Polysilane polymers conformation

The present calculations on po1y di-/j-hexylsilylene) are consistent with the hypothesis that the origin of thermochromlc behavior in selected polysilane polymers resides in a change in population of conformational states along the silicon backbone with temperature. 

It now seems evident that the solid state conformations of polysilanes are much more complicated and perplexing than had been thought. Determination of the actual conformations and solid state structures of polysilane polymers is likely to occupy chemists for many years. 

Solid-state multinuclear NMR experiments have successfully provided very useful information about inorganic-polymer conformations. The relationship between the solid-state conformations and NMR chemical shifts is of widespread interest. In this work, the structural behaviors of several substituted polysilanes in inorganic polymers were studied by means of Si CP/MAS... 

Owing to the extensive delocalization of silicon backbone, polysilanes display an intense absorption band in the near-UV region. This absorption band is also strongly coupled with the polymer conformation. Evidence of thermochromism in the solid... 

Soluble polysilane high polymers have recently attracted considerable attention because of their unusual electronic properties.(i, 2) Polysilanes absorb strongly in the UV spectral region, typically between 300-410 nm, and are strongly fluorescent. These transitions have been attributed to delocalized Si(o-a ) states. The spectroscopic properties of polysilanes are sensitive to the molecular weight, polymer conformations, and the substituents attached to the silicon backbone.(2-4)... 

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. 

Although main-chain chirality refers to both polymers with stereogenic centers in the main chain (configurational main-chain chirality) and polymers with main chains consisting of helical stereogenic bonds induced by chiral side groups or end termini (conformational main-chain chirality), in this chapter we mainly focus on polysilanes exhibiting the latter type of chirality. 

The conformational mobility of a chromophoric main-chain polymer is often connected to its electronic structure. Therefore, changes in the UV-visible absorption spectra and/or chiroptical properties are spectroscopically observable as thermo-, solvato-, piezo-, or electrochromisms. It is widely reported that o-conjugating polysilanes exhibit these phenomena remarkably clearly.34 However, their structural origins were controversial until recently, since limited information was available on the correlation between the conformational properties of the main chain, electronic state, and (chir)optical characteristics. In 1996, we reported that in various polysilanes in tetrahydrofuran (THF) at 30°C, the main-chain peak intensity per silicon repeat unit, e (Si repeat unit)-1 dm3 cm-1, increases exponentially as the viscosity index, a, increases.41 Although conventional viscometric measurements often requires a wide range of low-dispersity molecular-weight polymer samples, a size exclusion chromatography (SEC) machine equipped with a viscometric detector can afford... 

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