Nonpentacene Polarizing Agents with Improved Air Stability for Triplet Dynamic Nuclear Polarization at Room Temperature. 
Triplet dynamic nuclear polarization (triplet-DNP), a method to enhance nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) sensitivity using photoexcited triplet electrons, has great potential to hyperpolarize nuclear spins at room temperature.  Since the first report of room-temperature triplet-DNP in 1990, pentacene has been the only and best option of triplet polarizing agent.  However, the poor air stability of pentacene has severely limited the applicability of triplet-DNP.  We report the first example of polarizing agents with significant air stability as well as high polarizing ability comparable to pentacene.  The introduction of electron-withdrawing diaza-substitution to pentacene and tetracene reduces the lowest unoccupied molecular orbital level and endows much improved stability under the ambient conditions.  Importantly, the diaza-substituted pentacene and tetracene offer similar, or even slightly better, 1H NMR signal enhancement compared with pentacene in the prototypical triplet-DNP test using p-terphenyl crystals.  This work removes one of the largest obstacles toward the application of triplet-DNP for the hyperpolarization of biological molecules.