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Troponin (Tn) is a protein that consists of three subunits, troponin C (TnC), troponin I (TnI), and troponin T (TnT), and Tn controls cardiac muscle contraction by calcium ion binding and phosphorylation. The Ca2+-binding site is the E–F hand motif (C helix–loop–D helix) in the N-terminal domain of TnC, and the structural transition induced by Ca2+ is the opening of these helices and the interaction with TnI, probably at the A and B helices. In this paper, we studied structural changes in the TnC–TnI binary complex on Ca2+ binding by double quantum coherence (DQC) distance measurements. We used a binary complex of the cardiac troponin C and I (cTnC and cTnI) complexes, chose four positions of nitroxide spin label at helices A, B, C, and D in the N-terminal domain and chose the E helix in the C-terminal domain as the reference position to study the structural changes on Ca2+ addition. The label positions were (A22C/S98C), (M47C/S98C), (Q58C/S98C), and (C84/S98C) for the A, B, C, and D helices, respectively. The effects of phosphorylation of the cardiac-specific N-terminal region of cTnI were studied using a phosphomimetic cTnI mutant. Analysis of the modulation of the DQC echo signals provided the distribution of the spin–spin distance. The distances averaged over the distribution showed that the labels on the A, B, and C helices decreased, i.e., moved to the E helix, on Ca2+ binding, while the distance of the label on the D helix showed almost no change. Shoulders and/or small separate peaks were observed in the shape of the distribution and were analyzed as the sum of a few Gaussian functions. The Gaussian functions were grouped into two components, components 1 and 2, at the longer and shorter distances, respectively, separated by 0.7–1.5 nm. The fractions of component 2 were ca. 0.1–0.2 in the Ca2+-free state and increased by 0.2–0.3 on Ca2+ addition, suggesting that the increase in component 2 is related to physiological control of cardiac muscle contraction. The phosphomimetic-modification effects on the Ca2+-induced changes of the fraction of components and the distances of the C- and D-helix labels are small. On the other hand, in the A and B helices, there are significant effects on the Ca2+-induced changes in the distances of the components. The different behaviors of A/B and C/D helices support the current model of the phosphorylation effects in which both N-terminal region and regulatory domain of cTnI interact with the A and B helices of cTnC.

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...

Mobius aromatic molecules have attracted great attention as new functional materials because of their π-orbital cyclic conjugations lying along the twisted Mobius topology. To elucidate the electronic character of the lowest excited triplet (T1) state of a Mobius aromatic [28]hexaphyrin, we employed a time-resolved electron paramagnetic resonance (TREPR) method with applied magnetophotoselection measurements at 77 K. Analyses of the EPR parameters have revealed that the T1 state possesses intramolecular charge-transfer (CT) character together with local excitation character residing at one side in the Mobius strip ring. We have also demonstrated that the CT character between orthogonal unpaired orbitals triggers quick triplet deactivation by spin-orbit coupling. This deactivation can be an important barometer to represent the “antiaromaticity” because of a connection between the orthogonal CT character and instability by a weakened spin-spin exchange coupling in the T1 state.

We studied the effects of isotopic substitution (namely, deuterium substitution in solvents and 15N substitution in a spin label) on improving the sensitivity of distance measurements based on double quantum coherence (DQC) electron paramagnetic resonance (EPR) using cardiac troponin C. Deuteration of solvents increased the phase memory time, Tm, by a factor of only 1.7, whereas the DQC amplitude increased significantly at long pulse intervals. The two-pulse echo amplitude was 1.7 times larger for the 15N spin label than it was for the 14N label because of the limited bandwidth of the microwave pulses. In the case of DQC, the echo is 2.7 times greater for the 15N label than it is for the 14N label due to the accumulated effects of the finite bandwidth. The combination of the two isotopic substitutions is expected to increase the sensitivity by about a factor of 6 when measuring distances longer than 4–5 nm.

The lowest excited triplet state of free-base 5,10,15-trispentafluorophenylcorrole (H3Cor) was studied in rigid glass by time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy. Triplet sublevels were experimentally determined by using magnetophotoselection and liquid crystal. Quantum chemical calculations of zero-field splitting parameters D and E were used for the sublevel assignment. The out-of-plane sublevel Tz was found to be the lowest; namely, D is positive for H3Cor, in contrast to previous reports. The origin of the difference is discussed in detail. Preliminary TR-EPR experiments on rhodium corrole, accompanied by the quantum chemical calculations, emphasize the important contribution of spin-orbit couplings.

