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Naphthalene diimides (NDIs) are a promising material for n-type polymer acceptors in thin-film devices such as organic light-emitting diodes. The intersystem crossing (ISC) pathway of NDIs has been...

We describe the structural and magnetic properties of a tetranuclear [2 × 2] Co4 grid complex containing a ditopic arylazo ligand. At low temperatures and in solution the complex is comprised of Co3+ and singly reduced trianion-radical ligands. In the solid state we demonstrate the presence of valence tautomerization via variable temperature magnetic susceptibility experiments and powder-pattern EPR spectroscopy. Valence tautomerism in polynuclear complexes is very rare and to our knowledge is unprecedented in [2 × 2] grid complexes.

Trityl and nitroxide radicals are connected by π-topologically controlled aryl linkers, generating genuinely g-engineered biradicals. They serve as a typical model for biradicals in which the exchange (J) and hyperfine interactions compete with the g-difference electronic Zeeman interactions. The magnetic properties underlying the biradical spin Hamiltonian for solution, including J's, have been determined by multifrequency CW-ESR and 1H ENDOR spectroscopy and compared with those obtained by quantum chemical calculations. The experimental J values were in good agreement with the quantum chemical calculations. The g-engineered biradicals have been tested as a prototype for AWG (Arbitrary Wave Generator)-based spin manipulation techniques, which enable GRAPE (GRAdient Pulse Engineering) microwave control of spins in molecular magnetic resonance spectroscopy for use in molecular spin quantum computers, demonstrating efficient signal enhancement of specific weakened hyperfine signals. Dynamic nuclear polarization (DNP) effects of the biradicals for 400 MHz nuclear magnetic resonance signal enhancement have been examined, giving efficiency factors of 30 for 1H and 27.8 for 13C nuclei. The marked DNP results show the feasibility of these biradicals for hyperpolarization.

Three new mixed triradicals with small exchange coupling parameters (J ≪ AN) were obtained on the base of a coupling reaction between the derivative of spirofused 2,5-dihydroimidazol-type monoradical and the two mol. equivalents of carboxylic acid derivatives of PROXYL-, TEMPO- or 2,5-dihydro-1H-pyrrol-type nitroxides. Their intramolecular magnetic interactions were characterized in terms of comparison of the CW X, Q- and W-band EPR spectra with those of the monoradical precursors. The dipole–dipole coupling parameters of the triradicals were estimated on the base of the quantum chemical calculations at UB3LYP/6-31G(d) level of theory. Two types of the spin distances were found in the triradicals: short—with the distance of 13–17 A (D ≈ 11–24 MHz) and long—with the distance of 21–23 A (D ≈ 4–6 MHz). The longest spin–spin as well as spin–lattice relaxation times at 50 K were detected for the triradical carrying the two TEMPO fragments, indicating the potential usage of three-spin qubit models for quantum gate operations.

We describe the structural and magnetic properties of a tetranuclear [2 × 2] Co4 grid complex containing a ditopic arylazo ligand. At low temperatures and in solution the complex is comprised of Co3+ and singly reduced trianion-radical ligands. In the solid state we demonstrate the presence of valence tautomerization via variable temperature magnetic susceptibility experiments and powder-pattern EPR spectroscopy. Valence tautomerism in polynuclear complexes is very rare and to our knowledge is unprecedented in [2 × 2] grid complexes.

The function of proteins is linked to their conformations that can be resolved with several high-resolution methods. However, only a few methods can provide the temporal order of intermediates and conformational changes, with each having its limitations. Here, we combine pulsed electron–electron double resonance spectroscopy with a microsecond freeze-hyperquenching setup to achieve spatiotemporal resolution in the angstrom range and lower microsecond time scale. We show that the conformational change of the Cα-helix in the cyclic nucleotide-binding domain of the Mesorhizobium loti potassium channel occurs within about 150 μs and can be resolved with angstrom precision. Thus, this approach holds great promise for obtaining 4D landscapes of conformational changes in biomolecules. Copyright © 2021 The Authors. Published by American Chemical Society, Journal of the American Chemical Society, 143 (18), ISSN:0002-7863, ISSN:1520-5126

The mobility-lifetime ( μ τ) product is an important parameter that determines the performance of electronic and photonic devices. To overcome the previously reported difficulties in measuring the μ τ product at cryogenic temperatures, we implement a time-resolved cyclotron resonance method to determine the carrier lifetime τ. After clarifying the difference between the AC and DC mobilities measured by cyclotron resonance and time-of-flight methods, respectively, we demonstrate an inverse temperature dependence of the μ τ product. The highest recorded μ τ product of 0.2 cm2/V, which is approximately 100 times the room-temperature value, was obtained at 2 K for chemical-vapor-deposition diamond of the highest currently available purity.

