Covalent layer-by-layer type modification of electrodes using ferrocene derivatives and crosslinkers

A series of mono-substituted ferrocenes (Fecp₂R, R = CONHCH₂N(C₂H₄NH₂)₂ (4), R = CH₂NHCH₂N(C₂H₄NH₂)₂ (5)) and di-substituted ferrocenes (Fe(cpR)₂, R = COF (3)) have been prepared, with 4 and 5 representing new compounds. The ferrocenes were grown layer-by-layer on metal oxide electrodes and crosslinked. The underlying principle of synthesis is based on repetitive and fast amide forming reactions between trimesic acid chloride (2) or Fe(cpCOF)₂ (3) and tris(2-aminoethyl)amine (6), or one of the amine-substituted ferrocenes 4 or 5 on a 3-aminopropyl triethoxysilane (APS) (1) modified ITO, FTO or ATO electrode. Linear growth of the films electroactivity with the number of cascade steps is observed for up to 12 cascade steps yielding 1.1 × 10⁻⁸ ferrocene centers/cm² on a flat ITO electrode, whereas 3 cascade steps yield 2.2 × 10⁻⁷ ferrocene centers/cm² on a mesoporeous ATO electrode. In case of the ATO electrode, the inner walls of the mesopores exhibiting a very large surface are completely modified in the procedure. Beside this large coulometry fast electron transfer kinetics, high stability and low optical density in both oxidation states is observed.The electrodes were checked for their potential use as optically transparent counter electrodes in electrochromic devices.     

Ethylene polymerization with homogeneous nickel-diimine catalysts: effects of catalyst structure and polymerization conditions on catalyst activity and polymer properties

Ethylene polymerization was carried out using three nickel α-diimine catalysts ((ArNC(An)-C(An)NAr)NiBr₂ (1), (ArNC(CH₃)-C(CH₃)NAr)NiBr₂ (2) and (ArNC(H)-C(H)NAr)NiBr₂ (3); where An=acenaphthene and Ar=2,6-(i-Pr)₂C₆H₃) activated with modified methylaluminoxane (MMAO) in a slurry semi-batch reactor. We investigated the effects of ethylene pressure, reaction temperature, and α-diimine backbone structure variation on the catalyst activity and polymer properties. Changes in the α-diimine backbone structure had remarkable effect on the polymer microstructure as well as the catalyst activity. Catalyst 2 produced polymer with the highest molecular weight, while Catalyst 3 produced polymer with the lowest molecular weight. In addition, Catalyst 2 produced polymer with the lowest melting point, while Catalyst 3 produced the highest melting level exhibiting a melting behavior typical of high-density polyethylene (HDPE). With all the three catalysts, polymer molecular weight tended to decrease with increasing polymerization temperature due to the increase in chain transfer rates. In general, there was no clear and consistent trend observed for the effects of ethylene pressure on the polymer molecular weight. However, in polyethylene produced with Catalyst 2, the molecular weight was independent of ethylene pressure suggesting that chain transfer to ethylene may be a dominant mechanism for this catalyst.     

Synthesis and optical properties of three novel functional polyurethanes bearing nonlinear optical chromophoric pendants with different π electron conjugation bridge structure

High molecular weight functional polyurethanes bearing large π electron conjugated chromophoric pendants with different conjugation bridge structure, poly(1a), poly(1b), and poly(1c), were synthesized and characterized by FTIR, ¹H NMR and UV-vis absorption spectra. Their optical properties were evaluated by optical limiting and nonlinear optical analyses. The results show that these polymers possess good optical limiting and large nonlinear optical properties, which are attributed to the long D-π-A conjugated π electron structure of the NLO-chromophoric segment. Poly(1a) with CC double bond as π electron conjugation bridge shows better optical limiting property than poly(1b) and poly(1c) with CN or NN double bond as conjugation bridge structure under the same linear transmittance, while poly(1c) with NN double bond as π electron conjugation bridge of the NLO-chromophoric segment is superior on nonlinear optical properties to poly(1a) and poly(1b) with CC and CN double bonds as π electron conjugation bridge structure, respectively.     

