Electrochemical and Photophysical Properties of a Series of Group-14 Metalloles

A series of six group-14 dimethyl- or diphenyl−tetraphenylmetallacyclopentadienes were synthesized and characterized by their spectroscopic and electrochemical properties. The group-14 elements investigated were silicon, germanium, and tin. (The compounds are designated according to the heteroatom and the substituent on the heteroatom, i.e., SiMe, SiPh, ..., SnPh.) Five of the six compounds luminesce in both the solid state and in solution. The emission maxima of SiPh, GePh, and SnPh are invariant to a change in the heteroatom, while for SiMe, GeMe, and SnMe there is a strong dependence of the emission maxima on the identity of the heteroatom. SiMe emits at a longer wavelength than GeMe, while SnMe is not luminescent. The dramatic luminescence difference between the two tin compounds was investigated. ¹³C NMR coupling to ^119/117Sn, observed in both SnMe and SnPh, was used to make ¹³C NMR resonance assignments. Qualitative results of semiempirical molecular orbital calculations support the ¹³C NMR assignments. The crystal structure data for SnPh was obtained at 20 °C:  a = 10.353(2) Å, b = 16.679(2) Å, c = 9.482(1) Å, α = 99.91(1)°, β = 106.33(1)°, γ = 77.80(1)° with Z = 2 in space group P_1̄. It is proposed that the increased electron density at tin in SnMe is responsible for the deactivation of the emissive state. The presence of phenyl substituents in SnPh serves to stabilize the emissive state and luminescence is observed.