Oligosilane chain conformation strongly affects its ability to delocalize sigma electrons. A helical chain with an SiSiSiSi dihedral angle close to 180° (planar zig-zag, all-anti) is strongly sigma conjugated and an increase in its length reduces the HOMO-LUMO excitation energy almost as much as it does in polyenes. In contrast, a chain with a small backbone dihedral angle (close to 0°, all gauche) is not sigma conjugated at all. It behaves like an alkane and does not change its HOMO-LUMO excitation energy significantly when its length is increased. We provide a simple explanation of this dramatic difference between loose and tight helical conformations and examine its generality.

Sigma-electron delocalization: Why are loose oligosilane helices strongly conjugated and tight helices not at all?

253rd National Meeting (2017)

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