Activationless Multiple-Site Concerted Proton-Electron Tunneling.

Area of Science:

• Photochemistry
• Molecular Biophysics
• Quantum Chemistry

Background:

• Proton and electron transfer are fundamental to energy conversion and storage processes.
• Understanding and controlling these charge transfer mechanisms are critical for advancing technologies like artificial photosynthesis and fuel cells.

Purpose of the Study:

• To design and investigate a novel molecular system capable of fast, photoinduced multiple-site concerted proton-electron transfer (MS-CPET).
• To elucidate the mechanism and temperature dependence of this MS-CPET process, focusing on the roles of proton and electron tunneling.

Main Methods:

• Design of an anthracene-phenol-pyridine molecular triad.
• Utilizing fluorescence quenching and transient absorption spectroscopy.
• Conducting experiments across a wide temperature range (5.5 K to 350 K) in solutions and glasses.

Main Results:

• The designed molecular triad exhibits rapid MS-CPET with a rate constant of 3.2 × 10^10 s^-1 at 298 K.
• The reaction rate and kinetic isotope effect (KIE) are temperature-independent from 5.5 K to 90 K, indicating zero Arrhenius activation energy.
• Minimal temperature dependence observed from 145 K to 350 K, suggesting a dominant tunneling mechanism.

Conclusions:

• The observed MS-CPET reaction proceeds primarily through quantum tunneling of both protons and electrons in distinct directions.
• The rate constant directly reflects the probability of simultaneous proton and electron double tunneling.
• This study provides a molecular platform for exploring efficient, thermally independent charge transfer relevant to energy conversion and storage.