Site-directed spin labeling is a powerful tool for investigating the conformation and dynamics of biomacromolecules such as RNA. Here we introduce a spin labeling strategy based on click chemistry in solution that, in combination with enzymatic ligation, allows highly efficient labeling of complex and long RNAs with short reaction times and suppressed RNA degradation. With this approach, a 34-nucleotide aptamer domain of the preQ1 riboswitch and an 81-nucleotide TPP riboswitch aptamer could be labeled with two labels in several positions. We then show that conformations of the preQ1 aptamer and its dynamics can be monitored in the absence and presence of Mg2+ and a preQ1 ligand by continuous wave electron paramagnetic resonance spectroscopy at room temperature and pulsed electron–electron double resonance spectroscopy (PELDOR or DEER) in the frozen state.

The fictitious spin-1/2 (effective spin-1/2) Hamiltonian approach has been the putative method to analyze the conventional fine-structure/hyperfine ESR spectra of high spin metallocomplexes with sizable zero-field splitting (ZFS) tensors since the early 1950s, and the approach gives salient principal geff-values far from g = 2 without explicitly affording their ZFS values in most cases. The experimental geff-values thus determined, however, never agree with those (gtrue-values) of the true principal g-tensors, which are obtainable from reliable quantum chemical calculations. We have recently derived exact or extremely accurate analytical expressions for the geff/gtrue relationships for the spin quantum number S's up to S = 7/2 (T. Yamane et al., Phys. Chem. Chem. Phys., 2017, 19, 24769-24791). In this work, we have removed the limitation of the collinearity between g- and ZFS tensors and derived the generalized geff/gtrue relationships. To illustrate the usefulness of the present approach, we have revisited important typical high spin systems with large ZFS values such as picket fence metalloporphyrins with MnII (S = 5/2) (Q. Yu et al., Dalton Trans., 2015, 44, 9382-9390), a 6th ligand coordinated porphyrin with FeIII (S = 5/2) (Y. Ide et al., Dalton Trans., 2017, 46, 242-249) and a pseudo-octahedral CoII (S = 3/2)(hfac)2 complex (D. V. Korchagin et al., Dalton Trans., 2017, 46, 7540-7548), completing the ESR spectral and magnetic susceptibility analyses and gaining significant physical insights into their electronic structures. The off-principal axis extra peaks overlooked in the documented spectra of the picket fence MnII porphyrins have fully been assigned, affording their accurate true g-, hyperfine and ZFS tensors, for the first time. For the CoII complex, the occurrence of the non-collinearity between the g- and ZFS tensors has been discussed by using the generalized geff/gtrue relationships. We have attempted to carry out reliable DFT-based and ab initio quantum chemical calculations of their magnetic tensors, in which spin-orbit couplings are incorporated, reproducing the experimental true tensors. We emphasize that the incorporation of multi-reference nature in the electron configuration is important to interpret the magnetic tensors for the CoII complex.

Utilization of triplets is important for preparing organic light-emitting diodes with high efficiency. Very recently, both electrophosphorescence and electrofluorescence could be observed at room temperature for thienyl-substituted phenazines without any heavy metals ( Ratzke et al. J. Phys. Chem. Lett. , 2016 , 7 , 4802 ). It was found that the phosphorescence efficiency depends on the orientation of fused thiophenes. In this work, the thienyl-substituted phenazines are investigated in more detail by time-resolved electron paramagnetic resonance (EPR) and quantum chemical calculations. Spin dynamics, zero-field splitting constants, and electron-spin structures of the excited triplet states for the metal-free room-temperature triplet emitters are correlated with phosphorescence efficiency. Complete active space self-consistent field (CASSCF) calculations clearly show that the electron spin density distributions of the first excited triplet states are strongly affected by the molecular geometry. For the phosphorescent molecules, the electron spins are localized on the phenazine unit, in which the sulfur atom of the fused thiophene points upward. The electron spins are delocalized onto the thiophene unit just by changing the orientation of the fused thiophenes from upward to downward, resulting in the suppression of phosphorescence. Time-resolved EPR measurements and time-dependent density functional theory (TD-DFT) calculations demonstrate that the electron spins delocalized onto the thiophene unit lead to the acceleration of nonradiative decays, in conjunction with the narrowing of the singlet-triplet energy gap.

