We apply Density Functional Theory (DFT) and the DFT+U technique to study transition metal porphine molecules in condensed-matter settings, including Pd(II) and Mn(II) phosphines adsorbed on atomistically flat Au(111) surfaces and Mn(II) and Mn(III) porphines dispersed in liquid water. DFT calculations using the Perdew-Burke-Ernzerhof (PBE) exchange correlation functional correctly predict the palladium porphine (PdP) low-spin ground state. PdP is found to adsorb preferentially on gold in a flat geometry, not in an edgewise geometry, in qualitative agreement with `experiments on substituted porphyrins. The DFT+U technique, parameterized to B3LYP predicted spin state ordering of the Mn 3d-electrons, is found to be crucial for reproducing the correct magnetic moment and geometry of the isolated manganese porphine (MnP) molecule. Adsorption of Mn(II)P on Au(111) substantially alters the Mn ion spin state and electronic structure. Its strong binding to the gold surface can be partially reversed by applying an electric potential, which leads to significant changes in the electronic, magnetic, and structural properties of the adsorbed MnP.

We also apply DFT+U/ab initio molecular dynamics simulations to examine MnP dispersed in liquid water. Mn(III)P is on average planar, and the Mn ion coordinates to two water molecules, while in Mn(II)P it exhibits a strong tendency to displace out of the porphine plane and binds to a single water molecule. These results will be compared with nuclear magnetic resonance data for water-soluble manganese porphyrins. We conjecture that this DFT+U approach may be a useful general method for modeling first row transition metal ion complexes in condensed-matter settings.

This work was supported by the Department of Energy under Contract DE-AC04-94AL85000. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. Department of Energy.