Spin Transient Nutation Research Articles

Pulse-based electron spin transient nutation measurement of BaTiO3 fine particle: Identification of controversial signal around g = 2.00

Two dimensional pulse-based electron spin transient nutation (2D-ESTN) spectroscopy is a powerful tool for determining the spin quantum number and has been applied to BaTiO3 fine powder in order to identify the origin of the continuous wave electron spin resonance (CW-ESR) signal around g = 2.00. The signal is frequently observed in BaTiO3 ceramics, and the correlation between the signal intensity and positive temperature coefficient of resistivity (PTCR) properties has been reported to date. The CW-ESR spectrum of BaTiO3 fine particles synthesized by the sol-gel method shows a typical asymmetric signal at g = 2.004. The 2D-ESTN measurements of the sample clearly reveal that the signal belongs to the S = 5/2 high spin state, indicating that the signal is not due to a point defect as suggested by a number of researchers but rather to a transition metal ion. Our elemental analysis, as well as previous studies, indicates that the origin of the g = 2.004 signal is due to the presence of an Fe3+ impurity. The D value (second-order fine structure parameter) reveals that the origin of the signal is an Fe3+ center with distant charge compensation. In addition, we show a peculiar temperature dependence of the CW-ESR spectrum, suggesting that the phase transition behavior of a BaTiO3 fine particle is quite different from that of a bulk single crystal. Our identification does not contradict a vacancy-mediated mechanism for PTCR. However, it is incorrect to use the signal at g = 2.00 as evidence to support the vacancy-mediated mechanism.

Oxidation of allenes bearing 1,8‐diphenoxy or diaryloxyacridene moieties

New allene compounds bearing 1,8‐diphenoxy or bis(3,5‐di‐tert‐butylphenyl) oxyacridene (diaryloxyacridene) moieties were synthesized. These compounds were more easily oxidized than Cl‐substituted compounds. The allene compounds were oxidized to generate thermally stable triplet carbenes. Electron spin resonance and electron spin transient nutation spectra of the oxidation reaction products suggested the generation of triplet species; however, the main isolated product from the 1,8‐diphenoxy allene was a cyclized compound that was characterized by X‐ray analysis. The product derived from diaryloxy allene was also characterized by X‐ray analysis as a singlet species, which included a vinyl cation stabilized by a neighboring O(3,5‐di‐tert‐butylphenyl) group. This singlet species was stable even at room temperature and did not show any electron spin resonance signals in solution, indicating that the triplet species was not in equilibrium with the singlet. Copyright © 2016 John Wiley & Sons, Ltd.

Oxidation of an allene compound bearing 1,8‐dichloroacridene moieties and photolysis of the halogenated allene compound for the generation of triplet carbenes

New allene compounds, 5c and 5d, bearing 1,8‐dichloroacridene moieties were synthesized, and the oxidation of 5c and 5d was performed to generate thermally stable triplet carbenes. Since the oxidation resulted in the formation of decomposition products, the photolysis of brominated allene compound 18 was performed. UV–vis measurement at low temperatures indicated the generation of intermediates with decreasing starting material 18, although the efficiency of photolysis was quite low. ESR and electron spin transient nutation (ESTN) spectra suggested that the multiplicity of intermediates is triplet, probably the target triplet carbene 6d is generated. In terms of |D| values of the intermediates, ESR showed that unpaired electrons are more effectively delocalized in 6d by the introduction of nitrogen atoms in the skeleton (biradical character in Scheme 1). The observed triplet species 6d was not stable enough to isolate. Copyright © 2014 John Wiley & Sons, Ltd.

