External magnetic field and magnetic isotope effects on the dynamic behavior of radical pairs in solutions have been studied theoretically, where the relaxation of their electron spins was taken into account. The decay observed with some radical pairs in micellar solutions under high magnetic fields is successfully interpreted in terms of the relaxation mechanism, and the magnetic field dependence of the relaxation rates is calculated for a model system.

Theoretical study of relaxation mechanism in magnetic field effects on chemical reactions.

When light impinges on photosynthetic material – a plant leaf, an alga or a photosynthetic bacterium – it is absorbed by an array of lightcollecting pigments. Through resonant energy transfer the absorbed quantum of light is transported to a trap, the reaction centre. Within such a trap, a specialized (bacterio)chlorophyll complex is able to eject from its excited state an electron. This electron is ‘captured’ by an adjacent acceptor, which in turn donates the electron to a second acceptor, and so on. Thus, light energy is converted into chemical energy which is ultimately used in the metabolic processes of the cell.

Magnetic field effects on photosynthetic reactions.

The chlorophyll triplet state as a probe of structure and function in photosynthesis.

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

Measuring transient absorptions on the nanosecond time range, we observed the magnetic field ( H = 40 Oe) modulation of the pyrene triplet and pyrene anion concentrations formed through quenching of singlet excited pyrene molecules by N,N -diethylaniline in methanol at room temperature. We interpret this effect as being due to a magnetic field dependent triplet production in the geminate radical ion pair, induced by hyperfine interaction of the unpaired electrons. The discussion is based on a simple model related to the radical pair model in CIDNP theory. Moreover, the sign of the magnetic field modulation of triplets and radical ions allows to infer on the relative probability for the formation of excited and ground state molecules in ion recombination processes.

Magnetic field modulation of geminate recombination of radical ions in a polar solvent

The effect of a magnetic field on the singlet/triplet ratio in geminate ion recombination☆

The effect of an applied magnetic field on the fluorescence from radiolytic ion recombination has been studied for anthracene in some hydrocarbon solvents In pulse-irradiated anthracene (10−2 mol dm−3) in squalane, the fluorescence intensity following the pulse increases as a function of applied magnetic field in the range studied, 0 to 03 T (0 to 3000 G) At a constant magnetic field strength, the field-induced enhancement of the fluorescence intensity varies with time after the pulse At high field strengths (03 T) the enhancement reaches a maximum of ∼45% about 50 ns after the pulse Similar effects are observed in cyclohexane but the enhancement is smaller than that in squalane In benzene solutions the effect is extremely small These findings are confirmed by observations in continuously gamma-irradiated solutions In gamma-irradiated solutions of anthracene (10−2 mol dm−3) in squalane, the fluorescence intensity increases with increasing magnetic field and approaches ∼13% enhancement at high fie

Effect of a magnetic field on the fluorescence produced in irradiated anthracene solutions

The effect of a magnetic field on the light emission from squalane solutions of fluorene, pulse irradiated at the center of a large electromagnet, is described. The fluorescence intensity following pulse radiolysis was found to be 40 to 50 percent higher in an applied field of 0.3 T (3000 G) and the triplet yield showed a small decrease. The enhancement of the singlet emission is attributed to an increase in the fraction of geminate solute ion recombinations leading to excited singlet fluorene. The magnetic field effect was time dependent, being undetectable during the pulse and reaching a maximum value after about 100 ns. These observations are compared with the theory of Brocklehurst which describes the singlet to triplet conversion of geminate solute ion pairs quantitatively in terms of the electron--nuclear hyperfine interactions in the radical ions. After correction for the large in-pulse fluorescence and its decay, satisfactory agreement between experiment and theory was obtained. The results emphasize the importance of ion recombination in the generation of excited states in radiolysis and confirm the role of the electron--nuclear spin interaction in determining the ratio of singlet and triplet yields.

Pulse radiolysis in an applied magnetic field. The time dependence of the magnetic field enhancement of the fluorescence from solutions of fluorene in squalane

B. Brocklehurst

Papers

The effect of a magnetic field on the singlet/triplet ratio in geminate ion recombination☆

Effect of a magnetic field on the fluorescence produced in irradiated anthracene solutions

Pulse radiolysis in an applied magnetic field. The time dependence of the magnetic field enhancement of the fluorescence from solutions of fluorene in squalane