Monitoring ionic adducts to elucidate reaction mechanisms: Reduction of tetracyanoquinodimethane and oxidation of triphenylamine investigated using on-line electrochemistry/electrospray mass spectrometry

The products of the electrochemical reduction of 7,7′,8,8′- tetracyano-p-quinodimethane (TCNQ) and the electrochemical oxidation of triphenylamine (TPA) were studied using a thin-layer flow cell coupled upstream on-line with electrospray mass spectrometry. Linear sweep voltammetry was used to generate intermediates and products of reduction/oxidation that were monitored by mass spectrometry in negative/positive ion mode, respectively. During reduction of TCNQ the potential dependence of the radical anion, the single-charge and the double-charge charge-transfer complexes and the double-charge anion were determined. The data provided direct evidence that following the first electron transfer practically all radical anions turn into the [TCNQ ₂ ^2- +Li⁺]^- adduct. The adduct formation could be observed also in case of the double-charge anion, forming [TCNQ^2-+Li⁺]^- during/after the second electron transfer. Similarly, in the case of TPA the potential dependence of the radical cations of the monomer/dimer and the double-charge dimer were evaluated. Results on TPA oxidation suggested the formation of [TPA-H] ⁺ that can originate from TPA^•+ by consecutive proton and electron loss. The existence of [TPA-H]⁺ was confirmed by close inspection of the mass spectrometric peak shapes around m/z 244 and by simulation and mathematical evaluation of the measured data. However, monitoring the adducts of doubly (multiply) charged ions with oppositely charged ion(s) resulting in single-charge ions has proven to be a useful method to get relevant information about the doubly (multiply) charged ions when mass signal interference occurs between double-charge dimer and single-charge monomer molecules that have very close or equal m/z values. Moreover, in each case (reduction and oxidation) the results proved again that the electrochemistry/ electrospray mass spectrometry technique is capable of monitoring reactions with complex reaction paths.