Origin and Relevance of the Staggering in One-dimensional "Molecular Metals". A Density Functional Study of Metallophthalocyanine Model Dimers.

Electronic structure aspects of the interaction between the metallophthalocyanine units in the stacks forming a one-dimensional molecular metal are investigated for M(Pc)I (M = Co, Ni, Cu; Pc = phthalocyaninato; I is the dopant). Density functional calculations are carried out on the complete benzoporphyrin-like ring systems. An energy analysis in terms of electrostatic interaction, Pauli repulsion (steric hindrance), and occupied/virtual orbital interactions is presented. The following are concluded (a) The staggering angle of -40° between adjacent rings originates from steric hindrance effects that can be traced to Pauli repulsion between occupied orbitals mainly located on the benzo rings. (b) The highest occupied P ligand orbital, 2a1,, forms the conduction band in the Ni and Cu systems. In the Co system the metal d,Z plays a role. However, this system may not be a simple metal-spine conductor, but a two-band model (ligand P orbital (2al,) and metal d,Z (13a1,) bands) might be more appropriate. (c) The overlap between neighboring 2alu orbitals is zero at -22’. The overlap (Pauli) repulsion of the 2alu orbitals would therefore drive the system to -22O, at which angle the conductivity would be small. The steric hindrance of the benzo rings however is stronger and drives the angle to -40°, where the dispersion of the 2al, band and therefore the conductivity is considerable. The benzo rings therefore play a key role in the conductivity. (d) The oxidation (doping) removes electrons from the top of the 2alu band and, apart from creating holes to act as charge carriers, relieves some of the 2alu repulsion present at -40°, facilitating the change from slipped to metal-over-metal stacking.