Electronic Properties of Polysilanes

As explained in the introduction, the polysilanes (and related polygermanes and poly-stannanes) are different from all other high polymers, in that they exhibit sigma-electron delocalization. This phenomenon leads to special physical properties strong electronic absorption, conductivity, photoconductivity, photosensitivity, and so on, which are crucial for many of the technological applications of polysilanes. Other polymers, such as polyacetylene and polythiophene, display electron delocalization, but in these materials the delocalization involves pi-electrons. 

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. 

This section will describe the recent changes in the theory of rotational conformations, which has undergone a drastic revision in the past few years.56 

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. 

Consider a linear polymer, (MX2) . If the substituents X are small compared to the distance M-M between the atoms in the main polymer chain, then the conventional anti-gauche model is valid. This is the case for hydrocarbons, and probably for other (MH2) polymers. However, if the van der Waals radius of the X group becomes a little larger relative to the M-M distance, then repulsions arise between the X atoms on the M atoms in 1,3 positions along the chain. The critical value for such interference is about RX/RM M = 0.8. Beyond this value the anti conformation is no longer a minimum, 

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Applications based on the photophysical and electronic properties of polysilanes... 

The electronically excited states of the emitter can be populated via direct ex-citon formation at the emitter molecule. Alternatively, the electronic properties of polysilanes facilitate an energy transfer from excited states of the polysilane to the emitter molecule. 

Polysilanes are also applicable as matrix materials in phosphorescent OLEDs. Mixtures of polysilanes and triplet emitters are sufficient to effect an energy transfer from polysilane triplet states to emitter triplet states, thus amplifying the luminescence of the device. It has been shown that if polysilanes have electrophosphorescent side chains consisting of triplet emitters, the energy transfer from polysilane to emitter is most effective [124]. Thus the beneficial electronic properties of polysilanes are perfectly combined with the spectroscopic properties of transition metal based triplet emitters. The compounds described are derivatives of polymethylphenylsi-lanes, (Fig. 24) which are covalently attached to triplet emitters with iridium as metal centre. The polymers were applied in OLEDs with an ITO/active layer/Ca/Ag layer sequence. The active layer contained a fraction of 70% by weight of the... 

R. G. Kepler and Z. G. Soos, Electronic properties of polysilanes excitations of tr-conjugated chains, in Relaxation in Polymers (T. Kobayashi, ed.). World Scientific, Singapore, 1993, p. 100. 

Because the electronic properties of polysilanes are dependent on the conformation of the backbone,which may depend on the configuration of the substituents, it is necessary to prepare polymers with well-defined microstructures. In ring opening polymerization, the resulting polymer microstructure depends on two factors. The fust one is the configuration of the substituents in the cycfic monomers. The second factor is the mechanism of die ring opening polymerization. This paper is mainly focused on the discussion of stereochemistry in the cyclotetrasilanes. 

From these results, the electronic properties of the ladder polysilanes are found to be highly affected by their stereochemistry. 

Dr. Keisuke Ebata, and Dr. Kazuaki Furukawa are thanked for valuable discussions on the electronic structure and optoelectronic properties of polysilanes at NTT. Prof. Nobuo Matsumoto (Shonan Institute of Technology), and Drs. Masao Morita, Kei-ichi Torimitsu, and Hideaki Takayanagi are acknowledged for encouragement and generous support at NTT during the course of this study. 

An important property of the alkyl poly silanes is their electronic absorption at relatively low energy. This was particularly surprising because such molecules lack tt, d ot lone pair electrons they were the first substances containing only bonding cr-electrons to show such long-wavelength absorptions. The unusual electronic properties of the polysilanes have sparked considerable interest in the chemophysical properties of these molecules. The UV spectra of cyclic polysUanes follow a quite different pattern . 

The poiygermanes exhibit many of the same electronic properties as polysilanes, including near-uv photoabsorpilon. thermochroniism. photo-bleaching. as well as nonlinear optical activity, and have seen a fair amount of theoretical and experimental investigation, However, despiie similarities with polysilanes, poiygermanes appear to he unlikely candidates for commercial exploitation,... 

Recent studies of polymeric Group IV catenates (in particular, polysilanes and polygermanes, la,b have demonstrated that there is significant sigma electronic delocalization along the polymer backbone which is responsible for many of the curious electronic properties of this class of materials [3]. 

In polysilane polymers, the polymer backbone is made up entirely of silicon atoms. Therefore these materials differ from other important inorganic polymers, the siloxanes and phosphazenes, in which the polymer chain is heteroatomic. Structurally, they are more closely related to homoatomic organic polymers such as the polyolefins. However, because the units in the main chain are all silicon atoms, the polysilanes exhibit quite unusual properties. The cumulated silicon-silicon bonds in the polymer chain allow extensive electron delocalization to take place, and this delocalization of the sigma electrons in the Si-Si bonds gives the polysilanes unique optical and electronic properties. Many of the potential technical uses, as well as the remarkable properties, of polysilanes result from this unusual mobility of the sigma electrons. 

The polysilanes can be regarded as one-dimensional analogs to elemental silicon, on which, of course, nearly all of modem electronics is based. The photophysical behavior of polysilanes is not approached by any other materials, save for the less stable and more costly polygermanes and polystannanes. The remarkable properties of polysilanes have led to intense interest, and to numerous proposed high-tech applications. But the great promise of polysilanes as materials has yet to be realized. Their only commercial use at present is as precursors to silicon carbide ceramics, an application which takes no advantage of their optical or electronic properties. 

Ultraviolet Spectral Properties of Polysilanes Containing jt-Electron Systems... 

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). 

Properties such as photoconduc.tivityl l t l and photoluminescence of silicon polymers have been reported because of their wider optical band gap compared with crystalline silicon. Theoretical investigations of silicon polymers have been also reported ll2l-[21] Xakeda, Matsumoto and Fukuchi calculated the electronic structure of polysilane chains using the semi-empirical approach called the Complete Neglect of Differential Overlaps (CNDO) Molecular-Orbital (MO) method They discussed the dependence of the size and... 

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... 

The electronic properties of the polysilanes in discussion which might also be a reason for the differences in their behavior as HTLs the workfunctions and the hole conductivity are currently under investigation. 

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