Transient Nutation Research Articles

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.

Coherence times and Rabi oscillations in CaWO 4:Cr 5+ crystal

The coherence times of dopant pentavalent chromium ions in CaWO 4 single crystal (0.0006 at.% Cr 5+) were investigated both theoretically and experimentally. Temperature dependences of spin–lattice relaxation time T 1 and phase memory time T M were measured in the temperature range 6–30 K at high (94 GHz, W band) and low (3.5 GHz, S band) frequencies of electron spin resonance. It follows from T M calculations that phase relaxation of Cr 5+ ion arises mainly from magnetic dipole interactions between the chromium ions. Anomalously fast damping of Rabi oscillations is detected in both S- and W-band experiments. It is shown that this phenomenon is caused by microwave field inhomogeneity inside the resonator. Relations between the damping time of Rabi oscillations, Rabi frequency and the crystal sample size are obtained. Lumped-element resonators and smaller sample dimensions are suggested to lower spin dephasing during transient nutations.

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.

Biradical character of D-rich carriers in the insoluble organic matter of carbonaceous chondrites: A relic of the protoplanetary disk chemistry

The insoluble organic matter (IOM) of the Orgueil meteorite contains deuterium-rich radicals detectable by Electron Paramagnetic Resonance (EPR) techniques ( Gourier et al., 2008). However the structure of these deuterium carriers remains poorly constrained. In this work, their electronic structure is deduced from the measurement of the spin states S by transient nutation in pulsed-EPR. It is shown that these deuterium-rich radicals are dominated by biradicaloids (species with S = 0 ground state and thermally accessible S = 1 state) and biradicals (species with S = 1 ground state) representing ∼61% and ∼31% of the radicals in the IOM of Orgueil, respectively, while single radicals ( S = 1/2) contribute only to ∼8%. This is definitely different from mature terrestrial IOMs, which are known to contain almost exclusively S = 1/2 radicals. A structural model is proposed, whereby the occurrence of dominant biradicaloids and biradicals is the direct consequence of the structure of the IOM, made of a network of small aromatic moieties linked by branched and short aliphatic units. This implies that the formation of stable biradicaloids and biradicals by C–H breaking and their deuterium enrichment are produced after the formation of the IOM in the primitive solar system. These results reinforce the idea that the formation of the IOM and the deuterium-rich hotspots are the product of ion chemistry in the solar disk.

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.

Pulsed EPR of P1 centers in synthetic diamond under bichromatic excitation conditions

The dynamics of the interaction of P1 centers in synthetic diamond with a bichromatic radiation, representing microwave (MW) and radio frequency (RF) fields in a configuration characteristic of the stationary EPR spectroscopy with modulated magnetic field, has been studied using the transient nutation technique. It is demonstrated that a thermobaric treatment of the crystal leads to an increase in the phase relaxation time of P1 centers. Additional increase in this relaxation time is observed under the conditions of a nutation resonance, where the RF field frequency is close to the effective Rabi frequency in the MW field. These data are taken into account in considering the inversion of the EPR lines of P1 centers that was recently discovered in the stationary EPR.

Influence of disorder on electrically and optically detected electron spin nutation

A numerical study of the influence of disorder in semiconductors on spin-Rabi nutation observed with pulsed electrically or optically detected magnetic-resonance techniques (pEDMR and pODMR, respectively) is presented. It is shown that transient nutation signals of disordered spin ensembles differ from ordered ensembles as inhomogeneously broadened Land\'e-factor distributions are presented. In contrast to ordered systems, the magnitudes of spin-Rabi nutation and spin-Rabi beat nutation change significantly with a strong dependence of their ratio on the correlation of the Land\'e factors within the nearest-neighbor spin pairs. An interpretation of these results is given and their application for the investigation of disorder using pEDMR and pODMR is discussed.

On Room-Temperature Inversion of EPR Signals of P1 Centre in Synthetic Diamond

Room-temperature inversion of EPR absorption signals of P1 centre in synthetic diamond is studied by the transient nutation technique. Use of the bichromatic field, consisting of a transverse microwave field and longitudinal radio frequency field, allows to investigate the dynamics of P1 centres in the same field configuration as in cw EPR spectrometers. It is shown that the annealing decreases the P1 centre concentration and, respectively, increases the spin-spin relaxation time. As a result, the periodic inversion (nutation) of the P1 centre absorption signal is observed longer. It is assumed that the P1 centre signal inversion, which was previously observed by cw EPR, might be caused by the Bloch–Siegert effect in the bichromatic field.

A High-Spin (S = 2) Macrocycle

The nitroxide radicals in macro­cycle 2 form a quintett state (S = 2) as shown by two-dimensional pulse-based electron-spin transient nutation (2D-ESTN) spectroscopy. Compound 2 crystallizes in a 1,3-alternate conformation and was obtained from the tetrabromo derivative 1 by lithiation, reaction with (t-BuNO)2 and subsequent oxidation. The monomeric unit 4 was also prepared to study differences in the spin structures of 4 and 2.

