Electron Transfer Stopped-flow Research Articles

Amine-based organic high-spin systems; a room-temperature-stable one-dimensional oligoaryl triamine-based trication

A triamine, 2,2′,4,4′-tetrabromo-3″,4″-dimethyl-5,5′-bis(di(4-methylphenyl)amino)triphenylamine, was designed and synthesized as a model precursor for purely organic cationic high-spin systems. Cyclic voltammetry measurements revealed that the triamine gives stable mono-, di-, and tricationic states at ambient temperature. Novel electron transfer stopped-flow (ETSF) methods were invoked for characterizing the absorption spectra of the corresponding mono- and oligo-cationic states. The triamine-based trication was generated by chemical oxidation at room temperature, and the triamine was quantitatively recovered upon reduction. The electronic ground state of the trication is discussed.

Amine-based organic high-spin systems: Synthesis, electrochemical and spectroscopic studies of polyalkylated one-dimensional oligoaryl triamines

A polyalkylated triamine, 2,2′,4,4′-tetrabromo-4″-methyl-5,5′-bis(di(3,4-di-methylphenyl)amino)triphenylamine, was designed and successfully synthesized as a model precursor for purely organic cationic high-spin systems. Cyclic voltammetry measurement revealed that the triamine gives stable mono- and dicationic states and fairly unstable tricationic state. Novel electron transfer stopped-flow (ETSF) methods were invoked for characterizing the absorption spectra of the corresponding mono- and oligo-cationic states. The triplet state of the dication was confirmed by ESR spectroscopy.

Organic high-spin systems of oligoarylamines: Properties of tetraaryl-m-phenylendiamine oligocations as examined by electron transfer stopped-flow method

Abstract Three types of N,N,N′,N′-tetraaryl-1,3-phenylenediamines with bromo atoms as protecting groups were designed and synthesized. Electrochemical and novel electron transfer stopped-flow (ETSF) methods were invoked for characterizing the absorption spectra of the corresponding short-lived mono- and dicationic states. Useful molecular design rules for stabilizing the dicationic states of N,N,N′,N′-tetraaryl-1,3-phenylenediamines as precursors for positively charged high-spin systems were elucidated. An extended system, 3,3′-bis(diphenylamino)triphenylamine in the tricationic state with four bromo groups was also examined, being confirmed to give a ground-state triplet dication by ESR spectroscopy.

Kinetic analysis of the reactions of 1-substituted pyrene cation radicals with water in acetonitrile

Although the spectroscopic detection of electrochemically generated pyrene cation radical (PY +) is generally difficult at room temperature in acetonitrile (AN), we could successfully observe the time changes in the absorption spectra of short-lived PY + using an electron transfer stopped-flow (ETSF) method by adopting the tris(2,4-dibromophenyl)amine cation radical (TDBPA +) as a reaction initiator. The decrease of PY + was observed to be complete within 30 ms after generating PY + via the mixing of the AN solutions of PY and TDBPA +. This decay reaction of PY + can be attributed to the reaction with a trace amount of water in AN. Thus, the reactions of 1-substituted pyrene cation radicals (XPY +) with water were systematically analyzed using the ETSF method by adding a constant amount of water. Consequently, the reactions were confirmed to be second-order in the cation radicals in all the cases. Among the derivatives, the higher stability of 1-aminopyrene + was confirmed on the basis of the slow reaction rates, and the order of the reactivity toward water was clarified to be pyrene +>1-bromopyrene +>1-methylpyrene +>1-aminopyrene +. The present results of XPY + were compared with our previous results on the reactions of 9-substituted anthracene derivative cation radicals.

Kinetics and mechanisms of the reactions of 9-substituted anthracene cation radicals with water or methanol in acetonitrile

The reactions of 9-substituted anthracene cation radicals (XA + s) with water and methanol in acetonitrile were analyzed using an electron transfer stopped-flow (ETSF) method. By adopting the tris(2,4-dibromophenyl)amine cation radical (TDBPA + ) as a reaction initiator, five XA + s, i.e. phenyl, bromo, acetyl, benzyl, and methyl derivatives, could be generated quantitatively by mixing XA with TDBPA + , and the chemical reaction processes of XA + s with water or methanol (ROH) could then be followed by observing dynamic transformation processes of the absorption spectra. Consequently, the reaction orders of XA + and ROH as well as the reaction rates could be determined. The order of the reactivity was anthracene + >9-methylanthracene + >9-benzylanthracene + >9-acetylanthracene + , 9-bromoanthracene + >9-phenylanthracene + , and diverse changes in the reaction orders of both XA + and ROH were observed indicating some mechanistic changes. The order of the reactivity was difficult to explain only on the basis of the character of XAs or XA + s. However, as a hypothesis, it can be assumed that when the electron transfer between XA + and XA(ROH) + is favorable to formation of XA(ROH) 2+, the reaction order of [XA + ] is second, and the reaction rates become faster.

