No abstract

The synthesis and characterization of a novel family of positively charged fac-[Re(bpy)(CO)(3)(L)]PF(6) (bpy = 2,2'-bipyridine) complexes are reported, where L is a pyridine functionalized in para or meta position with a fulvene moiety, namely, 4-fluoren-9-ylidenemethyl-pyridine (pFpy) and 3-fluoren-9-ylidenemethyl-pyridine (mFpy). The complexes were prepared in high yield (86%) by direct addition at room temperature of the corresponding pyridine to the tetrahydrofuran (THF) adduct fac-[Re(bpy)(CO)(3)(THF)][PF(6)] precursor. Both ligand and complex structures were fully characterized by a variety of techniques including X-ray crystallography. The complexes did not exhibit the expected triplet mixed metal-ligand-to-ligand charge transfer (MLLCT) emission, because of its deactivation by the non-emissive triplet excited state of fulvene. The absorption profile shows that the MLLCT is overshadowed by the fulvene centered π-π* transition of higher molar absorptivity as shown by time dependent density functional theory (TD-DFT) calculations. The position of the fulvene on the pyridyl ring has a large effect on this transition, the para position displaying a much higher absorption coefficient (21.3 × 10(3) M(-1) cm(-1)) at lower energy (364 nm) than the meta position (331 nm, 16.0 × 10(3) M(-1) cm(-1)).

Protic ionic liquids composed of 2-methoxypyridinium and trifluoroacetate were used in their neat, undiluted form as an electrolyte to study the charge storage properties of MnO2. Potentiostatic and potentiodynamic methods were used to prepare MnO2 thin films on indium tin oxide substrates. The film morphology had an effect on the specific capacitance, with the potentiostatic deposition providing the most suitable structure to increase specific capacitance in the ionic liquid. The deposition time and number of cycles were varied to find optimal electrode preparation conditions in protic ionic liquid. A linear dependency of the specific capacitance on the amount of MnO2 deposited was observed for thin films up to ca. 4 μg·cm–2, which corresponds to a 70 nm thick film. The maximum specific capacitance of these thin films was 397 F·g–1 at 5 mV·s–1 and similar to the values obtained in a Na2SO4 aqueous solution. The pseudocapacitive behavior of MnO2 thin films in protic ionic liquid was investigated further by in situ UV–visible spectroscopy. While increasing the potential applied to the films, the measurements between 400 and 600 nm revealed an increase of absorbance that is a consequence of the higher absorptivity of Mn(IV) relative to Mn(III). The demonstrated redox transitions occurring in MnO2 with a protic ionic liquid electrolyte are associated to a proton exchange between the ionic liquid and the active material.

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