The Dual Roles of Phenylenediamines: Using their Voltammetric Behavior to Measure the Hydrogen Donor and Acceptor Abilities of Alcohols in Acetonitrile

Cyclic voltammetry experiments on 2,3,5,6-tetramethyl-1,4-phenylenediamine (P) in acetonitrile in the presence of varying concentrations of alcohols indicate that the oxidized forms of the compound (P^.+ and P²⁺ ) interact with the alcohols through a hydrogen-bonding mechanism where P^.+ and P²⁺ act as the hydrogen donor and the alcohols act as acceptors. However, the neutral form (P) largely acts as a hydrogen acceptor but only for strong hydrogen donors that do not undergo proton-transfer reactions with the phenylenediamine. These results were ascertained based on measuring the difference in potential of the two one-electron transfer reactions (ΔE_P^ox(1, 2) =|E_P^ox(1) -E_P^ox(2) |) in the oxidative electrochemistry of P, which thereby allows a simple measure of relative hydrogen bonding strengths.

Using Voltammetry to Measure the Relative Hydrogen-Bonding Strengths of Pyridine and Its Derivatives in Acetonitrile

The voltammetric behavior of 2,3,5,6-tetramethyl-1,4-phenylenediamine was found to be able to differentiate the hydrogen acceptor abilities of electroinactive pyridine compounds in acetonitrile. Weak and strong hydrogen acceptors were distinguished through the onset of a third oxidation process that came about at sub-stoichiometric amounts of strong hydrogen acceptors, but not in the presence of weak hydrogen acceptors. This additional oxidation reaction occurred at a potential between the two 1 e^- -oxidation reactions that phenylenediamines typically undergo (i.e. E_P^ox(1) <E_P^ox(3) <E_P^ox(2) , with E_P^ox(1) and E_P^ox(2) representing the electrochemical conversion of the neutral phenylenediamine into the radical cation and thereafter to the quinonediimine dication) as well as at the expense of the second electrochemical reaction. E_P^ox(2) and E_P^ox(3) were observed to shift towards less positive potentials with increasing concentrations of weak or strong hydrogen acceptors, respectively, whereas E_P^ox(1) remained virtually unaffected. This allows the electrochemical parameters ΔE_P^ox(1, 2) =|E_P^ox(1) -E_P^ox(2) | and ΔE_P^ox(1, 3) =|E_P^ox(1) -E_P^ox(3) | to be employed as measures of the hydrogen-bonding strengths within each category, to which they were found to be highly reproducible and responsive to steric, electronic, inductive, and mesomeric effects. The electrochemical findings concur with available aqueous pK_a data of the protonated pyridine compounds but were, however, in poor agreement with results obtained by density functional theory calculations.

Nano-impacts of bifunctional organic nanoparticles

The synthesis and characterization of Oil Blue Dye nanoparticles is reported along with their use for nano-impacts experiments in aqueous solution. The latter reveal current spikes corresponding to quantitative two electron reductions due to the reduction of the quinone groups and quantitative two electron oxidations from the 1,4-phenylenediamine groups presented in the Oil Blue Dye molecules within the nanoparticles. In both cases, the oxidation or reduction leads to size distributions in good agreement with independent measurements made using dynamic light scattering showing that the redox events accompanying the nano-impacts lead to the full dissolution of the nanoparticles.