Trityl and nitroxide radicals are connected by π-topologically controlled aryl linkers, generating genuinely

Pulsed EPR dipolar spectroscopy (PDS) offers several methods for measuring dipolar coupling and thus the distance between electron-spin centers. To date, PDS measurements to metal centers were limited to ions that adhere to the high-field approximation. Here, the PDS methodology is extended to cases where the high-field approximation breaks down on the example of the high-spin Fe3+ /nitroxide spin-pair. First, the theory developed by Maryasov et al. (Appl. Magn. Reson. 2006, 30, 683-702) was adapted to derive equations for the dipolar coupling constant, which revealed that the dipolar spectrum does not only depend on the length and orientation of the interspin distance vector with respect to the applied magnetic field but also on its orientation to the effective g-tensor of the Fe3+ ion. Then, it is shown on a model system and a heme protein that a PDS method called relaxation-induced dipolar modulation enhancement (RIDME) is well-suited to measuring such spectra and that the experimentally obtained dipolar spectra are in full agreement with the derived equations. Finally, a RIDME data analysis procedure was developed, which facilitates the determination of distance and angular distributions from the RIDME data. Thus, this study enables the application of PDS to for example, the highly relevant class of high-spin Fe3+ heme proteins.

Polymer chain morphology and its response to annealing were investigated in terms of g-tensor anisotropy in poly(3-hexylthiophene) (P3HT)/fullerene blend films fabricated by dip coating. The g-anis...

The fictitious spin-1/2 (effective spin-1/2) Hamiltonian approach has been the putative method to analyze the conventional fine-structure/hyperfine ESR spectra of high spin metallocomplexes with sizable zero-field splitting (ZFS) tensors since the early 1950s, and the approach gives salient principal geff-values far from g = 2 without explicitly affording their ZFS values in most cases. The experimental geff-values thus determined, however, never agree with those (gtrue-values) of the true principal g-tensors, which are obtainable from reliable quantum chemical calculations. We have recently derived exact or extremely accurate analytical expressions for the geff/gtrue relationships for the spin quantum number S's up to S = 7/2 (T. Yamane et al., Phys. Chem. Chem. Phys., 2017, 19, 24769-24791). In this work, we have removed the limitation of the collinearity between g- and ZFS tensors and derived the generalized geff/gtrue relationships. To illustrate the usefulness of the present approach, we have revisited important typical high spin systems with large ZFS values such as picket fence metalloporphyrins with MnII (S = 5/2) (Q. Yu et al., Dalton Trans., 2015, 44, 9382-9390), a 6th ligand coordinated porphyrin with FeIII (S = 5/2) (Y. Ide et al., Dalton Trans., 2017, 46, 242-249) and a pseudo-octahedral CoII (S = 3/2)(hfac)2 complex (D. V. Korchagin et al., Dalton Trans., 2017, 46, 7540-7548), completing the ESR spectral and magnetic susceptibility analyses and gaining significant physical insights into their electronic structures. The off-principal axis extra peaks overlooked in the documented spectra of the picket fence MnII porphyrins have fully been assigned, affording their accurate true g-, hyperfine and ZFS tensors, for the first time. For the CoII complex, the occurrence of the non-collinearity between the g- and ZFS tensors has been discussed by using the generalized geff/gtrue relationships. We have attempted to carry out reliable DFT-based and ab initio quantum chemical calculations of their magnetic tensors, in which spin-orbit couplings are incorporated, reproducing the experimental true tensors. We emphasize that the incorporation of multi-reference nature in the electron configuration is important to interpret the magnetic tensors for the CoII complex.

