Switching from Transition-State Theory to Solvent-Controlled Adiabatic Charge-Transfer Regime in Bis-Triarylamine Mixed Valence Radical Cations by Modification of the Bridge Electron Density

For a series of organic intervalence charge-transfer (IVCT) radical cations where two triarylamine redox centers are linked via donor-/acceptor-substituted fluorene bridges, the influence of bridge electron density on the thermally activated hole transfer and the associated energy barrier Δ<i>G</i>* was investigated. Comparison of barrier heights obtained from the two-state Mulliken Hush (MH) theory and from potential energy surfaces constructed by the three-state-generalized Mulliken Hush theory (GMH theory) revealed the influence of an accessible bridge-centered hole-transfer state on the activation barrier Δ<i>G</i>*. Comparison of these values with averaged barrier heights obtained from temperature-dependent EPR spectroscopy in DCM showed a continuous decrease of barrier height for electron-rich bridges. This was accompanied by a charge-transfer regime transition from the transition-state theory to a solvent-dynamic-controlled adiabatic charge-transfer regime in DCM. These findings show a strong dependence of hole-transfer dynamics and energy barriers on the electronic structure of the bridge and provide a guideline for tuning IVCT systems by a rational molecular design.