Molecular Metals with Widely Tunable Band Filling. Structure/Stoichiometry/Counterion Relationships in the Electrochemistry of a Cofacially Joined Polymeric Phthalocyanine Metal

Article abstract of DOI:10.1021/ja00196a037

The oxidative electrochemistry of the cofacially joined phthalocyanine polymer n to yield molecular metals/conductive polymers of the type <Xy>n is studied by combination of X-ray diffractometric and spectroscopic techniques.Electrochemical methodology includes controlled-potential coulometry and electrochemical potential spectroscopy (ECPS) applied to rapidly stirred slurries or to microcompactions of the solid polymer.For X(1-)= BF4(1-) in acetonitrile, oxidation ("dopping") of as-polymerized orthorhombic n to yield tetragonal<(BF4)y>n (y ca. 0.50) is accompanied by a significant overpotential, minimal tunability in y, and involves a first-order structural phase-transformation.Electrochemical undoping occurs smoothly and over a broader potential range (0.90 V) to afford tetragonal n, which is also accesible by thermally undoping I_1.1>n.Once in the more open tetragonal structure, both the electrochemical and diffraction data argue that y (hence, conduction band filling) can be homogeneously/continuously tuned between 0.0 and 0.50.This result verifies the crystal structural basis of the polymer electrochemical "break-in" phenomenon.It also represents the first case in which the band filling of a molecular metal is broadly tunable.In tetrahydrofuran, tetragonal n can also be reversibly n-doped to yield <0.09>n.Oxidative ECPS studies with a number of anions in acetonitrile (PF6(1-), SbF6(1-), tosylate, CF3(CF2)nSO3(1-), n=0,3,7) demonstrate that maximum doping stechiometries achievable (y, hence band filling) are largely a function of anion size, i.e., packing constraints within thetetragonal <Xy>n crystal structure.In contrast to these results, ECPS studies of solid Ni(Pc) (monoclinic slipped-stack β phase) reveal a first-order structural transformation to yield tetragonal Ni(Pc)(BF4)y (y ca. 0.48) upon oxidative doping, and a subsequent first-order transformation to another slipped-stack Ni(Pc) structure (monoclinic slipped-stack γ phase) upon undoping.Doping/undoping occurs over a relatively narrow potential range; consequently there is far less tunability in y than in the <Xy>n materials, and large overpotentials are observed.ECPS studies of n reveal irreversible oxidative processes, and polymer decomposition via Ge-O bond cleavage is implicated.