Dimerization of thianthrene radical cations as studied by in situ ESR and UV-Vis-NIR voltammetry at different temperatures

Electrochemically generated thianthrene radical cations (Th+.) are shown to be reversibly transferred into dimers. The ratio of radical ion to dimer can be easily controlled by the thianthrene concentration and the temperature. Using temperature-dependent in situ UV-Vis spectroelectrochemistry the dimer formation was followed and superimposed optical spectra of the electrochemical reaction products were separated into those of the free radical and the dimer. Expanding the study to in situ ESR/UV-Vis-NIR spectroelectrochemical measurements at different temperatures the formation of the spinless dimer was proved and the extent of dimerized radical was determined quantitatively. Besides the sigma-dimer of thianthrene radical cation (Th+- Th+), the existence of a charge-transfer resonance structure (Th, Th)(+.) was detected in NIR spectra.