Optically-generated Overhauser dynamic nuclear polarization: A numerical analysis

Abstract

Recently, an alternative approach to dynamic nuclear polarization (DNP) in the liquid state was introduced, using optical illumination instead of microwave pumping. By exciting a suitable dye to the triplet state which undergoes diffusive encounter with a persistent radical forming a quartet-doublet pair in the encounter complex, dynamic electron polarization (DEP) is generated via the radical-triplet pair mechanism (RTPM). Subsequent cross-relaxation generates nuclear polarization without the need for microwave saturation of the electronic transitions. Here, we present a theoretical justification for the initial experimental results, by means of numerical simulations. These allow investigation of the effects of various experimental parameters, such as radical and dye concentrations, sample geometry, and laser power, on the DNP enhancement factors, providing targets for experimental optimization. It is predicted that reducing the sample volume will result in larger enhancements, by permitting a higher concentration of triplets in a sample of increased optical density. We also explore the effects of pulsed laser rather than continuous-wave illumination, rationalising the failure to observe the optical DNP effect under illumination conditions common to DEP experiments. Examining the influence of illumination duty cycle the conditions necessary to permit use of pulsed illumination without compromising signal enhancement are determined, which may reduce undesirable laser heating effects. This first simulation of the optical DNP method therefore underpins the further development of the technology. I. INTRODUCTION Among all of the hyperpolarization techniques for overcoming the intrinsically low sensitivity of Nuclear Magnetic Resonance (NMR) spectroscopy, Dynamic Nuclear Polarization (DNP) is the most studied and widely applicable. 1 It was first proposed by Overhauser in 1953, 2 and demonstrated experimentally by Carver and Slichter, 3 that saturating Electron Paramagnetic Resonance (EPR) transitions results in an increase in NMR sensitivity. This effect, due to cross-relaxation between the electron and nuclear spins, was later extended from lithium to free radicals in solution, 4 but has never been widely applied due to technical challenges and problems with reducing Overhauser efficiency at the high magnetic fields necessary for high resolution NMR spectroscopy.

Viscosity effects on optically generated electron and nuclear spin hyperpolarization

“…In order to extract information on the efficiency of generation of electron spin-polarization the time evolution of the magnetization may be modelled using a kinetic approach based on the Bloch equations. 20,22,23 While previous work used a simple kinetic scheme we have found that excellent agreement between simulations and experiment can be obtained by using a more detailed photochemical reaction scheme arising from a recent comprehensive study of the photochemistry of Rose Bengal in water. 24 The full scheme is as follows: Dashed lines are fits to the data.…”

“…The scope for maintaining enhancements using a pulsed laser to produce much lower duty cycle trains of pulses has been discussed elsewhere and remains a target for future experimental investigation. 23 As noted elsewhere the build-up time for the optical enhancement is expected to be governed by the nuclear relaxation time T 1n , 23,37 values for which are given in Table 2. While one might therefore expect maximal polarization to be achieved with illumination for B5T 1n , corresponding to 3.5-7.0 s in the present case, illumination longer than 4 s reduced the enhancement obtained.…”

“…Details of the spectral density function used and discussion of this choice are provided elsewhere. 23 Considering the leakage and coupling factors both reduce on addition of glycerol it is clear that the increased NMR enhancement must arise from a significantly increased saturation factor. By rearranging eqn (14) we determined the effective saturation factor in optical DNP experiments (Fig.…”

Dynamic Electron Polarization Lasting More Than 10 μs by Hybridizing Porphyrin and TEMPO with Flexible Linkers

“…An alternative approach to facilitate this encounter is the "hybridized" system, where dyes and radicals are connected by covalent or noncovalent bonds. 34−56 This is promising for photodriven DNP [30][31][32][33][34][35]57 and EPR imaging. 58 The derivatives of quinones, 35−37 polyaromatics, 38−41 fullerene, 34,42 BODI-PY, 43 porphyrins, 44−52 and phthalocyanine 53−56 have been used as the chromophores for these hybridized systems.…”

“…The simplest system for inducing DEP is a solution of “unlinked” dyes and radicals. − However, since the encounter between the photoexcited species and the radicals is the rate-limiting step, intense laser irradiation and high concentration of dyes and radicals are necessary to obtain large DEP. An alternative approach to facilitate this encounter is the “hybridized” system, where dyes and radicals are connected by covalent or noncovalent bonds. − This is promising for photodriven DNP − , and EPR imaging . The derivatives of quinones, − polyaromatics, − fullerene, , BODIPY, porphyrins, − and phthalocyanine − have been used as the chromophores for these hybridized systems.…”