Electrochemical reactions of solutes and of electroactive polymer films in low dielectric media: toluene and heptane

Synthesis, electrochemistry and spectroelectrochemistry of tetraundecylporphyrins in nonaqueous media

Six meso-substituted tetraundecylporphyrins were prepared in high yield using an improved synthetic method and then characterized by electrochemistry and spectroelectrochemistry as well as UV-vis, FTIR and NMR spectroscopy. The examined compounds are represented as (TUP)M where M = H2, ZnII, CuII, NiII, PdII or MnIIICl and TUP is the dianion of a porphyrin having undecyl groups (-CH2(CH2)9CH3) on the four meso-positions of the macrocycle. Two oxidations are observed for each compound in CH2Cl2 and PhCN containing 0.1 M TBAP, leading to the stepwise formation of π-cation radicals and dications. The free-base and MnIIICl derivatives undergo two reductions but only one reversible reduction is seen for the other porphyrins under the given solution conditions. Comparisons are made between the electrochemical behavior of the (TUP)M derivatives and two related series of porphyrins, (TPP)M and (OEP)M. The effect of substituents on the meso-positions of the TUP macrocycle is also discussed.

Electrochemistry in Media of Exceptionally Low Polarity: Voltammetry with a Fluorous Solvent

This work demonstrates the first cyclic voltammetry in a perfluorocarbon solvent without use of a cosolvent. The novel electrolyte tetrabutylammonium tetrakis[3,5-bis(perfluorohexyl)phenyl]borate (NBu(4)BArF(104); 80 mM) allows for voltammetry of ferrocene in perfluoro(methylcyclohexane) by lowering the specific resistance to Ω268 k cm at 20.8 °C. Despite significant solution resistance, the resulting voltammograms can be fitted quantitatively without difficulty. The thus determined standard electron transfer rate constant, k°, for the oxidation of ferrocene in perfluoro(methylcyclohexane) is somewhat smaller than for many solvents commonly used in electrochemistry, but can be explained readily as the result of the viscosity and size of the solvent using Marcus theory. Dielectric dispersion spectroscopy verifies that addition of NBu(4)BArF(104) does not significantly raise the overall polarity of the solution over that of neat perfluoro(methylcyclohexane).

Ferrocene functional polymer brushes on indium tin oxide via surface-initiated atom transfer radical polymerization

The synthesis and electrochemical characterization of ferrocene functional polymethacrylate brushes on indium tin oxide (ITO) electrodes using surface-initiated atom transfer radical polymerization (SI-ATRP) is reported. SI-ATRP of ferrocene-containing methacrylate (FcMA) monomers from a phosphonic acid initiator-modified ITO substrate yielded well-defined homo- and block (co)polymer brushes of varying molar mass (4,000 to 37,000 g/mol). Correlation of both electrochemical properties and brush thicknesses confirmed controlled SI-ATRP from modified ITO surfaces. The preparation of block copolymer brushes with varying sequences of FcMA segments was conducted to interrogate the effects of spacing from the ITO electrode surface on the electrochemical properties of a tethered electroactive film.

Wire-like charge transport at near constant bridge energy through fluorene oligomers

The study of photoinitiated electron transfer in donor-bridge-acceptor molecules has helped elucidate the distance dependence of electron transfer rates and behavior of various electron transfer mechanisms. In all reported cases, the energies of the bridge electronic states involved in the electron transfer change dramatically as the length of the bridge is varied. We report here, in contrast, an instance in which the length of the bridge, and therefore the distance over which the electron is transferred, can be varied without significantly changing the energies of the relevant bridge states. A series of donor-bridge-acceptor molecules having phenothiazine (PTZ) donors, 2,7-oligofluorene (FL(n)) bridges, and perylene-3,4:9,10-bis(dicarboximide) (PDI) acceptors was studied. Photoexcitation of PDI to its lowest excited singlet state results in oxidation of PTZ via the FL(n) bridge. In toluene, the rate constants for both charge separation and recombination as well as the energy levels of the relevant FL(n)(+.) bridge states for n = 1-4 are only weakly distance dependent. After the initial photo-generation of (1)(PTZ(+.)-FL(n)-PDI(-.)), radical pair intersystem crossing results in formation of (3)(PTZ(+.)-FL(n)-PDI(-.)) that recombines to give (3.)PDI. The dependence of the (3.)PDI yield on an applied magnetic field shows a resonance, which gives the singlet-triplet splitting, 2J, of the radical ion pair. The magnitude of 2J directly monitors the contribution of coherent charge transfer (superexchange) to the overall electron transfer rate. These data show that charge recombination through FL(n) is dominated by incoherent hopping at long distances.

