Vacuum ultraviolet spectroscopy of the optical properties and electronic structure of seven poly(di-alkylsilanes)

We report the ultraviolet (UV) and vacuum ultraviolet (VUV) optical properties and electronic structure, up to 44 eV, of thin-film samples of seven poly(di-alkylsilanes) [alkyl=n-butyl, n-pentyl, i-hexyl, n-hexyl, n-octyl, and n-tetradecyl] with three types of Si-Si backbone conformations: helical, planar zigzag, and trans-gauche--tra~s-gauche' (TGTG'). The backbone conformation determines the UV transitions, with helical materials exhibiting one near-UV absorption, while two UV transitions are seen for the two-phase materials containing both the helical and planar zigzag backbone conformations. The TGTG' backbone exhibits a single UV absorption at 3.6 eV. At higher energies all materials show a prominent shoulder at 7.2 eV, with a doublet peak structure seen at ~9 and 12 eV. The 7.2 eV transition is unaffected by the backbone conformation or alkyl substitution, while comparison with the electronic transitions in polyethylene shows that the high-energy double-peak structure corresponds to transitions in the hydrocarbon sidechains. The ~ 12 eV transitions appear to shift to higher energy with increasing sidechain length and in tetradecyl these sidechaln transitions are noticeably narrowed, suggesting unusual sidechain crystallization in poly(di-n-tetradecylsilane). A hierarchy of electronic transitions can be developed whereby the UV transitions arise in the Si backbone, at intermediate energies backbone to sidechain transitions are observed, while the highenergy transitions are of the alkyl sidechains. This hierarchy in the electronic transitions demonstrates the ability of VUV spectroscopy of the electronic structure to serve as a microscopic probe of the bonding and structure of polysilanes, for example providing detailed insight into the properties of the sidechains in these polymers.