Understanding the gap in polyoxovanadate molecule-based magnets

We report a joint experimental and theoretical investigation of the transport gap, optical properties, and electronic structure of two chemically similar, inhomogeneously mixed-valent polyoxovanadate molecule-based magnets. We attribute the substantial gap in [NHEt3](4)[(V8V4As8O40)-V-IV-As-V(H2O)]center dot H2O to weak p-d hybridization and a large on-site Coulomb repulsion (U=6 eV). The reduced gap in [NHEt3](3)[(V6V6As8O40)-V-IV-As-V(HCO2)]center dot 2H(2)O is associated with a smaller value of U (4 eV), at least from a molecular point of view, although the transport properties also reflect subtle organization of the molecular structure and differences between direct and indirect intermolecular charge transfer. A detailed analysis of the vibrational response supports the important role of local molecular distortions and hydrogen bonding in the intramolecular and intermolecular charge transport of [NHEt3](4)[(V8V4As8O40)-V-IV-As-V(H2O)]center dot H2O.