The fictitious spin-1/2 Hamiltonian approach is the putative method to analyze the fine-structure/hyperfine ESR spectra of high spin metallocomplexes having sizable zerofield splitting (ZFS), thus giving salient principal g-values far from around g = 2 without explicitly providing their ZFS parameters in most cases. Indeed, the significant departure of the g-values from g = 2 is indicative of the occurrence of their high spin states, but naturally they never agree with true g-values acquired by quantum chemical calculations such as sophisticated DFT or ab initio MO calculations. In this work, we propose facile approaches to determine the magnetic tensors of high spin metallocomplexes having sizable ZFS, instead of performing advanced high-field/high-frequency ESR spectroscopy. We have revisited analytical expressions for the relationship between effective g-values and true principal g-values for high spins. The useful analytical formulas for the geff–gtrue relationships are given for S's up to 7/2. The genuine Zeeman perturbation formalism gives the exact solutions for S = 3/2, and for higher S's it is much more accurate than the pseudo-Zeeman perturbation approach documented so far (A. Abragam and B. Bleaney, Electron Paramagnetic Resonance of Transition Metal Ions, 1970; J. R. Pilbrow, J. Magn. Reson., 1978, 31, 479; F. Trandafir et al., Appl. Magn. Reson., 2007, 31, 553; M. Fittipaldi et al., J. Phys. Chem. B, 2008, 112, 3859), in which the E(Sx2 − Sy2) term is putatively treated to the second order. To show the usefulness of the present approach, we exploit FeIII(Cl)OEP (S = 5/2) (OEP: 2,3,7,8,12,13,17,18-octaethylporphyrin) and CoIIOEP (S = 3/2) well magnetically diluted in the diamagnetic host crystal lattice of NiIIOEP. The advantage of single-crystal ESR spectroscopy lies in the fact that the molecular information on the principal axes of the magnetic tensors is crucial in comparing with reliable theoretical results. In high spin states of metallocomplexes with sizable ZFS in pseudo-octahedral symmetry, their fine-structure ESR transitions for the principal z-axis orientation appear in the lower field far from g = 2 at the X-band, disagreeing with the putative intuitive picture obtained using relevant ESR spectroscopy. A ReIII,IV dinuclear complex in a mixed valence state exemplifies the cases, whose fine-structure/hyperfine ESR spectra of the neat crystals have been analyzed in their principal-axis system. The DFT-based/ab initio MO calculations of the magnetic tensors for all the high spin entities in this work were carried out.

Metal-free dual singlet–triplet organic light-emitting diode (OLED) emitters can provide direct insight into spin statistics, spin correlations and spin relaxation phenomena, through a comparison of fluorescence to phosphorescence intensity. Remarkably, such materials can also function at room temperature, exhibiting phosphorescence lifetimes of several milliseconds. Using electroluminescence, quantum chemistry, and electron paramagnetic resonance spectroscopy, we investigate the effect of the conjugation pathway on radiative and nonradiative relaxation of the triplet state in phenazine-based compounds and demonstrate that the contribution of the phenazine nπ* excited state is crucial to enabling phosphorescence.

Site‐directed spin labeling of RNA based on click chemistry is used in combination with pulsed electron‐electron double resonance (PELDOR) to benchmark a nitroxide spin label, called here dŲ . We compare this approach with another established method that employs the rigid spin label Çm for RNA labeling. By using CD spectroscopy, thermal denaturation measurements, CW‐EPR as well as PELDOR we analyzed and compared the influence of dŲ and Çm on a self‐complementary RNA duplex. Our results demonstrate that the conformational diversity of dŲ is significantly reduced near the freezing temperature of a phosphate buffer, resulting in strongly orientation‐selective PELDOR time traces of the dŲ ‐labeled RNA duplex.

Quantum effects in biological systems have recently been studied extensively in the fields of quantum computing (QC) and quantum information processing (QIP). The focus of this review is on quantum coherences and entanglement properties created in natural photosynthesis, whose understanding may be crucial for achieving the remarkable efficiency of its excitation energy transfer. In the beginning, an overview of electron and energy transfer in photosynthetic reaction centers (RCs) and light-harvesting complexes (LHCs) is given. Then the physical aspects of spin-correlated radical pairs (SCRPs) are described, which are ubiquitous intermediates in a wide range of biochemical reactions. Examples are given mainly with relation to quantum coherences in RCs and LHCs, which persist at room temperature, since such long-lived quantum coherences are crucial for quantum information storage and manipulation. Where appropriate, experimental observations of quantum coherences in artificial molecular assemblies are also briefly surveyed. In the second part, site-directed spin-labeling and pulsed electron-electron double resonance (PELDOR or DEER) are described, which are becoming important techniques in QC/QIP.

