Magnetic field effects upon photochemistry of bifuctional chain molecules

Abstract

External magnetic field effects are discussed for three typical examples of photoreactions involving biradical intermediates: photo-induced intramolecular electron transfer in chain molecules D-(CH*),-A consisting of an electron donor (D) and an acceptor (A), intramolecular hydrogen abstraction in bifunctional species :X-(CH2).-YH containing a hydrogen donor (YH) and an acceptor (:X in the excited triplet state) andphotochemical homolytic ring opening of a cyclic ketone which yields a biradical consisting of an acyl and an alkyl group. There has been progress in the elucidation of reaction mechanisms through the analysis of magnetic field effects on lifetimes of reaction intermediates and product yields in radical reactions involving biradicals. Extremely small interactions such as the Zeeman energy and hyperfine interaction may give rise to a large change in the distribution of photo-products derived from bifunctional chain molecules.

Photoinduced intramolecular electron-transfer reactions in carbazole-fullerene and phenothiazine-fullerene linked compounds in benzene and benzonitrile as studied by fluorescence, transient absorption, time-resolved EPR, and magnetic field effects

“…Previously, the lifetimes of the photogenerated triplet biradicals from do- nor-acceptor linked compounds were found to be remarkably extended in the presence of magnetic fields [7][8][9]. Similar MFEs were observed on the decay of the radical pair between a C6o cluster anion and a pyrene cation in a fluorinated micellar system [10].…”

“…This is consistent with the fact that the biradical was not generated in benzene because the electron-transfer reactions are endothermic. In benzonitrile, on the other hand, the decay of transient absorption at 520 nm for Ph (8)C6o was retarded in the presence of a magnetic field (Fig. 7), as in the case of Ph(10)C60 and Ph(12)C60 [18].…”

“…3) from the triplet to the singlet state of the biradicals by the Zeeman splitting of triplet sublevels. In other words, the MFEs in Ph(n)C60 can be explained in terms of the isotropic hf coupling and SLR mechanisms [4][5][6][7][8][9].…”

Magnetic field effects on photoelectrochemical reactions of modified electrodes with C60-phenothiazine nanoclusters

“…In the presence of magnetic field, Zeeman splitting of triplet sublevels occurs, and therefore the intersystem crossing process (k isc2 ) is retarded by the spin-lattice relaxation from triplet sublevels. Therefore, the MFEs are explained in terms of spin-lattice relaxation mechanism [10][11][12][13]. As a result, the escape process (k esc ) from the triplet radical pair increased with increasing magnetic field, giving larger numbers of the escaped (C 60 N + ) n À and MePH + .…”

“…Magnetic field effects (MFEs) on the reaction kinetics or yields of photochemical reactions in the condensed phase have been studied [10][11][12][13]. The MFEs have been powerful for verifying the mechanism of photochemical reactions including triplet states.…”