Multiple-frequency (X-band and W-band) time-resolved electron paramagnetic resonance spectra of Zn-tetraphenylporphyrin (ZnTPP) triplet states coordinated to two different stable nitroxide moieties are presented and discussed. The position of the pyrimidyl nitroxide relative to the ZnTPP plane is the only structural change made between the two complexes, from ortho to para. This changes the angle between the interacting orbitals of the three-spin system with only a minor change in the distance between the unpaired electron of the nitroxide and the TPP ring system. Changes in the electron spin polarization patterns at the two different frequencies of observation are discussed in terms of a radical–triplet pair spin-state mixing model, including the intersystem crossing processes. It is determined that the inclusion of an additional spin-selective relaxation process to the computational model for the spectral shapes gives adequate reproduction of the experimental results using the same parameters at each frequency.

The electronic structure of a water-soluble near-infrared (NIR)-absorbing complex was determined to be singlet diradical [PtII(LISQ)2]2– [(LISQ)2–· = o-diiminosulfobenzosemiquinonate radical] by X-ray diffraction and 1H NMR spectroscopy. Cyclic voltammetry (CV) showed redox reactions of [PtII(LISQ)2]2– to give [PtII(LISQ)(LIBQ)]– (E1 = –0.26 V vs. Fc+/Fc) and [PtII(LISQ)(LPDI)]3– (E2 = –1.46 V) [(LIBQ)– = o-diiminosulfobenzoquinonate, (LPDI)3– = o-phenylenediiminesulfonate]. The monoradical structure of the latter was confirmed by electron paramagnetic resonance (EPR) spectroscopy. Splitting of the reduction wave of [PtII(LISQ)(LIBQ)]– in CV and the weak EPR signal suggested the formation of a dimer. Because the rest potential of the solution exceeds Epa1 at pH 4.0, the decrease in the NIR absorption at pH

The lowest excited triplet (T(1)) states of magnesium and zinc tetraphenylporphines (MgTPP and ZnTPP) were studied by time-resolved (TR) high-frequency/high-field W-band electron paramagnetic resonance (hf-EPR) spectroscopy in rigid glasses at low temperatures. Inspections of the TR-hf-EPR spectra of the spin-polarized triplets revealed that the zero field splitting (ZFS) parameters, D and E, for MgTPP and ZnTPP triplets were nearly the same. At the same time, their g-tensors were found to be different. These results are interpreted quantitatively in terms of spin-orbit couplings (SOCs) and angular momenta among the excited states, giving a magnitude of SOC in the T(1) state of ZnTPP. For the first time, both the TR-hf-EPR spectra and corresponding time profiles were acquired on the ZnTPP's triplet at room temperature in liquid paraffin solution with the populations of the electron spin states being in Boltzmann equilibrium. Because of relatively fast paramagnetic relaxation in rotating triplet at room temperature, the spectra and time profiles were free from the effects of microwave saturation that allowed for the direct measurement of the absolute intersystem crossing ratios P(x):P(y):P(z) 0.085:0.085:0.83. All of these results have demonstrated advantages and new perspectives of the W-band EPR spectroscopy.

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.

We reported determination of the loaded quality factor (Q) of highly overcoupled (dielectric, loop-gap, and cavity) resonators used in time-domain electron paramagnetic resonance. We introduced a microwave absorber into resonators and achieved critical-coupling. Due to the deep “Q-dip” of critical-coupling, we can easily determine the loaded Q as low as 10. The loaded Q of resonators with and without the microwave absorber was examined under various overcoupling conditions. We found that the radiation Q (Q r) can be calculated from the loaded Q of the resonator that contains the microwave absorber. We proposed a simple model that represents the loaded Q of the overcoupled resonator in terms of two parameters, Q 0 and Q r. Q 0 is the effective unloaded Q of the resonator determined for the critically coupled resonator without the microwave absorber and is independent of a degree of coupling. The model can be applied to overcoupling in which the coupling parameter (Q 0/Q r) is in the range of 1 to ca. 20.

We propose a novel formation model of an intermolecular hydrogen bond between radicals and a diamagnetic matrix in the condensed phase. W-band and multi-frequency ESR studies revealed that the poly...

EPR spectra of the excited quartet and doublet molecular states of (tetraphenylporphinato)zinc(II)covalently bounded to 3-( N -nitronyl-notroxide) pyridine stable radical are modeled in terms of the spin-Hamiltonian given by the sum of the contributions from the radical and triplet moieties, and the interaction between them. The later is represented by anisotropic point dipolar and isotropic exchange electron spin–spin interactions. It is shown that the high field (W-band) EPR spectra depend on energy separation between the electronic doublet (D) and quartet (Q) states. This dependence was utilized to estimate the upper limit of the intensity of exchange interaction between the radical and porphyrin moieties.

