Determination of the intervalence charge-transfer rate in mixed-valence arylamino-fluorene derivatives using EPR spectroscopy

Thermally activated electron-transfer processes are of interest for a variety of theoretical and technological applications. In organic mixed-valence (MV) systems, the intramolecular charge transfer between two redox centers offers valuable insights about the kinetics and energetics of electron hopping. We investigated the intramolecular charge transfer process in a series of MV organic molecules where two arylamino units are linked together by a variable length bridge. The experimental investigation is carried out by means of Electron Paramagnetic Resonance (EPR) spectroscopy on MV radical cations. The EPR spectra, recorded in a series of solvents at different temperatures, were adequately simulated using a multiple-site random jump model implemented in a software routine, providing the intramolecular charge transfer rate as the output. The dependence of the free energy of the electron transfer process on the solvent allowed us to identify a correlation between the energetic of the charge transfer and some solvent properties.