Determination of the principal g-values of Type I or highly-anisotropic low spin (HALS) ferriheme centers in frozen solutions

Continuous wave (CW) electron paramagnetic resonance (EPR) spectroscopy of highly-anisotropic low spin (HALS) ferric heme centers in frozen solutions is not a very informative approach because usually only one feature is reliably observed in the spectra, that at the maximal principal g-value of, typically, 3.3–3.79. The other two EPR turning points are severely broadened by g-strain and are not easily observed in the first-derivative CW EPR spectra. In this work, we have explored the potential of alternative EPR techniques, the electron spin echo (ESE) field sweep and electron spin transient nutation (TN), for obtaining information about the g-tensors of such systems, using as an example a typical HALS ferric heme center, [FeIII(15N-coproporphyrin)(CN)2]. The analysis of the experimental g-tensor of [FeIII(15N-coproporphyrin)(CN)2]− has shown that the widths of the underlying energy distributions for this HALS center are comparable to those found for the rhombic bis-imidazole complex. The greater effect on the g-value distributions for HALS centers is determined by near degeneracy of two of the three lower-energy d-orbitals, dyz and dxz, which contain the unpaired electron.

Ferromagnetic Spin Coupling between Endohedral Metallofullerene La@C82 and a Cyclodimeric Copper Porphyrin upon Inclusion

The cyclic host cyclo-[P(Cu)](2) carrying two covalently connected Cu(II) porphyrin units can accommodate La@C(82), a paramagnetic endohedral metallofullerene, in its cavity to form the inclusion complex cyclo-[P(Cu)](2)⊃La@C(82), which can be transformed into the caged complex cage-[P(Cu)](2)⊃La@C(82) by ring-closing olefin metathesis of its side-chain olefinic termini. On the basis of electron spin resonance (ESR) and electron spin transient nutation (ESTN) studies, cyclo-[P(Cu)](2)⊃La@C(82) is the first ferromagnetically coupled inclusion complex featuring La@C(82), whereas cage-[P(Cu)](2)⊃La@C(82) is ferrimagnetic.

Decay of rabi oscillations induced by magnetic dipole interactions in dilute paramagnetic solids

Decay of Rabi oscillations of equivalent spins diluted in diamagnetic solid matrix and coupled by magnetic dipole interactions is studied. It is shown that these interactions result in random shifts of spin transient nutation frequencies and thus lead to the decay of the transient signal. Averaging over random spatial distribution of spins within the solid and over their spectral positions within magnetic resonance line, we obtain analytical expressions for the decay of Rabi oscillations. The rate of the decay in the case when the half-width of magnetic resonance line exceeds Rabi frequency is found to depend on the intensity of resonant microwave field and on the spin concentration. The results are compared with the literature data for E1' centers in glassy silica and [AlO4] centers in quartz.

Organic polyanionic high-spin molecular clusters: topological-symmetry controlled models for organic ferromagnetic metals

Trianionic spin-quartet and tetraanionic spin-quintet molecular clusters derived from m-dibenzoylbenzene in solution were identified by CW-ESR/pulse-ESR based two-dimensional electron spin transient nutation spectroscopy, and their spin and clustering structures in the ground state were determined in terms of a D-tensor based phenomenological approach and DFT calculations. The molecular structures obtained semiempirically are supported by DFT-based quantum chemical calculations. The DFT calculations have been tested for a sodium ion bridged fluorenone-based cluster, [fluorenone(-)˙ {Na(+)(dme)(2)}](2), whose crystal structure was reported in the literature [H. Bock, H.-F. Herrmann, D. Fenske and H. Goesmann, Angew. Chem., Int. Ed. Engl., 1988, 27, 1067], reproducing the experimentally determined moelcular structure of the dimer cluster. It is suggested that both the quartet and quintet clusters in the 2-MTHF glass and solution form the cross-typed structures with the two m-dibenzoylbenzene moieties in cis-configuration. A dianionic spin-triplet m-dibenzoylbenzene derivative was detected for the first time and its charge and spin densities were studied by the quantum chemical calculations. The high-spin states of the open-shell entities under study were confirmed by X-band pulse-ESR based electron spin nutation spectroscopy in organic frozen glasses. The D values and other spin Hamiltonian parameters of all the polyanionic high-spin species were determined by the hybrid eigenfield spectral simulation for fine-structure ESR spectra. m-Dibenzoylbenzene provides pseudo-degenerate π-LUMOs arising from its topological symmetry of the π-electron network and its dianion in the triplet ground state is a prototypical model for topologically-controlled genuinely organic ferromagnetic metals.