The oxidation of a redox-active tyrosine residue Y(Z) in photosystem II (PSII) is coupled with proton transfer to a hydrogen-bonded D1-His190 residue. Because of the apparent proximity of Y(Z) to the water-oxidizing complex and its redox activity, it is believed that Y(Z) plays a significant role in water oxidation in PSII. We investigated the g-anisotropy of the tyrosine radical Y(Z)(•) to provide insight into the mechanism of Y(Z)(•) proton-coupled electron transfer in Mn-depleted PSII. The anisotropy was highly resolved by electron paramagnetic resonance spectroscopy at the W-band (94.9 GHz) using PSII single crystals. The g(X)-component along the phenolic C-O bond of Y(Z)(•) was calculated by density functional theory (DFT). It was concluded from the highly resolved g-anisotropy that Y(Z) loses a phenol proton to D1-His190 upon tyrosine oxidation, and D1-His190 redonates the same proton back to Y(Z)(•) upon reduction.

A highly time-resolved high-frequency/high-field W-band electron paramagnetic resonance (EPR) (ν ~ 94 GHz) is a powerful technique to determine small g anisotropies of transient paramagnetic species. We applied this method to studies of the lowest excited triplet (T1)3 ππ* states in metal complexes such as a platinum (Pt) diimine complex and metal (Zn and Mg) porphines in rigid glasses. From the analyses of time-resolved EPR spectra, g anisotropies were obtained as g z = 2.0048, g x = g y = 2.0035 for Pt(b-iq)(CN)2 (b-iq = 3,3′bi-isoquinoline) and g z = 1.9968, g x = g y = 2.0022 for zinc tetraphenylporphine (ZnTPP). No measurable anisotropies were found for magnesium (Mg) TPP. The g values of the Pt complex are larger than g e (=2.0023, g value of free electron) and that g z of ZnTPP is smaller than g e. These results were interpreted in terms of the nature of the perturbed states: the higher triplet ππ′* state mixes with T1(ππ*) via spin–orbit coupling in ZnTPP. In contrast, the higher triplet dπ* state is involved in this coupling for the Pt complex. Thus, the nature of the perturbed state can be distinguished from the anisotropic g values of the T1(ππ*) state.

Pure water was sprayed by a pneumatic atomizer into a low-pressure gas cell, and the droplets were interrogated by Fourier transform infrared (FTIR) extinction spectroscopy in the region of 700−5000 cm−1. The spectral analysis using Mie scattering theory revealed that the particles have an average diameter of 170 nm and their phases are either supercooled liquid or ice, depending on the condition of atomization. Application of pneumatic atomization to an aqueous solution of BSA was also demonstrated.

Low-temperature generation of P-nitroxyl phosphane 2 (Ph2 POTEMP), which was obtained by the reaction of Ph2 PH (1) with two equivalents of TEMPO, is presented. Upon warming, phosphane 2 decomposed to give P-nitroxyl phosphane P-oxide 3 (Ph2 P(O)OTEMP) as one of the final products. This facile synthetic protocol also enabled access to P-sulfide and P-borane derivatives 7 and 13, respectively, by using Ph2 P(S)H (6) or Ph2 P(BH3 )H (11) and TEMPO. Phosphane sulfide 7 revealed a rearrangement to phosphane oxide 8 (Ph2 P(O)STEMP) in CDCl3 at ambient temperature, whereas in THF, thermal decomposition of sulfide 7 yielded salt 10 ([TEMP-H2 ][Ph2 P(S)O]). As well as EPR and detailed NMR kinetic studies, indepth theoretical studies provided an insight into the reaction pathways and spin-density distributions of the reactive intermediates.

