Electronic Spectra of Polysilanes

Most polysilanes absorb in the region between 300-400 nm [1-4, 26, 74-77]. The electronic absorption data of representative examples are summarized in Table 7.2. 

The main features of the electronic spectra of polysilanes can be summarized as follows  

The electronic transitions in polysilanes are of the a-a type. The high values of the extinction coefficients of these absorptions is consistent with the a-o nature of these transitions. One of the highest e values are observed for [( Bu-/ -C6H4)2Si]n (e = 26,600) [2,26] and [(nCioHi3)(5-CH2CH(CH3)(C2H5)Si] (8 = 55,000) [74] 

The Xmax and the e values depend on the chain length but seem to reach an optimum value for a chain length equaling [RRSi] (n=30). The X ax value also depends on the type of substituents present on the silicon. Thus, the highest energy absorption is observed for [(CF3CH2CH2)(CH3)Si] at 285 nm [28]. 

Poly(dialkylsilane)s have a Xrmx between 305-315 nm. Introduction of aryl groups causes a bathochromic shift. Thus, for poly(alkyl/arylsilane)s the X ax increases to 325-350 nm. The highest bathochromic shifts are observed for poly(diarylsilane)s [26,75]. This progressive shift of absorption to lower energy upon introduction of aryl groups has been attributed to c-n mixing. 

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