Polysilane nonlinear materials

Polysilanes are cr-conjugated polymers composed of Si-Si skeletons and organic pendant groups. They are insulators with filled intramolecular valence bands and empty intramolecular conduction bands. However, because of strong cr conjugation, they have rather narrow band gaps of less than 4 eV [24,25] and are converted to conductors by photoexcitation or by doping electron donors or acceptors. Recently they have attracted much attention because of their potential utility as one-dimensional conductors, nonlinear optical materials, and electroluminescent materials [26-28]. 

The optical and electronic functions of polysilanes owe to their delocalized highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) that are occupied by holes and conduction electrons, respectively. The polymer does not show high conductivity or optical nonlinearity if the electrons or holes are localized on a small part of the polymer chain. To elucidate the structure of HOMO and LUMO is therefore important for the molecular design of polysilanes as functional materials. 

Polysilane high polymers possessing fully saturated all-silicon backbone have attracted remarkable attention recently because of their unique optoelectronic properties and their importance in possible applications as photoresists, photoconductors, polymerization initiators, nonlinear optical materials etc. A number of review articles have been published on this topic4-9. The studies in this field have stimulated both experimental and theoretical chemists to elaborate on understanding the excited state nature of polysilanes and oligosilanes and of their mechanistic photochemistry. 

Applications. Some applications of polysilanes include their use as semiconductors, photoresists, photoinitiators, nonlinear optical materials, and ceramic precursors (9). Their use as ceramic precursors can be illustrated by the following reactions ... 

Polysilanes are air-stable Si-Si backbone polymers that exhibit efficient emission in the UV spectral region, high hole mobility, and high nonlinear optical susceptibility [28]. These properties arise from delocalization of a electrons along the Si-Si chain. Polysilanes have been employed as fluorescent materials for radiation detection, as electroluminescent materials for display devices, and as photorelractive materials for holographic data storage [28]. 

Studies of polygermanes indicate that the a delocalization is even more extensive than for polysilanes and that the a—a band-gap transition for the high polymers is significantly red-shifted by ca 20 nm in comparison to the silicon analogues (44,45) (Table 3). Other studies have shown that these materials possess semiconductive behavior upon oxidative doping (49) as well as significant nonlinear optical behavior (50) and thermochromicity (45). 

Polysilanes (Fig. 1) [1] have attracted considerable attention due to their usefulness as precursors for thermally stable ceramics [2, 3] or a material for microlithography [4, 5] and also due to their potentiality in preparation of new types of material showing semiconducting, photoconducting, or nonlinear optical property [6-8]. 

Both polysilanes and polygermanes contain a metal-metal backbone, which emits in the near UV spectral region, exhibits high hole mobility, and exhibits high nonlinear optical susceptibility. This makes these materials candidates for a variety of optoelectronic applications. ... 

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