Magnetic resonance techniques are successfully utilized in a broad range of scientific disciplines and in various practical applications, with medical magnetic resonance imaging being the most widely known example. Currently, both fundamental and applied magnetic resonance are enjoying a major boost owing to the rapidly developing field of spin hyperpolarization. Hyperpolarization techniques are able to enhance signal intensities in magnetic resonance by several orders of magnitude, and thus to largely overcome its major disadvantage of relatively low sensitivity. This provides new impetus for existing applications of magnetic resonance and opens the gates to exciting new possibilities. In this review, we provide a unified picture of the many methods and techniques that fall under the umbrella term “hyperpolarization” but are currently seldom perceived as integral parts of the same field. Specifically, before delving into the individual techniques, we provide a detailed analysis of the underlying principles of spin hyperpolarization. We attempt to uncover and classify the origins of hyperpolarization, to establish its sources and the specific mechanisms that enable the flow of polarization from a source to the target spins. We then give a more detailed analysis of individual hyperpolarization techniques: the mechanisms by which they work, fundamental and technical requirements, characteristic applications, unresolved issues, and possible future directions. We are seeing a continuous growth of activity in the field of spin hyperpolarization, and we expect the field to flourish as new and improved hyperpolarization techniques are implemented. Some key areas for development are in prolonging polarization lifetimes, making hyperpolarization techniques more generally applicable to chemical/biological systems, reducing the technical and equipment requirements, and creating more efficient excitation and detection schemes. We hope this review will facilitate the sharing of knowledge between subfields within the broad topic of hyperpolarization, to help overcome existing challenges in magnetic resonance and enable novel applications.

Spin Hyperpolarization in Modern Magnetic Resonance

A change in the sign of the ground-state electron spin polarization (ESP) is reported in complexes where an organic radical (nitronylnitroxide, NN) is covalently attached to a donor acceptor chromophore via two different meta-phenylene bridges, (bpy)Pt(CAT-m-Ph-NN) (mPh-Pt) and (bpy)Pt(CAT-6-Me-m-Ph-NN) (6-Me-mPh-Pt) (bpy = 5,5’-di-tert-butyl-2,2’-bipyridine, CAT = 3-tert-butylcatecholate, m-Ph = meta-phenylene). These molecules represent a new class of chromophores that can be photoexcited with visible light to produce an initial exchange-coupled, 3-spin (bpy•-, CAT+• and NN•), charge-separated doublet 2S1 (S = chromophore excited spin singlet configuration) excited state. Following excitation, the 2S1 state rapidly decays to the ground state by magnetic exchange enhanced internal conversion via the 2T1 (T = chromophore excited spin triplet configuration) state. This process generates emissive ground state ESP in 6-Me-mPh-Pt while for mPh-Pt the ESP is absorptive. It is proposed that the emissive polarization in 6-Me-mPh-Pt results from zero-field splitting induced transitions between the chromophoric 2T1 and 4T1 states, whereas predominant spin-orbit induced transitions between 2T1 and low-energy NN-based states give rise to the absorptive polarization observed for mPh-Pt. The sign difference in the ESP of these molecules is consistent with a smaller excited state 2T1 – 4T1 gap for 6-Me-mPh-Pt that derives from steric interactions with the 6-methyl group. These steric interactions reduce the excited state pairwise CAT+•-NN• exchange coupling compared to that in mPh-Pt.

/pdf/chromophore-radical-excited-state-antiferromagnetic-exchange-4r1n63qhvt.pdf

Chromophore-radical excited state antiferromagnetic exchange controls the sign of photoinduced ground state spin polarization

Magnetic resonance techniques are successfully utilized in a broad range of scientific disciplines and in a number of practical applications, with medical MRI being the most widely-known example. Currently, both fundamental and applied magnetic resonance are enjoying a major boost owing to the dramatic signal enhancement provided by the rapidly-developing field of spin hyperpolarization. Such techniques are able to enhance signal intensities in magnetic resonance by several orders of magnitude, and thus to largely overcome its major disadvantage of relatively low sensitivity compared to other analytical techniques. This provides new impetus for existing applications, and, even more importantly, this opens the gates to numerous novel and exciting possibilities in the broad fields of fundamental and applied magnetic resonance. There are many different techniques that fall under the umbrella term “hyperpolarization”. Existing reviews cover the various subfields, but they are mostly addressed separately, and are seldom perceived as integral parts of the same field. In this review we attempt to unify the many methods that are used to hyperpolarize nuclear spins into one picture. Specifically, before delving into the individual techniques, we provide a detailed analysis of the underlying principles of spin hyperpolarization. We attempt to uncover and classify the origins of hyperpolarization; to establish its sources and the specific mechanisms that enable the flow of polarization from a source to the target nuclear spins. After outlining the inner workings of hyperpolarization, we give a more detailed analysis of individual hyperpolarization techniques: the mechanisms by which they work, fundamental and technical requirements, characteristic applications, unresolved issues and possible future directions. While a substantial progress has been achieved in the field of spin hyperpolarization in recent years, the continuing growth of activity indicates that this is just the beginning. It is unlikely that fundamentally new sources of hyperpolarization will be uncovered in the near future, but we expect the field to flourish as new ways to improve and utilize current hyperpolarization techniques are identified and implemented. There is great scope for cross-fertilization between known methods, and developments in one area (e.g., prolonging polarization lifetimes, or creating more efficient excitation-detection schemes) can have a very broad impact across the entire field of hyperpolarization. We hope this review will facilitate this process, since advances in hyperpolarization will help to overcome existing challenges in magnetic resonance and enable novel applications. 

