No abstract

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-[CH,],-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, AEEhfi, between the unpaired electron spin and the nuclear spins in each radical and by the exchange interaction, J ( n ) , of the radical spini 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 =s 6), medium (6 < n < 12) and long ( n 3 12) polymethylene chains are discussed.It is well established that photoinduced electron-transfer reactions between electron-donor (D) and electron-acceptor (A) molecules in polar solvents produce radical-ion pairs (2A-+ 2D+) which, depending on the spin multiplicity of the pair,

No abstract

The magnetic field dependence of the geminate recombination triplet yield of radical ion pairs generated via photoinduced electron transfer in polar solvents is investigated for the systems pyrene/N,N-dimethylaniline (Py/DMA), pyrene/3,5-dimethoxy-N,N-dimethylaniline (Py/DMDMA), and the perdeuterated system Py-d10/DMA-d11. The magnetic field dependence characterized through its B1/2 value is found to be dependent on the sum of the hyperfine coupling constants in the radical pair in agreement with previous theoretical predictions. A drastic reduction of the B1/2 value is observed with the perdeuterated system. By means of measurements of the radical ion and triplet absorption signals with nanosecond time resolution, the influence of the solvent on the geminate singlet and triplet recombination yields is investigated. Complementary measurements of exciplex lifetimes and quantum yields are carried out in a series of solvents with different polarities in order to determine the rate constants of fluorescence emission and intersystem crossing in the exciplexes.

No abstract