Substituted Fullerenes for Enhanced Optical Nuclear Hyperpolarization in Random Orientations

Polarized electron spins in photoexcited triplet states enable dynamic nuclear polarization (DNP) at high temperatures, leading to increased sensitivity in magnetic resonance imaging (MRI). For a practical nuclear polarization of 10%, single crystals must be precisely oriented in a magnetic field to generate electron spin resonances in a narrow field range that satisfies resonance conditions for polarization transfer. However, this is not realistic for medical applications. Substituted fullerenes as triplet polarizing agents have enabled 1H polarizations above 10%, even for random molecular orientations. They have not been used as polarizing agents for triplet-DNP because of electron spin relaxation via pseudo-rotation. Here, the spin-lattice relaxation time was significantly increased by chemical modification of two sites on C60 fullerenes. Symmetry considerations revealed fullerenes that avoided pseudo-rotations. Di-substituted fullerenes were ideal polarizing agents with sharp linewidths and long relaxation times that enabled 14.2% 1H polarization in randomly oriented orientations. Optimized polarizing agents would enable ultra-sensitive MRI medical diagnostics under mild preparation conditions.