In this work, the cage structure of a family of europium metallofullerenes, Eu@C74 and three isomers of Eu@C82, were investigated. The analogy of the electronic states of three isomers of Eu@C82 with those of three isomers of Ca@C82 was confirmed by UV−vis−near-IR absorption spectra. The symmetries Cs, C2, and C2v were assigned for three isomers of Eu@C82 by comparing each corresponding isomer of Ca@C82. The D3h cage structure of Eu@C74, which is expected only for the C74 from the isolated pentagon rule, was also confirmed by comparing its photoabsorption spectrum with that of Ca@C74. Multifrequency electron paramagnetic resonance (EPR) spectroscopy was employed to determine the zero-field splitting (ZFS) parameters of the metallofullerenes, which are closely related to the surrounding cage structures. The experimental X- and W-band EPR spectra were completely reproduced by computer simulations based on a spin Hamiltonian considering the ZFS terms up to fourth order. The introduction of the fourth-order Z...

One of lanthanoid ions, Eu2± (8S7/2), doped in a CaF2 single crystal with cubic symmetry has been studied by an Electron Spin Transient Nutation (ESTN) method of pulsed ESR spectros-copy. In a two-dimensional ESTN spectrum of the Eu2± ion, the nutation frequencies indicated the dependence of hyperfine ESR transitions as well as of spin quantum number S and MS manifolds. It is demonstrated that the 2D-ESTN method discriminates the hyperfine allowed ESR transitions from the hyperfine forbidden ones. Such discrimination of the ESR transitions enables us to make facile assignments of complicated ESR spectra. The nutation frequencies observed for the hyperfine allowed / forbidden transitions were quantitatively understood with the help of reduced rotation matrix elements.

Molecular magnets Cu(hfac)(2)L(R) represent a new type of photoswitchable materials based on exchange-coupled clusters of copper(II) with stable nitroxide radicals. It was found recently that the photoinduced spin state of these compounds is metastable on the time scale of hours at cryogenic temperatures, similar to the light-induced excited spin state trapping phenomenon well-known for many spin-crossover compounds. Our previous studies have shown that electron paramagnetic resonance (EPR) in continuous wave (CW) mode allows for studying the light-induced spin state conversion and relaxation in the Cu(hfac)(2)L(R) family. However, light-induced spin dynamics in these compounds has not been studied on the sub-second time scale so far. In this work we report the first time-resolved (TR) EPR study of light-induced spin state switching and relaxation in Cu(hfac)(2)L(R) with nanosecond temporal resolution. To enhance spectral resolution we used high-frequency TR EPR at W-band (94 GHz). We first discuss the peculiarities of applying TR EPR to the solid-phase compounds Cu(hfac)(2)L(R) at low (liquid helium) temperatures and approaches developed for photoswitching/relaxation studies. Then we analyze the kinetics of the excited spin state at T = 5-21 K. It has been found that the photoinduced spin state is formed at time delays shorter than 100 ns. It has also been found that the observed relaxation of the excited state is exponential on the nanosecond time scale, with the decay rate depending linearly on temperature. We propose and discuss possible mechanisms of these processes and correlate them with previously obtained CW EPR data.

The lowest excited triplet (T(1)) ππ* states of gallium (Ga) and various rhodium (Rh) 5,10,15-trispentafluorophenyl corroles (Cors) were studied in the liquid crystal (LC) E-7 and in rigid glasses by time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy. The triplet sublevel energies were experimentally determined by the alignment of the molecules in the LC and by magnetophotoselection in the glass. The sublevel scheme of GaCor was determined by calculating the zero field splitting (ZFS) parameters. Axial ligand effects and quantum chemical calculations were used for the sublevel assignment of RhCors. The anisotropic EPR parameters were used to determine the important higher excited states and the magnitudes of their spin-orbit coupling (SOC) contributions were evaluated. On the basis of these results and analyses, the EPR parameters and triplet lifetime were discussed for each RhCor complex.

