Polysilanes spectra

Absorption and Emission Spectra. The excitation-emission spectrum of 1 (bottom half of Fig. 1) shows that the relatively narrow emission band is nearly independent of the excitation wavelength and that the excitation spectrum is not only nearly independent of the wavelength at which the emission is monitored, but is also very similar to the absorption spectrum, both being somewhat broader than the emission band. This leaves no doubt that the observed emission is due to the polysilane, and its shape, location and the mirror image relation to the absorption permit its assignment as fluorescence. 

Polymers with triflate groups react with alcohols to form alkoxysubstituted polysilanes. This reaction occurs readily in the presence of bases. The best results were obtained using triethylamine and hindered pyridine. In Fig. 3c the NMR spectrum of the reaction mixture containing the excess of triethylamine is shown, the methyl groups from the polymer chains absorb in the range typical for alkoxysilanes. Reaction in the presence of unsubstituted pyridine leads to the formation of insoluble polymer probably by attack at the p-C atom in the silylated pyridine. 

The spectrum of silicon based polymers is enriched by high tech ceramics like silicon nitride and carbide, respectively. These materials are produced by pyrolysis of appropriate polymeric precursors such as polysilanes, polycarbosilanes and polysilazanes (preceramics). These synthetic ceramics display a certain analogy to silicates, having SiC, SiN, or Si(C,N) as structural subunits instead ofSiO. 

Figure 61 UV spectrum of 31-Si polysilane dendrimer 139 with 13-Si longest chain.378 Reprinted with permission from Lambert, J. B. Wu, H. Organometallics 1998, 17, 4904-4909, 1998 American Chemical Society.

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... 

In the model compounds, this red shift has been ascribed to a combination of silicon backbone with the x-orbitals of the aromatic substituent coupled with a decrease in the LUMO energy due to x -(a, d) interactions (15,16). Further examination of the data in Table III shows that the absorption maximum of the cyclohexylmethyl derivative, 9, is also somewhat red-shifted relative to the other alkyl polymers suggesting that the steric bulk of the substituents and/or conformational effects may also influence the polysilane absorption spectrum. 

In a revealing experiment, Toyoda and Fujiki showed that the UV absorption and emission spectra of (PhMeSi) are virtually identical to those of the copolymer, (PhMeSi)o.95 [PhSiCH2C (H)(CH3)CH2CH3]005 . The latter is known to be helical in solution from its CD spectrum. The implication is that (PhMeSi) , which was for many years described in terms of the anti-gauche model, is actually helical in solution also.69 The evidence which is accumulating now suggests that nearly all polysilanes adopt helical conformations in solution. 

Helical polysilanes where the side groups are partly substituted with Rhodamine B dye molecules and chiral groups (Fig. 12) have been synthesized and spread onto quartz plates by vertical dipping.77 A weak absorption peak due to the dye is observed around 2 eV in addition to the sharp exciton peak at 3.85 eV. The PL spectrum shows a new peak at 2 eV, while the original peak at 4 eV for the polysilane without the dye is greatly decreased. Strong red PL is observed. The introduction of only a few percent of dye modifies the absorption... 

In the spectra for PMPS and PPS, broad peaks also exist around 472 nm. The FWHM (0.21 eV) of the sharp emission peak is similar for all polysilanes. The peak wavelengths of the sharp emissions of PMPS, PPS, PNPS, and PBPS are 357 nm, 378 nm, 409 nm, and 409 nm. The origin of the broad emission peak at 472 nm is the radiative transition of excitons localized at branching points or a CT from phenyl ir -orbitals to Si a-orbitals. The spectrum of PNPS is almost the same as that of PBPS, which may be due to the energetic equivalence of biphenyl and naphtyl substituents. Although both PMPS and PPS possess a phenyl substituent, their emission peaks differ due to the fact that PMPS has a linear structure and PPS has Si-branching. The EL spectra are... 

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