A series of ( E )-1,2-diaryldigermenes incorporating bulky Eind groups: structural characteristics and absorption properties.

A series of ( E )-1,2-diaryldigermenes, (Eind)ArGeGeAr(Eind) [Ar = phenyl (2), thiophen-2-yl (3), 9,9-dimethyl-2-fluorenyl (4) and 2,2'-bithiophen-5-yl (5)], supported by the fused-ring bulky 1,1,3,3,5,5,7,7-octaethyl- s -hydrindacen-4-yl (Eind) groups, have been obtained as yellow-orange to red crystalline solids by the reaction of 1,2-dibromodigermene, (Eind)BrGeGeBr(Eind) (1), with ArLi. In the crystals of 2-5, the digermene cores show a flexible nature adopting a trans -bent geometry with the trans -bent angles ( θ ) between the Ge-Ge vector and the C Eind -Ge-C Ar plane of 34.04(12)° (2), 38.3(3)° and 38.8(3)° (3), 33.69(12)° (4) and 39.30(13)° (5). In the UV-vis spectra, strong π-π* absorptions have been observed with an absorption maximum at 451 nm ( ε = 1.3 × 10 4 ) (2), 455 nm ( ε = 9.7 × 10 3 ) (3), 480 nm ( ε = 1.3 × 10 4 ) (4) and 497 nm ( ε = 1.4 × 10 4 ) (5), retaining the GeGe double bond in solution. The absorption data and DFT calculations provide evidence for the intrinsic π-conjugation between the GeGe chromophore and aromatic rings involving the narrowing of the HOMO-LUMO gaps (Δ E ) with the extension of the carbon π-electron systems.