Syntheses and magnetic functionalities of exchange-coupled magnetic systems in a controlled fashion of molecular basis have been the focus of the current topics in chemistry and materials science; particularly extremely large spins in molecular frames and molecular high-spin clusters have attracted much attention among the diverse topics of molecule-based magnetics and high spin chemistry. Magnetic characterizations of molecule-based exchange-coupled high-spin clusters are described in terms of conventional as well as high-field/high-frequency ESR spectroscopy. Off-principal-axis extra lines as a salient feature of fine-structure ESR spectroscopy in non-oriented media are emphasized in the spectral analyses. Pulse-ESR-based two-dimensional electron spin transient nutation spectroscopy applied to molecular high-spin clusters is also dealt with, briefly. Solution-phase fine-structure ESR spectroscopy is reviewed in terms of molecular magnetics. In addition to finite molecular high-spin clusters, salient features of molecule-based low-dimensional magnetic materials are dealt with. Throughout the chapter, electron spin resonance for high-spin systems is treated in a general manner in terms of theory. Hybrid eigenfield method is formulated in terms of direct products, and is described as a powerful and facile approach to the exact numerical calculation of resonance fields and transition probabilities for molecular high spin systems. Exact analytical expressions for resonance fields of high spin systems in their principal orientations are for the first time given.

Carbohydrate aqueous solutions of trehalose, glucose, and fructose were sprayed by using a pneumatic atomizer, and their infrared extinction spectra were recorded for the region from 700 to 5000 cm(-1). Analysis based on Mie scattering theory indicated that sprayed droplets transformed into nonvolatile amorphous nanoparticles by solvent evaporation. Average diameters of these particles were estimated to be about 100 nm, which was further confirmed by differential mobility analysis. The results demonstrate that nanoparticles can be created by spray drying of aqueous solutions, and that the sizes, phases, and structures of these particles can be well characterized by infrared extinction spectroscopy.

The multiline signal from the S2-state manganese cluster in the oxygen evolving complex of photosystem II (PSII) was observed in single crystals of a thermophilic cyanobacterium Thermosynechococcus vulcanus for the first time by W-band (94 GHz) electron paramagnetic resonance (EPR). At W-band, spectra were characterized by the g-anisotropy, which enabled the precise determination of the tensor. Distinct hyperfine splittings (hfs's) as seen in frozen solutions of PSII at X-band (9.5 GHz) were detected in most of the crystal orientations relative to the magnetic field. In some orientations, however, the hfs's disappeared due to overlapping of a large number of EPR lines from eight crystallographic symmetry-related sites of the manganese cluster within the unit cell of the crystal. Analysis of the orientation-dependent spectral features yielded the following g-tensor components: g(x) = 1.988, g(y) = 1.981, g(z) = 1.965. The principal values suggested an approximate axial symmetry around the Mn(III) ion in the cluster.

Syntheses and magnetic functionalities of exchange-coupled magnetic systemsin a controlled fashion of molecular basis have been the focus of the current topics in chemistry and materials science; particularly extremely large spins in molecular frames and molecular high-spin clusters have attracted much attention among the diverse topics of molecule-based magnetics and high spin chemistry. Magnetic characterizations of molecule-based exchange-coupled high-spin clusters are described in terms of conventional as well as highfield/high-frequency ESR spectroscopy. Off-principal-axis extra lines as a salient feature of fine structure ESR spectroscopy in non-oriented media are emphasized in the spectral analyses. Pulse-ESR-based two-dimensional electron spin transient nutation spectroscopy applied to molecular high-spin clusters is also dealt with, briefly. Solution-phase fine-structure ESR spectroscopy is reviewed in terms of molecular magnetics. In addition to finite molecular high-spin clusters, salient features of molecule-based low-dimensional magnetic materials are dealt with. Throughout the chapter, electron spin resonance for high-spin systems is treated in a general manner in terms of theory. Hybrid eigenfield method is formulated in terms of direct products, and is described as a powerful and facile approach to the exact numerical calculation of resonance fields and transition probabilities for molecular high spin systems. Exact analytical expressions for resonance fields of high spin systems in their principal orientations are for the first time given.

A lanthanoid ion, Eu2+(8S7/2), doped in a CaF2 single crystal with cubic symmetry has been studied by an Electron Spin Transient Nutation (ESTN) method based on pulsed ESR spectroscopy. In a two-dimensional ESTN spectrum of the Eu2+ ion, the nutation frequencies indicated the dependence of hyperfine ESR transitions as well as of spin quantum number S and MS manifolds. It is demonstrated that the 2D-ESTN method discriminates the hyperfine allowed ESR transitions from the hyperfine forbidden ones. Such discrimination of the ESR transitions enables us to make facile assignments of complicated ESR spectra. The nutation frequencies observed for the hyperfine allowed and forbidden transitions were quantitatively understood with the help of reduced rotation matrix elements.