Synthesis and Identification of a Trimethylenemethane Derivative π-Extended with Three Pyridinyl Radicals

A trimethylenemethane (TMM) derivative 1(2*), in which the parent TMM is pi-extended by the symmetric insertion of three pyridine rings into the C-C bonds of TMM, has been synthesized by the alkali metal reduction of the isolated corresponding dication. Although the frozen-glass X-band cw-ESR spectrum of 1(2*) gave unresolved fine structures due to the small ZFS parameters, pulsed ESR two-dimensional electron spin transient nutation (2D-ESTN) spectroscopy unambiguously can afford to identify diradical 1(2*) as a triplet species.

Electronic and molecular structures of C 60-based polyanionic high-spin molecular clusters: Direct spin identification and electron spin transient nutation spectroscopy for high-spin chemistry

C 60-based polyanionic high-spin clusters ( S = 1–3) in their ground state have been prepared by successive chemical reductions of pristine C 60 fullerene with potassium in the presence of dicyclohexano-18-crown-6-ether in solution. Intermolecular spin-triplet, quintet, and septet states arising from the C 60-based polyanionic molecular clusters have been generated at ambient temperature and identified by CW-ESR and pulse-ESR-based two-dimensional (2D) electron spin transient nutation (2D-ESTN) spectroscopy in organic rigid glasses, for the first time. Intermolecular exchange interactions between mono- and polyanionic C 60 fullerenes are ferromagnetic via bridging potassium metal cations. The molecular structures of the polyanionic high-spin C 60 clusters in solution have been proposed by a well-established phenomenological spin Hamiltonian approach in terms of the D -tensor-based calculations for the high-spin clusters. The findings of the C 60-based high-spin molecular clusters evidence the occurrence of an intramolecular triplet C 60 dianion in the ground state. Unequivocal spin identification for molecular high-spin entities by 2D-ESTN spectroscopy and its powerfulness have been illustrated, emphasizing that the 2D-ESTN spectroscopy is useful for mixtures of molecular species with different spin multiplicities, characterized by a small g-anisotropy, for which the powerfulness of advanced high-frequency ESR spectroscopy is hampered.

The Poly(Radical Cation) of a Star‐Shaped Oligoarylamine — Detection of Excited High‐Spin States

AbstractThe intramolecular spin–spin interaction in the highly oxidized species of a star‐shaped oligoarylamine, namely hexakis{4‐[bis(p‐methoxyphenyl)amino]phenyl}benzene (1), in which six diphenylamine units are introduced as redox‐active sites into a central hexaphenylbenzene core, has been investigated by EPR spectroscopy. The hexacation‐rich sample was prepared by chemical oxidation with 7.5 equivalents. of [bis(trifluoroacetoxy)iodo]benzene (PIFA) in the presence of 10 vol.‐% trifluoroacetic acid (TFA) in dichloromethane at 195 K. Variable‐temperature EPR measurements provided the non‐Curie temperature dependence of the signal intensity. The fine structure of the EPR spectrum also varies with temperature. In order to identify the spin multiplicity of the generated poly(radical cation), we carried out electron spin transient nutation (ESTN) measurements based on pulsed EPR spectroscopy at various temperatures. From the ESTN spectra, it was found that the high‐spin states are thermally populated with increasing temperature. These results indicate the low‐spin ground state of the poly(radical cation) of 1. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Spin-multiplicity of a moderately coupled triplet–doublet spin pair in a biphenylene-linked porphyrin dimer

Electron spin transient nutation spectroscopy has been applied to a moderately coupled excited triplet and doublet spin-pair. In a biphenylene-linked copper(II)-free base porphyrin dimer, laser irradiation of either the free base half or the copper half, leads to excitation to the excited states and relaxation to the lowest excited state of the dimer. In this lowest excited state, the free base moiety in the lowest triplet state is coupled to the copper porphyrin ground doublet state, and thus forming a triplet–doublet spin pair. Time-resolved electron paramagnetic resonance (TREPR) spectra of the dimer in frozen solution at 40 K show much wider spectral width and a prominent band in the centre of the spectra than either of those for the free base monomer triplet or that of the copper(II) monomer porphyrin. These features are quite different from the strongly-coupled and weakly-coupled triplet–doublet spin pairs. Transient nutation method by pulsed EPR is used to examine the spin-multiplicity of the excited state in the dimer at three magnetic field positions. It was found that spin-multiplicity depends largely on the magnetic field. This is rationalized in terms of an intermediate size of coupling. In such intermediate coupling cases, the degree of mixing of the triplet and doublet basis wavefunctions depends greatly on the relative orientation between magnetic field and molecular axis because the ratio of the coupling size to an energy difference between the unperturbed species, triplet and doublet, varies with magnetic field direction. A splitting of TREPR spectra in partially oriented samples suggests that exchange coupling |J| is around 10 mT, which reasonably matches the results of the pulsed experiments.

