Kosaku Suga

Electrocatalytic reduction of CO2 by nickel(II) cyclam: Study of the reduction mechanism on mercury by cyclic voltammetry, polarography and electrocapillarity

Abstract The electrocatalytic reduction of CO 2 on mercury with Ni(II)-cyclam was studied in detail by cyclic voltammetry, polarography and electrocapillarity of aqueous solutions with and without the catalyst under N 2 , CO and CO 2 . It is concluded from these electrochemical measurements that (i) the adsorbed complex (Ni(I)-cyclam ad ) on Hg, not the reduced complex in solution Ni(I)-cyclam, is the active catalyst for the CO 2 reduction; (ii) both Ni(II)- and Ni(I)-cyclam can adsorb on Hg, and the oxidation states of the adsorbed complexes depend on the electrode potentials; (iii) Ni(I)-cyclam adsorbs on Hg even at −2.0 V (vs. SCE) under N 2 , but the surface concentration of Ni(I)-cyclam ad decreases gradually with a decrease in the potential and desorbs completely at potentials more negative than −1.7 V under CO or CO 2 ; and (iv) the desorption of Ni(I)-cyclam ad under CO or CO 2 is due to the formation of unadsorbable Ni(I)-cyclam-CO by the reaction of Ni(I)-cyclam in solution with dissolved CO or electrocatalytically generated CO from CO 2 .

Alkali metal acetates as effective electron injection layers for organic electroluminescent devices

Abstract The effect of alkali metal (Li + , Na + , K + , Rb + , and Cs + ) acetates and fluorides at an aluminum/tris(8-hydroxyquinoline) aluminum (Alq 3 ) interface on performance of organic electroluminescent (EL) devices is described. We also studied the effect of acetates with different metals, such as Mg 2+ and Al 3+ , on the performance of the EL devices. EL characteristics of the devices with Al/alkali metal acetates were compared with those of devices with Al/alkali metal fluorides or an Al cathode itself. We found that the bilayer cathodes with alkali metal acetates exhibited better device performance than the bilayer cathodes with alkali metal fluorides. The improvement can be attributed to lowering of the work function of the Al cathode and doping of Alq 3 with alkali metals that were formed by the higher reactivity of alkali metal acetates than alkali metal fluorides with hot Al atoms impinging on these salts during the vapor deposition. The enhanced electron injection resulted in more balanced charge-carrier injection and thus higher EL efficiencies.

A study of topographic effects on chemical force microscopy using adhesive force mapping

Origins of peak broadening in a histogram of measured adhesive forces were studied. The adhesive forces were measured in water by pulsed-force-mode atomic force microscopy. One sample was prepared by a microcontact printing method on a sputtered gold film with fine grains, on which CH(3)- and COOH-terminated regions were produced. Gold surfaces of other samples were chemically modified homogeneously by a self-assembling method in solution. Their surfaces were, however, topographically different, i.e. (i) an Au(111)-terrace-rich gold film prepared by vacuum vapor deposition at high temperature and (ii) sputtered gold films on cover glass with different grain sizes obtained by different deposition time. These sample surfaces and the probe tip surface were all CH(3)-terminated by self-assembled monolayers with CH(3)(CH(2))(19)SH. The main origin of peak broadening in the histogram was the topographic effect. Namely, the change in the grain sizes and the change in multiplicity of contacts between the tip and convexities of the grains resulted in the distribution of the observed adhesive forces.

Electrochemical study on calix [4]quinone and calix[4]hydroquinone in N,N-dimethylformamide

The voltammograms of calix[4]quinone (C[4]Q) and calix[4]hydroquinone (C[4]HQ) were measured in N,N-dimethylformamide (DMF) solutions containing tetraethylammonium perchlorate (TEAP). The cyclic voltammogram of C[4]Q had four reversible or quasi-reversible small waves and a more negative quasi-reversible large wave. Comparison between the oxidation–reduction potentials of C[4]Q and those of linear polymeric quinones cited in the literature shows that the oxidation–reduction potentials of these compounds are mainly determined by the intrinsic oxidation–reduction potentials of each quinone moiety in the molecule and the coulomb interaction energies between the newly entering electron and the unpaired electrons in the ready-reduced quinone moieties. However, the cyclic voltammogram of C[4]HQ had only one large anodic peak and one cathodic peak showing that the oxidation products have no positive charge.

