Development and Application of NMR Signal Enhancement Technique by using Laser-Excited Triplet State

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In NMR, the low frequency leads to low sensitivity. This is the biggest remaining problem of NMR.We are investigating a possibility of improving the sensitivity using dynamic nuclear polarization (DNP). We transfer the big polarization of electron spins in a photo-excited triplet state to nuclear spins by cross polarization. Comparing with DNP by paramagnetic species, this approach has a potential for obtaining much larger polarization.Firstly, we exmained 13.6 MHz proton NMR for a single crystal (18 mg) of perdeuterated naphthalene (the residual proton content is 0.8%) doped with 0.015 mo1% pentacene. The pentacene molecules in the naphthalene lattice are photo-excited to the triplet state, from which the electron polarization is trabsfered to the ^1H spins. We could achieve the polarization up to 67.5% by DNP, which is essentially the same with the electron polarization of about 63%. Thus, the theoretically maximum proton polarization has been attained for the first time. The proton … More polarization is 3.4×10^5 times as large as the thermal equilibrium value. The buildup time constant is 420 s, being much shorter than that (7430 s) in normal naphthalene.Secondly, to examine the applicability of this approach for a powdered sample, we undertook a similar expariment for a powdered naphthalene sample doped with pentacene. For perdeuterated naphthalene, the proton polarization reaches to 10000 times as large as the thermal equilibrium value at 100 K.For non-deuterated naphthalene, the gain is ca. 3000 at 100 K.These result show that this approach may be hopeful to obtain high NMR sensitivity in interesting systems by applying the DNP technique to interesting guest/host systems or by transferring the big polarization created in such a system as pentacene-doped naphthalene to initially unpolarized interesting systems via the surfaces. As an interesting example of applications, the present DNP technique may be used to greatly improve the NMR sensitivity of a system used for NMR quantum computing and to initialize the computer. Less