Thiophene-containing poly(arylene-ethynylene)-alt-poly(arylene-vinylene)s: Synthesis, characterisation and optical properties

This work describes the synthesis and characterisation of two types of thiophene-containing poly(arylene-ethynylene)-alt-poly(arylene-vinylene)s (PAE-PAV) copolymers, whose repeating units (-Ph-CC-Th-CHCH-Ph-CHCH-)_n, 5, and (-Th-CC-Ph-CC-Th-CHCH-Ph-CHCH-)_n, 8a-c, consist, respectively, of a 1:2 and a 2:2 ratio of triple bond/double bond moieties. Comparison of their photophysical, electrochemical and photovoltaic properties has been carried out. Although similar electrochemical data (HOMO: −5.43 eV, LUMO: ∼−3.15 eV, ) as well as identical thin film absorption behaviour (λ_a=500 nm, ) were obtained for both types of materials, significant differences in their thin film photoluminescence behaviour and photovoltaic properties were observed. While polymer 5 shows a fluorescence maximum at λ_e=568 nm (with a fluorescence quantum yield of Φ_f=7%), a total fluorescence quenching was observed in 8. Far better photovoltaic performance was obtained from solar cells (set up: ITO/PEDOT:PSS/active layer/LiF/Al; active layer consisting of 5 or 8b as donor and PCBM as acceptor in a 1:3 ratio by weight) designed from 5 than from 8b. Open circuit voltage, V_OC, as high as 900 mV and power conversion efficiency, η_AM1.5, around 1.2% were obtained. This can be attributed to the 1:2 triple bond/double bond ratio as well as the grafting of shorter octyloxy and 2-ethylhexyloxy side chains in 5 and to its comparatively higher molecular-weight.     

Synthesis and characterization of liquid crystalline copolymers with dual photochromic pendant groups

In order to study the photoreactivity and the optical properties of liquid crystalline copolymers with multiple photochromic groups, a series of novel liquid crystalline binary and ternary polyacrylates consisting of one (CC or NN) or dual (CC and NN) photochromic segments were synthesized and characterized considering their liquid crystalline, optical, and photochromic properties and their thermal stability. Achiral homopolymer P1 shows a smectic A phase (fan-shaped texture), and all chiral copolymers CP1-CP6 exhibit chiral nematic phases (cholesteric, oily streaks textures). The polymers show excellent solubility in common organic solvents such as chloroform, toluene, and THF. These polymers also exhibit good thermal stability, with decomposition temperatures (T_ds) greater than 373 °C at 5% weight loss, and beyond 440 °C at 50% weight loss under nitrogen atmosphere. UV irradiation caused E/Z photoisomerization at NN and CC segments of the synthesized photochromic copolymers leading to reversible and irreversible isomerizations, respectively. The synthesized liquid crystalline ternary copolymer CP6, containing two different photochromic NN and CC groups, is sensitive to different UV wavelengths and is notably interesting from the viewpoint of photochromic copolymers.     

Theoretical study of the structural effects of polymethylene amines on corrosion inhibition of iron in acid solutions

Quantum chemical calculations were performed on azacyclo C5 to C14 amines, open chain C6 to C14 amines and phenylazacyclo C5 to C14 amines. Inspection of the calculated parameters and corrosion inhibition efficiencies were made to observe any clear links, which might exist between the two. Possible correlations between experimental inhibition efficiencies and parameters such as dipole moment (μ), highest occupied (E_HOMO) and lowest unoccupied (E_LUMO) molecular orbitals and the differences between them, HOMO-LUMO gap (Δ), as well as some structural characteristics were investigated. The models of the inhibitors were optimized with the Modified Neglect of Diatomic Overlap (MNDO) method. The Quantitative Structure Activity Relationship (QSAR) approach has been used and a composite index of some quantum chemical parameters were constructed in order to characterize the inhibition performance of the tested molecules. The inhibition effect of polymethylene amines is closely related to orbital energies and/or energy gap and dipole moment.     

Enhancement of ionic conductivity by mixing lithium borate with lithium aluminate

Several lithium borates (Salt A and Salt B) and a lithium aluminate (Salt C) with electron-withdrawing groups, OC₆F₅, OCOCF₃ or N(SO₂CF₃)₂, and oligoether chains (O(CH₂CH₂O)_nCH₃) directly bonded to the ate complex center, B or Al, were prepared. Lithium borate and lithium aluminate were mixed to get mix-salt electrolytes. Higher ionic conductivity was observed for the mix-salt than for the pure-salt. Conductivity as high as 1.1 × 10⁻⁴ S/cm at ambient temperature (25 °C) was achieved for the electrolyte in the optimized composition. The reason for such mixing effect on enhancement of ionic conductivity was discussed. Other electrochemical properties including electrochemical stability, compatibility with lithium anode and cyclic performance were also investigated for the mix-salt electrolytes.     

