Intramolecular Spin-spin Interactions Research Articles

A Heterometallic Porphyrin Dimer as a Potential Quantum Gate: Magneto-Structural Correlations and Spin Coherence Properties.

In the development of two-qubit quantum gates, precise control over the intramolecular spin-spin interaction between molecular spin units plays a pivotal role. A weak but measurable exchange coupling is especially important for achieving selective spin addressability that allows controlled manipulation of the computational basis states |00⟩ |01⟩ |10⟩ |11⟩ by microwave pulses. Here, we report the synthesis and Electron Paramagnetic Resonance (EPR) study of a heterometallic meso-meso (m-m) singly-linked VIV O-CuII porphyrin dimer. X-band continuous wave EPR measurements in frozen solutions suggest a ferromagnetic exchange coupling of ca. 8 ⋅ 10-3 cm-1 . This estimation is supported by Density Functional Theory calculations, which also allow disentangling the ferro- and antiferromagnetic contributions to the exchange. Pulsed EPR experiments show that the dimer maintains relaxation times similar to the monometallic CuII porphyrins. The addressability of the two individual spins is made possible by the different g-tensors of VIV and CuII -ions, in contrast to homometallic dimers where tilting of the porphyrin planes plays a key role. Therefore, single-spin addressability in the heterometallic dimer can be maintained even with small tilting angles, as expected when deposited on surface, unlocking the full potential of molecular quantum gates for practical applications.

Synthesis and structural evaluation of closed-shell folded and open-shell twisted hexabenzo[5.6.7]quinarene.

We describe the synthesis and characterization of hexabenzo[5.6.7]quinarene, which is composed of an anthraquinodimethane (AQD) central core that is end-capped with fluorenyl and dibenzosuberenyl groups. Due to strong intramolecular spin-spin interaction through the central AQD unit, this compound is obtained as a stable folded form. Isolation of the stable twisted dication by oxidation and reduction of the dication yields a twisted triplet species, which thermally reverts to the folded form. This is a spin-switching system based on a combination of chemical oxidation/reduction and thermal stimulation.

Symmetry mediated tunable molecular magnetism on a 2D material

Symmetries in nanoscale structures can be decisive for their structural, electronic, and magnetic properties, particularly in systems with reduced dimensions. Here we show that the symmetries of a flat metal-organic molecule adsorbed on a transition metal dichalcogenide, a 2-dimensional layered material, have a dramatic effect on the total spin and the intramolecular spin-spin interactions. Using a scanning probe microscope, we find two different molecular spin states by modifying the symmetry of the molecules via the twist angle to the substrate. Additionally, we observe significant non-collinear Dzyaloshinskii–Moriya interaction between two electron spins on the molecule induced by the spin-orbit coupling of the van der Waals coupled layered material with broken inversion symmetry. Our work opens a path for modifying the spin by exploiting symmetries and for studying the nature of surface-induced non-collinear spin-spin interaction within a single molecule which might allow the realization of more complex topological spin structures.

TEMPO Radical-Functionalized Supramolecular Coordination Complexes with Controllable Spin-Spin Interactions.

The topic of noncovalent spin-spin interactions is of increasing general interest in supramolecular radical chemistry. In this report, a series of exo- and endo-TEMPO radical-functionalized metallacycles 1-4 and metallacages 5 and 6 were constructed via coordination-driven self-assembly, wherein the number, location, and distance of the spins were precisely controlled. Their intriguing spin-spin interactions were systematically investigated by electron paramagnetic resonance (EPR) and were well interpreted at the molecular level assisted by X-ray crystallography analysis. The results revealed their distinct spin-spin interactions in the solution state, wherein the spin-spin interaction of metallacycle 3 was much stronger than that of the other five assemblies mainly due to its shorter intramolecular spin-spin distance. In the solid state, 1-6 exhibited obvious spin-spin (dipole-dipole) interactions because of the close arrangement of TEMPO units as indicated in their single crystals. Specifically, a large zero-field splitting (ZFS; D = 17.5 mT) was observed in the crystalline form of metallacycle 4, which arose from the strong intermolecular spin-spin coupling. Interestingly, when the counterion of PF6- in 4 was changed to BF4-, the BF4- counterion analog 4a also exhibited a large ZFS, but the ZFS originated from the intramolecular spin-spin interaction due to a small variation in its crystal conformation. Moreover, the reversible on-off switching of the ZFS in 4 and 4a via the crystal-to-amorphous transformation induced by mechanical grinding and solvent vapor stimuli was also successfully realized. The unique and controllable inter- and intramolecular spin-spin interactions in this work reveal new insights for the understanding and manipulation of spin-spin interactions and may open up a new way to develop organic spin materials in the future.