/pdf/spin-hyperpolarization-in-modern-magnetic-resonance-1ldx9a1d.pdf

Ground-state electron spin polarization (ESP) is generated in radical elaborated (bpy)Pt(CAT-NN) and (bpy)Pt(CAT-p-Me2PhMe2-NN) (bpy = 5,5'-di-tert-butyl-2,2'-bipyridine, CAT = 3-tert-butylcatecholate, p-Ph = para-phenylene, NN = nitronylnitroxide). Photoexcitation produces an exchange-coupled, three-spin, charge-separated doublet 2S1 (S = chromophore excited spin singlet configuration) excited state that rapidly decays to a 2T1 (T = chromophore excited spin triplet configuration) excited state. The SQ-bridge-NN bond torsions affect the magnitude of the excited state exchange interaction (JSQ-NN), which determines the 2T1-4T1 energy gap. Ground state ESP is dependent on the magnitude of JSQ-NN, and we postulate that this results from differences in 2T1 and 4T1 state mixing. Mechanisms that lead to the rapid transfer of the excited state ESP to the ground state are discussed. Although subnanosecond 2T1 state lifetimes are measured optically in solution, the ground state ESP decays very slowly at 20 K and is observable for more than a millisecond.

Excited State Exchange Control of Photoinduced Electron Spin Polarization in Electronic Ground States.

Photoinduced electron spin polarization (ESP) is reported in the ground state of a series of complexes consisting of an organic radical (nitronylnitroxide, NN) covalently attached to a donor-acceptor chromophore either directly or via para-phenylene bridges substituted with 0-4 methyl groups. These molecules represent a class of chromophores that undergo visible light excitation to produce an initial exchange-coupled, three-spin [bpy•-, CAT•+ (= semiquinone, SQ) and NN•], charge-separated doublet 2S1 (S = chromophore spin singlet configuration) excited state that rapidly decays by magnetic exchange-enhanced internal conversion to a 2T1 (T = chromophore excited spin triplet configuration) state. The 2T1 state equilibrates with chromophoric and NN radical-derived excited states, resulting in absorptive ESP of the recovered ground state, which persists for greater than a millisecond and can be measured by low-temperature time-resolved electron paramagnetic resonance spectroscopy. The magnitude of the ground state ESP is found to correlate with the excited state magnetic exchange interaction between the CAT+• and NN• radicals, which in turn is controlled by the structure of the bridge fragment.

Excited State Magneto-Structural Correlations Related to Photoinduced Electron Spin Polarization.

Both the sign and intensity of photoinduced electron spin polarization (ESP) in the electronic ground state doublet (2S0/D0) of chromophore-radical complexes can be controlled by changing the nature of the metal ion. The complexes consist of an organic radical (nitronyl nitroxide, NN) covalently attached to a donor-acceptor chromophore via a m-phenylene bridge, (bpy)M(CAT-m-Ph-NN) (1) (bpy = 4,4'-di-tert-butyl-2,2'-bipyridine, M = PdII (1-Pd) or PtII (1-Pt), CAT = 3-tert-butylcatecholate, m-Ph = meta-phenylene). In both complexes, photoexcitation with visible light produces an initial exchange-coupled, three-spin (bpy•-, CAT•+ = semiquinone (SQ), and NN•), charge-separated doublet 2S1 (S = chromophore excited spin singlet configuration) excited state that rapidly decays to the ground state via a 2T1 (T = chromophore excited spin triplet configuration) state. This process is not expected to be spin selective, and only very weak emissive ESP is found for 1-Pd. In contrast, strong absorptive ESP is generated in 1-Pt. It is postulated that zero-field-splitting-induced transitions between the chromophoric 2T1 and 4T1 states (1-Pd and 1-Pt) and spin-orbit-induced transitions between 2T1 and NN-based quartet states (1-Pt) account for the differences in polarization.

Metal Ion Control of Photoinduced Electron Spin Polarization in Electronic Ground States.

Electronic coupling through organic bridges facilitates magnetic exchange interactions and controls electron transfer and single-molecule device electron transport. Electronic coupling through alternant π-systems (e.g., benzene) is better understood than the corresponding coupling through nonalternant π-systems (e.g., azulene). Herein, we examine the structure, spectroscopy, and magnetic exchange coupling in two biradicals (1,3-SQ2Az and 1,3-SQ-Az-NN; SQ = the zinc(II) complex of spin-1/2 semiquinone radical anion, NN = spin-1/2 nitronylnitroxide; Az = azulene) that possess nonalternant azulene π-system bridges. The SQ radical spin density in both molecules is delocalized into the Az π-system, while the NN spin is effectively localized onto the five-atom ONCNO π-system of NN radical. The spin distributions and interactions are probed by EPR spectroscopy and magnetic susceptibility measurements. We find that J = +38 cm-1 for 1,3-SQ2Az and J = +9 cm-1 for 1,3-SQ-Az-NN (H=-2JS^SQ·S^SQorNN). Our results highlight the differences in exchange coupling mediated by azulene compared to exchange coupling mediated by alternant π-systems.

Rules for Magnetic Exchange in Azulene-Bridged Biradicals: Quo Vadis?

Oligomers of poly(thiophene) and poly(ortho-phenyleneethynylene) having pendant S = 1/2 semiquinone radicals (as complexes of cobalt(III)) have been prepared and characterized by electron paramagnetic resonance (EPR) spectroscopy and magnetic measurements (superconducting quantum interferences device = SQUID magnetometry). Our results show that exchange coupling of semiquinone groups along a polythiophene backbone is greater than the corresponding coupling along a poly(ortho-phenyleneethynylene) backbone.

In Search of Stable, High-Spin Polymers

Magnetic Exchange Coupling through the Nonalternant Cyclopentadienyl π-System of Ferrocene.

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