Polysilylenes conformation

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). 

As the sample is cooled, a second absorption band is observed in the range from 365 to 375 nm, which continues to grow upon further cooling. This behavior is completely reversible. This type of thermochromic transition is not observed in the solid-state absorption spectra of PDBS (iO) or PDPS 10,11). To understand this unusual absorption behavior of the polysilylenes in solution and in the solid state, a variety of studies have been directed toward the determination of the polymer chain conformation. 

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-... 

Solution and Solid-State Structures of PDBS and PDFS. A well-developed picture of the structure of PDHS has been obtained from the experimental work just discussed. A less complete picture is available for the structures of other polysilylenes. The structure of the dipentyl polymer, PDFS, was determined by Miller et al. (11) by using Raman and X-ray techniques. PDFS does not have an sl -trans conformation, unlike phase I of PDHS it is a 7/3 helix see previous discussion of conformational energy calculations and Figure 7). The same chain conformation has been found by Schilling et al. (10) for the dibutyl polymer, PDBS. 

Other Polysilylenes. The symmetrically substituted poly(di-n-tetra-decylsilylene) is reported to have a TGTG trans-gauche-trans-gauche ) conformation, and a bathochromic shift is observed in the UV spectrum at 54 °G with Xjnax shifting from 322 to 350 nm (35). The structures of many polysilylenes may be more complicated than what have been discussed thus far. As an example, the DSG data for PMHS are shown in Figure 24, The figure shows that Tg (glass transition temperature) is 220 K and that two... 

Although much has been learned about the structures of polysilylenes, a tremendous amount of work remains before a full understanding of these materials is developed. The microstructure of the polymers can be studied directly by solution NMR spectroscopic techniques. The determination of the chain conformation in solution is diflScult, particularly at low temperature. Light-scattering techniques may be able to establish the solution dimensions of the polysilylenes through the low-temperature thermochromic transition. The chain conformation in the solid state can be established by X-ray and electron difiraction methods. Solid-state Si NMR spectroscopy can become... 

In this chapter, the theory of conformation-dependent polymer-solvent interactions, which was developed in detail by Schweizer (20-22) for soluble TT-conjugated polymers, will be used to explain both qualitatively and quantitatively a large body of observations on the polysilylenes (23, 24). The same theory has been used recently to interpret qualitatively order-disorder phenomena and the electronic thermochromism of TT-conjugated-polymer solutions and films (25, 26). The study presented in this chapter represents part of an ongoing effort to understand in a unified fashion both the optical properties (27-30) and order-disorder transitions (20-24) of flexible, conjugated-polymer solutions. 

Another class of soluble polysilylenes exhibits essentially no or very weak thermochromism. This class includes poly(cyclohexylmethyl- 15, 38), poly(phenylmethyl- 15, 38), (polytrimethylsilylmethyl- 15), and poly(diarylsilylenes) 46), all of which appear to be conformationally locked over a wide range of temperatures. In terms of our theoretical perspective, this behavior would arise from the steric effects of bulky substituents, which imply a large value of e and, hence, a small coupling constant Vj /e. For aryl-substituted polysilylenes, the proximity of an aromatic group to the backbone could also stabilize a highly ordered rodlike conformation via enhanced dispersion interactions. 

Electronic transport of photoinjected holes, polysilylenes, fixed-electiic fields, mobility temperature correlations, transport behavior, bathochromic shifts, polymer conformational changes. 61... 

The polysilylenes continue to be of interest because of the novel electronic and optical properties of these macromolecules. These properties stem from the presence of a o-delocalized -Si-Si- polymer backbone. The interplay between conformational states, side-chain order/disorder transitions, and electronic states creates interesting temperature and pressure dependent optical phenomena. These novel properties have stimulated interest in improved synthetic methods. Thus, the first three papers in this symposium focus on understanding and improving catalytic methods for generating silylene chains. These efforts are followed by Worsfold s research on improving the traditional Wurtz coupling method of synthesis. 

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