Reduced and Oxidized Forms of the Pt-Organometallic Version of Polyaniline

This work represents an effort to synthesize all four forms of polyaniline (PANI) in its organometallic versions. Polymers containing substituted 1,4-benzoquinone diimine or 1,4-diaminobenzene units in the backbone exhibiting the general structure (C CC6H4-N=C6X4=N-C6H4C C-PtL2) and (C CC6H4NH-C6X4NHC6H4C C-PtL2) along with the corresponding model compounds (C CC6H4-N=C6X4=N-C6H4C C)-(PtL2Cl)(2) and (C CC6H4NH-C6X4-NHC6H4C C)-(PtL2Cl)(2) (L = PBu3; X = H, F, Cl) were synthesized. The polymers and corresponding model compounds were characterized (including H-1 and P-31 NMR, IR, mass spectra, elemental analysis, and X-ray structure determinations) and investigated for their redox properties in the absence and in the presence of acid. Their optical properties, including ns transient spectroscopy were also investigated. These properties were interpreted through density functional theory (DFT) and time-dependent DFT (TDDFT) computations. These materials are found to be oligomers (GPC) with thermal stability (TGA) reaching 350 degrees C. The greatest stabilities were found in the cases with X = F. Using a data bank of 8 X-ray structures of diimine derivatives, a relationship between the C=N bond distance and the dihedral angle between the benzoquinone ring and the flanking phenyl planes is noted. As the size of the substituent X on the benzoquinone center increases, the degree of conjugation decreases as demonstrated by the C=N bond length. The largest dihedral angles are noted for X = Cl. These polymers exhibit drastic chemical differences when X is varied (X = H, F, Cl). The completely reduced polymer (C CC6H4NH-C6H4-NHC6H4C C-PtL2) (i.e., X = H) was not chemically accessible whereas in the cases of X = F, Cl, these materials were obtained and represent the first examples of fully reduced organometallic versions of PANI (i.e., leucoemaraldine). For the (C CC6H4-N=C6X4=N-C6H4C C-PtL2) polymers, the completely oxidized form for X = H was isolated (pemigraniline), but for X = F and Cl, only the largely reduced mixed-valence form (i.e., emaraldine) was obtained via chemical routes. In acidic solutions, the chemically accessible polymer for X = H, (C CC6H4-N=C6H4=N-C6H4C C-PtL2), exhibits two chemically reversible waves indicating that the reduced form (C CC6H4NH-C6H4-NHC6H4C C-PtL2) can be generated. The absorption spectra of the highly colored diimine-containing species exhibit a broad charge transfer band (assigned based on DFT calculations (B3LYP); C6H4C C-PtL2-C CC6H4 -> N=C6X4=N) in the 450-800 nm window red shifting according X = H -> Cl -> F, consistent with their relative inductive effect. The largest absorptivity is measured for X = H because this polymer is fully oxidized whereas for the cases where X = F and Cl, these polymers exists in the mixed valence form. The ns transient absorption spectra of two polymers (X = F; reduced and mixed-valence polymers) were measured. The triplet excited state in the mixed-valence polymer is dominated by the reduced diamine residue and the T-1-T-n absorption of the diimine is entirely quenched

Hydrothermal Formation of W/Mo-Oxides: A Multidisciplinary Study of Growth and Shape

The hydrothermal formation of mixed nanoscale W/Mo-oxides with the hexagonal tungsten bronze (HTB) structure has been investigated by in situ EDXRD (energy dispersive X-ray diffraction). Compared to the binary oxide systems, they display intermediate kinetics with a nucleation-controlled mechanism dominated by the slow growing tungsten component. Furthermore, the thermal stability of nanostructured W/Mo-HTB compounds has been monitored through combined in situ X-ray absorption spectroscopy (XAS) and XRD in reducing and oxidizing atmospheres. Their transformation into other mixed nanostructures was only observed above 300 °C in O2- and H2-containing atmospheres. In addition, the shape of nanoscale hexagonal W/Mo-oxides can be expanded into a variety of morphologies via the use of alkali chlorides as hydrothermal additives. The alkali cations exert a two-fold role as internal stabilizers and external shape control agents. Their mobility within the channels of the W/Mo-oxide host framework has been investigated by solid state NMR spectroscopy

Electrochemical and spectroscopic methods for evaluating molecular electrocatalysts

© 2017 Macmillan Publishers Limited. Modern energy challenges have amplified interest in transition metal-based molecular electrocatalysts for fuel-forming reactions. The activity of these homogeneous electrocatalysts, and the mechanisms by which they operate, can be uncovered using state-of-the-art electrochemical methods. Catalyst performance can be benchmarked according to metrics obtainable from cyclic voltammograms by analysis of catalytic plateau currents and peak potentials, as well as by foot-of-the-wave analysis. The application of complementary spectroscopic techniques, including spectroelectrochemistry, stopped-flow rapid mixing and transient absorption, are also discussed. In this Review, we present case studies highlighting the utility of these analytical methods in the context of renewable energy. Alongside these examples is a discussion of the theoretical underpinnings of each method, outlining the conditions necessary for the analysis to be rigorous and the type of information that can then be extracted