Coherent manipulation of non-thermal spin order in optical nuclear polarization experiments

Abstract

Time resolved measurements of Optical Nuclear Polarization (ONP) have been performed on hyperpolarized triplet states in molecular crystals created by light excitation. Transfer of the initial electron polarization to nuclear spins has been studied in the presence of radiofrequency excitation; the experiments have been performed with different pulse sequences using different doped molecular systems. The experimental results clearly demonstrate the dominant role of coherent mechanisms of spin order transfer, which manifest themselves in well pronounced oscillations. These oscillations are of two types, precessions and nutations, having characteristic frequencies, which are the same for the different molecular systems and the pulse sequences applied. Hence, precessions and nutations constitute a general feature of polarization transfer in ONP experiments. In general, coherent manipulation of spin order transfer creates a powerful resource for improving the performance of the ONP method, which paves the way to strong signal enhancement in nuclear magnetic resonance.

Spin Hyperpolarization in Modern Magnetic Resonance

“…For instance, all three mechanisms (Overhauser, ONP, and t DNP) were observed in doped anthracene crystals . In the case of coherent polarization transfer, oscillations in nuclear polarization can occur under appropriate conditions, particularly for high B 1 and moderate B 0 fields. − Coherence transfer pathways have been analyzed in some detail …”

Recent advances in the application of parahydrogen in catalysis and biochemistry

“…A simple phenomenological model of the polarization transfer processes was given by Barskiy et al 115 Quantitative theoretical description of the SABRE process with rigorous consideration of spin dynamics, level anti-crossings (LACs) and chemical exchange was proposed by Knecht et al 116,11 A detailed discussion of these models is given in the review by Ivanov et al, 118 which includes an extensive list of SABRE active substrates. In the following the salient elements of this description are summarized.…”

“…Dynamic nuclear polarization [1][2][3][4] schemes solve this problem by employing stable radicals or optically created transient triplet states or noble-gases (see e.g., ref. [5][6][7][8][9][10][11]. Parahydrogen based hyperpolarization schemes employ a chemical reaction of a substrate with parahydrogen 12 for the creation of the hyperpolarization, the so-called parahydrogen induced polarization (PHIP) experiments, which were initially predicted by Bowers and Weitekamp.…”

“…, ref. 5–11). Parahydrogen based hyperpolarization schemes employ a chemical reaction of a substrate with parahydrogen 12 for the creation of the hyperpolarization, the so-called parahydrogen induced polarization (PHIP) experiments, which were initially predicted by Bowers and Weitekamp.…”

Optically enhanced solid-state 1H NMR spectroscopy

“…Today, the most common optical nuclear polarization methods in solids harness the electron spin polarization in the excited triplet state of a chromophore, using gated microwave irradiation, fast magnetic field sweeps, and/or low-field level anti-crossing (LAC) matching conditions to transfer the electron spin polarization to nearby nuclear spins. 27,38,[44][45][46][47][48][49][50][51] These optical hyperpolarization techniques are typically performed in single crystals, as they require alignment of the chromophore molecular frame with respect to the applied magnetic field because of the strong orientation dependence of the triplet zero-field splitting. Consequently, it is extremely challenging to generate optical nuclear polarization if the target cannot be formulated as a single crystal, although one example of 1 H hyperpolarization has been observed in pentacene-doped polycrystalline naphthalene under microwave irradiation and magnetic field sweeping.…”