Study on Band Structures of Acetylene-Methylacetylene Copolymers

The band structures of copolymers of acetylene and methylacetylene and their Li-doped systems are calculated by using LCAO-SCF ab initio crystal orbital method. The conduction band and valence band structures and energy gaps as well as the charge distribution in carbon-chain are studied to discuss the change of conducting properties of copolymers after doping. The influences of methyl group on the conducting properties are also discussed.
Both different acetylene concentration in copolymers and different dopant concentration are taken into account. The calculated systems are （C₇H₈）_x, (C₈H₁₀)_x, （C₇H₈Li）_x, (C₈H₁₀Li)_x, （C₇H₈Li₂）_x, and (C₈H₁₀Li₂)_x. The calculated results are compared with those of polyacetylene（CH）_x. On one hand, methyl（-CH₃） is a donor that charged a little, the π-electrons interact with π-orbitals in one-dimensional carbon chain through Baker-Nathan effect, which lowers conjugation affect in carbon chain and causes lowering of the energy gap. The conductivities of copolymers are therefore larger than those of polyacetylene; After doping with lithium, the charge in carbon chain partly disperses to methyl, so methyl group weakens the affect of dopant and the energy gap rises. The conductivities of doped copolymers are therefore smaller than those of doped polyacetylene on the other hand.