Reaction of the triphenylamine cation radical with pyridine in acetonitrile. Electrochemical responses vs. decay reactions in homogeneous solution

The reaction of the triphenylamine cation radical (TPA + ) with pyridine (Py) in acetonitrile (AN) was studied using cyclic voltammetry and an electron transfer stopped-flow (ETSF) method. The cyclic voltammograms of TPA in AN recorded in the presence of 5-fold excess of Py at conventional scan rates are almost identical to those obtained without Py. This indicates that, also in the presence of Py, the product in the electrochemical oxidation of TPA is the dimer, tetraphenybenzidine (TPB). While the identical electrochemical responses might imply that there are no interactions between TPA + and Py, diverse transformation of the decay profiles of TPA + depending on the concentration of Py were observed in homogeneous solutions using the ETSF method. For instance, in the reaction between 0.050 mM TPA + and 0.050 mM Py, a steep decrease of TPA + was observed in the first 10 s after the mixing without accompanying an increase of TPB + . This process was assigned to be due to the interaction between TPA + and Py, in which H + was extracted from TPA + by Py. The present case would be an interesting example that the voltammetric measurements at the conventional scan rates cannot give the remarkable differences, even though there are significant interactions between cation radical and coexisting base species.

Kinetics of the Decay Reactions of the N,N-Dimethyl-p-Toluidine Cation Radical in Acetonitrile. Acid−Base Interaction to Promote the CH2−CH2 Bonding

The decay reaction of N,N-dimethyl-p-toluidine (DMT) cation radical (DMT•+) in acetonitrile (AN) was analyzed using an electron-transfer stopped-flow (ETSF) method. In the ETSF method, DMT•+ is gen...

Remarkable 3-methyl substituent effects on the cyclization reaction of diphenylamine derivative cation radicals in acetonitrileElectronic supplementary information (ESI) available: results of kinetic analysis for the reactions in Figs. 4 and 5. See http://www.rsc.org/suppdata/p2/b2/b201796b/

Reactions of methyl substituted diphenylamine cation radicals in acetonitrile were observed using an electron transfer stopped-flow (ETSF) method. In the reactions of the 4-methylphenyl(phenyl)amine cation radical (4M–DPA˙+), the main reaction route was the formation of the benzidine dimer, which is similar to the case of the diphenylamine cation radical (DPA˙+). The dimerization rate of 4M–DPA˙+ was 2.3 × 104 M−1 s−1, which was slower than for DPA˙+ (dimerization rate 1.0 × 106 M−1 s−1), due to the 4-methyl substituent. Also, in the cases of 4-methyl and 4,4′-dimethyl DPA˙+, the formation of the cyclized dimer compounds was inferred from the presence of a large excess of the neutral molecules acting as a base. In contrast, the formation of the cyclized dimer compounds was characterized for 3-methyl- and 3,3′-dimethyl substituted DPA˙+ using cyclic voltammetry. In the ETSF method, the dimerization reaction was determined to be second-order in the cation radical and totally independent of the neutral molecule. The dimerization rates were ca. 1.0 × 107 M−1 s−1, which are faster than the reaction of DPA˙+. In spite of the complex pathway of the cyclization reaction, the 3-methyl substituent was found to promote the reaction between the 6-position of the phenyl ring and the nitrogen molecule of 3M–DPA˙+ or 3,3′M–DPA˙+.