Probing the Photoexcited States of Rhodium Corroles by Time-Resolved Q-Band EPR. Observation of Strong Spin−Orbit Coupling Effects

The photoexcited states of two 5,10,15-tris(pentafluorophenyl)corroles (tpfc), hosting Rh(III) in their core, namely Rh(pyr)(PPh 3)(tpfc) and Rh(PPh 3)(tpfc), have been studied by time-resolved electron paramagnetic resonance (TREPR) combined with pulsed laser excitation. Using the transient nutation technique, the spin polarized spectra are assigned to photoexcited triplet states. The spectral widths observed for the two Rh(III) corroles crucially depend on the axial ligands at the Rh(III) metal ion. In case of Rh(PPh 3)(tpfc), the TREPR spectra are found to extend over 200 mT, which exceeds the spectral width of non-transition-metal corroles by more than a factor of 3. Moreover, the EPR lines of the Rh(III) corroles are less symmetric than those of the non-transition-metal corrroles. The peculiarities in the TREPR spectra of the Rh(III) corroles can be rationalized in terms of strong spin-orbit coupling (SOC) associated with the transition-metal character of the Rh(III) ion. It is assumed that SOC in the photoexcited Rh(III) corroles effectively admixes metal centered (3)dd-states to the corrole centered (3)pipi*-states detected in the TREPR experiments. This admixture leads to an increased zero-field splitting and a large g-tensor anisotropy as manifested by the excited Rh(III) corroles.

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.

Electron spin–lattice and spin–spin relaxation study of a trinuclear iron(iii) complex and its relevance in quantum computing

Electron spins of molecular magnets are promising candidates for large scale quantum information processing because they exhibit a large number of low-lying excited states. In this paper X-band pulse electron paramagnetic resonance spectroscopy is used to determine the intrinsic relaxation times T1 and T2 of a molecular magnet with an S = 1/2 ground state, namely the neutral trinuclear oxo-centered iron (III) complex, [Fe3(micro3-O)(O2CPh)5(salox)(EtOH)(EtOH)(H2O)]. The temperature dependence of the spin-lattice relaxation time T1 between 4.5 and 11 K shows that the Orbach relaxation process is dominant with the first excited state lying 57 cm(-1) above the ground state, whereas the phase memory time T(M) is of the order of 2.6 micros and exhibits a modest temperature dependence. These results together with previous magnetic measurements give further insight into the magnetic properties of the complex. The coherent manipulation of the electron spins is also examined by means of transient nutation experiments.

Electron paramagnetic resonance and non‐resonant microwave absorption of single wall carbon nanotubes

AbstractSingle wall carbon nanotubes (SWNT) prepared by the “super growth” method exhibit electron paramagnetic resonance (EPR) signals, which originate from temperature activated detrapping of spins. Non‐resonant microwave absorption in the temperature range below 20 K is indicative for the opening of a small gap at the Fermi level for tubes of metallic character, which might indicate a transition into a superconducting (SC) state. The expected concurrent microwave field inhomogeneity has been detected by a transient nutation experiment. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Multiphoton transitions in a spin system driven by strong bichromatic field

EPR transient nutation spectroscopy is used to measure the effective field (Rabi frequency) for multiphoton transitions in a two-level spin system bichromatically driven by a transverse microwave (MW) field and a longitudinal radio-frequency (RF) field. The behavior of the effective field amplitude is examined in the case of a relatively strong MW field, when the derivation of the effective Hamiltonian cannot be reduced to first-order perturbation theory in ω1/ωrf(ω1 is the microwave Rabi frequency, ωrf is the RF frequency). Experimental results are consistently interpreted by taking into account the contributions of second and third order in ω1/ωrf evaluated by Krylov-Bogolyubov-Mitropolsky averaging. In the case of inhomogeneously broadened EPR line, the third-order correction modifies the nutation frequency, while the second-order correction gives rise to a change in the nutation amplitude due to a Bloch-Siegert shift.

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.

Two-photon nutation excited in a two-level spin system by microwave and RF fields

Two-photon transient nutation is observed in a two-level spin system (E′1 centers in crystalline quartz) using a transverse microwave field and a linearly polarized rf field oriented along a static magnetic field in the electron paramagnetic resonance. Nutation is excited when the sum of the energies of a microwave photon and a rf photon is equal to the energy difference between two spin states. The two-photon nature of nutation is confirmed by measuring its frequency as a function of the amplitude and frequency of the rf field as well as the amplitude of the microwave field. The amplitude of the effective field of two-photon transitions is measured. It is shown that the decay rate of two-photon nutation is close to the decay rate for one-photon nutation and is determined by the spin-spin interaction between E′1 centers.

Transient nutation of dressed spin states of E′1 centers in a quartz crystal

The transient nutation of states dressed by a microwave field in a two-level system (E′1 centers in a quartz crystal) is observed in pulsed electron paramagnetic resonance (EPR) in the course of an additional pulse of a linearly polarized radio-frequency (rf) field that has an amplitude 2B2 and is applied parallel to the static magnetic field. It is shown that, when the frequency of the rf field coincides with the frequency of nutation of the bare spin system, the signal of this nutation is modulated by the nutation of dressed states at the frequency ω2=γB2, where γ is the electron gyromagnetic ratio. The decay time of nutation of dressed states is considerably (no less than four times) longer than that of bare states of E′1 centers due to spin-spin relaxation and correlates with the spin-lattice relaxation time in the rotating coordinate system.

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.