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Conformational effects, polysilanes

In the model compounds, this red shift has been ascribed to a combination of silicon backbone with the x-orbitals of the aromatic substituent coupled with a decrease in the LUMO energy due to x -(a, d) interactions (15,16). Further examination of the data in Table III shows that the absorption maximum of the cyclohexylmethyl derivative, 9, is also somewhat red-shifted relative to the other alkyl polymers suggesting that the steric bulk of the substituents and/or conformational effects may also influence the polysilane absorption spectrum. [Pg.297]

The recent interest in substituted silane polymers has resulted in a number of theoretical (15-19) and spectroscopic (19-21) studies. Most of the theoretical studies have assumed an all-trans planar zig-zag backbone conformation for computational simplicity. However, early PES studies of a number of short chain silicon catenates strongly suggested that the electronic properties may also depend on the conformation of the silicon backbone (22). This was recently confirmed by spectroscopic studies of poly(di-n-hexylsilane) in the solid state (23-26). Complementary studies in solution have suggested that conformational changes in the polysilane backbone may also be responsible for the unusual thermochromic behavior of many derivatives (27,28). In order to avoid the additional complexities associated with this thermochromism and possible aggregation effects at low temperatures, we have limited this report to polymer solutions at room temperature. [Pg.61]

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... [Pg.622]

Another interesting chiral chain end effect is exhibited by the helical polymer block co-polymer, poly(l,l-dimethyl-2,2-di-/z-hexylsilylene)- -poly(triphenylmethyl methacrylate), reported by Sanji and Sakurai (see Scheme 7) and prepared by the anionic polymerization of a masked disilene.333 The helical poly(triphenylmethyl methacrylate) block (PTrMA) is reported to induce a PSS of the same sign in the poly(di- -propylsilylene) block in THF below — 20 °C, and also in the solid state, by helicity transfer, as evidenced by the positive Cotton effect at 340 nm, coincident with a fairly narrow polysilane backbone UV absorption characteristic of an all-transoid-conformation. This phenomenon was termed helical programming. Above 20°C, the polysilane block loses its optical activity and the UV absorption shifts to 310 nm in a reversible, temperature-dependent effect, due to the disordering of the chain, as shown in Figure 45. [Pg.622]

Many polysilanes, both in the solid state and in solution, display a striking thermo-chromism—the absorption wavelength depends upon temperature. Before discussing this and the other chromotropic effects of polysilanes, it will be necessary to outline the modem theory of rotational conformations, described in the following section. [Pg.217]

The conformation of the polysilane chain, and hence the amount of electron delocalization and the absorption wavelength, may change with temperature, solvent, pressure and so on. The result is that many polysilanes are chromotropic.65 The effect of temperature changes, leading to thermochromism, have been most thoroughly investigated. [Pg.220]

In addition to photoconductivity, polysilanes have been found to exhibit marked nonlinear optical properties,95-97 suggesting that they may eventually be useful in laser and other optical technology. The third-order non-linear susceptibility, X3, is a measure of the strength of this effect. The non-linear properties of polysilanes, like the absorption spectra, seem to be dependent on chain conformation and are enhanced for polymers having an extended, near anti conformation (Table 5.5). The value of 11 x 10 12 esu observed for (n-Hex2Si) below its transition temperature is the largest ever observed for a polymer which is transparent in the visible region. [Pg.232]

The conformational structures of polysilane main chains at the macro-and microscopic levels are controllable by suitable choice of the side chain structures. Similarly, it is also the side chain which controls the optoelectronic properties by effecting the optical band gap. In the case of phenyl-substituted polysilanes, electronic interaction between the delocalized Si chain cr-bonding orbitals and the it-orbitals of the aryl groups causes a dramatic modification of both the band gap and conformational properties [61,83]. These aryl-containing polysilanes may be potential candidates for applications in a molecular-based chiroptical switch and memory in the UV/visible region. On the other hand, the precise control of helical polymers is now a subject of great interest and importance, due to the tech-... [Pg.159]


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