The sign of the exchange interaction between triplet excited fullerene and nitroxide free radicals

The sign of the exchange interactionJ in a series of radical triplet pairs (RTPs), formed by a nitroxide free radical and a triplet excited fullerene, has been determined from the spin polarization of time-resolved electron paramagnetic resonance spectra. Radical and fullerene are linked together by covalent bonds in different geometries. It is shown that the sign ofJ depends on the overlap between the orbital of nitroxide unpaired electron and the LUMO of fullerene, which is singly occupied in the excited triplet state. When the overlap does not vanish, a negative contribution toJ arises from the admixing of a charge transfer structure in the wave function of the excited doublet state D* of the RTP, which does not take place in the excited quartet state Q*. The mixing of D* and Q* states lowers the energy of the former spin state and gives antiferromagnetic coupling.     

Excited quartet and doublet states in the complex of tetraphenylporphine zinc (II) and a nitroxide radical in solution: X- and W-band time-resolved EPR studies

The photoexcited quartet (Q₁) and doublet (D₁) states of the complex of tetra-phenyl-porphine zinc (II) and a nitroxide radical have been studied in toluene solution by X-(9.4 GHz) and W-band (94 GHz) time-resolved electron paramagnetic resonance spectroscopy. The spin-polarized signals of the Q₁ and D₁ states are observed and assigned by analysis of theg-values. Line broadening and spin polarization mechanisms in this system are discussed.     

Stimulated Electron Spin Polarization in Strongly Coupled Triplet–Doublet Spin Pairs

The possibility of stimulating electron spin polarization in a system consisting of a stable paramagnetic center and a chromophore that can be excited into its triplet state is discussed. In such systems, the doublet state of the paramagnetic center couples to the excited triplet state of the chromophore and if the coupling is larger than the difference in the precession frequencies of the doublet and triplet, the eigenstates of the coupled system are quartet and doublet states. The quartet state is usually the lowest energy excited state. Following light excitation, the initial electronic relaxation to the quartet state generates strong multiplet polarization if it is governed by the spin–orbit coupling that follows the molecular symmetry. It is shown that application of a selective π-pulse to the ±3/2 ↔ ±1/2 transitions of the quartet converts this multiplet polarization into net polarization. The magnitude and orientation dependence of the generated polarization is estimated on the basis of a simple analytical model. The experimental conditions required for this net polarization to be retained in the ground state after decay of the quartet state are discussed. The viability of using this as a method to enhance the signal strength of a spin label or metal center in selective excitation experiments is considered.     

Orientation Information from the Dipolar Interaction Between a Complex in the Excited Quartet State and a Doublet Spin Label

An algorithm is proposed for deriving the position of a stable radical relative to a photoexcited quartet state from the electron spin–spin interactions measured by double resonance methods. Intersystem crossing generates multiplet polarization in the quartet state and microwave excitation of the ±3/2 ? ±1/2 transitions converts the multiplet polarization into net polarization of the ±1/2 levels. The dependence of the electron spin echo envelope modulation (ESEEM) of the +1/2 ? ?1/2 transition on the field/frequency of the stimulation pulse is demonstrated. The algorithm is tested by comparing the predicted ESEEM patterns to those from explicit numerical calculations of the spin evolution (so-called numerical experiments), which act as a model for experiment results. The comparison demonstrates that within the point-dipole approximation it is feasible to obtain not only the distance between the two paramagnetic centers but also the orientation of the distance vector relative to the principal axes of the quartet state.     

Theoretical spin density in nitroxides

The evolution of the hyperfine tensors in the nitroxide series with increasing alkyl substitution on the NO group has been studied theoretically for radicals from H₂NO to C₅H₁₀NO. A projection technique has been applied to the UHF wave-functions in order to correct spin densities for quartet contamination. The magnitude of the isotropic and anisotropic couplings reflects the substitution effect already observed on the spin distribution maps, that is a spin transfer from oxygen to nitrogen when alkyl groups are substituted to hydrogens in H₂NO. The alternation of the signs of the couplings along the chain as well as the cos² γ law for the coupling constants of atoms (C or H) in β position are verified in the series. The orientation of the anisotropic tensors with respect to the chemical bonds depends on the position of the atoms in the molecule (radical site, α, β positions); it is not affected by further substitution, except for strongly asymmetric configurations.     

