SYNTHETIC, STRUCTURAL, AND PHYSICAL ASPECTS OF ORGANO-OLIGOGERMANES

Absorbance and emission studies in solution and the solid state and band gap determination of Pri3Ge(GePh2)4GePri3

The hexagermane Pri3Ge(GePh2)4GePr i3 (1) can adopt four different conformations by rotations about its germanium –germanium single bonds that differ in energy across an energy range of 31.63 kJ/mol, with the trans-coplanar arrangement having the lowest energy. Conformational changes can occur among these four structures resulting in the observation of thermochromic absorbance spectra both in solution and in the solid state. Bathochromic shifts of 5 nm and 15 nm were observed in solution and in the solid state with increasing temperature. Compound 1 is also luminescent both in solution and in the solid state. The solution emission spectra are solvent dependent and the solid state emission maxima were shown to be temperature dependent. When 1 is excited at 300 nm in the solid state at 80 K its emission spectrum contains a broad emission peak in the visible region and this emission can be observed with the naked eye. The indirect band gap of 1 was determined to be 3.25 eV, which is consistent with investigations on other related oligogermane systems.

Incorporating Methyl and Phenyl Substituted Stannylene Units into Oligosilanes. The Influence on Optical Absorption Properties

Molecules containing catenated heavy group 14 atoms are known to exhibit the interesting property of σ-bond electron delocalization. While this is well studied for oligo- and polysilanes the current paper addresses the UV-absorption properties of small tin containing oligosilanes in order to evaluate the effects of Sn–Si and Sn–Sn bonds as well as the results of substituent exchange from methyl to phenyl groups. The new stannasilanes were compared to previously investigated oligosilanes of equal chain lengths and substituent pattern. Replacing the central SiMe₂ group in a pentasilane by a SnMe₂ unit caused a bathochromic shift of the low-energy band (λ_max = 260 nm) of 14 nm in the UV spectrum. If, instead of a SnMe_2, a SnPh₂ unit is incorporated, the bathochromic shift of 33 nm is substantially larger. Keeping the SnMe₂ unit and replacing the two central silicon with tin atoms causes shift of the respective band (λ = 286 nm) some 26 nm to the red. A similar approach for hexasilanes where the model oligosilane [(Me₃Si)₃Si]₂(SiMe₂)₂ (λ_max = 253 nm) was modified in a way that the central tetramethyldisilanylene unit was exchanged for a tetraphenyldistannanylene caused a 50 nm bathochromic shift to a low-energy band with λ_max = 303 nm.

Synthesis, structure, and properties of the hexagermane Pri3Ge(GePh2)4GePri3

The synthesis of the hexagermane Pri3Ge(GePh2)4GePri3 was achieved starting from the cyclotetragermane (Ph2Ge)4. Ring-opening of (Ph2Ge)4 with Br2 yielded Br(GePh2)4Br that was converted to H(GePh2)4H, and this material was treated with two equiv. of Pri3GeNMe2 to furnish Pri3Ge(GePh2)4GePri3 via the hydrogermolysis reaction. The X-ray crystal structures of (Ph2Ge)4, Br(GePh2)4Br and Pri3Ge(GePh2)4GePri3 were determined. The hexagermane Pri3Ge(GePh2)4GePri3 represents the longest structurally characterized linear oligogermane reported to date and exhibits physical properties that resemble those of the larger polygermane systems. The hexagermane is luminescent and interacts with polarized light, appearing pale yellow under one orientation of polarized light and deep blue under the opposite orientation. The electrochemistry of Pri3Ge(GePh2)4GePri3 was also explored, and this species exhibits the expected five irreversible oxidation waves.