H. Staerk

A quantitative interpretation of the magnetic field effect on hyperfine-coupling-induced triplet fromation from radical ion pairs

Abstract Experimentally determined B1/2 values characterizing the magnetic field dependence of molecular triplet production from radical ion pairs originating from photoinduced electron transfer are compared with semi-empirical values obtained according to B1/2(hfi) = 2(B12 + B22)/(B1 + B2) from the root-mean-square values for the hyperfine coupling of the two radicals, B1 and B2. The very good agreement is discussed.

Solvent, isotope, and magnetic field effects in the geminate recombination of radical ion pairs

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 e...

Magnetic field dependence of intramolecular exciplex formation in polymethyelene-linked A–D systems

Abstract An extremely large increase of exciplex fluorescence intensity has been observed with photoexcited compounds pyrene-(CH2)n - DMA in acetonitrile when an external magnetic field of several hundred gauss is applied. The effect is readily explained as originating from the radical ion pair which can undergo a magnetic-field-dependent singlet-triplet conversion due to nuclear hyperfine coupling, or an association to form the exciplex. The increasing influence of the exchange interaction in the radical ion pair with decreasing chain length (CH2)n is evident.

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.

Electron transfer reactions and inhibition of triplet state formation in mixed fluorescence quencher experiments

Abstract It is shown that mixed electron transfer fluorescence quenching experiments in which formation of the fluorescer triplet state is effectively inhibited allow new electron transfer reactions to be investigated. The rate constants obtained are discussed with respect to the free-energy changes involved in the reactions.

Radical ion and triplet formation in electron transfer fluorescence quenching studied by nanosecond laser spectroscopy

Abstract Application of nanosecond laser flash photolysis led to the detection of delayed triplet production (from initially produced radical ions) in electron transfer fluorescence quenching. From both, the second order radical ion decay and the triplet growing-in, a diffusion-controlled recombination rate constant in acetonitrile of (4.3 ± 0.3) × 10 10 M −1 sec −1 (ca. 1.2 × 10 10 M −1 in n -propanol) is obtained.

Solvent influence on the magnetic field effect of polymethylene-linked photogenerated radical ion pairs

The solvent viscosity and polarity dependence of the magnetic field effect in polymethylene‐linked radical ion pairs, which were generated by photoinduced intramolecular electron transfer in compounds of the type pyrene–(CH2)n–N,N‐dimethylaniline, has been studied. A stochastic Liouville equation is used, in which the dynamics of the polymethylene chain, the spin Hamiltonian as a function of the varying radical distance (exchange interaction), and a distance‐dependent back electron transfer rate are incorporated. The results are compared with predictions made on the basis of the (static) subensemble approximation.

Life uncertainty broadening in photoinduced electron transfer

Abstract Two types of energy-level broadening, one caused by electron exchange between radicals and ground-state molecules, the other by selective sampling at short times, are investigated. Both lead, in accordance with the uncertainty principle, to an increase in the B 1/2 value.

Solvent effects on the magnetic-field-dependent reaction yields of photogenerated radical ion pairs

Abstract A pronounced solvent viscosity and polarity dependence of the magnetic field effect was found for polymethylene-linked radical ion pairs generated by photoinduced intramolecular electron transfer in compounds of the type pyrene-(CH 2 ) n -N,N-dimethylaniline, with n = 7–16. This is interpreted in terms of the general radical pair mechanism, i.e. the nuclear hyperfine interaction mechanism including a spin-exchange interaction, modulated by the stochastic folding motion of the linking CH 2 chain which leads to a “motional deformation” of the reaction yield spectra.

Triplet-doublet electronic energy transfer from benzophenone to the ketyl radical: viscosity and magnetic field effects

Abstract The influence of solvent viscosity and an external magnetic field on the rate constant of electronic energy transfer from triplet bonzophenone to the ketyl radical is studied; it is concluded that the transfer is mediated by electron exchange.