Ionization energies of linear and cyclic polysilanes. Application of the Green's function method coupled with semiempirical molecular orbital calculations

The ionization energies of the parent and permethylated linear [R(3)Si(SiR(2))(n)SiR(3)] (n = 1-3; R = H, CH3) and cyclic [(c-SiR(2))(n)] (n = 3-6; R = H, CH3) polysilanes have been calculated with the outer valence Green's function (OVGF) technique, using as the zeroth order approximation the wave functions obtained with semiempirical AM1, PM3, and MNDO methods. It is found that the OVGF(AM1) method gives Significantly better agreement with the experimental ionization potentials than semiempirical calculations using Koopmans' theorem. Furthermore, the OVGF(AM1) results are of comparable quality to those of OVA and EPT ab initio calculations. The mean deviation, for all the studied molecules, between the experimental and the OVGF(AM1) values is only 0.21 eV. The ionization energies of the studied polysilanes behave as follows: (1) The first ionization energies of the permethylated polysilanes are significantly (by 1-1.7 eV) lower than those of the corresponding parent polysilanes. (2) In the cyclic silanes the first IP is higher when the number of silicon atoms in the polysilane increases, while in the open-chain silanes the first IP decreases with the number of silicon atoms. The crossover between the two series occurs between the Si-4 and Si-5 polysilanes.