The coordination of small molecules and ions at the sixth position of ferric myoglobins (Mb) from the heart muscle of two species of shark,Lamna ditropis (LD) andPrionace glauca (PG), was studied in frozen solution by electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy. The results were compared with those of horse heart Mb (EQMb). From our preliminary studies of the absorption spectra and partial amino acid sequences of PGMb and LDMb, we expected PGMb to have glutamine (Gln E7) in place of the distal His E7 that is highly conserved in most animals and present in LDMb. Here, we have used proton ENDOR to show that a water molecule coordinates to the ferric ion in both LDMb and PGMb at pH 6.3. The observed proton hyperfine coupling constant of the coordinating water molecule of PGMb is smaller than those of LDMb and EQMb, which suggests weaker coordination of the water molecule in PGMb, consistent with the absent distal His (E7). Coordination of several exogenous ligands at the distal site was examined by EPR. PGMb showed a coordination behavior different from that of LDMb and EQMb for 1-methylimidazole, 4-methylimidazole and SCN−, suggesting that its distal site is less sterically hindered. On the basis of the ENDOR and EPR results, we assigned the distal residues of PGMb and LDMb as Gln and His, respectively.

Bis-(quinoline-TEMPO)-yttrium-tetraphenylporphyrin complexes having potassium and silver counter cations have been studied by cw-EPR in their ground states and by time-resolved (TR) EPR and two-dimensional (2D) nutation spectroscopy in the excited states after laser excitation. Simulations of the TR-EPR spectra at 30 K and at room temperature were made, yielding zero-field splitting parameters D and E, and g values. The parameters obtained together with the nutation frequencies clearly assigned the excited states of the bis-radical complexes having counter cations of potassium and silver as quartet and quintet states, respectively. This result is explained by modulation of radical–triplet interactions by the counter ion due to different sizes of metal ions.

The interplanar interactions for fully overlapped (FO) and partially overlapped (PO) carbazole dimers were investigated using zero field splitting (ZFS) parameters D and E in the excited triplet (T1) states. The observed D value decreases from the monomer to the dimers due to electron delocalization in the T1 dimers. The highest electron delocalization was observed within the two planes of the FO dimer. The interplanar interactions were analysed in terms of exciton (EX) and charge resonance (CR) types interactions by comparing the D values of monomer and the dimers. It is found that about 90% of excitation is delocalized over the two carbazole units in the T1 state of the FO dimer whereas for PO this contribution is 40%. In a model system, DCR and ECR were calculated using the two approximations, point charge and half point charge, for the evaluation of the ZFSs obtained.

Multifrequency (X-, Q-, and W-band) electron spin resonance (ESR) spectroscopy has been used to characterize the phenoxyl radical produced from alpha-(3,5-di-tert-butyl-4-hydroxyphenyl)-N-tert-butylnitrone, which is a new spin-trapping reagent. The X-band measurement did not resolve the powder-pattern ESR spectrum. Because of its higher resolution with g value, the Q-band ESR study revealed that the g factor has an axial-like symmetry and that the observed hyperfine structure in the Z-direction is caused by the nitrogen nucleus at the para-position. Furthermore, the results of the W-band ESR experiment more clearly distinguished the perpendicular components from the parallel component, resolving the perpendicular components into x and y components. The X-band powder spectrum was similar to the X-band ESR spectrum of the radical in a frozen solution of toluene. The computer simulation spectra performed using the obtained parameters fitted the experimental spectra well. A comparison of the amplitude of g( perpendicular)(gx, gy) with that of gz showed that the unpaired electron is delocalized over the pi-conjugated framework. Considering the hyperfine coupling constant, it was concluded that about 16% of the unpaired electron distributed over the nitrogen nucleus at the para-position. This study thus showed the significant potential of a multifrequency ESR approach to a powder sample radical in terms of its high resolution with g value.

Double coaxial carbon nanotubes with nitrogen (N)-doped and boron (B)-doped multiwalls possess composite Raman characteristics, originating not only from the outer N-doped but also from inner B-doped layers. Both N and B dopings result in substantial shifts of the characteristic D band and G band of sp(2) carbon constituting nanotube walls but in different ways. The downshift of the G band is correlated with the decreases of electrical resistivity of carbon nanotubes regardless of N or B doping.

Some of 1,6-(N-substituted) aza-[60] fulleroids (1) and their corresponding photochemical reaction products, 1,2-(N-substituted) aziridino-[60] fullerenes (2) showed peculiar transient absorption peaks depending on the N-substituents. Although these bands are considered to concern with very slow photochemical rearrangements 1 → 2 in the excited triplet state, no definitive evidence has been obtained yet. We report on the character of the excited triplet states in these compounds by means of a time-resolved electron paramagnetic resonance technique. Definitive evidence on the involvement of intra-molecular charge transfer states is shown in this Letter.

