. Germa N pericyclynes were initially obtained by shot-gun synthesis, and later, the product yields were improved by stepwise synthesis. Extended germa N pericyclynes composed of butadiynes as alkyne units were also synthesized by coupling with acetylides and germanium chlorides. The obtained pericylcynes were characterized by X-ray crystallography, UV-vis, and fluorescence and phosphorescence emission spectroscopy. The molecular orbitals are estimated by DFT calculations. A unique fluorescence emission was observed for the large germa 8-and 10 pericyclynes compared to the appropriate acyclic germanium polymers. Additionally, these germanium cyclic polymers showed phosphorescence emission without transition metal components.