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

Charge transfer processes in conjugated triarylamine-oligothiophene-perylenemonoimide dendrimers

The synthesis and charge transfer properties of triarylamine-oligothiophene-perylenemonoimide dendrimers, TPA(T2-PMI)3 and TPA(T4-PMI)3, are described. The fluorescence quantum yields indicate strong emission quenching by electron transfer [phi(THF) = 0.004 for TPA(T2-PMI)3, phi(THF) = 0.003 for TPA(T4-PMI)3, and phi(THF) = 0.8 for PMI]. Moreover, with the increase of the solvent polarity, the quantum yields decrease indicating that the A+* D-* (acceptor/donor) couple is more stabilized. The femtosecond transient absorption spectra show a very fast charge separation process (approximately 2 ps; k(cs) approximately 5 x 10(11) s(-1)) and a charge recombination of more than 1 order of magnitude slower (approximately 50 ps; k(cr) approximately 2 x 10(10) s(-1)), as observed from the rise time and decay of the radical anion and radical cation absorption bands. The analysis of the transient absorption spectroscopy and of the energetics of the process using Marcus theory indicates that in the electron transfer process the thiophene unit is the first electron donor. The triarylamine is not functioning as a second electron donor, as also substantiated by the absence of an effect of the addition of acid on the emission intensity of the dendrimers. The presence of the triarylamine and/or the proximity of the oligothiophenes does improve the donor capabilities of the oligothiophene unit slightly and enhances its conjugation as seen in the absorption spectra and the transients of the radial cations. These results can be used for a better design of molecular materials for, e.g., photovoltaic applications.

Voltammetric and Electrochemical ESR Studies of Oxidation Reactions Mediated by Tris(4-bromophenyl)amine in Acetonitrile

The electrochemical oxidation of tris(4-bromophenyl)amine in the presence of 2,6-lutidine is examined in acetonitrile. Voltammetric and spectroscopic investigations suggest that the electrogenerated triaryl aminium radical cation oxidizes 2,6-lutidine in an EC' mechanism, and an equilibrium constant for this homogeneous electron transfer is estimated. The mediated oxidation of a protected phenyl selenoglycoside by this reaction mixture is studied by the use of electrochemical ESR, employing a tubular flow cell, and signal intensity data is found to be consistent with the proposed mechanism, allowing the determination of kinetic parameters by computational simulation. Products of the mediated glycoside oxidation are determined by proton NMR and mass spectrometry.

Ionization of Aniline and Its N-methyl and N-phenyl Substituted Derivatives by (Free) Electron Transfer to n-butyl Chloride Parent Radical Cations

The electron transfer from aniline and its N-methyl as well as N-phenyl substituted derivatives (N-methylaniline, N,N-dimethylaniline, diphenylamine, triphenylamine) to parent solvent radical cations was studied by electron pulse radiolysis in n-butyl chloride solution. The ionization results in the case of aniline (ArNH2) and the secondary aromatic amines (Ar2NH, Ar(Me)NH) in the synchronous and direct formation of amine radical cations, as well as aminyl radicals, in comparable amounts. Subsequently, ArNH2*+ deprotonates in a delayed reaction with the present nucleophile Cl-, and forms further ArNH*. In contrast, tertiary aromatic amines such as triphenylamine and dimethylaniline yield primarily the corresponding amine radical cations Ar3N*+ or Ar(Me2)N*+, only. The persistent Ar3N*+ forms a charge transfer complex (dimer) with the parent amine molecule, whereas Ar(Me2)N*+ deprotonates to carbon-centered radicals Ar(Me)NCH2*.