Influence of axial ligands, MeCN, H2O, py, and piperidine (pip), on distortion of (2,3,5,10,12,13,15,20-octaphenylporphinato)chromium(III), [Cr(OPP)]+, was investigated by X-ray crystallography and UV–vis and ESR spectroscopies. In crystal structures of [Cr(OPP)(MeCN)(H2O)]ClO4 · MeCN and [Cr(OPP)(H2O)2]ClO4 · 3THF, the OPP2− ligand had a planar structure. On the other hand, crystal structures of [Cr(OPP)(pip)2]ClO4 · 2CH2Cl2 and [Cr(OPP)(py)2]ClO4 exhibited a waved structure and a saddle-shaped structure with ruffling, respectively. In the UV–vis spectrum of [Cr(OPP)(py)2]ClO4 in CH2Cl2, the large red shift of the Soret band was observed. Furthermore, the small D value of 0.10 cm−1 was obtained for [Cr(OPP)(py)2]ClO4 from the ESR spectrum in frozen 1,2-dichloroethane solution at 5 K. These results suggest that the OPP2− ligand is distorted both in solid and in solution, and that the axial ligand would exert some effects on the porphyrin distortion.

Heme-regulated eukaryotic initiation factor 2alpha kinase (HRI) regulates the synthesis of hemoglobin in reticulocytes in response to heme availability. HRI contains a tightly bound heme at the N-terminal domain. Earlier reports show that nitric oxide (NO) regulates HRI catalysis. However, the mechanism of this process remains unclear. In the present study, we utilize in vitro kinase assays, optical absorption, electron spin resonance (ESR), and resonance Raman spectra of purified full-length HRI for the first time to elucidate the regulation mechanism of NO. HRI was activated via heme upon NO binding, and the Fe(II)-HRI(NO) complex displayed 5-fold greater eukaryotic initiation factor 2alpha kinase activity than the Fe(III)-HRI complex. The Fe(III)-HRI complex exhibited a Soret peak at 418 nm and a rhombic ESR signal with g values of 2.49, 2.28, and 1.87, suggesting coordination with Cys as an axial ligand. Interestingly, optical absorption, ESR, and resonance Raman spectra of the Fe(II)-NO complex were characteristic of five-coordinate NO-heme. Spectral findings on the coordination structure of full-length HRI were distinct from those obtained for the isolated N-terminal heme-binding domain. Specifically, six-coordinate NO-Fe(II)-His was observed but not Cys-Fe(III) coordination. It is suggested that significant conformational change(s) in the protein induced by NO binding to the heme lead to HRI activation. We discuss the role of NO and heme in catalysis by HRI, focusing on heme-based sensor proteins.

The electron paramagnetic resonance (EPR) signals of photoexcited quartet (Q1) states for zinc(II) tetra-tert-butyl-phthalocyanine (ZnPc) ligated by 3- and 4-(N-nitronyl-nitroxide) pyridine radicals (3-NOPy, 4-NOPy) were observed in toluene solution at room temperature by means of X-band (9.4 GHz) time-resolved EPR (TREPR) spectroscopy. Theg values of Q1 in the ZnPc-3-NOPy and ZnPc-4-NOPy complexes were found to beg=2.0025 andg=2.0036, respectively. The obtainedg value (2.0036) for ZnPc-4-NOPy is in good agreement with the value (g=2.0037) of the Q1 state calculated under the strong-exchange limit. Theg value (2.0025) is just an average of the Q1 and D1 (g=2.0013) states for ZnPc-3-NOPy. Theg value of Q1 for zinc(II) meso-tetraphenylporphine (ZnTPP) ligated by 3-NOPy showed a slight shift (g=2.0027) at X-band and no shift (g=2.0031) at W-band from the calculatedg value (g=2.0031) (J. Fujisawa, Y. Iwasaki, Y. Ohba, S. Yamauchi, K. Koga, S. Karasawa, M. Fuhs, K. Mobius, S. Weber, Appl. Magn. Reson. 21, 483–493, 2001). These changes in theg value were found to originate from an averaging of the TREPR spectra over the Q1 and photoexcited doublet (D1) states via a fast intersystem crossing (ISC) process. The ISC rates between these two states were estimated by means of numerical calculations with the modified Bloch equations as 1.2·108 and 6·107 s−1 for the ZnTPP-3-NOPy complex at the X- and W-bands, respectively. The lower limit of the ISC rate was obtained as 109s−1 for the ZnPc-3-NOPy complex and the higher limit was found to be 3.1·108 s−1 for the ZnPc-4-NOPy complex.

Time-resolved EPR spectra of the lowest excited triplet state of boron subphthalocyanine chloride have been measured in toluene at various temperatures (5−360 K). On the basis of the observed and simulated spectra, electronic structure and molecular motion of the triplet state (1T) were analyzed, both in solid and fluid solution. The simulations were carried out using a model, which considers a temperature-dependent exchange between sites having different zero-field splitting (ZFS) parameters and molecular orientations. The ZFS parameter, D, was nearly the same at all temperatures examined. At very low temperatures, below 20 K, the spectrum was analyzed by a static model. At 40−120 K, two conformers with different ZFS parameter, E, have been found. The population ratio between the two conformers showed strong temperature dependence. These conformers were attributed to Jahn−Teller states and were identified by their different ZFS parameters. The exchange rate and activation energy of the conformers were co...