High-spin Co(II) complexes (S = 3/2) with hydrogen bonding network were synthesized as models for magnetic quantum wires or 2D magnetic network systems. Their magnetic properties were characterized in terms of the SQIUD measurement. An analysis for the magnetic susceptibilities assuming an effective angular momentum L'=1 reproduced the experimental temperature dependences of the susceptibilities. In order to evaluate intermolecular magnetic interactions, it turned out that models beyond Heisenberg-Dirac exchange Hamiltonian are required for the analysis.

Regiospecific synthesis of 4-substituted pyridines is attained via alkylation of diisopropyl 1-ethoxycarbonyl-1, 4-dihydropyridine-4-phosphonate followed by treatment with butyllithium. The phosphonate is prepared directly from pyridine in one pot.

A mixture of molecular high-spin species is frequently observed in a variety of model systems for molecule-based magnetics. Two-Dimensional Electron Spin Transient Nutation (2D-ESTN) spectroscopy based on pulsed ESR is useful to discriminate between high-spin species in spin mixtures and identify unequivocally the spin quantum number of them. In this work, we have performed the spectral simulation of the 2D ESTN spectrum including forbidden transitions for Eu 2+ ion (S=7/2, I=5/2) in a CaF 2 single crystal, which has two isotopes ( 151 Eu (47.9%) and 153 Eu (52.1%)), based on the transition moment analysis including nuclear spin transitions. The simulated spectrum is reasonably consistent with the experimental one, indicating that the transition moment spectroscopy-based simulation is a powerful tool for the unequivocal identification of the 2D-ESTN spectra from a mixture of electronic and nuclear high-spin systems.

This work discusses an issue caused by omitting the fourth-order zero-field splitting (ZFS) terms in the analysis of random-orientation EPR spectra. Eu(II)-doped strontium aluminates were employed as a model system of theS-state lanthanoid ions, which have relatively large second-rank ZFS components, comparable to the conventional X-band microwave energy. The initial estimation of second- and fourth-rank ZFS components was acquired uniquely from the W-band spectrum based on a perturbation treatment of the spin Hamiltonian, and they were refined by a hybrid-eigenfield simulation approach. A parallel simulation of the X- and W-band spectra with the refined parameters reasonably reproduced both the observed spectra. The reliability of the parameters was also verified by reproducing the off-principal-axis extra lines arising from the Ms = ½ ↔ -½ transition observed in the W-band spectrum. By comparison with an analysis based on the spin Hamiltonian omitting the fourth-order terms, it was concluded that omitting these terms results in an erroneous determination of the rhombic parameter (E) of the second-rank ZFS tensor. The obtained reliable second-rank ZFS tensors enabled us to invoke the superposition model, determining their principal-axis orientation. This is the first report of such relative orientation determined for Eu(II)-doped strontium aluminates.

This work demonstrates the usefulness of pulsed electron spin resonance (ESR)-based two-dimensional electron spin transient nutation (2-D ESN) spectroscopy for complete assignments of complicated fine-structure hyperfine ESR spectra including hyperfine forbidden transitions from electronic and nuclear high-spin systems. The 2-D ESN spectroscopy is termed transition moment spectroscopy as spectra are acquired as a function of transition moment instead of transition energy used in conventional spectroscopy. We have applied the novel spectroscopic technique to Eu2+ ion (S=7/2,I=5/2), which has two isotopes (151Eu [47.9%] and153Eu [52.1%]), in a CaF2 single crystal as a model system. We have completely identified the complicated fine-structure hyperfine ESR spectra by invoking the spectral simulation of the 2-D ESN spectra on the basis of transition moment analyses. The analyses are based on exact numerical calculations of the transition moments as well as a perturbation-based analytical approach combined with reduced rotation matrices for the nuclear part of the transition moment. This is the first example of the spectral simulation for 2-D ESN spectra including the hyperfine allowed and forbidden transitions in high-spin systems. In addition, we have made simulation of the fine-structure forbidden transitions, which reproduces the angular variations of the observed spectra at liquid helium temperatures.