Kinetics and Energetics of Electron Transfer to Dimer Radical Cations

Area of Science:

• Physical Chemistry
• Photochemistry
• Spectroscopy

Background:

• Dimer cations play a crucial role in charge transport phenomena.
• Understanding the energetics and dynamics of dimer cations is essential for molecular electronics and materials science.

Purpose of the Study:

• To investigate the spectral properties and charge transfer dynamics of naphthalene dimer cation (Nap2•+) and ethene dimer cation (Ethene2•+) in dichloromethane.
• To determine the thermodynamic stability of dimer cations and elucidate the factors influencing their charge transfer rates.

Main Methods:

• UV-Vis absorption spectroscopy was employed to measure the spectra of dimer cations.
• Charge transfer reactions were studied to determine rate constants and reaction energetics.
• Dimerization equilibrium constants were used to calculate Gibbs free energy of dimerization (ΔGd°).

Main Results:

• Spectra of Nap2•+ and Ethene2•+ peaked at 1.2 eV and 3.3 eV, respectively.
• Dimerization stabilizes Nap2•+ by ΔGd° = -218 meV.
• Charge transfer rate constants for both dimers rose more gradually with reaction energetics compared to previously studied systems.
• Hole transfer from Nap2•+ to phenanthrene was significantly slower than from biphenyl•+ monomer to naphthalene, despite similar reaction free energy.

Conclusions:

• The observed slower charge transfer rates are attributed to a significant reorganization energy (λ(M2)) associated with the separation of dimer partners.
• Dimerization of ethene is likely more stable and involves a larger reorganization energy than naphthalene dimerization.
• These findings provide insights into the fundamental processes governing charge transport in molecular dimers.