Electron transfer driven by proton fluctuations in a hydrogen-bonded donor-acceptor assembly

Area of Science:

• Chemical Physics
• Photochemistry
• Molecular Dynamics

Background:

• Proton-coupled electron transfer (PCET) is crucial for many chemical and biological processes.
• Understanding the temperature dependence of PCET is key to elucidating reaction mechanisms.
• Molecular dyads provide model systems for studying electron and proton transfer events.

Purpose of the Study:

• To investigate the temperature-isotope dependence of PCET in a noncovalent molecular dyad.
• To analyze the transition from reverse to normal kinetic isotope effects.
• To correlate observed isotope effects with underlying charge-transfer dynamics.

Main Methods:

• Utilized a molecular dyad comprising an excited-state Zn(II) porphyrin donor and a naphthalene diimide acceptor.
• Studied electron transfer across an amidinium-carboxylate interface.
• Measured temperature-dependent kinetic isotope effects (kH/kD) over a range of temperatures.

Main Results:

• Observed a reverse kinetic isotope effect (kH/kD < 1) below 160 K, with kH/kD = 0.9 at 120 K.
• Observed a normal kinetic isotope effect (kH/kD > 1) above 160 K, with kH/kD = 1.2 at 300 K.
• Identified a smooth transition with a crossover temperature around 160 K.

Conclusions:

• The temperature-dependent isotope effects are consistent with charge-transfer dynamics influenced by bath-induced fluctuations.
• Fluctuations in the proton coordinate play a significant role in modulating PCET rates.
• The study provides insights into the complex interplay of electron transfer, proton transfer, and environmental factors.