CIDEP Created by the Quenching of Photo-Excited Tryptophan at Protein Surface: A Challenge to CIDEP Probing of Protein Structural Changes

Quenching of the triplet excited state of molecular tryptophan by nitroxide radical in 1,4-dioxane and water solutions was investigated by means of time-resolved electron paramagnetic resonance (EPR) and Fourier-transform (FT)-EPR. The chemically induced dynamic electron polarization (CIDEP) signals with net emissive phase were recorded at these quenching events and were analyzed through radical-triplet pair mechanism. The CIDEP time profiles were well reproduced by Bloch and kinetic equations, assuming radical-triplet pair mechanism with the appropriate quenching rate constants. From a comparison of the simulation and the experiment, CIDEP enhancement factor in 1,4-dioxane was determined to be −30 × P _eq, where P _eq is the spin polarization of nitroxide at thermal equilibrium. Net emissive CIDEP was also observed by FT-EPR measurements on the nitroxide quenching of the triplet excited state of tryptophan residue in α-lactalbumin. Magnitude of CIDEP created in α-lactalbumin/nitroxide system depends on the pH condition of α-lactalbumin solution, which is related to protein folding dynamics. We argue the CIDEP mechanism at the α-lactalbumin surface and propose a possibility of a novel CIDEP method to probe a protein surface and structural changes.     

LiNbO3∶Fe3+晶体的光谱精细结构、零场分裂参量及Jahn-Teller效应

应用不可约张量方法和群的理论构造了三角对称晶场中3d⁵组态离子的252阶可完全对角化的微扰哈密顿矩阵，利用该矩阵计算了LiNbO₃∶Fe³⁺晶体的光谱精细结构、零场分裂、晶体结构、Jahn-Teller(J-T)效应，其理论计算值与实验值相符合，并研究了自旋四重态、自旋二重态分别对基态能级的影响，证明了自旋四重态对基态能级的贡献是主要的，自旋二重态对基态能级的贡献虽很小，但却是不可忽略的.在此基础上,进一步研究了自旋-轨道耦合作用、自旋-自旋耦合作用对LiNbO₃∶Fe³⁺晶体的光谱精细结构和零场分裂参量的影响，发现自旋-轨道耦合作用是最主要的，自旋-自旋耦合作用也是不可忽略的. 研究表明，该种物质的四重态光谱结构中含有J-T效应. 其产生原因是自旋-轨道耦合及三角畸变的共同作用的结果，两者缺一不可.     

Infra-red emission spectrum of the quartet state of the F3 center in NaCl and KCl

The infra-red emission spectrum of the metastable ground spin quartet state of the F₃ center has been measured in NaCl and KCl. In KCl, it exhibits a zero-phonon line, followed by a broad band. The quartet to doublet energy value can be well explained from the F₂ center exchange energy.     

Residual polarization of negative muons implanted in C60 and K3C60

Negative muons captured by atoms with zero nuclear spin usually retain about 1/6 of their initial spin polarization P_0 after cascading down to their lowest Bohr orbital. This is particularly observed for μ^- implanted into graphite and diamond. However, μ^- implanted into a powder of the fullerene C₆₀ are found to show essentially zero residual polarization. A residual polarization of \sim P_0/6 is again seen in metallic K₃C₆₀. We speculate that these results are to be explained on the basis of hyperfine effects after completion of the muonic cascade. This revised version was published online in August 2006 with corrections to the Cover Date.     

Chemically induced dynamic electron polarization of C60 nitroxide bisadducts and spin multiplicity of their excited states

A series of biradicals consisting of a C₆₀ linked to two nitroxide addends have been studied by time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy. The series includes all four trans bisadducts and the equatorial isomer. After a visible laser pulse, the bisadducts in toluene liquid solution show chemically induced dynamic electron polarization (CIDEP) effects with a complex pattern of narrow lines in emission and enhanced absorption. From the knowledge of the sign of exchange interaction between the two nitroxide moieties, it was possible to account for the CIDEP effect in terms of an intramolecular triplet-triplet annihilation. A broad signal attributed to the quintet excited state was observed at short time delay after the laser pulse. The TR-EPR spectra recorded at low temperature in glassy matrix are assigned to an excited triplet state localized on the C₆₀, with a small interaction with the spins of the two nitroxide fragments. The assignment is based on spectral simulation and analysis of the transient nutation frequency. The small exchange interaction between excited triplet C₆₀ and nitroxide is shown to depend on the nature of the nitroxide addend.     

TR-EPR of single and double spin-labeled C60 derivatives in frozen matrices

Time-resolved electron paramagnetic resonance spectra of two single spin-labeled and two double spin-labeled C₆₀ derivatives in frozen solution are recorded with pulsed laser excitation and 100 ns time resolution. Quartet and quintet excited species are detected which arise from the electron spin coupling of the triplet excited fullerene moiety with the unpaired spin(s) of the nitroxide label(s). Despite the similar molecular structure, in both series of single and double labeled derivatives a different behavior was found, which is due to substantial difference of the energy of exchange coupling.     

