Electron spin relaxation in radical pairs: Beyond the Redfield approximation

Area of Science:

• Chemical Physics
• Quantum Chemistry
• Spectroscopy

Background:

• Relaxation processes significantly influence spin-selective electron transfer reactions in radical pairs.
• Current models often employ phenomenological superoperators or Bloch-Redfield-Wangsness theory for microscopic relaxation mechanisms.

Purpose of the Study:

• To present an alternative perturbative relaxation theory based on the Nakajima-Zwanzig equation.
• To combine the Nakajima-Zwanzig approach with the Schulten-Wolynes semiclassical method for efficient modeling.
• To investigate spin-dependent electron transfer and intersystem crossing in dimethyljulolidine-naphthalenediimide (DMJ-NDI) radical ion pairs.

Main Methods:

• Application of the Nakajima-Zwanzig equation as a perturbative relaxation theory.
• Integration with the Schulten-Wolynes semiclassical method for simulating spin dynamics.
• Analysis of spin-dependent electron transfer and intersystem crossing in DMJ-NDI radical ion pairs.

Main Results:

• The Nakajima-Zwanzig equation avoids the positivity problem of Redfield theory in the static disorder limit.
• An efficient method was developed for modeling spin dynamics with numerous hyperfine-coupled nuclear spins.
• Simulations revealed a field-independent contribution to triplet quantum yields in DMJ-NDI radical ion pairs.

Conclusions:

• The developed method offers advantages over Redfield theory for modeling radical pair spin dynamics.
• The observed field-independent contribution suggests mechanisms beyond simple electron spin relaxation.
• This work provides a more robust framework for understanding complex spin-dependent reactions.