Gauche polysilane

The value of mh greatly depends on the helical angle 6, whereas that of m does not. The mh of gauche polysilane by DZ is 8mo, being 66 times heavier than the mh of the trans polysilane, 0.12/no. 

This finding is interpreted physically as follows. The valence band edge of the polysilane consists of pure 3p orbital of the silicon atom. The orbital interaction is maximum at the trans conformation because the orbital stands in a coplane. On the other hand, the two 3p orbital lobes in gauche polysilane are not arranged in a coplane. The interaction of these orbitals, therefore, is less than that for the trans polysilane. 

Figure 4. The INDO/S first singlet excitation energies of polysilanes, (SiH2)n full line, all trans broken line, all gauche.

Research on polysilane oligomers and polymers has been the major driving force for the paradigm shift to a new model of rotational conformations.57 58 Until very recently, the structures of polysilanes were mainly described in terms of the older anti-gauche model. This is now known to be incorrect, and as a result nearly 20 years of research will probably need to be reinterpreted. 

In a revealing experiment, Toyoda and Fujiki showed that the UV absorption and emission spectra of (PhMeSi) are virtually identical to those of the copolymer, (PhMeSi)o.95 [PhSiCH2C (H)(CH3)CH2CH3]005 . The latter is known to be helical in solution from its CD spectrum. The implication is that (PhMeSi) , which was for many years described in terms of the anti-gauche model, is actually helical in solution also.69 The evidence which is accumulating now suggests that nearly all polysilanes adopt helical conformations in solution. 

Until recently, a common tendency has been to account for the behavior of polysilanes in terms of anti (often called trans) and gauche states, following the pattern familiar from hydrocarbons. From the discussion above it follows that the exact A conformation... 

The seven-carbon polysilane ( -Hep2Si)n has not been carefully studied but is reported also to adopt the anti form at room temperature. For the next member of the series, (n-OcbSi),. two low-temperature forms were originally described, all-A and a form with 7-max near 350 nm43. The latter was attributed to a i GTCi (TG+TG ) structure, with alternating handedness for the gauche turns, claimed to exist for all the di-n-alkylpolysilanes with linear sidechains from Cs to C14. 

EflFect of Conformation on Band Structure. The trans and gauche parent polysilanes are shown in Figure 15. Thermochromism is one of the most controversial subjects in polysilanes. However, the following basic questions have not been answered yet Which conformation is more stable, trans or gauche What is the essential difference in their band structures ... 

The local-density functional approach was used to compare the band structures of the sW-trans conformation of unsubstituted polysilane with a 4/1 helical conformation and with an dll-trans conformation of dimethyl-substituted poly silane. In line with previous theoretical studies, the electronic wave functions in the vicinity of the Fermi level are primarily silicon-back-bone states, with the major effect of methyl substitution being a decrease in the gap. The predicted absorption spectra for the dll-trans conformations of unsubstituted and dimethyl-substituted polysilane are similar for nearthreshold absorption. Given this similarity, we believe that the shift in energy and strong anisotropy of threshold absorption that we predict for the two extremes of the dll-trans conformation and the dll-gauche model will also occur in alkyl-substituted systems, which are currently under investigation. 

The parent polysilane chain is extremely flexible, and for tetrasilane, the energy barriers between the gauche—gauche and gauche—anti rotation were calculated to be only 6.3 0.8 and 2.9 + 0.4 kJ mol 1, respectively.117 For permethylated silanes, still no distinct conformational preferences could be determined however, for the more bulky n-hexyl substit-... 

These effects have been explained in terms of temperature- and solvent-dependent conformational changes in the polysilane chains. It is probable that the random-coil polysilanes are conformationally flexible, with tram, gauche and intermediate torsional angles coexisting in solution. It is also likely that the excitation energy depends on conformation. INDO/S calculations on (SiH2)w support the model that polysilane... 

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.)...

In polysilanes, the primary motivation has been to identify the chain conformational sequences and, to a lesser extent, orientation of the side chains. PDF analysis [356] of three atactic poly(methyl- -alklysilanes) in bulk is indicative of a near-planar transoid/ deviant sequencing of the skeletal backbone with relatively low concentrations of either ortho- or gauche-type conformations. Figure 17.24 shows data for atactic poly(methyl-n-propylsilane) and a single-chain model fit. The difference at lower q, <5 A , is due to scattering from interchain correlations. 

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