A Computational Investigation of the Substituent Effects on Geometric, Electronic, and Optical Properties of Siloles and 1,4-Disilacyclohexa-2,5-dienes.

Thirty two differently substituted siloles 1a-1p and 1,4-disilacyclohexa-2,5-dienes 2a-2p were investigated by quantum chemical calculations using the PBE0 hybrid density functional theory (DFT) method. The substituents included σ-electron donating and withdrawing, as well as π-electron donating and withdrawing groups, and their effects when placed at the Si atom(s) or at the C atoms were examined. Focus was placed on geometries, frontier orbital energies and the energies of the first allowed electronic excitations. We analyzed the variation in energies between the orbitals which correspond to HOMO and LUMO for the two parent species, here represented as ΔεHL, motivated by the fact that the first allowed transitions involve excitation between these orbitals. Even though ΔεHL and the excitation energies are lower for siloles than for 1,4-disilacyclohexa-2,5-dienes the latter display significantly larger variations with substitution. The ΔεHL of the siloles vary within 4.57-5.35 eV (ΔΔεHL = 0.78 eV) while for the 1,4-disilacyclohexa-2,5-dienes the range is 5.49-7.15 eV (ΔΔεHL = 1.66 eV). The excitation energy of the first allowed transitions display a moderate variation for siloles (3.60-4.41 eV) whereas the variation for 1,4-disilacyclohexa-2,5-dienes is nearly doubled (4.69-6.21 eV). Cyclobutadisiloles combine the characteristics of siloles and 1,4-disilacyclohexa-2,5-diene by having even lower excitation energies than siloles yet also extensive variation in excitation energies to substitution of 1,4-disilacyclohexa-2,5-dienes (3.47-4.77 eV, variation of 1.30 eV).