Phenylene-bridged Porphyrin meso-Oxy Radical Dimers.

Stable meta- and para-phenylene bridged porphyrin meso-oxy radical dimers and their NiII and ZnII complexes were synthesized. All the dimers exhibited optical and electrochemical properties similar to the corresponding porphyrin meso-oxy radical monomers, indicating small electronic interaction between the two spins. Intramolecular spin-spin interaction through the π-spacer was determined to be J/kB =-15.9 K for m-phenylene bridged ZnII porphyrin dimer. The observed weak antiferromagnetic interaction has been attributed to less effective conjugation between the porphyrin radical and linking π-spacer due to large dihedral angle. In the case of ZnII complexes, both para- and meta-phenylene bridged dimers formed 1D-chain in solutions and in the solid states through Zn-O coordination.

Extreme nuclear magnetic resonance: Zero field, single spins, dark matter….

An unusual regime for liquid-state nuclear magnetic resonance (NMR) where the magnetic field strength is so low that the J-coupling (intramolecular spin-spin) interactions dominate the spin Hamiltonian opens a new paradigm with applications in spectroscopy, quantum control, and in fundamental-physics experiments, including searches for well-motivated dark-matter candidates. An interesting possibility is to bring this kind of "extreme NMR" together with another one-single nuclear spin detected with a single-spin quantum sensor. This would enable single-molecule J-spectroscopy.

From Open-Shell Singlet Diradicaloid to Closed-Shell Global Antiaromatic Macrocycles.

A dithieno[a,h]-s-indacene- (DTI-) based diradicaloid DTI-2Br was synthesized and its open-shell singlet diradical character was validated by magnetic measurements. On the other hand, its macrocyclic trimer DTI-MC3 and tetramer DTI-MC4 turned out to be closed-shell compounds with global antiaromaticity, which was supported by X-ray crystallographic analysis and NMR spectroscopy, assisted by ACID and 2D-ICSS calculations. Such change can be explained by a subtle balance between two types of antiferromagnetic spin-spin coupling along the π-conjugated macrocycles. The dications of DTI-MC3 and DTI-MC4 turned out to be open-shell singlet diradical dications, with a singlet-triplet energy gap of -2.90 and -2.60 kcal mol-1 , respectively. At the same time, they are both global aromatic. Our studies show that intramolecular spin-spin interactions play important roles on electronic properties of π-conjugated macrocycles.

Spin-Lattice Relaxation of Hyperpolarized Metronidazole in Signal Amplification by Reversible Exchange in Micro-Tesla Fields.

