Triplet J-Driven DNP─A Proposal to Increase the Sensitivity of Solution-State NMR without Microwave

Dynamic nuclear polarization (DNP) is an important method to enhance the limited sensitivity of nuclear magnetic resonance (NMR). Using the existing mechanisms such as Overhauser DNP (ODNP) is still difficult to achieve significant enhancement of NMR signals in solutions at a high magnetic field. The recently proposed J-driven DNP (JDNP) condition (when the exchange interaction J_ex of two electron spins matches the electron or the nuclear Larmor frequency ω_E and ω_N) may enable signal enhancement in solution as it requires only dipolar interaction between the biradical polarization agent and the analyte. However, likewise ODNP, the current JDNP strategy still requires the saturation of the electron polarization with high microwave power which has poor penetration and is associated with heating effects in most liquids. The replacement of high-power microwave irradiation is possible if the temporal electron polarization imbalance is created by an electron electromagnetic (EM) irradiation at different wavelengths such as the visible light. Here, we propose a triplet-JDNP mechanism which first exploits the light-induced singlet fission process (i.e., a singlet exciton is converted into two triplet excitons). As the JDNP condition J_ex ≈ ± ω_E is fulfilled, a triplet-to-triplet cross-relaxation process will occur with different rates and consequently lead to the creation of hyperpolarization on the coupled nuclear spin states. This communication discusses the theory behind the triplet-JDNP proposal, as well as the polarizing agents and conditions that will enable the new approach to enhance NMR's sensitivity without the need of microwave irradiation.