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

The oxidative electrochemistry of the cofacially joined phthalocyanine polymer [Si(Pc)O]_n to yield molecular metals/conductive polymers of the type {[Si(Pc)OJX_y}_n is studied by a 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^- = BF₄^- in acetonitrile, oxidation (“doping”) of as-polymerized orthorhombic [Si(Pc)O]_n to yield tetragonal {[Si(Pc)O](BF₄)_y}_n(y = 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 [Si(Pc)0]„, which is also accessible by thermally undoping ([Si(Pc)O]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 [Si(Pc)O]_n can also be reversibly n-doped to yield |[N(n-butyl)₄]_0.09[Si(Pc)O]}_n. Oxidative ECPS studies with a number of anions in acetonitrile (PF₆^~, SbF₆^~, tosylate, CF₃(CF₂)_nSO₃^-, n = 0, 3, 7) demonstrate that maximum doping stoichiometries achievable (y, hence band filling) are largely a function of anion size, i.e., packing constraints within the tetragonal {[Si(Pc)O]X_y}_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 =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 {[Si(Pc)O]X_y}_n materials, and large overpotentials are observed. ECPS studies of [Ge(Pc)O]_n reveal irreversible oxidative processes, and polymer decomposition via Ge—O bond cleavage is implicated.