Photobehavior of (alpha-diimine)dimesitylplatinum(II) complexes

The photobehavior of complexes of the type Pt(diimine)(mes)2 is investigated (where diimine = 2,2'-bipyridine (bpy), 1,10-phenanthroline (phen), 3,4,7,8-tetramethyl-1,10-phenanthroline (tmp), 2,9-dimethyl-1,10-phenanthroline (2,9-dmp), 5,6-dimethyl-1,10-phenanthroline (5,6-dmp), and 4,7-diphenyl-1,10-phenanthroline (dpp) and mes = the mesityl (2,4,6-trimethylphenyl) anion). For all compounds studied, solution RT emission is observed to be weak and excited-state lifetimes are found to be short (< or = 20 ns) regardless of solvent choice. Evidence is presented for energy-transfer quenching of Pt(dpp)(mes)2 luminescence in toluene by dissolved O2 (primarily producing singlet oxygen) with an observed quenching rate constant of kq > or = 1.3 x 10(9) M-1 s-1. Electron-transfer quenching is also observed in the presence of 3,5-dinitrobenzonitrile, yielding a quenching rate constant of kq > or = 1.6 x 10(9) M-1 s-1. The latter observation suggests that phase Pt(II) systems may have future value as excited-state reductants. All of the complexes display a much more intense and longer-lived luminescence in the solid state at room temperature. Several possible explanations for this dependence on phase are proposed, with the most probable mechanism involving radiationless deactivation in solution via rotation of the o-methyl groups of the mesityl ligands.

Chronopotentiometric analysis of highly resistive media

Reciprocal derivative chronopotentiometry is shown to be well suited for performing electrochemistry in highly resistive solutions in connection to conventional-sized electrodes. The fundamentally different operational principles of reciprocal derivative chronopotentiometry (compared to controlled-potential voltammetry) reduces its susceptibility to solution resistance effects. The influences of the electrolyte concentration and constant current upon the reciprocal derivative chronopotentiometric peak area and potential are examined in different solvents. Well-defined, analytically useful peaks are observed in highly resistive media (with low electrolyte/analyte ratios). Narrower peaks, shifted to higher potential values, are observed upon increasing the solution resistance. Yet, the analytical signal (square root of the peak area) is nearly independent of the ohmic drop. The experimental results are compared with the theoretical predictions. Considerable ohmic drop distortions are observed in analogous voltammetric experiments. The defined chronopotentiometric peaks permit convenient quantitation of micromolar analyte concentrations in highly resistive media. Such observations open the door to new electrochemical applications of conventional electrodes that were previously restricted to ultramicroelectrodes.

Probing the limits: ultraslow diffusion and heterogeneous electron transfers in redox polyether hybrid cobalt bipyridine molten salts

This paper describes microelectrode voltammetry measurements of self-diffusion coefficients and of heterogeneous Co(II/III) electron-transfer rate constants (ko) in undiluted molten salts of three cobalt tris(bipyridine) perchlorate complexes in which the bipyridine ligands are "tailed" with poly(propylene oxide) and poly(ethylene oxide) oligomers. The self-diffusion coefficients are measured with potential step chronoamperometry and range from 10(-12) to 10(-17) cm2/s, while the quasi-reversible reaction rate constants are measured using cyclic voltammetry and small potential steps and range from 10(-7) to 10(-12) cm/s. The ko measurements are unusual in that when rate constants become smaller, the reaction remains quasi-reversible, because of concurrently decreasing self-diffusion rates. The measurements are, furthermore, accomplished in the face of uncompensated resistances that range from mega- to gigaohms, which is made possible by the combination of microelectrode properties and small diffusivities. The melt in which self-diffusion and ko values are smallest is at a temperature below its nominal glassing transition and in the regime of molecule-scale diffusion profiles.

Voltammetry with Microscopic Electrodes in New Domains

Voltammetric electrodes of microscopic dimension, termed ultramicroelectrodes, can be used to make measurements that are difficult or impossible with conventional electrochemical techniques. Measurements of chemical concentration can be made with these electrodes on a microsecond time scale and with micrometer spatial resolution. In addition, measurements can be made in highly resistive solutions.