Hideto Matsuoka

W-band time-resolved electron paramagnetic resonance study of light-induced spin dynamics in copper-nitroxide-based switchable molecular magnets.

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.

Proton-Coupled Electron-Transfer Processes in Photosystem II Probed by Highly Resolved g-Anisotropy of Redox-Active Tyrosine YZ

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

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.

Time-Resolved Electron Paramagnetic Resonance and Phosphorescence Studies of the Lowest Excited Triplet States of Rh(III) Corrole Complexes

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.

Midinfrared extinction spectra of submicron carbohydrate particles generated by a pneumatic atomizer.

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

Spin Characterization of Lanthanoid Ion Eu2± (8S7/2) in a CaF2 Single Crystal by Electron Spin Transient Nutation Spectroscopy

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

Molecule-Based Exchange-Coupled High-Spin Clusters

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

Molecule-Based Exchange-Coupled High-Spin Clusters: Conventional, High-Field/High-Frequency and Pulse-Based Electron Spin Resonance of Molecule-Based Magnetically Coupled Systems

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

Spin characterization of lanthanoid ion Eu2+(8S7/2) in a CaF2 single crystal by two-dimensional electron spin transient nutation spectroscopy

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

Transition moment spectroscopy-based simulation of 2D electron spin transient nutation spectra for high spin chemistry

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