Simultaneous reversible chemical exchange of parahydrogen and to-be-hyperpolarized substrate on metal centers enables spontaneous transfer of spin order from parahydrogen singlet to nuclear spins of the substrate. When performed at sub-micro-Tesla magnetic field, this technique of NMR Signal Amplification by Reversible Exchange in SHield Enables Alignment Transfer to Heteronuclei (SABRE-SHEATH). SABRE-SHEATH has been shown to hyperpolarize nitrogen-15 sites of a wide range of biologically interesting molecules to a high polarization level (P > 20%) in one minute. Here, we report on a systematic study of 1H, 13C and 15N spin-lattice relaxation (T1) of metronidazole-13C2-15N2 in SABRE-SHEATH hyperpolarization process. In micro-Tesla range, we find that all 1H, 13C and 15N spins studied share approximately the same T1 values (ca. 4 s at the conditions studied) due to mixing of their Zeeman levels, which is consistent with the model of relayed SABRE-SHEATH effect. These T1 values are significantly lower than those at higher magnetic (i.e. the Earth's magnetic field and above), which exceed 3 minutes in some cases. Moreover, these relatively short T1 values observed below 1 micro-Tesla limit the polarization build-up process of SABRE-SHEATH- thereby, limiting maximum attainable 15N polarization. The relatively short nature of T1 values observed below 1 micro-Tesla is primarily caused by intermolecular interactions with quadrupolar iridium centers or dihydride protons of the employed polarization transfer catalyst, whereas intramolecular spin-spin interactions with 14N quadrupolar centers have significantly smaller contribution. The presented experimental results and their analysis will be beneficial for more rational design of SABRE-SHEATH (i) polarization transfer catalyst, and (ii) hyperpolarized molecular probes in the context of biomedical imaging and other applications.

Ambient-Stable Bis-Azoaromatic-Centered Diradical [(L•)M(L•)] Complexes of Rh(III): Synthesis, Structure, Redox, and Spin–Spin Interaction

Bis-azoaromatic electron traps, viz. 2-(2-pyridylazo)azoarene 1, have been synthesized by colligating electron-deficient pyridine and azoarene moieties, and they act as apposite proradical templates for the formation of stable open-shell diradical complexes [(1•-)RhIII(1•-)]+ ([2]+), starting from the low-valent electron reservoir [RhI]. The less stable monoradical [RhIII(1•-)Cl2(PPh3)3] (3) has also been isolated as a minor product. These π-radical complexes are multiredox systems, and the electron transfer processes occur exclusively within the pincer-type NNN ligand backbone 1. Molecular and electronic structures of the diradicals and monoradicals have been ascertained with the aid of X-ray diffraction, electrochemical, spectroelectrochemical, and spectral (electronic, IR, NMR, and EPR) studies. In the diradicals [2]+, the orthogonal disposition of two ligand π orbitals linked via a closed-shell metal center (t26) impedes significant coupling between the radicals. Indeed, the observed magnetic moment of [2a]+ lies near ∼2.3 μB over the temperature range 50-300 K. A very weak antiferromagnetic (AF) intramolecular spin-spin interaction between two ligand π arrays in [(1•-)RhIII(1•-)]+ have been found experimentally (J ≈ -5 cm-1), and this is further substantiated by density functional theory (DFT) calculations at the (U)B3LYP/6-31G(d,p) level.

Fluorenyl Based Macrocyclic Polyradicaloids.

Synthesis of stable open-shell polyradicaloids including control of intramolecular spin-spin interactions is a challenging topic in organic chemistry and materials science. Herein, we report the synthesis and physical characterization of two series of fluorenyl based macrocyclic polyradicaloids. In one series (FR-MCn, n = 4-6), the fluorenyl radicals are directly linked at 3,6-positions; whereas in the other series (MC-FnAn, n = 3-5), an additional ethynylene moiety is inserted between the neighboring fluorenyl units. To access stable macrocyclic polyradicaloids, three synthetic methods were developed. All of these stable macrocycles can be purified by normal silica gel column chromatography under ambient conditions. In all cases, moderate polyradical characters were calculated by restricted active space spin-flip method due to the moderate intramolecular antiferromagnetic spin-spin interactions. The excitation energies from the low-spin ground state to the lowest high-spin excited state were evaluated by superconducting quantum interference device measurements. Their physical properties were also compared with the respective linear fluorenyl radical oligomers (FR-n, n = 3-6). It is found that the geometry, i.e., the distortional angle and spacer (w or w/o ethynylene) between the neighboring fluorenyl units, has significant effect on their polyradical character, excitation energy, one-photon absorption, two-photon absorption and electrochemical properties. In addition, the macrocyclic tetramers FR-MC4 and MC-F4A4 showed global antiaromatic character due to cyclic π-conjugation with 36 and 44 π-electrons, respectively.

