Poly(diaryl)stannanes:  Influence of Substituents on the <i>σ</i>−<i>σ</i>* Transition Energy

Poly(diaryl)stannanes were prepared by the dehydropolymerization of secondary hydrostannanes Ar2SnH2 with dimethylzirconocene catalyst. At room temperature, H(Ar2Sn)nH polystannanes (Ar = p-tBu-C6H4, p-nHex-C6H4, o-Et-C6H4, o-Et-p-nBuO-C6H3, p-nBuO-C6H4, p-(Me3Si)2N-C6H4) exhibit λmax values attributed to σ → σ* transitions in the range 430−506 nm. These values vary according to the polymer molecular weights and are independent of temperature between −20 and 90 °C. A sample of poly[bis(o-ethyl-p-nbutoxylphenyl)]stannane exhibits a strong UV−vis absorption at 506 nm (tailing to ca. 550 nm), which corresponds to the smallest band gap yet observed for a group 14, σ-conjugated polymer. The H(Ar2Sn)nH polystannanes undergo photodecomposition in solution to give a mixture of the cyclo-hexamer and the cyclo-pentamer. Thermal gravimetric analyses for the polymers suggest that H(Ar2Sn)nH polystannanes are thermally stable and have onset temperatures for thermal decomposition in the range 180−320 °C, under both nitrogen and air. The H(Ar2Sn)nH polystannanes undergo clean thermal decompositions to elemental tin (under nitrogen) or tin oxide (under oxygen or air), as shown by bulk pyrolysis.