Molecular metals with widely tunable band filling. Response of the collective properties of a phthalocyanine molecular metal to drastic excursions in partial oxidation state and charge-compensating counterions

Abstract

The electrical, optical, and magnetic properties of the partially oxidized, cofacially joined phthalocyanine polymers ((Si(Pc)O)X{sub y}){sub n} are investigated for X{sup {minus}} = BF{sub 4}{sup {minus}}, y = 0.00-0.50; X{sup {minus}} = p-toluenesulfonate (TOS{sup {minus}}), y = 0.00-0.67; and X{sup {minus}} = SO{sub 4}{sup 2{minus}}, y = 0.040, 0.095. As a function of increasing y, the physical properties of the BF{sub 4}{sup {minus}} and TOS{sup {minus}} salts evidence a transition at y {approx} 0.20 from a localized carrier semiconductor or insulator to a molecular metal. Thus, the dc electrical conductivity increases from low values having a temperature dependence most characteristic of disorder and/or hopping transport between localized states to values characteristic of a molecular metal with fluctuation-induced carrier tunneling between relatively large metallike particles. Beyond y {approx gt} 0.25, the conductivity is only weakly dependent on y. At y {approx} 0.20, the thermoelectric power (S(T)) also changes from behavior characteristic of a p-type semiconductor or insulator to that of a p-type molecular metal. Differences in S(T) between X{sup {minus}} = BF{sub 4}{sup {minus}} and TOS{sup {minus}} polymers appear to be due largely to minor structural variations. Optical reflectivity measurements reveal the appearance of a metal-like plasma edge at y {approx}more » 0.20, followed by an incremental shift of this feature to higher energy with further increase in y. The static magnetic susceptibility of ((Si(Pc)O)(BF{sub 4}){sub y}){sub n} evidences an abrupt transition at y {approx} 0.20 from a large concentration of localized, Curie-like spins to Pauli-like behavior characteristic of a molecular metal. Beyond y {approx gt} 0.30, the Pauli-like susceptibility is nearly independent of y. ESR studies indicate a ligand-centered {pi}-radical-cation electronic structure.« less