Polysilanes, radical anions

IR bands of polysilane radical anions and the location of conduction electrons. Except for poly(diarylsilanes), the conduction electron is confined either in the skeleton or in one of the pendant groups. 

Irie et al (7, 8) have reported that the radical cation of phenyl-substituted polysilanes exhibit a similar band in the IR region and assigned it to the CR band between two phenyl groups. They also reported that no IR absorption was observed for the radical anion. In the present study, however, we observed this IR band also for PMPrS(+) and for two polysilane radical anions. Since PMPrS has no aromatic groups, the optical transition should be related with the delocalized positive charge on the a-conjugated polymer main chain. 

The ladder polysilanes are highly a-conjugated systems, and they are easily oxidized and reduced to give unique oxidation products and persistent radical anions. 

The radical anions of polysilanes have been studied as unique a-conjugated radical ion species.2,4,56,57 Although many radical anions of polysilanes have been reported, most of them are unstable species that can only be observed at low temperatures. Quite recently, we found that the radical anions of longer ladder polysilanes are persistent at room temperature.58... 

The radical anions 1, anti-2, anti,anti-3, and anti,anti,anti-4 were generated by the reduction of the corresponding ladder polysilanes with potassium (Scheme 8). When the ladder poly silanes were treated with potassium in tetrahydrofuran (THF) at ca. — 70 °C, the solutions were immediately colored 1, purple anti-2, brown anti,anti-3, blue and anti,anti,anti-4, black. In the UV-visible-NIR spectra, several absorption bands appeared (Fig. 15). The intense absorption of anti,anti-3 and anti,anti,anti-4 in the near-infrared region is noted because it has been... 

The ESR spectra of the radical anions of the ladder polysilanes show a relatively broad signal with satellites (Fig. 16). From the intensity, the satellites are attributed to the 13C nuclei at the -positions of the isopropyl groups rather than the 29Si nuclei. The number of spin couplings is equal to the number of equivalent carbon atoms at the a-positions, indicating that the spin is highly delocalized. These results resemble those of the radical anions of other cyclopolysilanes observed by ESR57 and can be explained by... 

The structures of the radical anions were confirmed by the following experiment (Scheme 9). The reduction of the ladder polysilanes was monitored by UY-visible-NIR spectroscopy. When the absorption of the ladder polysilanes was completely replaced by the absorption of the radical anions, the sealed tube was opened. The radical anions were immediately oxidized, and the starting ladder polysilanes were recovered in high isolated yields. It is reasonable to conclude that the radical anions of the ladder polysilanes retain the ladder structure, and the Si-Si bond cleavage or skeletal rearrangement does not occur. 

Fundamental knowledge on the structures and properties of the ladder polysilanes has accumulated in our research for the past 15 years. Some results were unpredictable, including the silicon double helix structure, the domino oxidation, the formation of persistent radical anions, the Diels-Alder reactions at the 1,4-positions of anthracene, etc. These results let us recognize that the construction of novel structures will open the new chemistry. 

Radical anions, polysilane, 135, 150-153, 163 Rate-determining steps, oligomerization, 209, 210... 

Analyses of the electronic and electron spin resonance (ESR) spectra of the radical cation and anion of polysilanes make it possible to elucidate the structure of HOMO and LUMO, because an unpaired electron in the radical anion or cation occupies HOMO or LUMO, respectively. As schematically depicted in Fig. 10, the radical ions of polysilanes show absorption bands in UV and near-IR regions [29 31]. The former band corresponds to intraband transitions between valence and conduction bands. The latter band corresponds to transitions within the valence or the conduction band [32,33]. Because the near-... 

Replacing all the pendant groups with aryl groups attains the delocalization of the unpaired electron. Figure 23 compares the absorption spectra of the radical anions of poly-(4-ethylphenyl-phenylsilane) and poly(dicyclohexylsilane) in MTHF at 77 K. Although the molecular structures of these polysilanes are similar, the absorption spectra are different. The absorption spectrum of the poly(dicyclohexylsilane) radical anions is similar to that of... 

The most intriguing difference between the chemical properties of cyclopolysilanes and those of cycloalkanes is the ability of the former to form either anion or cation radicals upon one-electron reduction or oxidation, respectively. For example, the cyclic pentamer (Mc2Si)5 is reduced to the corresponding radical anion by sodium-potassium alloy in diethyl ether [see eqn (4.1) in Section 4.1.3], whereas the hexamer (Me2Si)6 is oxidised by aluminium trichloride in dichlor-omethane to the corresponding cation radical. In both cases the EPR spectra of the radical ions can be interpreted in terms of a-electron delocalisation over the entire polysilane ring (see Section 10.1.4.1). In this respect, the cyclosilanes resemble aromatic hydrocarbons rather than their aliphatic analogues. 

One of the clearest indications of the presence of acceptor orbitals on silicon is the ability of polysilanes to be reduced to radical anions (73). Although early work assigned the structure (Me2Si)6T to the radical anion produced by reduction of dodecamethylcyclohexasilane (75), subsequent investigations showed that cyclic (Me2Si)5, (Me2Si)6, and (Me2Si)7 all... 

Recently, several studies have been made of the photolysis of disilanes or polysilanes in the presence of an electron-deficient alkene using a photosensitizer (such as phenanthrene) and acetonitrile as solvent. These conditions result in the addition of silyl groups to one end of the alkene double bond and hydrogen to the other end (equation 18) and evidently involve the reaction of the radical anions of the electron-deficient silene with silyl radicals67 (see also Section VIII.A). 

The photoinduced electron transfer between Cgo and polysilane was investigated by laser flash photolysis9. The transient absorption spectra of the Cgo triplet (3Cgo ) (2), the Cgo radical anion (3) and the poly(methylphenylsilylene) radical cation (4, M = Si) (Scheme 2) were observed in the region 600-1600 nm in a polar solvent. 

The cyclic polysilanes can be reduced to radical anions either electrolytically or by reaction with alkali metals quite similar to aromatic rings. Numerous substituted... 

Carberry E, West R, Glass GE (1969) Cyclic polysilanes. IV. Anion radicals and spectroscopic properties of the permethylcyclopolysilanes. J. Am. Chem. Soc., 91 5446... 

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