THEORETICAL STUDIES OF INTRAMOLECULAR ELECTRON TRANSFER IN BIMETALLIC MIXED-VALENCY MOLECULES AND PHTHALOCYANINE-BASED LINEAR CHAIN CONDUCTORS (POLYMERS).

摘要

Electron transfer in isolated binuclear transition-metal complexes and phthalocyanine "molecular metals" is investigated. In the former systems, the process is discussed in terms of the usual two-site model and also in terms of a superexchange mechanism where the role of the bridging ligand is important. In the latter systems, the electron transfer (actually electronic conductivity) is discussed in terms of tight-binding band theory, and the actual temperature dependence is calculated.;A limitation of the standard vibronic model for electron transfer in mixed-valence molecules is addressed. This model predicts an intervalence transfer (IVT) frequency which, in the limit of delocalized states, is unchanged upon reduction by one further electron. This is in sharp disagreement with experiment, but is remedied in the present work by adding one-site Coulomb repulsions which shifts the analog of the IVT into the near ultra- violet, and an exchange term which produces the correct ordering of the energy levels, thus making the transition analogous to IVT spin-forbidden.;The role of bridge-assistance in the electron transfer in these molecules is investigated. The results of semiempirical electronic structure calculations for several bridging ligands suggest that bridge assistance is important, and that the bridge determines the magnitude of the effective tunneling integral t.;The first quantitative understanding of the temperature dependence of the conductivity (sigma) in the low-dimensional material, NiPCI (Pc = phthalocyanine), is achieved using a transport theory for noninteracting electrons scattered by one- and two-phonon processes. The lattice motions found to be the dominant sources of resistivity along the chain are the longitudinal stretch and an inter- planar twist (libron), with the former being a first-order scattering process ((sigma)(,1p) (PROPORTIONAL) 1/T) and the latter a second-order mechanism ((sigma)(,2L) (PROPORTIONAL) 1/T('2)). It is found that at low temperature, one-phonon scattering is most important, while at room temperature one-phonon and two-libron scattering contribute nearly equally to the resistivity. The effects of peripheral substitution of strongly electron-withdrawing groups on the electronic properties of phthalocyanines are also studied, and the possible effect on conductivity is discussed.