Substituted Fullerenes for Enhanced Optical Nuclear Hyperpolarization in Random Orientations

Polarized electron spins in photoexcited triplet states enable dynamic nuclear polarization (DNP) to enhance magnetic resonance imaging (MRI) sensitivity. For a practical nuclear polarization of 10%, single crystals must be precisely oriented in a magnetic field to generate narrow electron spin resonance lines, which is not appropriate for actual medical applications. Here, substituted fullerenes as triplet polarizing agents enable ¹H polarizations above 10%, even for random molecular orientations. While they have not been used as polarizing agents for triplet-DNP because of electron spin relaxation via pseudo-rotation, we overcome this by chemical modification of two sites on C₆₀ fullerenes. Symmetry considerations reveal fullerenes that avoided pseudo-rotations. Di-substituted fullerenes are ideal polarizing agents with sharp linewidths and long relaxation times that enabled 14.2% ¹H polarization in randomly oriented orientations. Optimized polarizing agents represent a promising approach for ultra-sensitive MRI medical diagnostics without the need for DNP under cryogenic temperatures and severe orientation control.