Electronic Structure of Chainlike Polysilane

Energy band structures are calculated for -(SiXY-)_npolysilane, model compounds in which X and Y are H, Me, Et, Pr, and Ph (phenyl) substituents. Polysilane has a directly allowed type band structure, and a a-a interband optical transition is allowed. Band-edge states are formed mainly of skeleton Si AOs (atomic orbitals), which results in a skeleton band gap. This skeleton band gap tends to be reduced when larger alkyl groups are substituted for side chains. The a and a* band-edge states are well delocalized on the skeleton axis. Polysilane with Ph side chains [poly(arylsilane)] exhibits a characteristic band-edge structure due to a~π mixing between the Si skeleton and Ph side chains. This interaction removes the doubly degenerated Ph π-Homostates. One state is mixed with the skeleton σ-valence band state and delocalized along the skeleton axis. The other remains strongly localized in the individual Ph side chains. However, no a*-π* mixing occurs at the conduction band edge, and no change is seen in the conduction band structure. This results in the intrusion of unoccupied localized states in the skeleton band gap.