Electron transfer reaction and mechanism of oxidation of Inosine

Electrochemical oxidation of Inosine has been studied in the phosphate buffers of pH range 3.3-10.9 at pyrolytic graphite electrode. In the entire pH range a single well-defined oxidation peak (I_a) was observed, when the sweep was initiated in the positive direction. In the reverse sweep no cathodic peak was obtained. The peak potential of the oxidation peak was dependent on pH and shifted to less positive potential with increase in pH. The kinetics of the UV absorbing intermediate was followed spectrophotometrically and the decay occurred in a pseudo first order reaction having k values in the range 0.50-0.92 × 10⁻³ s⁻¹ in the entire pH range studied. The value of n was found to be 2.95 ± 0.3. The products of oxidation were silylated and characterized by using GC-Mass. Two tetramers having CC, CN, NN, CON and COOC linkages were identified. A plausible mechanism for the electrooxidation of Inosine has been suggested.     

Oxidation of single-walled carbon nanotubes in dilute aqueous solutions by ozone as affected by ultrasound

A clean and simple wet chemical process using dilute aqueous ozone (O₃) solution with or without ultrasound (US) was used to functionalize single-walled carbon nanotubes (SWCNTs). Both O₃ and O₃/US treatments greatly increased the stability of SWCNTs in water. Results of X-ray photoelectron spectroscopy (XPS) showed that the surface oxygen to carbon atomic ratio increased by more than 600% after 72 h of O₃ treatment. Moreover, the effective particle size of SWCNTs was reduced from the initial 4400 to ∼300 and ∼150 nm, after 24 h of O₃ and O₃/US treatment, respectively. The zeta potential of treated SWCNTs decreased from 3.0 to −35.0 mV (at pH 4) after 2 h of treatment with both O₃ and O₃/US. Based on the XPS results, the oxidation pathway was proposed: at the onset of the oxidation reaction, the CC double bond was first converted to COH which was then oxidized to CO and OCOH concurrently. Oxidation reactions could be described well with first order expressions. Treatment time controlled the extent of surface oxidation and subsequently the stability and dispersion of SWCNTs in water.     

Electrochemical sensors for detection of hydrogen in air: model of the non-Nernstian potentiometric response of platinum gas diffusion electrodes

The potentiometric response of three different platinum gas diffusion electrodes deposited on H₃PO₄ doped polybenzimidazole (PBI) was investigated under humidified atmospheres that contained H₂ or mixtures of H₂ and O₂. Continuum modelling was used to analyse the response. It is shown that the non-Nernstian response under H₂H₂ON₂ mixtures can be explained by a difference of water activity on both sides of the membrane. Under H₂O₂N₂ mixtures, the oxygen mass transport parameters have a strong effect on the electrode sensitivity.     

Estimation of standard reduction potentials of alkyl radicals involved in atom transfer radical polymerization

The redox properties of some alkyl radicals, which are important in atom transfer radical polymerization both as initiators and mimics of the propagating radical chains, have been investigated in CH₃CN by an indirect electrochemical method based on homogeneous redox catalysis involving alkyl halides (RX) and electrogenerated aromatic or heteroaromatic radical anions (D⁻). Dissociative electron transfer between RX and D⁻ yields an intermediate radical (R), which further reacts with D⁻ either by radical coupling or by electron transfer. Examination of the competition between these reactions, which depends on ED/D−°, allows determination of the standard reduction potential of R as well as the self-exchange reorganization energy λR/R⁻. The standard reduction potentials obtained for the radicals CH₂CN, CH₂CO₂Et and CH(CH₃)CO₂Me are −0.72 ± 0.06, −0.63 ± 0.07 and −0.66 ± 0.07 V vs. SCE, respectively. Quite high values of λR/R⁻ (from 122 to 164 kJ mol⁻¹) were found for all radicals, indicating that a significant change of structure accompanies electron transfer to R.     

Low temperature preparation of carbon-supported PdCo alloy electrocatalysts for methanol-tolerant oxygen reduction reaction

Carbon-supported PdCo alloy electrocatalysts of different Pd/Co atomic ratios were simply prepared in an aqueous solution at room temperature with NH₄F as a complexing agent and H₃BO₃ as a buffer, followed by NaBH₄ reduction. As-prepared PdCo bimetallic nanoparticles show a single-phase face-centered-cubic (fcc) disordered structure, and the mean particle size is found to decrease with increase in Co content. TEM images demonstrated that the as-prepared PdCo alloy nanoparticles are well dispersed on the surface of the carbon support with a small particle size and a relatively narrow particle size distribution. For example, the average particle size of a Pd₂Co₁/C catalyst is ca. 3.0 nm, which is much smaller than that of the PdCo/C bimetallic nanoparticles reported by others. An activity evaluation of the oxygen reduction reaction (ORR) on as-prepared PdCo/C catalysts with a rotating disk electrode (RDE) technique indicated that the maximum ORR mass activity was observed for a Pd:Co atomic ratio of 4:1, but the highest specific activity was found on a Pd:Co atomic ratio of 2:1. Kinetic analysis reveals that the ORR on PdCo/C catalysts follows a four-electron process leading to water. Moreover, the PdCo/C catalyst exhibited much higher methanol tolerance during the ORR than the Pt/C catalyst, assessing that it may function as a methanol-tolerant cathode catalyst in a direct methanol fuel cell (DMFC).     

