Excitation dependence of Raman spectra of various polydialkylsilane conformations and σ–σ conjugation

Excitation dependence of the Raman spectra in a wide region (from blue to infrared) was investigated for hmw polysilanes [nHex2Si]n and [nPent2Si]n at various temperatures. The results obtained along with analogous data for other polydialkylsilanes lead to the conclusion that among all modifications formed by these polymers, there is only one that exhibits pre-resonance Raman scattering, that is, intensity enhancement and its excitation dependence. This is the crystalline modification with the σ–σ conjugated all-A (anti) conformation of the silicon backbone. The Raman spectrum of this modification is enhanced by pre-resonance even when (νe−ν), the difference between the frequency of the lowest energy σ–σ* electronic transition νe and that of irradiating light ν is as much as ∼17 300 cm−1, whereas Raman spectra of less-ordered modifications do not exhibit such enhancement even at much smaller (νe−ν) values, e.g. ∼9400 cm−1. These facts are discussed in terms of Shoryginʹs semi-classical treatment of Raman intensity problem. The Raman patterns observed below 800 cm−1 allow clear distinction between polysilane modifications with all-A, helical (all-D) and TGTG′ (AD+AD−) main chain conformations, whereas helical (all-D) and disordered conformations are hardly distinguishable.