Magnetic circular dichroism in the luminescence of F-centres in KI

Magnetic field induced circular polarization of the F-centre emission has been measured in KI at 1.9 K in fields up to 80 kG. Both the diamagnetic (field dependent) and the paramagnetic contributions (dependent upon the spin polarization of the relaxed excited state) have been detected. The spin polarization is produced by a pump beam modulated between left and right circular polarization. The expected behaviour of the spin polarization as a function of the frequency of the modulation has been observed. Knowledge of these small dichroic effects gives further insights into the nature of the relaxed excited state.     

EPR of photo-excited triplet states: A personal account

A sketch is presented of the path that has led from Zavoisky’s pioneering experiments to modern investigations by electron paramagnetic resonance (EPR) of the phosphorescent (S = 1) triplet state of polyatomic molecules or ions. The group-theoretical method first introduced by Wigner in his analysis of the multiplets of atomic spectroscopy, likewise provides a key for understanding the zero-field splitting and selection rules for radiative decay of the phosphorescent triplet state. Examples to illustrate the progress made through EPR experiments are selected from three fields. (i) Conformational instability on excitation. Both the zero-field splitting and the electron spin density distribution provide unique fingerprints of a triplet state’s geometry — structural information of a kind that is nonexistent for singlet states! Illustrations are provided by benzene C₆H₆ and fullerene C₆₀. (ii) The optical pumping cycle. The spin selectivity of singlet-to-triplet intersystem crossing and radiative decay of the individual spin components of the triplet state is discussed. In practice this selectivity is put to advantage by performing EPR on triplet states in zero-field by means of optical detection. In turn, such experiments have led to a detailed insight into the spin-orbit coupling mechanisms responsible for the spin selectivity of the above processes. The high sensitivity attainable with optical detection has recently culminated in EPR experiments on single molecules. (iii) Quantum interference. In a triplet state of low symmetry two of the spin sublevels may decay to the ground state by the emission of photons of a common polarization (i.e., out of plane for an aromatic hydrocarbon). In such a situation quantum interference between the two decay channels can be induced by an appropriate preparation of the excited state. An example is shown where flash-excitation in the singlet manifold followed by rapid intersystem crossing causes theS = 1 spin angular momentum to be created in a spin state which is not an eigenstate of the zero-field splitting tensor. This nonstationary character of the initial triplet state, which reflects the spin-orbit coupling pathway, is observed through the detection of a spontaneous microwave signal following the 25 ps laser flash.     

Missing fractions in heavy gases: Xe and CCl4?

The spin polarization of positive muons thermalized in Xe has been measured as a function of pressure up to 4660 Torr (6.1 atm) by the muon spin rotation (MSR) technique. At 4660 Torr, triplet muonium (F=1, M=1) accounts for about 40% of the initial muon polarization and no significant signal from diamagnetic muons has been observed. The unexpectedly slow recovery of the polarization in Xe at high pressures is discussed in conjunction with similar results seen in CCl₄ and CHCl₃ vapors.     

Excited high spin states of novel π conjugated verdazyl radicals: photoinduced spin alignment utilizing the excited molecular field

This paper reports the excited quartet (S = 3/2) and quintet (S = 2) states arising from the intramolecular radical-triplet pair in the purely organic π conjugated spin systems. A previous paper reported the excited quartet and quintet states of 9-anthracene-(4-phenyliminonitroxide) and 9,10-anthracene-bis(4-phenyliminonitroxide), respectively, in which iminonitroxide radicals are linked to the phenyl- or diphenylanthracene moiety (a spin-coupler) through the π conjugation. The similar excited quartet and quintet states were observed for the 9-anthra-cene-(4-phenylverdazyl) radical (1) and 9,10-anthracene-bis(4-phenylverdazyl) diradical (2) by time resolved electron spin resonance (TRESR). The TRESR spectrum was analysed by the ordinary spin Hamiltonian with the Zeeman and fine structure terms. For the quartet state of 1, the g value, fine structure splitting, and relative population of the Ms sublevels have been determined to be g = 2.0035, D = 0.0230 cm^?1, E = 0.0, P _1/2′ = P _?1/2′ = 0.5 and P _3/2′ = P _?3/2′ = 0.0, respectively, by spectral simulation. The spin Hamiltonian parameters of the quintet state of 2 were determined to be g = 2.0035, D = 0.0128 cm^?1, E = 0.0, P _2′ = P _?2′ = 0.0, P _1′ = P _?1′ = 0.37 and P _0′ = 0.26, respectively. Direct observation of the excited high spin state showed that photoinduced intramolecular spin alignment is realized between the excited triplet state (S = 1) of the phenyl- or diphenylanthracene moiety and the doublet spin (S = 1/2) of the dangling verdazyl radicals. Ab initio MO calculations (DFT) were carried out in order to clarify the mechanism of the photoinduced spin alignment.     

