Fourier transform-electron paramagnetic resonance spectroscopy of correlated radical pairs

Utilizing the high time and phase resolution of Fourier transform electron paramagnetic resonance, the buildup and decay kinetics of the transient radical pair Zn tetraphenylporphyrin+/Duroquinone− in ethanol could be studied by detecting the dispersive component in the Fourier transformed free induction decays of the fully separated radical anions. The dispersive line components originate from the exchange coupling J of the radical pair in its metastable state trapped by an attractive temperature dependent Coulomb interaction U(rc,T) at an equilibrium distance rc. The decay rate constant kd into free ions was studied over the temperature range of 225–298 K and varied from kd=3.2×106 to 10×106 s−1. This change in kd could be fully rationalized in terms of the temperature dependence of the solvent dielectric constant (leading to a variation of the cage potential from 0.07 to 0.12 eV) and the diffusion constant. The buildup rate constants were consistent with the assumption of a diffusion-controlled electron transfer process as a primary step.