Time-resolved EPR spectra were observed by means of the transient EPR technique for two kinds of fullerene dimers C120 and C120O in the lowest excited triplet (T1) states and compared with those of mono-adducts of fullerene (C60). It was found that zero-field splitting (zfs) parameters of both dimers are nearly the same as those of the monomers, indicating that the excitation is almost completely localized at one of the two fullerene units in the T1 state. © 2002 Elsevier Science B.V. All rights reserved.

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.

Sublevel properties of the lowest excited triplet (T1) npstates of ¬-silyl and ¬-germyl ketones were examined by means of ODMR, time-resolved EPR and optical spectroscopy. The EPR parameters, D, E and g, population ratios, and triplet lifetimes were obtained. The D value and the triplet lifetime varied among the molecules. In contrast the E value and population ratio remains nearly the same. These properties together with their solvent dependence and emission properties are interpreted in terms of spin± orbit couplings between T1 (np ¤ ) and higher S1 (np ¤ ), T2 (pp ¤ ) and S2 (pp ¤ ) states. An origin of the remarkable red-shifts of 1;3 (np ¤ ) are discussed based on a model of delocalized n, p and pelectrons over the Si and Ge atoms. This model is also consistent with all the triplet properties obtained.

EPR signals of the triphenylsilyl and triphenylgermyl radicals were observed for the first time in fluid solution by direct photolysis of hexaphenyldisilane and hexaphenyldigermane. Observed electron spin polarization was interpreted in terms of a triplet mechanism (TM). Along with the main signal, the 29Si isotope signal was also observed and the polarization was tentatively explained by combination of a TM and a radical pair mechanism. An electron density on the silicon atom of the silyl radical was determined as 0.52 from the isotropic hyperfine coupling constant of 29Si. The triphenylstannyl radical was not observed with the time resolution (ca. 80 ns) of our system.

The lowest excited triplet (T1) state of 3,3′-biisoquinoline(dicyano)platinum(II), [Pt(CN)2(i-biq)] was examined by time-resolved electron paramagnetic resonance (TREPR) and optical techniques in comparison with that of a ligand 3,3′-biisoquinoline (i-biq). Trans and cis i-biqs were assigned from obtained and calculated zero-field splittings (zfs') in the T1 state. The zfs parameter D increased with complex formation from 2.66 to 4.66 GHz in a rigid dimethylformamide solution; the polarization pattern of the spectrum was very different between the ligand and the complex. The triplet lifetime (ca. 1 s) of i-biq is also remarkably shortened in the complex (ca. 2 ms). These facts were analyzed in terms of the spin-orbit coupling (soc) between T1 (ππ*) and 1,3dπ*. From the analyses, the soc matrix element and the magnitude of delocalization of π* electrons over the Pt atom are obtained as 65 cm-1 and 0.02, respectively. The solvent dependences of the D value and the triplet lifetime also support the importanc...

The porphyrinato copper cation radical complexes were examined by EPR spectroscopy. The cation radical complexes having bulky peripheral substituents are present as monomers in fluid solution and show EPR spectra due to intramolecular spin coupling between the spins on copper and on the porphyrin ring. The cation radical complexes, having no such substituents, form a dimer in solution. On the other hand, when they are oxidized in frozen solution by γ-ray irradiation, the cation radical complexes are always formed as a monomer, showing a triplet state EPR due to the intramolecular spin coupling. The zero field splitting and spin exchange parameters, |D| and 2J, obtained from EPR measurements were classified into two groups by their magnitudes. The tetraphenylporphyrinato copper(II) cation radical complex, [CuII(tpp)]+, has a small |D| and a small negative 2J value, while the octaethylporphyrinato copper(II) cation radical complex, [CuII(oep)]+, has a large |D| and a large negative 2J value. The other porph...

An electron paramagnetic resonance (EPR) study of 3,4-fulleropyrrolidine-2-spiro-4′[2′,2′,6′,6′-tetramethyl]piperidine-1′-oxyl (1) was performed on the photoexcited quartet state in toluene glass. The spectrum of the |S,Ms〉=|3/2,±3/2〉⇔|3/2,±1/2〉 transitions was observed selectively by using a two-dimensional (2D) nutation method and analyzed with a spectral simulation in a randomly oriented system. A position of the nitroxide moiety was determined with respect to the zero-field splitting (zfs) axes of excited triplet fullerene (3C60) by taking into account of the dipolar–dipolar interaction between the radical and 3C60, the hyperfine coupling, the anisotropic g-value of the nitroxide radical, and the zfs of the 3C60 moiety. It was found that none of the zfs axes of the 3C60 moiety coincide with the local C2 axis of the molecule which is defined by the position of addition. A symmetry of the electronic structure in 3C60 is discussed on the basis of the result.

Electron spin polarizations were studied for a system of excited triplet (T1) phthalocyanine (MTNPc; M = H2, Zn) and a nitroxide radical (TEMPO) in toluene solution by means of time-resolved EPR (T...

