Investigation of solvent dynamic effects on the electron self-exchange in two thianthrene couples with large inner reorganization energies

Abstract

The large structural difference between thianthrene radical cations and their neutral parent molecules can possibly affect their electron self-exchange reactions. Before this can be investigated experimentally, it is necessary to first understand the influence of the solvent on such electron transfer reactions. To achieve this, the rate constants of the electron self-exchange reactions of the Th˙⁺/Th and MTh˙⁺/MTh (Th = thianthrene, MTh = 2,3,7,8-tetramethoxythianthrene) couples were investigated by means of ESR line broadening experiments in different solvents at 293 K. The diffusion corrected rate constants cover a range of 7.2 × 10⁸ ≤ k_et ≤ 44 × 10⁸ M⁻¹ s⁻¹ for Th˙⁺/Th and 2.0 × 10⁸ ≤ k_et ≤ 11.6 × 10⁸ M⁻¹ s⁻¹ for MTh˙⁺/MTh, respectively. The results were analysed within the framework of the Marcus Theory and the characteristic reorganization energy, λ, was determined. Both couples clearly show a solvent dynamic effect controlled by the longitudinal relaxation time τ_L of the solvents. However, the influence of the structural changes, in terms of λ, was smaller than expected at room temperature.