Stannoles – unseen heterocycles with unique optoelectronic properties

In the present work, a range of different substituted stannoles were synthesised and examined with respect to their optoelectronic properties from an experimental and theoretical point of view. Stannoles belong to the group 14 metalloles and are of great interest, because they show a special form of conjugation compared to their cyclopentadiene analogues, namely cross-hyperconjugation or also called σ*-π*-conjugation. This leads to a lower LUMO compared to their carbon analogue, while the HOMO remains almost constant and the HOMO-LUMO gap is reduced overall. Stannoles are thus potential building blocks for semiconducting polymers with interesting optoelectronic properties.
Stannoles can be synthesised in various ways. The method of choice in this work was the transmetallation from zirconium to tin. Precursor molecules for this were zirconacyclopentadienes, which were prepared by cyclisation of diynes or two monoynes with Rosenthal's zirconocene as “Cp2Zr(II)” source. In an experimental study, two of the major “Cp2Zr(II)” sources, Rosenthal's zirconocene and Negishi's reagent, were compared with respect to the synthesis of zirconacyclopentadienes. Overall, Rosenthal's zirconocene led to a higher product yield, a faster conversion of the starting material and a higher product stability in the reaction mixture. Overall, it could be shown that the stannoles follow the theory of Tamao, though there are exceptions where the tin is not involved in the LUMO but a diene structure is present. The influence of different substituents on the planarity of the molecular structure, the HOMO-LUMO energy gap, the electronic structure and on the absorption behaviour were investigated. Various fluorescence measurements proofed that non-fused stannoles show Aggregation Induced Emission (AIE), and for the first time, high quantum yields of up to 11.1% in the solid state and up to 24.4% in thin film were detected for this class of compounds. Tin-selective Stille cross-coupling reactions have successfully led to various polymers with stannole-thiophenyl units, which were analysed by UV-Vis, GPC, MALDI and TGA. Their small optical band gaps of 1.61-1.79 eV (thin film) indicate that they do indeed seem to exhibit semiconducting properties. Furthermore, attempts have been made to transmetalate from a stannole to a borole, which is also of great importance for semiconducting materials due to its anti-aromaticity and unique electronic structure.