Using W-band time-resolved EPR (TREPR) (95 GHz), the lowest excited doublet (D1) state was observed in solution for the first time together with the excited quartet (Q1) and doublet ground (D0) states in a complex of MgTPP (TPP; tetraphenylporphyrin) and an axial-ligating p-pyridylnitronylnitroxide radical (nit-p-py). The g value obtained for D1 was in good agreement with the calculated value. The spin polarization of D1 varied with time from an emission to an absorption as well as that of Q1 and D0. The result shows that the spin polarizations are produced by a radical−triplet pair mechanism with a ferromagnetic coupling (J > 0). The sign of the exchange coupling is consistent with that obtained in the X-band TREPR experiment (Fujisawa, J.; Ishii, K.; Ohba, Y.; Yamauchi, S.; Fuhs, M.; Mobius K. J. Phys. Chem. A 1997, 101, 5869).

Photoinduced electron and energy transfers between ZnTPP- d 20 (TPP; tetraphenylporphyrin) and fullerene (C 60 ) were studied in benzonitrile solution at room temperature by means of pulsed-Fourier transform and cw-time-resolved EPR. Two kinds of EPR signals were observed with absorptive spin polarizations and assigned to those of the ZnTPP- d 20 cation radical (ZnTPP + ) and the lowest excited triplet state of C 60 . Quantitative analysis of the time-evolution of the 3 C 60 and ZnTPP + signals demonstrates that electron transfer occurs from 3 ZnTPP to C 60 and ZnTPP to 3 C 60 and triplet–triplet energy transfer from 3 ZnTPP to C 60 . The induced absorptive polarizations of the 3 C 60 and ZnTPP + signals are explained by electron spin polarization transfer.

The excited multiplet states of (tetraphenylporphinato)zinc(II) (ZnTPP) coordinated by p-pyridyl nitronyl nitroxide (p-nitpy), ZnTPP−p-nitpy, and of ZnTPP coordinated by m-nitpy, ZnTPP−m-nitpy, were studied by time-resolved electron paramagnetic resonance (TREPR). The TREPR spectra observed at 20 K and 0.5 μs after laser excitation were assigned to the lowest excited doublet (D1) and quartet (Q1) states for both the para and meta complexes. The TREPR spectra of the Q1 state with electron-spin polarization (ESP) were well simulated for the first time. From the spectral simulation, it was established in general that the ESP in the Q1 state was interpreted by selective intersystem crossing (ISC), which was generated by spin−orbit coupling (SOC) between the excited doublet states and the eigenfunctions of the Q1 state in zero magnetic field. The TREPR spectra of the ZnTPP−nitpy systems were interpreted by selective ISC to the |±1/2〉 spin sublevels of the Q1 state, which originated from SOC due to the zinc ion...

Time-resolved electron paramagnetic resonance (EPR) has been used to investigate electron spin polarization transfer in triplet–triplet (T–T) energy transfer between porphyrins (MOEP and MTPP; M=H 2 , Zn) and fullerene (C 60 ) in toluene solution. EPR spectra of excited triplet (T 1 ) states of both a donor (porphyrins) and an acceptor (C 60 ) were directly observed. The induced spin polarizations of T 1 C 60 were found to be the same as those of T 1 porphyrins. From the analyses of time-profiles of both triplet signals, it was shown that T 1 polarizations of porphyrins are transferred to T 1 fullerene with conservation of spin polarizations via T–T energy transfer.

A chemically induced dynamic electron spin polarization (CIDEP) study has been accomplished on radical-excited triplet pairs (RTP) in systems of metalloporphyrins, MgTPP, ZnTPP, and ZnOEP, and pyridine-substituted nitronyl nitroxide radicals, nit-R (R = o-py, m-py, and p-py), by X- (9.5 GHz) and W-band (95 GHz) time-resolved electron paramagnetic resonance (TREPR) in solution. Axial-ligations between the porphyrins and the radicals were ascertained from visible absorption spectra except for the nit-o-py system. The TREPR spectra were composed of two signals, which were assigned to those of the ground (D0) state of the radical and the excited quartet (Q1) state of the RTP. These components showed two kinds of CIDEPs in different time regions. The polarizations of the Q1 state were attributed to radical−triplet pair mechanisms (RTPMs) with singlet and triplet precursors. In the nit-p-py and the nit-m-py systems, the polarizations of the radical were generated via an electron spin polarization transfer (ESPT...

