Cr-conjugation

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]. 

Bond homoconjugation via a,cr-type overlap is identical with cr-conjugation, and if the latter occurs it cannot be distinguished from the former (Scheme 5). As such it is not reasonable to use the term cr,c-bond homoconjugation. 

It is some kind of cr-conjugation that also plays a role in delocalization (or releasing) of an unpaired electron in ion radicals. 

The incorporation of siloles in polymers is of interest and importance in chemistry and functionalities. Some optoelectronic properties, impossible to obtain in silole small molecules, may be realized with silole-containing polymers (SCPs). The first synthesis of SCPs was reported in 1992.21 Since then, different types of SCPs, such as main chain type 7r-conjugated SCPs catenated through the aromatic carbon of a silole, main chain type cr-conjugated SCPs catenated through the silicon atom of a silole, SCPs with silole pendants, and hyperbranched or dendritic SCPs (Fig. 2), have been synthesized.10 In this chapter, the functionalities of SCPs, such as band gap, photoluminescence, electroluminescence, bulk-heterojunction solar cells, field effect transistors, aggregation-induced emission, chemosensors, conductivity, and optical limiting, are summarized. 

Another way to make an ether easier to remove is to make it benzylic 31 as cr-conjugation of the C-0 bond with the benzene ring weakens it enough for it to be cleaved by catalytic hydrogenation using various transition metals.5... 

Figure 2 Planar 7r-systems of representative conjugated molecules and polymers, and cr-conjugated polysilanes. Polyenes, acenes and rylenes have variable length. Polydiacetylenes, polythiophenes, polyparaphenylenevinylenes and polysilanes give families with similar backbones and different nonconjugated side groups.

The synthesis and characterization of polyacetylene (PA) provided new incentive for understanding 7r-electronic spectra, electron-phonon interactions and electronic correlations[l, 2, 14]. The electrical conductivity of chemically doped PA rivals that of metals. Families in Fig. 2 such as polydiacetylenes (PDAs), poly thiophenes (PTs), cr-conjugated polysilane (PSs) and polyparaphenylene vinylene (PPVs), among... 

A unique treatment of cyclopropane has been advanced by Dewar, who introduced the concept of a aromaticity, which explains some of the anomalous chemical and physical properties of cyclopropane. The notion of cr-conjugation implies that three cr-bonds form a cyclic system of six electrons thus cyclopropane is aromatic by the (4n -1- 2) rale. This explanation well accounts for the strain energy of cyclopropane. The actual value (27.5 kcal mol ) is much lower than the predicted value of 104 kcal mol (1 cal = 4.2 J), calculated from the C—C—C bending force constants obtained spectroscopically. - A similar comparison for cyclobutane (antiaromatic by the above notion and the 4n rule) underestimated the strain energy. o -Aromaticity also accounts for such observations as H NMR chemical shifts and the reactivity of cyclopropane toward electrojAiles. 

The positions of the Cl—C, HO—C, and RO—C bonds allows them to conjugate with the Hg—C bond ( cr,cr-conjugation ). Hence, an electrophile (with the latter two substrates), or an agent which can bind Hg-+, or a reagent which combines both properties, such as HCl, will break the conjugated cr-bonds, create at the center of the molecule the new 77-bond, and eliminate olefin or acetylene. This is formally similar to the 1,4-addition occurring in 77,77-conjugated systems. 

In order to produce polysilanes with controlled optical properties, the substituents have to be picked up properly thus, poly(alkyl(methoxyphenyl)silane) homo and copolymers are reported by Nakashima and Fujiki [40], who found that poly(alkyl(alkoxyphenyl)silane) with methyl, ethyl and n-hexyl moieties showed a typical absorption peak around 340-360 nm atUibuted to the delocalized cr-conjugation. 

Interestingly, cr-conjugated polymers such as polysilanes (see Chart 3.3) also exhibit remarkably large third-order susceptibilities... 

It must be noted that the values reported in the literature vary over broad ranges. Therefore, the values listed here reflect only the general behavior of several classes of compounds. It can be seen in Table 3.5 that trans-polyacetylenes (PAs) and polydiacetylenes (PDAs) exhibit the largest third-order NLO susceptibilities. The x value of cis-PA (not shown) is more than an order of magnitude smaller than that of trans-PA. Derivatives of poly-p-phenylene, poly(phenyl-ene vinylene), and polythiophene also exhibit NLO activity, but to a much lesser extent than PAs and PDAs. As pointed out above, polysilanes also possess quite large x values. This is explained by the cr-conjugation of the silicon chain, which implies a pronounced delocalization of cr-electrons. A very large x value... 

A) The coupling of 7t-conjugation in polyacetylene (and related poly-phenylenevinylenes) and cr-conjugation in polysilanes to produce hybrid materials with potentially new or superior optical or optoelectronic properties, and/or to generate efficient ceramic precursors, has been an important... 

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