Synthesis and characterization of low- and medium-molecular-weight hyperbranched polyethylenes by chain walking ethylene polymerization with Pd-diimine catalysts

We report in this article the synthesis and characterization of a range of hyperbranched polyethylenes having various low and medium molecular weights by chain walking ethylene polymerization with Pd-diimine catalysts of reduced ligand steric crowdedness, which are intended for potential applications as novel synthetic base stocks. Four Pd-diimine catalysts featured with different ligand crowdedness, ([(RC₆H₃NC(R′)-C(R′)NC₆H₃R)Pd(CH₃) (NCMe)]SbF₆) (1, R = 2,6-(iPr)₂, R′ = CH₃; 2, R = CH₃, R′ = H; 3, R = 2,6-(iPr)₂, R′ = H; 4, R = CH₃, R′ = CH₃), were employed herein for ethylene polymerizations at different conditions. Generally, reducing ligand steric crowdedness (in the order 1 > 4 > 3 > 2) leads to decreased catalyst activity and dramatically reduced polymer molecular weight. As opposed to high-molecular-weight polymers (weight-average molecular weight (M_w): about 150 kg/mol) obtained with catalyst 1, low-molecular-weight polymers (M_w: below 1.0 kg/mol) were obtained with 2 and 3, and medium-molecular-weight polymers (M_w: about 25 kg/mol) were produced with 4. Despite their various reduced molecular weights, the polymers are all featured with highly branched chain structures with a total branching density of above 100 branches per 1000 carbons. The low- and medium-molecular-weight hyperbranched polymers synthesized with 2-4 display good potential for applications as synthetic base stocks. In comparison with three commercial poly(α-olefin) based synthetic base stocks, they exhibit similar thermal and viscosity properties. Meanwhile, it is also discovered that a subsequent one-pot hydrogenation step can be incorporated in the process after the Pd-diimine catalyzed polymerization step to render nearly fully saturated hyperbranched polymers without the use of additional hydrogenation catalysts.     

Selective activation of metallic center in heterobinuclear cobalt and nickel complex in ethylene polymerization

Heterobinuclear cobalt and nickel complex {2-[2,6-R₂-C₆H₃NC(CH₃)-(CH₃)CN-(3,5-R₂′)C₆H₂-CH₂-(3′,5′-R₂)C₆H₂NC(CH₃)]-6-[2,6-R₂-C₆H₃NC(CH₃)] pyridine}CoCl₂NiBr₂ (R = isopropyl) (N₅CoNi) was prepared by reaction of pentadentate nitrogen ligand containing 2,6-bis(imino)-pyridine and α-diimine moieties with CoCl₂ and NiBr₂(DME) in turn. The complex was applied as catalyst for ethylene polymerization activated by AlEt₃, MMAO and AlEt₃/[PhMe₂NH] [B(C₆F₅)₄] respectively. The performance of the heterobinuclear complex in ethylene polymerization was compared with corresponding mononuclear complexes (α-diimine nickel bromide and 2,6-bis(imino)-pyridine cobalt chloride) and their equivalent mixture (binary complexes). When the complex N₅CoNi was activated by AlEt₃ or MMAO, its ethylene polymerization activity was lower than its control, the binary complexes. Both heterobinuclear complex and binary complex produced PE with bimodal molecular weight distribution. The amount of high-molecular-weight polyethylene produced by nickel center of N₅CoNi was less than the binary complexes, which reveals that productivity of nickel center of N₅CoNi is selectively suppressed. When the heterobinuclear complex N₅CoNi is activated by AlEt₃/[PhMe₂NH][B(C₆F₅)₄], the relative productivity of nickel center increased, although the total activity of catalyst decreased compared with AlEt₃ as cocatalyst. With respect to AlEt₃, [PhMe₂NH][B(C₆F₅)₄] can preferably activate nickel center of heterobinuclear complex. The results suggest that metal site in the heterobinuclear complex is selectively activated by cocatalyst.