A Fourier transform (FT) EPR technique has been used to study the photoinduced electron transfer from zinc-substituted myoglobin (ZnMb) to 1,4-benzoquinone (BQ). Both porphyrin cation and BQ anion radicals (BQ - ) were observed, which provide direct evidence of an electron transfer occurring between a porphyrin moiety (ZnPP) in myoglobin and BQ. Electron transfer rate constants, spin–lattice relaxation times of triplet porphyrin and BQ - and magnitudes of chemically induced dynamic electron polarization (CIDEP) due to the triplet mechanism (TM) and the radical pair mechanism (RPM) were evaluated from decay analyses of the FT-EPR signals of BQ - . These results were compared with those for a [5,10,15,20-tetrakis(4-sulfonatophenyl)porphinato]zinc(II) (ZnTPPS)–BQ system and discussed in terms of protein effects in ZnMb. We concluded that electron transfer occurs from the ZnPP moiety to BQ which is located at the surface of myoglobin.

Time-resolved EPR studies were made on CIDEP mechanisms in the system of excited triplets (T) and doublets (D) using various kinds of porphyrins and nitroxide radicals. The CIDEPs were investigated by observing both signals of T and D and discussed in terms of an electron-spin polarization transfer (ESPT) and a radical−triplet pair mechanism (RTPM). It is demonstrated for the first time that ESPT is actually existent by comparing the triplet and doublet CIDEP signals. The ESPT process was found to be diffusion-controlled in toluene and 2-butanol, and an effective distance for ESPT was estimated as

New mono- and bis[4-(3-hydroxy-2-methyl-4-quinolinoyloxy)-2,2,6,6-tetramethylpiperidin-1-oxyl](meso-tetraphenylporphyrinato)yttrium(III) complexes have been synthesized, and the properties of the excited states generated by photoexcitation of porphyrin were studied by time-resolved (TR) and pulsed two-dimensional electron paramagnetic resonance (EPR) spectroscopy. A TR-EPR spectrum was observed in the quartet (S = 3/2) or quintet (S = 2) states generated from interactions of one or two radicals with the photoexcited triplet state of the porphyrin. The zero-field splitting D values of these states were analyzed in terms of those of the triplet and the radical-triplet pair. The spin states of the excited states were definitely assigned by measuring the nutation frequencies with pulsed EPR.

The characteristic features of the electronic structure of some trigonal bipyramidal (tbp) copper complexes of the quadridentate tripodal nitrogen donor ligands, tris(2-aminoethyl)amine (tren) and tris(2-dimethylaminoethyl)amine (trenMe6), with different ligands at the fifth coordination position of tbp were studied in a powder form doped in diamagnetic zinc complexes by EPR and ENDOR. EPR shows that the copper complexes of tren and trenMe6 have a ground state with a dz2 unpaired electron orbital, and that some of the complexes are distorted from a trigonal symmetry to rhombic, to the contrary to expectations from the molecular formula. It is shown that a small structural or ligand-field distortion would give a recognizable spectral deviation from trigonal symmetry. It should be noticed that the hyperfine couplings of all the coordinating nitrogen atoms in each complex were observed by ENDOR, and that they were useful for calrifying the characteristic features of the electronic structures of the tbp compl...

A photoinduced electron transfer reaction in a model system of the photosynthesis was investigated by means of pulsed EPR in one (1D)- and two-dimensions (2D). EPR spectra of a spin correlated radical ion pair(SCRP) and a spin polarized radical anion were observed in the system of tetrakis(4-sulfonatophenyl) porphyrinozinc(II) (ZnTPPS)/duroquinone (DQ)/cetyltrimethylammonium chloride micellar solution. In order to investigate the dynamics and structures of the intermediate species, a non-linear least square fitting (NLLSQ), instead of linear prediction (LP), was examined for analyses of free induction decays (FID’s) in a 1D experiment. FID’s were fairly well reproduced with the NLLSQ method as a sum of the signals of the DQ anion and the SCRP. For the DQ anion, the linewidth gradually decreased indicating movement of the anion across the micelle/water interface. For a definitive assignment of the SCRP, a 2D nutation experiment was performed. Different flip angle dependences were observed for the SCRP and ...

Optical and time-resolved EPR(TREPR) studies were made on the excited states of fluorine-substituted magnesium tetraphenylporphines (MgF−TPPs) including related Mg porphyrins. From the analyses of TREPR spectra it was found that the zero-field splitting parameter, D, increased with an increasing number of substituents, e.g. fluorine atoms. This result is interpreted by the fact that 3(a2ueg) and 3(a1ueg) configurations are heavily mixed via vibronic couplings in the lowest excited triplet (T1) states of MgF−TPPs. Other T1 properties such as a T1 energy, a triplet lifetime, and an appearance of the b1g band in the phosphorescence supported the mixings. The mixings due to configuration interactions were studied in the lowest excited singlet (S1) states of MgF−TPPs by measuring absorption coefficients and intensities of vibronic bands both in the absorption and fluorescence. The magnitudes of the mixings were also evaluated in the S1 states of MgF−TPPs.