Electron-Transfer Stopped-Flow Method: Its Validity for Spectrochemical Analysis of Electrogenerated Cation Radicals

The reaction of 4-bromo-N,N-dimethylaniline cation radical was studied using an electron-transfer stopped-flow (ETSF) method. In the ETSF method, was generated via the electron-transfer reaction between DMABr and tris (p-bromophenyl)amine cation radical. Thus, the influence of the concentration of DMABr on the reaction of was easily observed by controlling the concentrations in the mixed solutions. As a consequence, diverse changes in the absorption spectra were observed involving the formation of the cation radical and dication of the dimer compound, tetramethylbenzidine (TMB), depending on the concentration of DMABr. When only was formed, the reaction was assigned to the simple formation of This reaction was first order in and the rate constant was determined to be 0.20 s−1. In contrast, when a large excess of DMABr was also present, the rapid and quantitative formation of was observed via the reaction expressed by The validity of the ETSF method for spectrochemical analysis of electrogenerated cation radicals is demonstrated by analyzing the present reactions. © 2001 The Electrochemical Society. All rights reserved.

Mechanistic discrimination of the reaction of 1-aminopyrene cation radical using an electron transfer stopped-flow method. Decay reaction accelerated by neutral molecules

Using an electron transfer stopped-flow (ETSF) method, the decay kinetics of 1-aminopyrene cation radical (AP + ) in acetonitrile was studied. When AP + was formed quantitatively via the electron transfer with tris( p-bromophenyl)amine (TBPA) cation radical, it was found that the decay reaction proceeded according to a rate law of −d[AP + ]/d t= k[AP + ] 2, indicating the cation radical–cation radical coupling (RRC) mechanism. In contrast, in the presence of AP, the decay reaction was accelerated remarkably, and the kinetics has been changed to adhere to a rate law of −d[AP + ]/d t= k[AP + ] 2[AP], which indicated the two step interactions as follows: AP + + AP⇌( AP) 2 + ( AP) 2 + + AP + →( AP + ) 2+ AP While the reactions of aromatic amine cation radicals have been revealed as the RRC mechanism mainly, the present type of interaction can be regarded as a possible reaction mechanism of cation radicals, and would be important to control the formation of conducting polymer materials. The ETSF method has permitted the clarification of the effect of the neutral molecules on the reaction kinetics of cation radicals, which would be very difficult to reveal using electrochemical techniques.

Spectroscopic detection and kinetic analysis of short-lived aromatic amine cation radicals using an electron transfer stopped-flow method

Using an electron transfer stopped-flow (ETSF) method, the reactions of short-lived methyldiphenylamine and diphenylamine cation radicals (MDPA˙+ and HDPA˙+) in acetonitrile (AN) were analysed by generating them quantitatively by electron transfer with tris(p-bromophenyl)amine cation radical (TBPA˙+). The absorption spectra of short-lived MDPA˙+ and HDPA˙+ could be successfully observed in a time frame of less than a few tens of milliseconds using a rapid-scan spectrophotometer, and as a result, the kinetics of the dimerization reaction could be analyzed in spite of the follow-up oxidation reactions of the dimer compounds, which are affected by the concentration of the neutral monomer. Both reactions are found to be second order in concentration of cation radicals, and the dimerization rate constants were determined to be 1.4 × 106 M−1 s−1 for MDPA˙+, and 1.0 × 106 M−1 s−1 for HDPA˙+. Compared with electrochemical methods, the ETSF method permits direct detection and straightforward kinetic analysis of short-lived cation radicals, as well as the elucidation of the effect of precursor molecules on subsequent reactions.

A concept of an electron transfer stopped-flow method

A new concept of an electron transfer stopped-flow (ETSF) method is proposed. In the ETSF method, unstable cation radicals are formed via the electron transfer reactions with stable cation radicals whose oxidation potential is positive to that of unstable ones. Thus, by combining the electron transfer reactions with the stopped-flow operation, absorption spectroscopic measurements for short-lived cation radical become possible with a reduced dead time from the point of the generation of cation radicals to the point of the optical measurement, in particular, with a help of a rapid-scan spectrophotometer. By mixing an acetonitrile (AN) solution of tris(p-bromophenyl)amine cation radical (TBPA ⋅+) with an AN solution of methyldiphenylamine (MDPA) or diphenylamine (HDPA), the kinetic analyses could be carried out successfully for the dimerization reactions of MDPA ⋅+ and HDPA ⋅+ as well as the observations of their absorption spectra. It was found that fast reactions whose second order rate constants are around 10 7 M −1s −1 can be analyzed using the ETSF method.