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

Abstract

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- trans-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 sidechain 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 high-energy 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.