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

Abstract

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) (A n −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 A n −R·, we considered the following two factors. (i) In the quenching of 3MPTZ* by A n −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•+ A n •-−R·]). However, this factor was found to have only a small contribution to the observed MFEs on the free ion yield. (ii) The ET reactions generate 4,2[MPTZ•+ A n •-−R·] in quartet (Q) and doublet (D) states, which decay through either the spin-selective back ET from the D states or the separation to free ions. At B = 0 T, the spin conversion between the Q and D states is efficient. The increase in the free ion yield with increasing B from 0 to 2 T can be attributed to the spin relaxation from the Q± 3/2 states to the Q± 1/2 and D± 1/2 states due to the dipole−dipole interaction of 3(A n •-−R·). The slight decrease in the free ion yield with increasing B from 2 to 10 T can be attributed to acceleration of the relaxation induced by the anisotropic Zeeman interaction and/or enhancement of the Q± 1/2 -D± 1/2 conversion induced by the difference in the isotropic g-factor between MPTZ•+ and A n •-−R·. Effects of solvents and additives on the present MFEs were also studied.

Influence of nitroxides on CIDNP of micellised radical pairs and short-lived biradicals

“…Nitroxides also have wide application as spin labels in biology [4,5] andas pH probes in biological systems [6]. A number of papers have been published to date on the influence of nitroxides on magnetic and spin effects [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] in chemical reactions. There are two theoretical models to account for this influence.…”

Pulse-shift measurement: a direct observation of radical pair dynamics in a micelle

“…Application of a magnetic field is both a traditional [1] and modern [2] technique employed for controlling reactions. A magnetic field is capable of modifying the intersystem crossing rate of the spin states of a radical pair [3] ora radical-biradical pair [4], resulting in a degree of control over the reaction products. By the same means, it is also possible to separate isotopes on the basis of their nuclear spin [5].…”

Magnetic field-enhanced photoinization of 6-methylpurine

“…Recently, magnetic field studies have been extended to molecular systems with total spin greater than unity [9]. MFE has also been observed during the ionization of Cu(CN) 2À 2 [10].…”