A Bindschedler's Green-Based Arylamine: Its Polycations with High-Spin Multiplicity

Intramolecular high-spin correlation in a series of the successively generated polycationic species of Bindschedler's green-based arylamine, N,N,N‘,N‘,N‘‘,N‘‘-hexakis[4-(dimethylamino)phenyl]-1,3,5-benzenetriamine (1), has been investigated by continuous wave (CW) and pulsed EPR spectroscopy. Cyclic voltammetry shows multiredox behavior of 1 that can be reversibly oxidized from monocation to hexacation. Depending on the quantity of the added oxidant, the characteristic EPR spectra are observed for polycations of 1 in frozen solution. Unequivocal determination of the spin state at each oxidation stage of 1 is given by a pulsed EPR technique, that is, electron spin transient nutation spectroscopy.

Spin States of C603-and C120On-(n= 2, 3, 4) Anions Using Electron Spin Transient Nutation Spectroscopy

Electron spin transient nutation (ESTN) experiments show that the spin multiplicity of the ground state of C-60(3-) in frozen solution is a doublet with S = 1/2. In purified samples, there is no evidence for excited states or other species with higher multiplicity. In the anions Of C120On- (n = 2, 3, 4), where the CW EPR experiments have shown that a mixture of species is present, ESTN experiments confirm that a doublet with S = 1/2 is associated with the 3- anion and triplets with S = 1 are associated with the 2- and 4- anions. A weak nutation peak attributable to m(s) = -1/2 1/2 transitions within a quartet state may arise from association of anions with spins of 1/2 and 1 in solute aggregates.

2-D electron spin transient nutation spectroscopy of Lanthanoid ion Eu2+(8S7/2) in a CaF2 single crystal on the basis of FT-pulsed electron spin resonance spectroscopy: Transition moment spectroscopy

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.

A cw/pulsed ESR study of a calix[4]arene biradical with two nitroxides

A calix[4]arene based biradical with two tert-butyl nitroxide radicals has been studied by cw and pulsed ESR spectroscopy. Two nitroxide radicals antiferromagnetically interact with each other, generating a thermally accessible triplet state. The spin multiplicity of the biradical was identified by a pulsed ESR-based 2D-electron spin transient nutation (ESTN) spectroscopy. Based on spin Hamiltonian parameters of the biradical determined from a spectrum simulation, a salient electronic local structure of the radical sites with π-orbitals on the nitrogen atoms facing each other was determined.

Electronic and Molecular Structures of Dendrimeric High-spin Polycarbenes as Studied by CW and Pulsed ESR-based Electron Spin Transient Nutation Spectroscopy

Highly symmetric high-spin polycarbenes of π-conjugation, which are the first and second generations of the spin-mediated dendrimers based on phenylacetylenes, have been studied by cw and pulsed ESR spectroscopy. We measured pulsed ESR-based two-dimensional electron-spin transient nutation (2D-ESTN) spectra of the stabilized polycarbene systems connecting the phenylacetylene moieties at the meta-positions of benzene rings, and we determined their spin multiplicity with the help of a transition moment analysis. A septet ground state of the triscarbene system, which is the first generation of the phenylacetylene dendrimers, was unequivocally discriminated. It is indicated that the stable diphenylcarbene serves as a useful building block and the phenylacetylene dendrimer as a ferromagnetic coupling unit with robust spin polarization for super high-spin polymeric systems and super paramagnetic materials.

