Expanded Analogs of Three-Dimensional Lead-Halide Hybrid Perovskites.

Replacing the Pb-X octahedral building unit of AI PbX3 perovskites (X=halide) with a pair of edge-sharing Pb-X octahedra affords the expanded perovskite analogs: AII Pb2 X6 . We report seven members of this new family of materials. In 3D hybrid perovskites, orbitals from the organic molecules do not participate in the band edges. In contrast, the more spacious inorganic sublattice of the expanded analogs accommodates larger pyrazinium-based cations with low-lying π* orbitals that form the conduction band, substantially decreasing the band gap of the expanded lattice. The molecular nature of the conduction band allows us to electronically dope the materials by reducing the organic molecules. By synthesizing derivatives with AII =pyridinium and ammonium, we can isolate the contributions of the pyrazinium-based orbitals in the band gap transition of AII Pb2 X6 . The organic-molecule-based conduction band and the inorganic-ion-based valence band provide an unusual electronic platform with localized states for electrons and more disperse bands for holes upon optical or thermal excitation.

Reactivity of NO2 with Porous and Conductive Copper Azobispyridine Metallopolymers.

We report the reactivity of copper azobispyridine (abpy) metallopolymers with nitrogen dioxide (NO2). The porous and conductive [Cu(abpy)]n mixed-valence metallopolymers undergo a redox reaction with NO2, resulting in the disproportionation of NO2 gas. Solid- and gas-phase vibrational spectroscopy and X-ray analysis of the reaction products of the NO2-dosed metallopolymer show evidence of nitrate ions and nitric oxide gas. Exposure to NO2 results in complete loss of porosity and a decrease in the room-temperature conductivity of the metallopolymer by four orders of magnitude with the loss of mixed-valence character. Notably, the porous and conductive [Cu(abpy)]n metallopolymers can be reformed by reducing the Cu-nitrate species.

Carving Out Pores in Redox-Active One-Dimensional Coordination Polymers.

Reduction of the insulating one-dimensional coordination polymer [Cu(abpy)PF6 ]n , 1 a(PF6 ), (abpy=2,2'-azobispyridine) yields the conductive, porous polymer [Cu(abpy)]n , 2 a. Pressed pellets of neutral 2 a exhibit a conductivity of 0.093 S cm-1 at room temperature and a Brunauer-Emmett-Teller (BET) surface area of 56 m2 g-1 . Fine powders of 2 a have a BET surface area of 90 m2 g-1 . Cyclic voltammetry shows that the reduction of 1 a(PF6 ) to 2 a is quasi-reversible, indicative of facile charge transfer through the bulk material. The BET surface area of the reduced polymer 2 can be controlled by changing the size of the counteranion X in the cationic [Cu(abpy)X]n . Reduction of [Cu(abpy)X]n with X=Br (2 b) or BArF (2 c; BArF =tetrakis(3,5-bis(trifluoromethyl)phenyl)), affords [Cu(abpy)]n polymers with surface areas of 60 and 200 m2 g-1 , respectively.

Electronic Conductivity in a Porous Vanadyl Prussian Blue Analogue upon Air Exposure.

Exposure to humid O2 or ambient air affords a 5-order-of-magnitude increase in electronic conductivity of a new Prussian blue analogue incorporating CoII and VIV-oxo units. Oxidation produces a mixed-valence framework, where the O2 exposure time controls the VIV/VV ratio and thereby the material's conductivity. The oxidized framework shows an intervalence charge-transfer band at ca. 4200 cm-1, consistent with mixed valence. The mixed-valence frameworks show semiconducting behavior with conductivity values of 10-5 S·cm-1 at room temperature and 10-4 S·cm-1 at 100 °C and activation energies of ca. 0.3 eV. N2 adsorption measurements at 77 K show that these materials possess permanent porosity before and after oxidation with Brunauer-Emmett-Teller surface areas of 340 and 370 m2·g-1, respectively.