Hisaharu Hayashi

ESR Spectra of the Charge‐Transfer Triplet States of Some Molecular Complexes

ESR spectra were measured with various solutions containing 1,2,4,5‐tetracyanobenzene (TCNB) and benzene or methyl‐substituted benzenes. We succeeded in finding the ESR absorption lines due to the Δmu2009=u2009±u20092 and Δmu2009=u2009±u20091 transitions of the lowest triplet excited states of TCNB complexes and also in measuring their lifetimes. From the analysis of the ESR spectral data, we determined the fine‐structure constants D and E for the TCNB–toluene complex in ether–toluene mixed solvent, and D* for the TCNB complexes with benzene, toluene, xylenes, mesitylene, durene, pentamethylbenzene, and hexamethylbenzene. From the theoretical consideration on the D* value, an equation representing its dependence upon the charge‐transfer degree (x) in the lowest triplet excited state of a molecular complex was derived. The combination of the equation with the observed D* values gave the x values for the TCNB complexes, for example 95%, 72%, and 42% for TCNB complexes with hexamethylbenzene, durene, and mesitylene, respectively. T...

Determination of the singlet-triplet separation of a weakly interacting radical pair from the E.S.R. spectrum

An anomaly in the hyperfine structure of the electron spin resonance spectrum of the triplet state is expected to occur for a weakly interacting radical pair whose singlet-triplet separation is comparable to or smaller than the electronic Zeeman energy. This is caused by the mixing between the singlet and triplet states. We develop a general theory for the E.S.R. spectrum of a weakly interacting pair and derive the effective nuclear spin hamiltonian. Applying the theory to the radical pair of dimethylglyoxime, we succeeded in determining the singlet-triplet separation to be -0·100 ± 0·010 cm-1.

The E.S.R. and phosphorescence spectra of some dicyanobenzene complexes with methyl-substituted benzenes

The E.S.R. and phosphorescence spectra of the lowest excited triplet states of some charge-transfer complexes containing phthalonitrile, isophthalonitrile and terephthalonitrile as electron acceptors have been measured. Hexamethylbenzene, pentamethylbenzene, durene and mesitylene were used as donors. From the observed zero-field splitting constants, D*, the charge-transfer degree, x, in the lowest excited triplet states of these complexes was obtained. For example, x for the phthalonitrile-hexamethylbenzene complex is 0·48. For a fixed acceptor a decreasing order in x was found to go with an increasing order in the ionization potential of the donor.

Magnetic field effects on the dynamic behavior of a radical pair as studied by a sub-nanosecond laser flash photolysis technique

The hydrogen abstraction reaction of triplet 4-methoxy-benzophenone with thiophenol at 265 K was studied by a sub-nanosecond laser flash photolysis technique in the absence and presence of a magnetic field of 1.75 T. The decay rate of the generated radical pair at zero field was found to be 1.1 = 10 9 s y1 and that at 1.75 T to be 1.8 = 10 9 s y1 . The observed magnetic field effect can be explained by the acceleration of the triplet-singlet spin conversion process due to the D g mechanism. q 2000 Published by Elsevier Science B.V.

External magnetic field effects upon photochemical reactions in solutions

Abstract External magnetic field effects upon photochemical reactions in solutions were classified into four cases by considering the singlet-triplet energy separation of a radical pair as a reaction intermediate and the singlet-triplet mixing through the electronic Zeeman and hyperfine coupling terms. Reaction systems corresponding to the respective cases have hitherto been found experimentally. The magnetic field effect was enhanced in micelles and was observed for the decay of the ketyl radicals of benzophenone, 4,4-difluorobenzophenone, and others in sodium 1-dodecyl sulfate. The magnetic isotope effect was found for the decay of the benzophenone ketyl radical in micelles.

Re-examination of the magnetic field effects on the thermal reactions of benzyl and pentafluorobenzyl halides with n-butyllithium in hexane

The magnetic field effects reported in 1973 for the thermal reaction of benzyl and pentafluorobenzyl chlorides with n-butyllithium in hexane have been re-examined under various reaction conditions. Using a new reaction apparatus, which has an electromagnet (up to 1.49 T) in a glove box under an argon atmosphere (H2O and O2 < 1 ppm), a study has been made on the effects on the yields of asymmetrical products (phenylpentane and pentafluorophenylpentane) and symmetrical ones (diphenylethane and decafluorodiphenylethane) in the absence and presence of external magnetic fields. No appreciable magnetic field effects have been observed with change of sample concentration, reaction temperature, or magnetic field strength, or with various benzyl halides.

Optical detection ESR for the nπ* triplet state of benzil

Abstract We have observed the ESR spectrum of the nπ * triplet state of benzil in a benzophenone single crystal with the optical detection method. The principal values and axes of its fine structure tensor are obtained. The fine structure parameters are | D | = 0.092 cm −1 and | E | = 0.021 cm −1 .

Magnetic field effects on the hydrogen abstraction reaction of triplet 4-methoxybenzophenone with thiophenol as studied by a picosecond laser flash photolysis technique

The hydrogen abstraction reaction of triplet 4-methoxybenzophenone with thiophenol at 265 K has been studied with a newly developed picosecond laser flash photolysis apparatus under magnetic fields of 0–1.7 T. The decay rate constant of the radical pair generated was found to increase from 3.42 × 109 s−1 to 4.15 × 109 s−1 with increasing the magnetic field from 0 to 1.7 T. The observed magnetic field effects can be explained by the Δg mechanism. Using the simple kinetics model with the Δg mechanism, the rate constant of the escape process from the pair (k esc) and two rate constants for the T-S spin conversion process (k T-S) at 0 and 1.7 T were found to be 1.97 × 109 s−1, 1.45 × 109 s−1, and 2.12 × 109 s−1, respectively. From the magnetic field dependence on k T-S, the difference in the g values of the 4-methoxybenzophenone ketyl and phenylthiyl radicals was estimated to be 0.0087.

Can an external magnetic field affect the thermal reaction of pentafluorobenzyl chloride with n-butyllithium in hexane?

The magnetic field effects reported in 1973 for the thermal reaction of pentafluorobenzyl chloride with n-butyllithium in hexane were re-examined at 23±3°C with modern techniques. Using a new reaction apparatus, which has an electromagnet (up to 14.9 kG) in a glove box under argon atmosphere (H2O and O2<1 ppm), we studied the magnetic field effects on the yields of final products of pentafluorophenylpentane and decafluorodiphenylethane in the absence and presence of an external magnetic field of 14.9 kG. No significant magnetic field effect was observed upon the yields of pentafluorophenylpentane and decafluorodiphenylethane under well-controlled conditions with sufficient repetitions.

New magnetic field effects observed in micellar and fluid solutions under ultrahigh fields of up to 30 T

With magnetic fields of up to 30 T, we have found the saturation and reversion of magnetic field effects (MFEs) on radical pair lifetimes and escape radical yields due to the relaxation mechanism for the photoreduction of carbonyl compounds in micellar solutions as well as the saturation of MFEs on the escape radical yield due to the Δg mechanism for the photoreduction of 4-methoxy-benzophenone with thiophenol in fluid solutions. We have also found large MFEs for several reactions involving three odd electrons in fluid solutions.