High-frequency dynamic nuclear polarization using biradicals: a multifrequency EPR lineshape analysis

Area of Science:

• Magnetic Resonance
• Chemical Physics
• Materials Science

Background:

• Dynamic Nuclear Polarization (DNP) experiments at high magnetic fields rely on cross-effect (CE) and thermal mixing (TM) mechanisms for signal enhancement.
• These mechanisms depend on electron-electron interactions, specifically interelectron distance (R) and electron paramagnetic resonance (EPR) frequency separation.
• Biradicals, molecules with two electron spins, offer a way to control these parameters.

Purpose of the Study:

• To investigate the effect of tether length and orientation in bis-TEMPO biradicals on DNP enhancement.
• To optimize biradical structures for efficient CE and TM mechanisms in DNP.

Main Methods:

• Synthesis of a series of bis-TEMPO biradicals (BTnE) with varying ethylene glycol tether lengths.
• Characterization of interelectron distances and g-tensor orientations using 9 and 140 GHz continuous-wave EPR.
• Measurement of DNP enhancements at 90 K and 5 T for monomeric TEMPO and synthesized biradicals.

Main Results:

• DNP enhancement increased with decreasing tether length in BTnE biradicals.
• BT2E biradical with R ≈ 13 Å showed a DNP enhancement of ~175, compared to ~40 for monomeric TEMPO (R ≈ 56 Å).
• Biradicals with shorter, rigid tethers (BT2E, TOTAPOL) exhibited the largest DNP enhancements, satisfying the CE matching condition.

Conclusions:

• Molecular design of biradicals, controlling interelectron distance and orientation, is crucial for maximizing DNP enhancement.
• Shorter tethers and specific TEMPO orientations in biradicals effectively optimize CE and TM mechanisms.
• The findings are applicable to DNP experiments on biological systems, particularly with water-soluble biradicals like TOTAPOL.