High resolution NMR study of T1 magnetic relaxation dispersion. IV. Proton relaxation in amino acids and Met-enkephalin pentapeptide

Nuclear Magnetic Relaxation Dispersion (NMRD) of protons was studied in the pentapeptide Met-enkephalin and the amino acids, which constitute it. Experiments were run by using high-resolution Nuclear Magnetic Resonance (NMR) in combination with fast field-cycling, thus enabling measuring NMRD curves for all individual protons. As in earlier works, Papers I-III, pronounced effects of intramolecular scalar spin-spin interactions, J-couplings, on spin relaxation were found. Notably, at low fields J-couplings tend to equalize the apparent relaxation rates within networks of coupled protons. In Met-enkephalin, in contrast to the free amino acids, there is a sharp increase in the proton T1-relaxation times at high fields due to the changes in the regime of molecular motion. The experimental data are in good agreement with theory. From modelling the relaxation experiments we were able to determine motional correlation times of different residues in Met-enkephalin with atomic resolution. This allows us to draw conclusions about preferential conformation of the pentapeptide in solution, which is also in agreement with data from two-dimensional NMR experiments (rotating frame Overhauser effect spectroscopy). Altogether, our study demonstrates that high-resolution NMR studies of magnetic field-dependent relaxation allow one to probe molecular mobility in biomolecules with atomic resolution.

Conversion of Nuclear Spin Isomers of Ethylene

A theoretical model of the nuclear spin isomer conversion in C2H4 induced by the intramolecular spin-spin interaction between hydrogen nuclei has been developed. In the ground electronic state, C2H4 has four nuclear spin isomers in contrast to two isomers in the molecules studied so far in this field of research. At the gas pressure of 1 Torr, the rate of conversion between isomers with the nuclear spin symmetries B1u and B2u was found to be 5.2 × 10(-4) s(-1), which coincides within experimental uncertainties with the rate recently measured by Sun et al. (Science 2005, 310, 1938). It was determined that at low gas pressures the conversion is induced mainly by the mixing of only one pair of rotational states. The calculated pressure dependence of the conversion rate predicts that conversion slows down with increasing pressure at pressures higher than 300 Torr.

Synthesis of Trityl Radical-Conjugated Disulfide Biradicals for Measurement of Thiol Concentration

Measurement of thiol concentrations is of great importance for characterizing their critical role in normal metabolism and disease. Low-frequency electron paramagnetic resonance (EPR) spectroscopy and imaging, coupled with the use of exogenous paramagnetic probes, have been indispensable techniques for the in vivo measurement of various physiological parameters owing to the specificity, noninvasiveness and good depth of magnetic field penetration in animal tissues. However, in vivo detection of thiol levels by EPR spectroscopy and imaging is limited due to the need for improved probes. We report the first synthesis of trityl radical-conjugated disulfide biradicals (TSSN and TSST) as paramagnetic thiol probes. The use of trityl radicals in the construction of these biradicals greatly facilitates thiol measurement by EPR spectroscopy since trityls have extraordinary stability in living tissues with a single narrow EPR line that enables high sensitivity and resolution for in vivo EPR spectroscopy and imaging. Both biradicals exhibit broad characteristic EPR spectra at room temperature because of their intramolecular spin-spin interaction. Reaction of these biradicals with thiol compounds such as glutathione (GSH) and cysteine results in the formation of trityl monoradicals which exhibit high spectral sensitivity to oxygen. The moderately slow reaction between the biradicals and GSH (k(2) ∼ 0.3 M(-1) s(-1) for TSSN and 0.2 M(-1) s(-1) for TSST) allows for in vivo measurement of GSH concentration without altering the redox environment in biological systems. The GSH concentration in rat liver was determined to be 3.49 ± 0.14 mM by TSSN and 3.67 ± 0.24 mM by TSST, consistent with the value (3.71 ± 0.09 mM) determined by the Ellman's reagent. Thus, these trityl-based thiol probes exhibit unique properties enabling measurement of thiols in biological systems and should be of great value for monitoring redox metabolism.

