Seigo Yamauchi

Temperature dependence of the CIDEP spectrum of acetone

The CIDEP spectrum of acetone has been studied by means of time‐resolved EPR over a wide range of temperature. It was found that the spectrum varied remarkably depending on temperature indicating that different CIDEP mechanisms are involved at different temperatures. The CIDEP mechanisms operating in each temperature region are determined quantitatively mainly from the simulations of the spectra and their dependence on the diffusion constant of the solvent; from room temperature to −90u2009°C, the radical pair mechanism involving S–T0 mixing (ST0M)is always dominant, but the triplet mechanism and the radical pair mechanism involving S–T−1 mixing (ST−1M)become important above and below −45u2009°C, respectively. The relative importance of ST−1M as compared with ST0M determined below −60u2009°C is in good agreement with the prediction of the theory. We have also found two interesting phenomena that relatively strong E/A peaks appeared in the center portion of the complete E/A spectrum around −45u2009°C, and that the line sh...

A chemically induced dynamic electron polarization study on the acetone ketyl radical and radical pair in an alcohol solution

A quantitative study of the chemically induced dynamic electron polarization (CIDEP) spectra of acetone and deuterated acetone ketyl radicals in i‐propanol over a wide temperature range has been done to understand the properties of the radical pairs in solution, e.g., the interactions between the radicals, the diffusional and rotational motions of the radicals of the geminate radical pairs, and the microscopic solvent structure. The spin polarization of the separated radical is analyzed in terms of the stochastic‐Liouville equation. It is shown that the observed polarization cannot be explained on the basis of the normal diffusion of the radicals in the pair. The modified diffusion models which presuppose slow diffusion of the radicals in a microscopic solvent structure can account for the observed polarization. The radical pair electron paramagnetic resonance (EPR) spectra observed at very low temperatures (<∼−70u2009°C) are explained by assuming that a considerable fraction of the radicals are held together...

The CIDEP spectrum of a radical pair of the acetone ketyl radical in solution. Effect of deuteration

We report the CIDEP spectrum of a geminate radical pair of the acetone-d6 ketyl radical in solution. Deuterated acetone shows a radical pair spectrum more clearly than protonated acetone and is found to be suitable for an investigation of the radical pair. It is also found that the radical pair spectrum of acetone-d6 shows less M1-dependent line broadenings than protonated acetone.

A time-resolved EPR study of the magnetic and decay properties of short-lived non-phosphorescent 3nπ* pyridazine

Abstract The properties of 3 nπ * pyridazine were investigated by time-resolved EPR. The zfs ( X = 0.092 cm −1 , Y = −0.162 cm −1 , Z = 0.071 cm −1 ), the sublevel decay rate constants ( k x ≈ k z ≈ 2.0×10 5 s −1 , k y = 1.7×10 6 s −1 ) and the relative populating rates ( P x : P y : P z ≈ 0.1 : 1 : 0.1) were determined. The lack of phosphorescence is ascribed to a very large radiationless decay rate constant ( k nr ≈ 10 6 s −1 ).

Properties of the short‐lived triplet states of pyridazine and 3,6‐dichloropyridazine studied by a time‐resolved EPR method

The magnetic and decay properties of the short‐lived triplet (T1) states of pyridazine and 3,6‐dichloropyridazine (DCP) were studied by means of time‐resolved EPR with laser excitation. The zfs of DCP in the p‐dibromobenzene (DBB) host are X=0.094 cm−1, Y=−0.164 cm−1, and Z=0.070 cm−1, and the nitrogen hfcc are Ax=27 G, Ay=18 G, and Az=25 G, which confirms the nπ* character of the T1 state. Similar zfs were obtained for DCP and pyridazine in other environments. The sublevel decay rate constants and the relative populating rates were determined from the analysis of the transient EPR signals measured at 3.0 K where the effect of the spin lattice relaxation can be neglected. It was found in both systems that ky is much greater than kx, kz as predicted by a simple group theoretical consideration. ky of DCP and pyridazine were determined to be on the order of 106 s−1, being larger than that of pyrimidine by a factor of ∼104. The reasons for the large ky are discussed, particularly, in terms of the distorted po...

