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The reversible one-electron-transfer reactions of the following cation radicals were studied by temperature-jump and stopped-flow methods: phenothiazine (PhZ)/N,N,N′,N′-tetramethyl-p-phenylenediamine cation (TMPD\underset.+), PhZ/N,N-dimethyl-p-phenylenediamine cation (DMPD\underset.+), DMPD/TMPD\underset.+ and DMPD/N,N,N′,N′-tetramethylbenzidine cation (TMBZ\underset.+). The solvent used was either 1 : 3 ethanol-water or acetonitrile. For PhZ/TMPD\underset.+, PhZ/DMPD\underset.+ in ethanol-water and also for DMPD/TMPD\underset.+ in acetonitrile, the backward reaction corresponding to the negative free energy change of reaction (ΔF°

The electron transfer reaction between Wurster’s Blue cation (TMPD\underset.+) and N,N-dimethyl-p-phenylenediamine (DMPD) in aqueous solution is investigated by the temperature-jump method. The forward and backward rate constants of TMPD\underset.+ + DKPD \ightleftharpoons TMPD + DMPD\underset.+ are determined to be (4.2±2)×106 and (1.5±0.5)×108 M−1 s−1 at 10±2°C, respectively.

The kinetics of the protonation of sodium salt of 7,7,8,8-tetracyanoquinodimethane (Na+TCNQ\ewdot) by hydrogen chloride in alcohols and acetonitrile was investigated, using a stopped-flow apparatus. In methanol, the protonation rate is first-order in HCl and second-order in Na+TCNQ\ewdot, while in ethanol, the rate is one-half-order in HGl and second-order in Na+TCNQ\ewdot. The protonation rate in acetonitrile is first-order in HCl, but it deviates from second-order in Na+TCNQ\ewdot. These results are interpreted in terms of the rapid formation of a protonated intermediate in the initial stage of the reaction, according to TCNQ\ewdot+H+\ightleftharpoonsHTCNQ\ewdot The reversibility of this reaction is well demonstrated by the second-order dependence of the rate on Na+TCNQ\ewdot. The rate is also compared with the protonation rate of semiquinones, such as sodium salts of p-chloranil and 2,3-dicyano-l,4-benzoquinone, from the view point of the difference in reactivity between carbon and oxygen acids.

The rates of the electron-transfer reaction between 7,7,8,8-tetracyanoquinodimethane anion radical and 2,3-dichloro-5,6-dicyano-p-benzoquinone in acetonitrile have been determined with a temperature-jump apparatus. The forward and backward rate constants are (4±1) ×109M−1s−1 and (1±0.5) ×104M−1s−1 at 15°C, respectively. Addition of 10 per cent water to acetonitrile decreases the forward rate constant by a factor of 100.

The reversible dimerization of the anion radical of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) has been studied in water, acetonitrile (MeCN), and other organic solvents. The analysis of the thermodynamic parameters of dimerization equilibria reveals that a large negative entropy term plays a primary role in determining the equilibrium constants. The temperature-jump measurement shows that the forward rate constant in water containing 5.8 M urea is nearly a diffusion controlled one, (1±0.5)×108 M−1 s−1 at −6°C. In MeCN-water mixtures, the backward reaction rate increases with the square of the MeCN concentration. This result suggests that a dimer is effectively decomposed by the successive attacks of the two MeCN molecules.

The complex formation between Fe(III) and 10 kinds of phenols was investigated using a stopped-flow apparatus. It was confirmed that the reaction rate is acid-independent in the acid region 0.01–1.0 M. A mechanism for the aquation of the iron (III)-phenol complex is proposed in which a proton transfers intramolecularly from a co-ordinated water molecule to a phenolate ion, viz., Fe(OH2)5A2+→Fe(OH2)4(OH)·AH2+. From the observed isotope effects it was concluded that the state of a proton of HA in Fe(OH2)4(OH)·AH2+ differs from that in a free phenol.

The complex formation reaction among amylose, iodine and iodide ion in aqueous solution was investigated with a stopped-flow apparatus. The forward reaction rate was found to be expressed as v=k[Amylose]([I3−]–θ) for [I3−]>θ, where θ is a critical value determining the mechanism. For [I3−]>θ, the rate was considered to be determined by the process between a free amylose unit and free I3−. For [I3−]

The temperature dependence of dark- and photo-currents was studied for the hole injection into anthracene crystals between 0 and 30°C. It was observed that the photocurrent or, as has been previously established, the electron-transfer rate from an excited anthracene crystal to an acceptor had a small temperature dependence, while the dark current or the electron-transfer rate from the crystal in its ground state to an acceptor had a definite value of activation energy. The observed small activation entropy for the dark reaction was interpreted in terms of Marcus’ theory, and the small polarizability of the organic crystal was, accordingly, suggested to increase the probability of the electron transfer.

The reversible dimerization of the anion radical of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) has been studied in water, acetonitrile (MeCN), and other organic solvents. The analysis of the thermodynamic parameters of dimerization equilibria reveals that a large negative entropy term plays a primary role in determining the equilibrium constants. The temperature-jump measurement shows that the forward rate constant in water containing 5.8 M urea is nearly a diffusion controlled one, (1±0.5)×108 M−1 s−1 at −6°C. In MeCN-water mixtures, the backward reaction rate increases with the square of the MeCN concentration. This result suggests that a dimer is effectively decomposed by the successive attacks of the two MeCN molecules.

The kinetics of the protonation of sodium salt of 7,7,8,8-tetracyanoquinodimethane (Na+TCNQ\ewdot) by hydrogen chloride in alcohols and acetonitrile was investigated, using a stopped-flow apparatus. In methanol, the protonation rate is first-order in HCl and second-order in Na+TCNQ\ewdot, while in ethanol, the rate is one-half-order in HGl and second-order in Na+TCNQ\ewdot. The protonation rate in acetonitrile is first-order in HCl, but it deviates from second-order in Na+TCNQ\ewdot. These results are interpreted in terms of the rapid formation of a protonated intermediate in the initial stage of the reaction, according to TCNQ\ewdot+H+\ightleftharpoonsHTCNQ\ewdot The reversibility of this reaction is well demonstrated by the second-order dependence of the rate on Na+TCNQ\ewdot. The rate is also compared with the protonation rate of semiquinones, such as sodium salts of p-chloranil and 2,3-dicyano-l,4-benzoquinone, from the view point of the difference in reactivity between carbon and oxygen acids.

1. The d.c. photoconductivity of organic molecular crystals was studied using liquid electrodes containing model compounds of the coenzymes to elucidate the effect of these additives on the rate of electron transfer from the crystals to the electrodes. The number of injected holes per unit time is the measure of the rate of the electron transfer. 2. Compounds containing the pyridinium cation nucleus enhanced the rate of the electron transfer reaction between excited anthracene molecules and the electrodes. On the addition of N-benzylnicotinamide chloride to the aqueous electrode, the photocurrent was observed to increase several fold. 3. Ag+ had similar effects on the photoconductivity of anthracene crystals. 4. Excited riboflavin molecules act as electron acceptors for crystals of the aromatic hydrocarbons. When riboflavin molecules and silver ions were added to the aqueous electrode, a remarkable increase in the hole current was observed. The effect was more than additive.