フェナレニルを基盤とする非局在型一重項ビラジカルの化学

Main issue described in this article is clarification of spin-spin interactions in delocalized singlet biradical species, especially co-existence of intra- and inter-molecular interactions. For the end, we designed and prepared some singlet biradical molecules thermodynamically stabilized by utilizing spin-delocalizing character of phenalenyl radical. These species were found to be stable in air and could be isolated in a crystalline form. X-ray crystallographic analysis revealed the abnormally short π-π contact between molecules, suggesting a strong intermolecular spin-spin interaction, even though the species had a strong intramolecular spin-spin interaction. Co-existence of both the interactions led to lower energy shift of HOMO-LUMO absorption bands in a solid state with respect to solution bands. These findings indicate that covalent bonding character is operative within and between molecules in molecular aggregates of delocalized singlet biradicals. This new aspect was supported by variable-temperature X-ray crystallography and reflectivity measurement on single crystals which had one-dimensional π-π chains of the delocalized singlet biradicals, and we concluded that the electronic structure of the 1-D chain could be represented by the Resonating Valence Bond model proposed by Pauling.

Nuclear spin conversion in the gaseous phase in the presence of a static electric field: Intramolecular magnetic interactions and the role of collisions

When a gaseous sample of $^{13}\mathrm{CH}_{3}\mathrm{F}$ is prepared with a spin-isomer population ratio (ortho and para forms) far from the equilibrium given by nuclear spin statistics, it relaxes towards this equilibrium with an exponential decay rate. This phenomenon, called nuclear spin conversion, is mainly governed by intramolecular spin-spin and spin-rotation interactions which couple two pairs of quasidegenerate ortho-para levels ($J=9,K=3;$ ${J}^{\ensuremath{'}}=11,{K}^{\ensuremath{'}}=1$) and ($J=20,K=3;$ ${J}^{\ensuremath{'}}=21,{K}^{\ensuremath{'}}=1$). The presence of a static electric field can induce the degeneracy for Stark sublevels and yields an increase of the conversion rate. Such a ``conversion spectrum'' has been recorded experimentally. The intensities of the peaks are directly related to the intramolecular magnetic interaction strengths, and their widths depend on how the collisions break the coherence between ortho and para levels which is created by the interactions. Such collision-induced rates are directly determined and compared to the rate of rotationally inelastic molecular collisions.

Experimental determination of spin-rotation and spin-spin magnetic interactions in by nuclear spin conversion

When a gas sample of 13CH3F is prepared with a population of isomers (ortho and para forms) far from the equilibrium given by nuclear spin statistics, it relaxes towards this equilibrium with an exponential decay rate. This phenomenon called nuclear spin conversion is mainly governed by intramolecular spin-spin and spin-rotation interactions. In the quantum relaxation model [P.L. Chapovsky, Phys. Rev. A 43, 3624 (1991)], two pairs of ortho-para levels (J = 9, K = 3; J' = 11, K' = 1) and (J = 20, K = 3; J' = 21, K' = 1) are principally responsible for the conversion. The levels of the second pair are coupled by both spin-spin and spin-rotation interactions. The application of an electric field (up to 10 kV/cm) induces a crossing of the Stark components of this pair, which is observed for the first time. A specific experimental set-up based on an electric field of alternating triangular shape is used, which allows the determination of the strength of both interactions via the measurement of the spin conversion decay rates. This work yields the first experimental value for the electronic contribution to the spin-rotation interaction in 13CH3F.

