Trityl biradicals and 13C dynamic nuclear polarization

Area of Science:

• Magnetic Resonance
• Chemical Physics
• Materials Science

Background:

• Dynamic Nuclear Polarization (DNP) enhances nuclear spin polarization for improved Magnetic Resonance Imaging (MRI) sensitivity.
• Trityl radicals are commonly used polarizing agents in DNP, but their efficiency can be limited.
• Investigating novel radical structures, like biradicals, is crucial for advancing DNP techniques.

Purpose of the Study:

• To evaluate the efficiency of novel trityl biradicals for Dynamic Nuclear Polarization (DNP) of low gamma nuclear spins.
• To compare the DNP performance of trityl biradicals against a trityl monoradical at low temperatures (approx. 1 K).
• To understand the impact of radical structure and concentration on DNP efficiency and relaxation times.

Main Methods:

• Synthesis and characterization of three novel trityl biradicals with varying sizes.
• Electron Paramagnetic Resonance (EPR) spectroscopy (X-band and W-band) to determine intramolecular electron-electron distances.
• Measurement of steady-state DNP levels and build-up times for [1-(13)C]pyruvic acid hyperpolarization at 3.35 T and 4.64 T.
• Analysis of electron longitudinal relaxation times (T(1e)) and (13)C longitudinal relaxation times.

Main Results:

• Trityl biradicals achieved similar maximal DNP levels as the monoradical, but the monoradical yielded twice the polarization.
• Optimal biradical concentrations were lower than the monoradical, but resulted in significantly longer polarization build-up times.
• Increasing magnetic field strength from 3.35 T to 4.64 T approximately doubled polarization for both biradicals and the monoradical.
• A considerable shortening of the (13)C longitudinal relaxation time was observed for biradicals, correlating with shorter build-up times but lower DNP levels.

Conclusions:

• Trityl biradicals do not offer improved DNP efficiency over monoradicals for the studied low gamma spins at low temperatures.
• Shorter (13)C relaxation times in biradicals, while beneficial for build-up, likely contribute to reduced overall DNP levels.
• Further research into radical design is needed to optimize DNP performance for specific applications.