Polysilane chains

The localization of the HOMO is also important for another reason. Since it describes the distribution of a hole in a radical cation, it relates to the hindrance that a positive charge will encounter as it propagates along the chain. There is indeed experimental evidence (9) that the hole states of the polysilane chain are localized and that they move by a hopping mechanism. 

The next-nearest-neighbor-orbital resonance integrals, /JI3, also remain unaffected by the pure twisting motion. We conclude that a pure twisting motion can therefore represent at best only a relatively small perturbation of the electronic structure of the polysilane chain, suitable for treatment by first-order perturbation theory. The perturbation is represented by changes in the resonance integrals between more distant hybrid orbitals, among which / 14 clearly is the most important. 

The molecular weight distributions were determined only in terms of the polystyrene equivalent from the polystyrene calibration of the GPC columns. This can only be regarded as semiquantitative, as the method determines only the relative hydrodynamic volumes. If the polysilane chain was significantly stiffer than polystyrene, the molecular weights would be estimated too high. If the chains are... 

When anti-2 was oxidized with 3 equivalents of MCPBA, the trioxidation product 11 was obtained in 81% yield (Scheme 7). Similarly, the oxidation of anti,anti-3 and anti,anti,anti-4 with 4 and 5 equivalents of MCPBA gave the tetraoxidation product 12 and the pentaoxidation product 13, respectively, in moderate yields. Therefore, these ladder polysilanes were found to be oxidized in a unique manner one of two polysilane main chains was oxidized selectively, and novel ladder compounds consisting of polysiloxane and polysilane chains were formed. 

Figure 4.24 Superhelix with helix angle < > governing handedness of packing between preferential helical main chains of contiguous polysilane chains for origin of Cotton effect of polysilane aggregates.

Figure 15 Schematic representation of the mechanism of polysilane chain scission.

The photoreaction of polysilanes with Ceo has also been investigated [35]. Reaction (8.16) shows an example in which the irradiation in benzene with a low-pressure mercury-arc lamp afforded a product that contains 14wt% of Ceo into the polysilane chain. The incorporation of Ceo into the polysilane backbone has not been observed upon irradiation with X > 300 nm, when the cleavage of Si—Si bond does not take place. The adduct obtained from Reaction (8.16) has a lower oxidation potential than C6o( + 0-77 vs + 1.21 V) and a lower reduction potential than polysilane (—1.24 vs — 2 V). 

Bond-order changes upon photoexcitation obtained from Sandorfy SCF-CI calculations have been employed to predict the ease of photoextrusion of a silylene from polysilane chains or rings236. Smaller bond-order changes with longer polysilane chains suggested that they are less efficient silylene sources. Similar conclusions were reached for branched and cyclic polysilanes. This approach focuses on a to a excitation without participation by Si 4s orbitals in descriptions of excited states237. 

Linear polysilane polymers, properly called poly(silylene)s, can be obtained as homopolymers or copolymers. Continuation of the polysilane chain consumes two of the four valences of each silicon atom the other two are taken up by pendent groups, which may be the same (5.1) or different (5.2). Copolymers (5.3), which contain two or more kinds of silicon atoms, can be made up from units like those in 5.1 or 5.2. A typical example is the copolymer of Me2Si and PhMeSi units,... 

RR SiCr, Na+ (equation (7)). The propagation step (equation (8)), which is rapid but nevertheless rate determining, is the reaction of anion-terminated chains with dichlorosi-lane to add one silicon unit and produce a chlorine-ended chain. The latter is thought to be reduced rapidly to the anionic form by reaction with sodium. The polysilane chains will alternately be terminated with silyl anions or Si-Cl. This is consistent with evidence from chemical derivatization that both may be present in the products of the reaction. ... 

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. 

For some uses it is important to form bonds that link different polysilane chains, to transform soluble, meltable polysilanes into insoluble resins. This process is vital if the polysilanes are to be used as precursors to silicon carbide ceramics, since, if cross-linking is not carried out, most of the polymer is volatilized before thermolysis to silicon carbide can take place. Several methods have therefore been developed to bring about cross-linking of polysilanes.109 110... 

Silane dendrimers with longer polysilane chains (e.g. with thirteen silicon atoms in the longest chain) were prepared by Lambert and Wu via a convergent route [73]. These molecules also assume an almost spherical shape. 

The foregoing facts suggest that the unique spectral properties of polysilanes are due to the silicon-silicon bond acting as a chromophore, probably through the use of vacant d orbitals of the silicon atom. In addition, it seems likely that there exists an enhanced conjugation through overlap of d and it orbitals between substituents with 7r-electron systems, such as phenyl and vinyl, and polysilane chains (53, 81,153). 

FIGURE 5. The cr-symmetry orbitals of a long, all-trans polysilane chain showing backbone orbitals (left), orbitals of the SiH bonds (right), and the result of their mutual interaction (center). (Reprinted from Ref. 63.)... 

The first step (Fig. 29a) is the evaporation of the polysilane material and recrystallization on the substrate. The film is then irradiated by the electron beam (Fig. 29b). In the irradiated area of the film, C-O-C, Si-O-C, and Si-O-Si bonds are formed between nearest-neighbor polysilane chains. Thus the... 

In phenyl-substituted polysilanes, the problems become more complicated because the benzene n electron system is mixed with the a (Si-Si) orbitals and the highest occupied molecular orbital is assigned to a linear combination of 7r (C6H5) and a (Si—Si) orbitals452. Similar properties are found in mixed-substituted methyl-phenylpolysilanes, where the effect of extension of the 3 d molecular orbitals of the polysilane-chain is found to be minimal453. ... 

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