Electron transfer quenching of an excited amphiphilic Ru(bpy)32+ derivative in langmuir-Blodgett films containing various ferrocene derivatives as electron donors

Photoinduced electron transfer kinetics in Langmuir-Blodgett (LB) films was studied in connection with photoelectric conversion devices. Four kinds of amphiphilic ferrocene derivatives as electron donors with different standard redox potentials E0 and with the same alkyl chain spacer were newly synthesized. By using nanosecond laser photolysis, the luminescence decay curves were recorded for a sensitizer Ru(bpy)32+ derivative (where bpy is 2,2′-bipyridine) confined in one monolayer in heterogeneous LB films containing also one of the ferrocene derivatives as the other monolayer. The layer of ferrocene moieties in the heterostructures is located a fixed distance from the sensitizer layer. The comparison of the dependence of the electron transfer quenching rate on the standard free energy difference ΔG0 of the reaction between the sensitizer and the donors for three different systems, i.e. the LB film, the micellar, and the solution systems, implies the presence of local electrical fields in these ionic LB films.

Electrocatalytic reduction of CO2 on modified electrodes with alkylcyclam-metal complex Langmuir-Blodgett films

Abstract Two amphiphilic nickel(II) complexes with the long-alkyl-substituted 1,4,8,11-tetraazacyclodecane cyclam were synthesized, i.e. nickel tetrakis(N-hexadecyl)cyclam and nickel N-hexadecyclam. Monolayers of these complexes were immobilized onto glassy carbon (GC) disc electrodes using the Langmuir-Blodgett (LB) technique, and the electrocatalytic activities of these electrodes were examined by cyclic voltammetry in aqueous solutions saturated with nitrogen or carbon dioxide. The redox waves of the LB films of the nickel alkylcyclams observed in nitrogen were shifted in the anodic direction relative to those of nickel cyclams in a homogeneous solution. In carbon dioxide, the increase in the reduction current on these electrodes compared with that in nitrogen was observed at more negative potentials than - 1.3 V vs. a saturated calomel electrode in a solution with a constant pH value. This result shows that the LB films of these complexes possess sufficient electrocatalytic activity to reduce carbon dioxide despite their small amounts when compared with the amounts of homogeneous catalyst.

A voltammetric study of the metal—ligand interaction in bis(arene)chromium(I) complexes

Abstract Cyclic voltammetric and ESR measurements are made on nine bis(arene)-chromium(I) (arene  hydrocarbons with condensed benzene rings) complexes. Every cyclic voltammogram exhibits a reversible one-electron reduction step. The half-wave potentials of these complexes are independent of the ligands and form a striking contrast with those of the complexes with alkyl- and aryl- substituted benzenes, which depend largely on the ligands. This finding suggests that the redox orbitals should be of pure σ-character which partly originates from ligand σ-orbitals. The ESR spectrum of each [Cr(arene) 2 ] + exhibits, besides 53 Cr hyperfine lines, only 1 H hyperfine lines due to a pair of benzene rings bonded to Cr, irrespective of the number of the rings. Consequently, the findings of previous workers concerning the bis(naphthalene) complexes of Cr 1 and V 0 are also true of complexes with greater aromatic ring systems. Our experimental results support the conclusion of the previous ESR and NMR studies that the HOMO of [Cr(arene) 2 ] 0 is mainly composed of the metal 3 d z2 and the ligand σ-orbitals. However, the previously reported formation of [Cr(naphthalene) 2 ] - by chemical reduction is not substantiated by our voltammetric experiments.

