Radical pairs (RPs) are important reaction intermediates generated whenever two radicals encounter one another, a bond is cleaved homolytically or electron transfer between non-radical species take...

Radical Pairs in Solution

A new method in radical chemistry: Generation of radicals by photo-induced electron transfer and fragmentation of the radical cation

Magnetic field effects (MFEs) on reactions of biradical radical ion pairs (BRIPs, S = 3/2 and 1/2) were investigated for the electron transfer (ET) reactions of the triplet state of 10-methylphenothiazine (3MPTZ*) (S = 1) and electron acceptors linked with 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) (An−R·) (S = 1/2) in various solvents by means of a nanosecond laser photolysis technique. In 2-propanol, the yield of free ions dramatically increased with increasing magnetic field (B) from 0 to 2 T, but slightly decreased from 2 to 10 T. The magnitude of the MFEs was much larger than those observed in ET reactions of 3MPTZ* with acceptors without TEMPO (S = 0). To interpret the MFEs observed in the reactions with An−R·, we considered the following two factors. (i) In the quenching of 3MPTZ* by An−R·, not only the ET reaction but also triplet−doublet (T−D) quenching takes place. The T−D quenching efficiency decreases with increasing B, resulting in a higher yield of BRIPs ([MPTZ•+ An•-−R·]). However, this fa...

Magnetic Field Effects on Chemical Reactions of Biradical Radical Ion Pairs in Homogeneous Fluid Solvents

The effect of magnetic field on the intensity of electroluminescence from devices made of a poly-$p$-phenylenevinylene (PPV) copolymer was investigated. The emission intensity was enhanced by the application of magnetic field, and the magnitude of the increase depended on operational voltages. When the device was operated under application of low voltages, the intensity increased with magnetic field and reached an 8.5% increase at about $100\phantom{\rule{0.3em}{0ex}}\mathrm{mT}$. With the increase of the operational voltage, the effect of magnetic field was lessened. In addition, when measured at high voltages with increasing magnetic field, the emission intensity started to decrease after passing a maximum, then leveled off. This saturation value was slightly higher than that observed in the absence of magnetic field. These findings suggest that two processes sensitive to magnetic field are included in the emission processes. They are assigned to the charge recombination (CR) of anion and cation radicals and triplet-triplet annihilation (TTA) processes. From the analysis of the effects of magnetic field on the emission intensity based on a kinetic model, we quantitatively determined the fractions of singlet and triplet excitons generated through the CR process to be 0.17 and 0.83, respectively. With the increase of the concentration of triplet excitons in the organic layer, production of singlet excitons through the TTA process was enhanced, and the total yield of the singlet excitons exceeded 0.5 under normal device operational conditions. We conclude that this high yield is responsible for the high emission efficiency observed in the light-emitting devices based on PPVs.

Fractions of singlet and triplet excitons generated in organic light-emitting devices based on a polyphenylenevinylene derivative

We investigated the emission properties of a photostable luminescent organic radical, (3,5-dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl radical (PyBTM), doped into host molecular crystals. The 0.05 wt %-doped crystals displayed luminescence attributed to a PyBTM monomer with a room-temperature emission quantum yield of 89 %, which is exceptionally high among organic radicals. The 10 wt %-doped crystals exhibited both PyBTM monomer and excimer-centered emission bands, and the intensity ratio of these two bands was modulated drastically by applying a magnetic field of up to 18 T at 4.2 K. This is the first observation of a magnetic field affecting the luminescence of organic radicals, and we also proposed a mechanism for this effect.

Magnetoluminescence in a Photostable, Brightly Luminescent Organic Radical in a Rigid Environment

Effects of high magnetic fields on the dynamic behavior of radical pairs generated in micellar solutions

Effects of large magnetic fields on the dynamic behavior of radical ion pairs in a non-viscous solution at room temperature

Photo-induced electron transfer reaction between hexamethyldisilane and quinones as studied by a CIDNP technique

Magnetic field effects (MFEs) on the dynamic behaviour of radical pairs generated by the photochemical hydrogen abstractions of 2′, 3′, 4′, 5′, 6′-pentafluoroacetophenone and decafluorobenzophenone in micellar solutions were investigated at large magnetic fields up to 10 T using a superconducting magnet. These MFEs were compared with the MFEs observed in the reactions of acetophenone and benzophenone. The decay rate of a radical pair generated in the reaction of acetophenone in a Brij 35 micellar solution decreased with increasing magnetic flux density (B) from 0 T, and the decrease was saturated at 0·34 T. On the other hand, the decay rate of the pair in the reaction of pentafluoroacetophenone decreased steeply with increasing B from 0 to 2 T, but increased gradually from 2 to 10 T. Such reversions of MFEs were observed for benzophenone and decafluorobenzophenone in Brij 35 micellar solutions, and the degree of the reversion for decafluorobenzophenone was larger than that of benzophenone. These new rever...

Reversion of magnetic field effects under large magnetic fields observed in the photochemical hydrogen abstraction reactions of fluorinated acetophenone and benzophenone in micellar solutions

Magnetic field effects (MFEs) on the photochemical hydrogen abstraction of 2′,3′,4′,5′,6′-pentafluoroacetophenone in a Brij 35 micellar solution were investigated by a nanosecond laser flash photolysis technique and compared with acetophenone. The decay rate of a radical pair generated in the reaction of acetophenone decreased with increasing magnetic field strength B from 0 T to 0·34 T, but was saturated by 0·34 T. On the other hand, the decay rate of the pair in the reaction of pentafluoroacetophenone decreased with increasing B from 0 T to 1·34 T. The difference in these MFEs is explained in terms of the electron spin relaxation of the radical pairs and the escape rate of the ketyl radicals from these pairs. The magnetic field dependence of these rate constants was obtained by an analysis of the time profiles of the transient absorbance of the ketyl radicals generated.

Masatoshi Igarashi

Papers

Effects of high magnetic fields on the dynamic behavior of radical pairs generated in micellar solutions

Effects of large magnetic fields on the dynamic behavior of radical ion pairs in a non-viscous solution at room temperature

Photo-induced electron transfer reaction between hexamethyldisilane and quinones as studied by a CIDNP technique

Reversion of magnetic field effects under large magnetic fields observed in the photochemical hydrogen abstraction reactions of fluorinated acetophenone and benzophenone in micellar solutions

Magnetic field effects on the photochemical hydrogen abstraction of 2′,3′,4′,5′,6′-pentafluoroacetophenone in a Brij 35 micellar solution