Electronic and Molecular Structures of Dendrimeric High-spin Polycarbenes as Studied by CW and Pulsed ESR-based Electron Spin Transient Nutation Spectroscopy

Highly symmetric high-spin polycarbenes of π-conjugation, which are the first and second generations of the spin-mediated dendrimers based on phenylacetylenes, have been studied by cw and pulsed ESR spectroscopy. We measured pulsed ESR-based two-dimensional electron-spin transient nutation (2D-ESTN) spectra of the stabilized polycarbene systems connecting the phenylacetylene moieties at the meta-positions of benzene rings, and we determined their spin multiplicity with the help of a transition moment analysis. A septet ground state of the triscarbene system, which is the first generation of the phenylacetylene dendrimers, was unequivocally discriminated. It is indicated that the stable diphenylcarbene serves as a useful building block and the phenylacetylene dendrimer as a ferromagnetic coupling unit with robust spin polarization for super high-spin polymeric systems and super paramagnetic materials.

Extremely π-Delocalized Nature of Dianthrylcarbene-Based High Spin Systems as Studied by CW and Pulsed 2D Electron Spin Transient Nutation Spectroscopy

Electronic and molecular structures of bis(9,9′-dianthrylcarbene) and tris(9,9′-dianthrylcarbene) coupled by 1,4-diethynylbenzene and 1,3,5-triethynylbenzene, respectively, have been investigated by cw and pulsed ESR spectroscopy. Their spin multiplicities were identified by two-dimensional electron spin transient nutation (2D-ESTN) method based on the pulsed ESR technique. The fine-structure ESR spectra observed after UV-irradiation of the corresponding diazo precursor were analyzed with the help of a hybrid eigenfield method. The biscarbene is spin-singlet in the ground state and a thermally accessible triplet state with an axial symmetry was detected, showing that the biscarbene undergoes a linear and perpendicular conformation of the two 9,9′-dianthrylcarbene moieties. Such a conformation gives rise to an extremely large amount of π-spin delocalization in ethynylbenzene-based 9,9′-dianthrylcarbene derivatives. The electronic and molecular structures of tris(9,9′-dianthrylcarbene) in the septet ground state are also discussed in this context. The oligocarbenes under study are a novel series of Skell-type high-spin carbenes characterized by extremely π-delocalized nature.

Electronic states of polyanionic high-spin C 60 fullerene and their clusters: anomalous spin relaxation and Jahn-Teller effects

Tri- and dianionic C 60 fullerenes and polyanionic C 60 clusters were prepared by the successive chemical reduction of C 60 with potassium metal in solution. Trianionic spin quartet C 60 and polyanionic high-spin C 60 clusters were detected by cw-ESR and pulsed ESR-based 2D Electron Spin Transient Nutation (2D-ESTN) spectroscopy. The high spin states of annealed polyanionic C 60 show a spin relaxation anomaly. A phonon bottleneck phenomenon is responsible for the anomaly and the phenomenon is attributable to the occurrence of Jahn-Teller hot phonon molecular states of polyanionic C 60 fullerenes. Wiggling behaviours of the T 1 relaxation curves were observed for both the annealed tri- and dianionic C 60 . The peculiar time profiles were identified to arise from a quantum spin tunnelling between the Jahn-Teller assisted vibronic states. Polyanionic C 60 -based molecular clusters have high-spin ground states (S=1, 3/2, 2, 3). Their clustering structures were proposed in terms of tensorial analyses of the fine structure parameters.

Pulsed ESR/2D-electron spin transient nutation study of stable high-spin polycarbenes

Electronic and molecular structures of high-spin polycarbenes with dendrimeric structure, which are topologically extended using ethynyl and phenyl groups, have been studied by cw- and pulsed ESR spectroscopy. Two-dimensional electron spin transient nutation method based on pulsed ESR spectroscopy was applied to the polycarbenes in order to identify their molecular spin multiplicity. We determined the fine-structure parameters of the triscarbene and the molecular structure based on a semiempirical treatment of a fine-structure tensor. It is shown that the stable brominated diphenylcarbenes with the triplet ground state serve as useful building blocks for the super high-spin molecules and the superparamagnetic materials.