Albert Weller

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.

Mechanism and Spindynamics of Photoinduced Electron Transfer Reactions

Photoinduced electron transfer in polar solvents generates exciplexes and radical pairs. The specific reactions of these intermediates have been studied for systems with pyrene as the primarily excited acceptor and various aromatic and aliphatic amines as electron donors by measuring exciplex decay times and by evaluating time-resolved extinction measurements carried out with and without an external magnetic field of 500Gauß. The time-independent rate

Triple complex formation in the excited state

Abstract Two flourescence transformations observed with the system, naphthalene (D) - 1.4-dicyanobenzene (A), when first the acceptor and then the donor concentration is increased are shown to be due to charge-transfer complex (D+A− and subsequent triple complex (DD.A− formation in the excited state. Both excited complexes have high dipole moments. The oxidation potential of the naphthalene dimer (in the ground state) is estimated as E(DD/DD+) = 1.1 volt versus SCE.

Intramolecular excimer fluorescence as a probe of fluidity changes and phase transitions in phosphatidylcholine bilayers

Abstract Intramolecular excimer formation of 1,3-di(1-pyrenyl)propane has been used to investigate, as a function of temperature, the fluidity in multibilayer dispersions and sobicated vesicles of dimynstoyl and dipalmitoyl phosphatidylcholine. The main phase transitions are detected at temperatures corresponding to those reported in the literature. The results show that the lipid structure in the ordered phase is strongly perturbed (and hence relatively Quid) in the immediate vicinity of the probe.

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.

Intramolecular excimer formation with non-planar molecules: 1,3-dibiphenylpropane. Kinetic and thermodynamic results

Abstract Intramolecular excimer formation with dibiphenylpropane in several alkane solvents is reported. By a combination of fluorescence decay time and quantum yield measurements the rate constants describing the excimer formation and deactivation and the values for Δ H and Δ S of the excimer formation equilibrium could be determined. These data are compared with results obtained with di(2-naphthyl) propane and di(1-pyrenyl) propane. It is argued that the difficulty to observe excimer emission with biphenyl under photoexcitation results from a very large value of −Δ S . A comparison between the excimer fluorescence of dibiphenylpropane and [2.2] biphenylophane points to an unusually weak transannular interaction in the phane.

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.