Quantum-chemical approach to the solvatochromic transition in polysilane derivatives

We approached the solvatochromic transition observed in polysilane derivatives (poly[bis(4-propoxybutyl)silylene] (PPBS)) from the standpoint of various quantum chemical treatments. It was found from conventional geometry optimizations at the levels of semiempirical and ab initio molecular orbital methods that a protonation to polysilane oligomers with side chain R = -OCH ₃ results in the conformational change of Si-backbone to a trans-zigzag structure. Using the Elongation method, which was developed for efficient calculations of huge systems, it was demonstrated that a protonation could change the conformation of Si-backbone to a trans-zigzag structure over 10-14 Si atoms. In addition, ab initio calculations showed that the positive charge of a proton can delocalize into the Si-backbone through a long side chain in PPBS. Positively charged polysilane oligomers provide a rotational barrier that prefers a trans-zigzag structure, whereas neutral oligomers have a barrier that results to a random structure. This unique behavior of the charged polysilane oligomers should not be disregarded in understanding the mechanism of the solvatochromic transition in PPBS. In ab initio configuration interaction/Møller-Plesset through-space/bond interaction analysis, it was found that such a unique behavior of the rotational barrier in polysilane oligomers could be explained by the effect of orbital delocalization through σ-conjugation on the Si-backbone.