Observation of surprisingly small zero-field splittings in the lowest emitting state of [Ru(bpy)3]2+

Optical detection of magnetic resonance (ODMR) and time-resolved EPR (TREPR) techniques were used to study the properties of the lowest excited triplet state of [Ru(bpy)3]2+ in several solvents at liquid-helium temperatures. Surprisingly small zero-field splittings (zfs ≈0.1 cm−1) were observed in all cases in contrast to the large zfs (10–100 cm−1) predicted by the generally accepted energy level scheme.

A well resolved phosphorescence spectrum of [Ru(bpy)3]2+ in a dilute system

Abstract A well resolved emission spectrum of [Ru(bpy) 3 ] 2+ was observed for a mixed crystal of [Zn(bpy) 3 ] (ClO 4 ) 2 . The spectrum was analyzed in terms of emissions from three states separated by 11 and 63 cm −1 . The decay of the emissions followed the same single-exponential curves for the three states, indicating thermal equilibrium among them. The energy differences between the states, decay rate constants and spectra in the mixed crystal were found to be essentially identical to those in the neat crystal of [Ru(bpy) 3 ] (ClO 4 ) 2 . It is concluded that these properties reflect the intrinsic nature of monomeric [Ru(bpy) 3 ] 2+ .

Geometries and energies of the excited states of pyridazine studied by sac and sac CI calculations

Abstract The geometries and energies of the ground and excited states of pyridazine are calculated by an ab initio calculation which includes electron correlation. It is found that electron correlation plays an essential role in determining the geometries of the excited states. The optimized geometry of the T 1 state as well as that of the ground state are planar. On the other hand, the force constant along the twisted mode of the Nue5f8N bond in the T 1 state is much smaller than that in the ground state. It is suggested that this distorted potential produces a large Franck-Condon factor between the T 1 and S 0 states, which leads to a very large radiationless decay rate constant of the T 1 state. The location of the S 2 origin is discussed on the basis of the present results.

A time‐resolved EPR study of short‐lived triplet states: Host and temperature dependences of triplet properties of phthalazine

The triplet properties of phthalazine are investigated by means of the time‐resolved EPR technique with laser excitation in various hosts over a wide range of temperature. The host and temperature dependences are interpreted satisfactorily in terms of the vibronic mixing between the 3nπ* and 3ππ* states. From the host dependences of the EPR parameters the zfs of the pure 3nπ* state of phthalazine is deduced to be Xn =0.027 cm−1, Yn =−0.058 cm−1, and Zn =0.031 cm−1. The fraction of the 3nπ* character of the T1 state is estimated to be ∼0% in ethanol, benzoic acid (BAC), and 1, 4‐dichlorobenzene (DCB), 14% in 1, 2, 4, 5‐tetrachlorobenzene (TCB), 60% in biphenyl (BP), and 84% in durene. The temperature dependences of the zfs, linewidth, phosphorescence spectrum, and lifetime are investigated and the results are explained using a continuum model. From the simulation of the temperature dependence of the triplet lifetime in the DCB host the decay rate constant of the second excited triplet (3nπ*) state is estim...

Mechanisms of external heavy atom effects on the lowest excited triplet states: Naphthalene and biphenyl X traps

An external heavy atom effect was investigated on the phosphorescent states of naphthalene X traps and biphenyl X traps using a variety of heavy atom perturbers at liquid helium temperatures. Phosphorescence spectra, triplet lifetimes, sublevel properties, and detrapping rates were measured. The well‐resolved phosphorescence spectra were obtained in almost all cases. The intensities of the vibronic bands were extremely weak in the heavy atom systems compared with those of the light atom systems. The triplet lifetimes range from 10 to 2300 ms in the naphthalene X traps and from 240 to 4200 ms in the biphenyl X traps. Selective enhancements were observed in the radiative decay rate constants of the triplet sublevels. Correlation between the intensities of phonon bands and the magnitudes of detrapping rates was found in the naphthalene X traps. From these results we conclude that: (i) The shortening of the triplet lifetimes is mostly due to the radiative enhancements of the T1→S0 transitions; (ii) the molecu...