The temperature dependence of EPR spectrum of La@C82 in the powder of empty C60 and C70 mixed crystals was studied by EPR spectroscopy employing X- and Q-band microwave frequencies. The rigid limit spectra (at 4.2 K for the X-band and at 132 K for the Q-band) could be analyzed by static spectral simulation which yielded the EPR parameters,g ‖=2.0021,g ⊥=1.9970,La A ⊥=7.8 MHz,La A ‖∼0 MHz and an isotropic13C coupling value of about 3 MHz. For higher temperatures an appreciable motional averaging effect was observed and the spectra were analyzed by using dynamic spectral simulation based on the stochastic Liouville equation, where we assumed an isotropic rotational motion with the Brownian diffusion. The calculated spectra reproduced the dominant feature of the temperature dependence of the spectra almost satisfactorily for both the X-and Q-band frequencies with the appropriate rotational correlation times. The Arrhenius plots of the correlation time gave two activation energies of 0.9 kcal/mol and 2.9–3.8 kcal/mol for the temperatures below and above 200 K, respectively.

Two pulsed EPR methods for studying spin-correlated radical pairs (SCRPs) transiently generated in photochemical reactions are presented. One is two dimensional (2D) EPR nutation method which uses anomalous flip angle dependence of SCRPs for microwave (MW) pulses and the other is application of the so called 2D J spectroscopy in NMR to studies of paramagnetic SCRPs. The methods were applied to observation of the SCRP generated in the photoinduced electron transfer reaction between zinctetraphenylporphyrinsulphonate (ZnTPPS) and duroquinone (DQ) in cetyltrimethylammonium chloride (CTAC) micellar solution in which SCRP alives for a relatively long period at the early stage of the reaction processes.

EPR parameters such as zero-field splittings (zfs) and anisotropic g values of rare-earth metal centered porphyrin monomers, Y(TPP)+ and Y(OEP)+, and double-decker dimers, [Y(TPP)2]-, [Y(OEP)2]-, and [La(TPP)2]-, were measured in the lowest excited triplet (T1) states by a time-resolved electron paramagnetic resonance (TREPR) technique. The zfs parameter D of Y(OEP)+ was smaller than that of ZnOEP, and its gzz was smaller than that of Y(TPP)+. From the analyses of these data and the temperature dependence of the steady state EPR spectra, it was found that the decreases in the D and gzz values of Y(OEP)+ were due to a spin−orbit coupling between the Jahn−Teller split T1 and T2 states. By comparing the D values of the dimers with those of the corresponding monomers, we estimated a charge resonance character, b2, as 0.34, 0.26, and

The results of Chemically Induced Dynamic Electron Polarization (CIDEP) studies of acetone in 2-propanol and triethylamine are reviewed to show how CIDEP studies can reveal interesting details of the initial processes of photochemical reactions. After a brief description of various mechanisms that give rise to CIDEP, the following topics are discussed: a) spectral pattern and the main reacting state, b) CIDEP mechanisms and the temperature and frequency dependences of the spectra, c) direct EPR observation of the radical pair spectra in solution, d) time-developments of the CIDEP signals and spin and reaction dynamics of the initial processes of photochemical reactions, e) quantitative analyses of the magnitudes of CIDEP in terms of a stochastic Liouville equation, and f) some related applications. An overview of the remaining problems and the future outlook is also presented.

The anomalous flip-angle dependence of the Fourier-transform EPR spectrum of photochemically generated spin-correlated radical pair (SCRP) was verified by means of a two-dimensional FT EPR nutation technique. The SCRP signals were partially separated from free-radical signals in the 2D EPR spectrum; SCRP showed additional higher and lower nutation frequencies compared with those of the free radical. Theoretical analyses, including off-resonance effects, are made to explain the experimental results.

Two‐dimensional (2D) Fourier‐transform (FT) electron paramagnetic resonance (EPR) nutation experiments were done in order to verify anomalous flip‐angle dependence of a FT–EPR spectrum of a photochemically generated spin‐correlated radical pair (SCRP). It was demonstrated by both the experiment and theory that the signals of SCRP have additionally higher and lower nutation frequencies than that of a free radical. This spectroscopy provides a direct spectroscopic evidence of spin‐correlation within SCRP, and enables one to partially separate SCRP signals from free radical signals in the 2D EPR spectrum.

On pulsed laser photolysis of bis(2-bromobenzoyl) peroxide (2-BrBPO) a unique absorption band was observed around 400 nm as well as a broad band due to 2-BrC 6 H 4 CO 2 · radicals in the 500–800 nm region. The 400 nm band was also attributed to 2-BrC 6 H 4 CO 2 · on the basis of the similarity of both absorption in time profiles and behavior towards quenchers. Time-resolved EPR measurements provided clear evidence for some interaction between the CO 2 · radical center and the ortho-bromine atom in 2-BrC 6 H 4 CO 2 · and for the formation of ortho-substituted phenyl radicals, 2-XC 6 H 4 by way of CBr bond cleavage in excited state 2-BrBPO.

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.