Intramolecular Electron Transfer And Its Dependence On The External Electric Field In π-σ-π Type Of Bistable Molecule

By using UHF/6-31G method, the intramolecular electron-transfer (ET) properties of the selected Ï€-Q-Ï€ system have been studied. This work includes two parts. Firstly, the two Ï€-electron moieties of biphenyl anion radical Bp and neutral naphthalene Np are set coplanar. Theoretical investigations on the ET characteristics in THF solvent and comparisons with the experimental observations are accomplished. After that, by searching for the least-coupling conformation, we obtain a modified system. Its weak coupling and energy barrier ensure that the excess electron of this system is hard to be transferred from one end of the molecule to the other by tunneling or thermal fluctuations. However, an external electric field could easily prompt an ET reaction. Its rate constant k_et would reach a maximum at the electric field of F_thresh=0.001458 au. Our study indicates that the electron transfer for the modified molecule is dominated by a through-bond rather than a through-space mechanism. These investigations indicate that the modified molecule exhibits characteristics as a bistable system and therefore appropriate for molecular device applications. The dependencies of ET properties on the electric field are simply derived.In addition, Influences of the axial ligands on the geometric features and magnetic properties in six-coordinated Os(IV) porphyrin complexes [Os(por)L₂](L=CH₃, OCCH₃, SCCH₃, SeCCH₃, Cl, Br, I) have also been investigated by density functional theory and a qualitative orbital interaction analysis has been presented in chapter 6. The results show that the influences of different axial ligands on the two degenerate E_g d-orbital of osmium, i.e. d_xz and d_yz, are not the same, which results in different electronic configurations of osmium. When the axial ligand is CCH₃ possessing strong trans-influence property, L-Os-L will bend. This bending leads to the larger splitting of d_xz and d_yz. Therefore, the corresponding complex presents bending diamagnetic properties. When axial ligands are respectively OCCH₃, SCCH₃ SeCCH₃, Cl, Br and I, the corresponding complexes adopt linear L-Os-L structure owing to the weaker trans-influence property of those species. On the other hand, for the stronger single-faced 7i-donor ligands SCCH₃ and SeCCH₃, the splitting of d_xz and d_yz is large. The corresponding complexes also present diamagnetic property. For ligand OCCH₃, which is a weaker single-faced Ï€-donor comparing with SCCH₃ and SeCCH₃, the splitting of d_xz and d_yz is very small, hence the corresponding complex shows paramagnetic property. For halide ligands, the d_xz and d_yz keep degenerated because of the equality of their double-faced Ï€-donor. The corresponding complexes exhibit paramagnetic property.