Helical conformation polysilanes

Similarly, use of the chiral anionic initiator (+) or (—) potassium menthoxide with the same masked disilene afforded a polysilane with a PSS helical conformation. Optically active polysilanes are discussed further in Section 3.11.6. 

Recently, the first example of chiral solvation of a polysilane was demonstrated dissolution of the inherently optically inactive poly(methylphenylsilyene), PMPS, and poly(hexylmethylsilylene), PHMS, in the optically active solvents (V)-2-methyl-l-propoxybutane and (V)-(2-methylbutoxymethyl)benzene induced the polymer chains to adopt PSS helical conformations as evidenced by (positive-signed) Cotton effects almost coincident with the UV a-a transition at 340 and 305 nm, respectively.332... 

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. 

An LCAO (linear combination of atomic orbitals) local-density functional approach was used to calculate the band structures of a series of polymer chain conformations unsubstituted polysilane in the all-trans conformation and in a 411 helical conformation, and all-trans poly(dimethylsilane). Calculated absorption spectra predict a highly anisotropic absorption for the all-trans conformation of polysilane, with the threshold absorption peak arising strictly from polarizations parallel to the chain axis. The absorption spectrum for the helical conformation is much more isotropic. Results for the dimethyl-substituted polysilane chain suggest that the states immediately surrounding the Fermi level retain their silicon-backbone a character upon alkyl-group substitution, although the band gap decreases by I eV because of contributions from alkyl substituent states both below the valence band and above the conduction band to the frontier states. 

In this study, we investigated a set of model polysilane chain systems that illustrate the basic physics and chemistry of some optical properties of these materials. In particular, we looked at the band structure for unsubstituted polysilane in an all-trans conformation, as well as in a 4/1 helical conformation with four silicon atoms contained in one translational repeat unit. In addition, we compared results for the dimethyl-substituted polysilane in an dl -trans conformation with the results for the unsubstituted poly silane. 

Figure 3. Calculated absorption spectra for all-trans unsubstituted polysilane (a), all-trans poly(dimethylsilane) (b), and helical unsubstituted polysilane (c). Solid lines denote absorption for polarizations parallel to chain axis, dashed-and-dotted lines denote absorption for polarizations perpendicular to the plane of the silicon backbone (for the all-trans conformations), and dashed lines denote the remaining polarization. All curves have been broadened with a 0.7-eV full width at half-maximum Gaussian.

This dominant feature is essentially the same for both the unsubstituted and dimethyl-substituted all-trans polysilane chains, and an equivalent feature is found when a smaller basis set is used for the dimethyl-, diethyl-, and dipropyl-substituted poly silanes. For the helical conformation, however, along with the larger band gap in this conformation (Figure 3c), a pronounced shift of the direct-gap absorption peak to higher energy is observed, with a trend toward a less anisotropic absorption. 

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. 

Absorption spectra of polysilanes anisotropic absorption for helical conformation, 549 bathochromic shifts, 549 effect of conformation on extinction coefficient, 547, 548/ low-energy feature for al -trans conformations, 547, 548/, 549 Acetylenes... 

The helical structure of polychloral was proposed by Vogl in 198028 and was demonstrated by Ute, Hatada, and Vogl via a detailed conformational analysis of chloral oligomers.29 As an example of a helical polymer with an inorganic backbone, polysilanes bearing a chiral side chain were synthesized and their conformational aspects were studied. A helical conformation with an excess screw sense for this class of polymers in solution was found in 1994 independently by Fujiki30a and by Moller.300 Matyjaszewski had pointed out such a conformation for chiral polysilanes in the solid state in 1992.30c... 

Polysilanes adopt a helical conformation. This class of polymers has the Si o conjugating backbone, which allows the conformational study by means of photophysical analysis of the polymers.30-220 226 Two polysilanes, 130 and 131, were synthesized by the Na-... 

In addition to the polymers described above, the polysilanes having aromatic side groups222 225 and the copolymers of a chiral monomer and an achiral monomer224-228 have been shown to adopt a helical conformation. A water-soluble, helical polysilane having an ammonium moiety has also been prepared.229... 

Reversible control over the helix sense in polysilanes was achieved in the case of poly(diaryl-silanes)128 as well as in (co)polymers of ((S)-3,7-dimethyloctyl)(3-methylbutyl)silane,129 which both showed helix reversal upon heating. For the latter polymer (Figure 12a), it was calculated that the potential curve has a double-well ( W j shape (Figure 12b) with a slight preference for the M-helix over the P-helix. CD spectroscopy indeed revealed that above the transition temperature the ordered (low-entropy) M-helical conformation becomes less stable than the entropically more favored P-helical state (Figure 12c,d). 

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.)...

Fluorescence spectroscopy in combination with circular dichroism (CD), optical rotatory dispersion. X-ray crystallography, UV and NMR spectroscopy of the main chain is a powerful probe for identifying helical conformation, uniformity, and rigidity in polymers. In recent years, these techniques have been applied extensively to investigate the structures of polysilanes in both the solid state and in solution and it is now clear that after electronic structure main chain helicity is the principal determinant of the properties of polysilanes. In... 

Figures 11.9 and 11.10, relating to PMPS and poly(methyl- -hexylsilane) (PMHS) respectively, show that the chiral influence on main chain helicity of polysilanes need not be an internal influence. In each case, the upper plots depict the UV spectra of the relevant polymer in THF solution and in the chiral solvent, (5)-2-methylbutylbenzyl ether. The lower plots depict the CD spectra overlaid on the corresponding UV spectra for solutions of the polymers in the chiral solvent. From the absorption spectra, a very slight change in solution-phase conformation is evident for PMPS on switching solvents but not so in the...

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

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