The transient EPR spectra and spin dynamics of coupled three-spin systems in photosynthetic reaction centres

The concept of introducing an additional, stable paramagnetic species into photosynthetic reaction centres to increase the information content of their spin polarized transient EPR spectra is investigated theoretically. The light-induced electron transfer in such systems generates a series of coupled three-spin states consisting of sequential photoinduced radical pairs coupled to the stable spin which acts as an “observer”. The spin polarized transient EPR spectra are investigated using the coupled three-spin system P⁺I⁻Q⁻ _A in pre-reduced bacterial reaction centres as a specific example which has been studied experimentally. The evolution of the spin system and the spin polarized EPR spectra of P⁺I⁻Q⁻ _A and Q⁻ _A following recombination of the radical pair (P = primary donor, I = primary acceptor, Q_A = quinone acceptor) are calculated numerically by solving the equations of motion for the density matrix. The net polarization of the observer spin is also calculated analytically by perturbation theory for the case of a single, short-lived, charge-separated state. The result bears a close resemblance to the chemically induced nuclear polarization (CIDNP) generated in photolysis reactions in which a nuclear spin plays the role of the observer interacting with the radical pair intermediates. However, because the Zeeman frequencies of the three electron spins involved are usually quite similar, the polarization of the electron observer spin in strong magnetic fields can reflect features of the CIDNP effect in both, high and low magnetic fields. The dependence of the quinone spin polarization on the exchange couplings in the three-spin system is investigated by numerical simulations, and it is shown that the observed emissive polarization pattern is compatible with either sign, positive or negative, for a range of exchange couplings, J_PI, in the primary pair. The microwave frequency and orientation dependence of the spectra are discussed as two of several possible criteria for determining the sign of J_PI.     

Structural and electronic properties of endohedral phosphorus fullerene P@C60: an off-centre displacement of P inside the cage

Single P-doped endohedral P@C₆₀ is investigated via semiempirical and first-principles calculations. Unlike the encased N atom, which is situated on the centre of the C₆₀ cage and not covalently bound to the carbon atoms of the fullerene cage, static geometric optimization shows that the encased P atom occupies an off-centre position and is bound to the carbon atoms of the fullerene cage. The electronic ground state of the doped system is the spin quarter state, with spin density distribution significantly compressed by the cage.     

EPR investigation of endofullerenes in solution

82 and Lu@C₈₂ were prepared by arc burning and subsequent HPLC purification. EPR spectra of Lu@C₈₂ could be interpreted as arising from unresolved hyperfine interaction with the I=7/2 nuclear spin of ¹⁷⁵Lu. At temperatures above 250 K, a thermally activated process, which is tentatively attributed to a hopping process of the encapsulated ion or to time-dependent population of close-lying electronic states, leads to pronounced line broadening. For the Ho@C₈₂ sample, no EPR signals could be detected, indicating a high spin state of this molecule. Spin relaxation data of N@C₆₀, which was prepared by ion bombardment, could be interpreted by assuming that collision-induced deformation of the carbon shell leads to a fluctuating zero-field splitting, sensed by the quartet spin state of the central encapsulated nitrogen atom. Received: 25 August 1997/Accepted: 6 October 1997     

Corrigendum

Theoretical calculations of g-tensor components for the spin–orbit quartet, which arises as the ground state in three-coordinate d⁹ complexes and low-spin d⁷ complexes of D_3h symmetry, have been made on the assumption that the spin–orbit interaction is commensurable with the electron-vibrational interaction. The calculations were carried out within the framework of crystal field theory using representations of the hole formalism. The analytical expressions for g-tensor components were obtained limited to first-order terms. It was shown that the account of the electron–vibrational interaction in the excited quartet only provides three-axial anisotropy for the g-tensor. It was shown that the g-tensor rotates in the plane of the three-coordinate structure with consensual motion of the atoms. The resulting expressions for the g-factor components are in good agreement with experimental data. Being universal for a wide range of contributions of the vibronic and spin–orbit interactions, these expressions essentially fill the gap in studying structures of coordination compounds.     

The electron paramagnetic resonance of Dy in single crystals of Pd

We report the observation of the electron paramagnetic resonance spectrum of Dy³⁺ in Pd single crystals. The observed resonance spectrum contains four main lines, one of which varies strongly with crystal orientation suggesting the ion ground state to be a Γ₈ quartet. The weak hyperfine lines due to the isotopes Dy¹⁶¹ and Dy¹⁶³ have also been observed. The results are discussed in terms of Bleaney's Hamiltonian for a Γ₈ quartet.