Marcus Relationship Maintained During Ultrafast Electron Transfer Across a Supramolecular Capsular Wall

Area of Science:

• Supramolecular Chemistry
• Photochemistry
• Electron Transfer

Background:

• Photoinduced electron transfer (PET) is crucial in chemical and biological processes.
• Controlling PET in confined environments is challenging but offers unique reactivity.
• Supramolecular hosts can pre-organize donors and acceptors for efficient electron transfer.

Purpose of the Study:

• To investigate PET across an organic capsular wall between encapsulated donors and external acceptors.
• To determine the rate constants and thermodynamic parameters of the electron transfer process.
• To understand the role of the confined environment and Coulombic attraction in PET.

Main Methods:

• Synthesis of an anionic supramolecular capsule to host a donor.
• Utilizing a cationic acceptor for Coulombic attraction and proximity to the donor.
• Kinetic measurements in aqueous buffer solution to determine rate constants.
• Analysis using Marcus theory to extract reorganization energy (λ) and electronic coupling (V_el).
• ¹H NMR spectroscopy to confirm donor-acceptor interaction.

Main Results:

• PET rate constants were measured in the range of 0.32–4.0 × 10¹¹ s^-1.
• Electron transfer occurred without diffusion due to the pre-organized structure.
• Marcus relation with inversion was observed, yielding λ = 1.918 eV and V_el = 0.0058 eV.
• The reorganization energy was primarily attributed to internal molecular factors, not solvent effects.
• ¹H NMR spectra indicated electronic interaction through the capsular wall.

Conclusions:

• The study demonstrates efficient, diffusion-free PET across an organic capsular barrier.
• The confined, nonpolar environment within the capsule influences the electron transfer dynamics.
• The results highlight the potential of supramolecular capsules for controlling molecular reactivity and probing radical ion behavior.