Spin-spin interaction between phenyl nitroxides through the - system

The spin states of a series of silicon- and carbon-bridged phenyl nitroxides were examined with respect to the temperature dependent ESR and SQUID measurements. Of those, a linear relationship between the ESR signal intensity and 1/T (T = absolute temperature) and an increase of χmolT along lowering T were observed for a compound having disilanylene-bridged m- and p-phenyl nitroxide units (Si2mp), indicating ferromagnetic spin-spin interaction in this molecule. In contrast to this, no clear intramolecular spin-spin interaction took place in the monosilanylene analogue (Si1mp). Mono- and disilanylene-bridged phenylnitroxides with p-, p- or m-, m-substitution (Si1pp,Si1mm, Si1pp, and Si2mm) exhibited the singlet ground state. The trisilanylene and siloxanylene bridges did not play an obvious role in the spin interaction, in either a ferro- or antiferromagnetic fashion, regardless of the substitution modes of the phenylenes. MO calculations on the model compounds provided a mechanistic interpretation for the high-spin interaction through the σ-π system.

2D-Electron Spin Transient Nutation (2D-ESTN) Study of a Stable High- Spin Carbene (S=3)

Stable high-spin polycarbene, polybrominated 1,3,5-tris[4-(phenylcarbeno)-phenylethy-nyl]benzene in a glass matrix was studied by Electron Spin Transient Nutation (ESTN) spectroscopy. 2D-ESTN spectra of the polycarbene were measured in order to identify a molecular spin multiplicity of the high-spin ground state. The observed nutation frequencies were assigned to those arising from the allowed ESR transitions of the septet ground state (S=3). It indicates that the high-spin state is generated by ferromagnetic interaction between stable bromo-substituted diphenylmethylenes (S=1) through a 2D topological spin coupling unit with C 3 symmetry, 1,3,5-tris(ethynyl)benzene. The observed fine-structure parameters of the septet state, D = −0.040 cm −1 and |E| = 0.007 cm −1, were well interpreted in terms of an intramolecular spin-spin interaction model. The observed small and negative D value was shown to arise from the C 3 symmetric molecular structure. The high-spin polycarbene has survived after annealing of the glass matrix. The stable triplet carbene unit is supposed to serve as a useful building block for extended super high-spin molecular systems and organic superparamagnets.

Nuclear spin conversion inCH3Fat elevated temperatures

Experimental data on the temperature dependence of the ortho-para nuclear spin conversion rate of gaseous ${\mathrm{CH}}_{3}\mathrm{F}$ are presented. In the range 300--750 K, the ${}^{12}{\mathrm{CH}}_{3}\mathrm{F}$ conversion rate increases by one order of magnitude. By contrast, the conversion rate for ${}^{13}{\mathrm{CH}}_{3}\mathrm{F}$ decreases with increasing temperature (for $Tl600\mathrm{K}$). These results are in qualitative agreement with a theoretical model based on mixing of states due to intramolecular spin-spin interactions between the molecular nuclei. Above 600 K, the rate in ${}^{13}{\mathrm{CH}}_{3}\mathrm{F}$ starts to increase again, indicating a possible additional mechanism for the conversion.

Double electron-electron resonance in electron spin echo: Conformations of spin-labeled poly-4-vinilpyridine in glassy solutions

Double electron-electron resonance in electron spin echo has been used to study the glassy solutions of poly-4-vinylpyridine doped by nitroxyl radicals frozen in liquid nitrogen. The phase relaxation of spin labels due to spin-spin interaction of unpaired electrons has been studied. The intramolecular and intermolecular contributions of the dipole-dipole interaction of spin labels into relaxation process have been separated. It has been established that both the intramolecular and intermolecular spin-spin interaction of spin labels lead to the dependence of echo signal on timeT of the exp (−aT q ) type. It is shown that for the intramolecular interaction the experimentalq value is 0.3, for the intermolecular one it is 2. The assumption has been made of the linear structure of polymeric molecules due to the presence of a sufficiently high density of an electric charge on polymeric molecules.