Polygermanes conformation

Since the electronic absorption spectra of polvsilanes and polygermanes also depend strongly on conformation (3), these materials provide a unique opportunity to study the effect of backbone conformation on the NLO properties, particularly since varying alkyl substituents can dramatically influence the backbone structure through intermolecular interactions (e.g., side chain crystallization) while causing... 

There has been a great deal of interest in the UV-visible spectroscopy of the polygermanes, particularly in comparison with the analogous chains that have silicon or tin backbones.41,42 Both conventional and Raman spectroscopy have been employed. One interesting observation is that the symmetrically disubstituted polyfdi- n - a I k y I g e n nanes) exhibit thermochromic transitions at temperatures below those of their polysilane analogues. Another is the conclusion that in poly(di-n-hexylgermane) the side chains adopt trans-planar conformations as in the polysilane counterpart. The two chains are also similar in that both backbones can, under certain circumstances, also adopt planar zig-zag conformations. 

Intramolecular energy calculations have been made on polygermane chains, so additional information on their conformational preferences is available 43 More specifically, molecular orbital calculations have been carried out as a function of rotations about the backbone bonds. For poly(dimethylgermane), the results predict a broad energy minimum located at the trans conformation, with two symmetrical, steeper and somewhat shallower minima near the usual gauche locations. The results are very similar to those for poly(dimethylsilane), except that the barriers are considerably lower.43 This is apparently due to the fact that Ge-Ge bonds are approximately 0.30 A longer than Si-Si bonds.44 6 Unfortunately, no relevant experimental data might be used to test the validity of the calculations are available at the present time. 

SCHEME 9. Chain conformations in linear polygermane segments anti and gauche conformation77... 

Figure 31 Band structures for polysilanes, polygermanes, and polystannanes (top, frans-planar conformation below, gauche-helical conformation). (Adapted with permission of Elsevier Ltd. from Takeda, K. and Shikaishi K., Chem. Phys. Lett 1992,195,121.)...

Fig. 6.5 Band structures for polysilanes (PSi), polygermanes (PGe), and polystannanes (PSn) (top, trans-planar conformation below, gaucfie-helical conformation). (Adapted from [15])...

ABSTRACT. Band calculated results for electronic structures of sigma-conjugated polymers are reviewed. Conformational and substitutional effects for polysilanes are calculated theoretically and are discussed in the light of experimental data from UV absorption and photoluminescence. The electronic structures of hetero-copolymers of polysilane and polygermane, corresponding to the 1-dimensional superlattice structure, are described. Two-dimensional silicon network polymers are studied theoretically and experimentally. 

The optoelectronic properties result from 0-0 delocalization and confinement of the conjugated electrons along the backbone. Polygermanes resemble polysilanes in this regard. The latter have been studied in more detail. Polysilanes exhibit UV absorption " and their exhibits a red shift as the silicon chain becomes longer. The UV spectra of some polysilanes were found to be thermochromic." This is due to conformational changes in the backbone. Conformational dependence of... 

As noted above, varies with the conformational distributions of the polygermane. Thus in solution polydihexylgermane has a X ax of 327 nm while the... 

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