Absorption spectra of polysilanes

Figure 14 UV absorption spectra of polysilanes 83-86 in THF at 30°C.204 Reprinted with permission from Fujiki, M. Macromol. Rapid Commun. 2001,22, 539-563, 2001 Wiley-VCH. 

The optical absorption spectra of polysilane radical ions also support the validity of the above-mentioned mechanism of charge migration. Figure 21 compares the absorption... 

FIGURE 16. Absorption spectra of polysilane films on quartz substrates at room temperature. (Reprinted from Ref. 91.)... 

Absorption spectra of polysilanes anisotropic absorption for helical conformation, 549 bathochromic shifts, 549 effect of conformation on extinction coefficient, 547, 548/ low-energy feature for al -trans conformations, 547, 548/, 549 Acetylenes... 

The absorption spectra of polysilanes have been found to be dependent... 

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... 

Examination of the absorption spectra of the new polysilane materials reveals a number of interesting features (14). As shown in Table III, simple alkyl substituted polymers show absorption maxima around 300-310 nm. Aryl substitution directly on the silicon backbone, however, results in a strong bathochromic shift to 335-345 nm. It is noteworthy that 4, which has a pendant aromatic side group that is buffered from the backbone by a saturated spacer atom, absorbs in the same region as the peralkyl derivatives. This red shift for the silane polymers with aromatic substituents directly bonded to the backbone is reminiscent of a similar observation for phenyl substituted and terminate silicon catenates relative to the corresponding permethyl derivatives... 

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. 

PMPS, poly(phenylsilyne) (PPS), poly(2-naphtyl-phenylsilane) (PNPS), and poly(phenyl-p-biphenylsilane) (PBPS) were used in EL diodes.91 The absorption spectra of the polysilanes all exhibited strong absorption in the UV region, except for PPS as, shown in Figure 16. Their PL spectra exhibited a sharp peak, as shown in Figure 17. 

Fig. 2 Comparison of UV absorption spectra of four optically active polysilanes in THF at 30 °C Poly methyl-(S)-2-methylbutylsilane (1, a of 0.59), poly n-hexyl-(S)-4-methylpentylsilane (2, a of 0.75), poly n-hexyl-(S)-3-methylpentylsilane (3, a of 0.92), poly n-hexyl-(S)-2-methyl-butylsilane (4-S, a of 1.25)...

Figure 2 shows the UV absorption spectra of four optically active poly (dialkylsilane)s bearing different chiral side groups in THF at 30 °C [45]. It is evident that as the value of a increases from 0.59 to 1.25, the UV absorption intensity increases, whereas the full width at half maximum (fwhm) decreases. These results led to the finding of the semi-empirical relationship between the main chain absorption characteristics and the global conformation of various polysilanes in solution. 

Figure 11.7. Absorption and luminescence spectra of polysilanes (a) typical spectra in which Ea and Ep are the peaks of absorption and emission respectively, and 5E and AE are respectively the Stokes shift and the width at half height of the absorption band (b), (c) and (d) depict the spectra of a single screw-sense rod-like polysilane, poly(di- -butylsilane), and PMPS, respectively (Reprinted from ref. l Copyright 2000 Kluwer Academic Publishers).

Compared to the C-C bond, the Si-Si bond has a more electropositive nature and heterolytic dissociation occurs more easily due to the low energy o-o excitation [85]. The absorption bands of polysilanes in the near UV range between 290 and 410 nm and they are temperature and structure dependant [86], Since the o-o transition is allowed, the absorption coefficients of their optical absorption spectra are large [42]. Although the Si-Si bond has a similar bond strength to the C-C bond [87], cleavage of the Si-Si bond by UV excitation can easily occur in the solid state as well as in solution. Both may lead to decomposition of the polymeric species [3, 88, 89]. 

Polysilanes have unique optical, electronic and photophysical properties due to the delocalization of the a-electrons along the silicon chain (see above). This delocalization was first discovered while studying the absorption spectra of oligo- and polysilanes. Polysilanes show distinctly red shifted absorption bands compared to the homologous series of polyolefins. Furthermore, the absorption bands depend... 

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