The role of solvent reorganization dynamics in homogeneous electron self-exchange reactions

Abstract

The homogeneous electron self-exchange reaction of N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPPD/TMPPD^˙+) shows the classical solvent dependence according to Marcus' theory. The rate constants are a linear function of γ=(1/n²–1/ε)(n= refractive index, ε= dielectric constant of the solvent). For 0.27 ⩽γ⩽ 0.53, log k_ex= const. ×γ. However, the reactions of tetracyanoquinonedimethane (TCNQ/TCNQ^˙–) and tetracyanoethylene (TCNE/TCNE^˙–) are dominated by the solvent dynamics, including the longitudinal relaxation time τ_L of the solvent. This is expressed by log (k_exτ_Lγ^–1/2)= const. ×γ, for 0.05 ⩽γ⩽ 0.53. The role of solvent reorganization dynamics is discussed together with the decision equation of Ovchinnikova.