ΔG0-dependence of the chemical transfer coefficient and the potential dependence of the electrochemical transfer coefficient

Summary Kinetic data of the homogeneous electron transfer reactions of the polycyclic aromatic hydrocarbons measured by electron spin resonance, pulse radiolysis and quenching of fluorescence methods are examined in the light of the theory of Marcus. The logarithms of the rate constants are found to depend quadratically on the free energy of reaction, as predicted theoretically. This may be experimental evidence for the linear dependence of the chemical transfer coefficient upon the free energy of reaction. The possibility of experimental verification of the potential dependence of the electrochemical transfer coefficient is discussed in connection with the behaviour of the chemical transfer coefficient.

Phase separation of mixed LB films containing spin probe molecules

Abstract Polyion complexed mixed Langmuir–Blodgett (LB) films of amphiphiles with long fluorocarbon chains (PFECA [CF3(CF2)8(CH2)2O(CH2)2COOH] or perfluorodecanoic acid (PFDA) [CF3(CF2)8COOH]) and those with long hydrocarbon chains (stearic acid (STA) or doxylstearic acids (5-, 10- or 16-doxyslstearic acid) deposited on Si, glass substrates or polyethylene terephthalate (PETP) sheets were characterized by atomic force microscopy/friction force microscopy (AFM/FFM) and electron spin resonance (ESR) spectroscopy. AFM/FFM images of 1:1 mixed LB films of PFECA (or PFDA) and 16-DXSTA showed clear phase-separated structure, while those of PFECA (or PFDA) and 5-DXSTA did not. All the values of the frictional force for the LB films, which did not show clear phase-separated structure, were intermediate between those for PFECA and STA. ESR spectra of mixed LB films of PFECA and 16-DXSTA were almost the same as that of a polyion complexed LB film of pure 16-DXSTA. On the contrary, the ESR spectra of mixed LB films of PFECA, STA and 5-DXSTA were very different from those of mixed LB films of 5-DXSTA and STA. From these results it was concluded that in the polyion complexed 1:1 mixed LB films of 5-DXSTA and PFECA, 5-DXSTA and PFDA, and 10-DXSTA and PFDA, the amphiphilic molecules with fluorocarbon chains and those with hydrocarbon chains are mixed with each other on the molecular level. This finding shows that phase separation of mixed LB films containing amphiphiles with fluorocarbon chains and those with hydrocarbon chains can be controlled by the selection of the structure of amphiphiles.

Electric and spectroscopic properties of Langmuir-Blodgett films containing tetracyanoquinodimethane with various mixed valent states

Abstract The electric conductivities of Langmuir-Blodgett (LB) films and cast films of octadecyltetracyanoquinodimethane (C18TCNQ), N-docosylpyridinium-TCNQn (C22PyTCNQn; n=1,2) and N-octadecylpyridinium-TCNQ2 (C18PyTCNQ2) with various mixed valent states were measured using patterned gold microelectrodes as the substrates of these thin films. The conductivity of the 1:1 mixed LB film of C18TCNQ and methyl arachidate (MA), whose mixed valency was controlled by the constant-potential electrolysis, changed drastically with the mixed valency of the film, and reached a maximum value when it was electrolyzed at -0.1 V vs. a saturated calomel electrode (SCE), i.e. the oxidation-reduction potential of the LB film. The electric conductivity of the cast films of C22PyTCNQ2 and (1⩽x⩽2), which were prepared by the mixed solution of C22PyTCNQ2 and C22PyTCNQ, increased about four orders from x=1 to x=2, and has its largest values at x=2. The electron spin resonance spectra of the 1:1 mixed LB films of C18TCNQ and MA electrolyzed at several electrode potentials were measured. The dependences of the linewidths of the LB films on the angle between the direction of the static magnetic field and that normal to the film were different from one anther, and depended on the temperature. The result was discussed in terms of the electron exchange rate in the LB films.