Silicon polysilylenes

Polysilanes (or polysilylenes) consist of a silicon-catenated backbone with two substituents on each silicon atom. The two groups attached to the silicon chain... 

The history and development of polysilane chemistry is described. The polysilanes (polysilylenes) are linear polymers based on chains of silicon atoms, which show unique properties resulting from easy delocalization of sigma electrons in the silicon-silicon bonds. Polysilanes may be useful as precursors to silicon carbide ceramics, as photoresists in microelectronics, as photoinitiators for radical reactions, and as photoconductors. 

Polysilanes (or polysilylenes) consist of a silicon-catenated backbone with two substituents on each silicon atom (Structure 1). The groups R and R attached to the silicon chain can be of a large variety. Polysilanes with alkyl and/or aryl substituents have been the most thoroughly investigated [1-3], whereas polysilanes having at least a heteroatom substitution such as H, Cl, OR, NR2 have received much less attention [4]. The number of silicon atoms is usually from several hundreds to several thousands. 

Soluble disubstituted polysilylenes are a class of polymers that recently has generated great interest. These polymers have the structure [-SiRR -] , in which R and R may be aryl or alkyl groups and R may be the same as R. The substituted polysilylenes exhibit a wide variety of physical properties, depending on the nature of R and R. Of particular interest is their intense UV absorption at 300-400 nm both in solution and in the solid state, a property conferred by the silicon backbone and accompanied... 

Relationship to Electronic Properties. As a result of the close connection between bond conformation and electronic properties (4), the analysis of chain conformation in the polysilylenes has been of interest to researchers in this field, both from the experimental and theoretical viewpoints. As reported by Trefonas et al. (5), most asymmetrically substituted alkyl polysilylenes in solution at room temperature display an electronic absorption with ranging from 303 to 309 nm. The variable-temperature absorption spectrum of PMHS is shown in Figure 4 (4). At room temperature, max is 308 nm, and as the solution is cooled, there is a continuous red shift with the X x reaching 328 nm at -95 °C. Some workers 4, 6) suggest that this observation is a reflection of an increasing population of trans rotational states in the silicon backbone as the temperature is lowered. This suggestion is supported by the finding that these spectra can be adequately modeled by a rotational isomeric-state treatment (4). 

PDHS Structures in Solution. The determination of the chain conformation of polysilylenes in solution, particularly the conformations at temperatures just above or below the low-temperature thermochromic transition, is of great interest. NMR spectroscopy is one of the most useful techniques for probing chain conformation in solution (2i), and NMR is especially effective because of the large sensitivity of the carbon chemical shift to bond conformation (22). Silicon nuclei are also very sensitive to chain conformation, but a good correlation between silicon chemical shift and bond conformation has not been established yet. Unfortunately, both of these nuclei suffer from low sensitivity, primarily because of their low natural abundance. In contrast, protons have an essentially 100% natural abundance, but compared with the carbon or silicon chemical shift, the proton chemical shift is not very sensitive to bond conformation. Efforts to use NMR to probe the low-temperature dilute-solution conformation of the polysilylenes have been unsuccessful thus far. The diflSculty is that PDBS and PDHS precipitate from solution in 20-30 min after cooling through the thermochromic tran-... 

The predicted intrinsic width of the order-disorder transition of a mono-disperse, flnite-molecular-weight polymer solution was also tested. The average molecular weights of dialkyl-substituted polysilylenes are in the order of 6 X 10, which implies that N is 3000-5000 silicon atoms. With equation 9, the theory predicts that ATq/Tc is 0.004-0,006, which for Tc = -30 corresponds to an intrinsic width of roughly 1 or 2 C. This result is in good agreement with the experimental observations summarized in Table II. 

In this chapter, a selective overview of technological and historical background is followed by a general discussion of the microscopic details of the transport phenomenon and experimental techniques. Key results of earlier studies on carbon-based systems are presented and then compared with corresponding data on poly(methylphenylsilylene) (PMPS), which has been taken as the prototype for studies of transport system in polymers with silicon backbones. Key points are then summarized. Those wishing to omit the extensive background section may proceed directly to the section on electronic transport in polysilylenes (page 492). 

The design of single-component polymer transport materials continues to interest researchers in this field. The use of such materials will completely eliminate solvent extraction, diffusional instability, and crystallization of the small molecules. One obvious route that has not been successful to date is the design of yet another aromatic-amine-containing carbon-backbone polymer. An alternative may be to explore the large class of glassy silicon-backbone polymers, such as polysilylenes (14) and polyphosphazenes (iS). 

Crystalline silicon is the most widely used semiconductor material today, with a maiket share of above 90%. Because of its indirect electronic band structure, however, the material is not able to emit light effectively and therefore carmot be used for key applications like light-emitting diodes or lasers. Selected one- or two-dimensional silicon compounds like linear or branched polysilylenes [1] or layered structures like siloxene [2], however, possess a direct band gap and therefore exhibit intense visible photoluminescence. Siloxene, a solid-state polymer with a sheet-like layered structure and an empirical formula Si H (OH) , in particular, is considered as an alternative material for Si-based liuninescent devices. Detailed studies of stmctural and photophysical properties of the material, however, are strraigly impeded by its insolubility in organic solvents. 

While it is only recently that high molecular weight silicon backbone polymers have become available for detailed study, compounds containing the silicon-silicon bond. Including oligomeric polysilylenes, have attracted considerable Interest for many years. Thus there exists a number of papers of fundamental Importance with particular relevance to the polysilylenes. Paramount among these are the contributions of Pitt and coworkers on the optical spectroscopy (18,23,2k)... 

Presentation and discussion of the experimental results will proceed in the following manner. First the absorption spectra of the polysilylenes will be described and a conq)arative analysis of the spectra in room temperature fluid solvent media and rigid low temperature glasses at 77°K made. This will be followed by a description of the rather remarkable emission properties of these materials with emphasis on results obtained at 77°K. Included as part of the emission spectroscopic properties are the results of photoselection or polarization of emission measurements obtained in a rigid glass at 77°K. Based on these results a model is developed which describes individual chains of these silicon polymers in terms of a distribution of all-trans sequences with variable effective conjugation lengths. 

Polysilanes (alternative denotations polysilylenes, poly-catena-silicons) of the general structure shown in Chart 7.11 exhibit an absorption band in a relatively long-wavelength region, i.e. between 300 and 400 nm, reflecting the cr-conjuga-tion of electrons in the silicon chain. 

Polysilylenes (polysilanes) (34b) have received widespread interest. Their electronic properties are associated with a-electron conjugation in the silicon backbone which allows a significant delocalization of electrons along the chain. In the usual synthesis of polysilylenes, diorganodichlorsilanes (34a) are treated with sodium metal in a hydrocarbon diluent [173]. In order to recreate the surface of the sodium metal permanently ultrasound is used in these reactions [174,175]. 

Polysilylenes are chains, rings and 3D network polymers of silicon that contain the Si-Si bond in the polymer backbone. The tetravalency of silicon is generally completed with hydrogen or organic (aliphatic or aromatic) moieties. The basic structural unit of a polysilylene has formula 1. 

The molecular weights of polysilylenes (see Table 1) depend on the method of synthesis and the substituents on silicon. Although size-exclusion chromatography (SEC) may be used to estimate molecular weight and polydispersity, more accurate values of M , and radius of gyration (RG) are obtained from light-scattering... 

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