Magnetic-field effects on geminate radical-pair recombination
Pairs of radical ions, 2A–+2D+, generated in acetonitrile by nanosecond laser flashes have been investigated in the presence of external magnetic fields. The pairs are produced in the overall singlet state via photoinduced electron transfer between electron-donor (D, dimethylaniline) and electron-acceptor (A, pyrene) molecules and they recombine geminately by back electron transfer to form the molecular triplet state of pyrene.
The results obtained with both freely diffusing systems and polymethylene-linked compounds (A—[CH2]n—D, n= 6–12) can be interpreted quantitatively on the basis of the assumption that the spin realignment in the initial radical ion pair is governed by the hyperfine interaction, ΔEhfi, between the unpaired electron spin and the nuclear spins in each radical and by the exchange interaction, J(n), of the radical spins in the pair. The latter increases with decreasing chain length, n.
The differences in behaviour with respect to geminate triplet formation of the linked compounds with short (n⩽ 6), medium (6 < n < 12) and long (n 12) polymethylene chains are discussed.