Peter P. Levin

Magnetic field effect on the recombination of the triplet radical ion pair state of the semirigid Zn-porphyrin-viologen dyad

Abstract Laser flash photolysis technique was used to record the recombination kinetics of the triplet radical pair state of the new semi-rigid Zn-porphyrin-viologen (P-Ph-Vi 2+ ) dyad in which Zn-meso-4-methoxyphenyl-tris (4-tolyl) porphine (P) and methylbenzylviologen (Vi 2+ ) are linked by the 1,4′-phenylene spacer (Ph). In polar solvents the 3 [P +• -Ph-Vi +• ] decay kinetics fits fairly into the first-order law and the corresponding rate constants ( k 2 ) equal (1.8–3.3) x 10 6 s −1 and do not correlate to a solvent polarity. Application of an external magnetic field ( B ≤0.24 T) decreases the k 2 value to (0.3–0.7) x 10 6 s −1 . The B 1 2 values (defined as the B values at which the magnetic field effect reaches half of its saturated value) are 15–30 mT. The largest magnetic field effect on the k 2 value is observed in glycerol, k 2 ( B =0)/ k 2 ( B =0.24 T)= 11. In ethanol solution the quenching rate constant of 3 [P +• -Ph-Vi +• ] by 3 O 2 equals (8±0.8) × 10 9 M −1 s −1 and is close to 1 3 of the diffusion limit. The mechanism of the triplet-singlet (T-S) evolution of RP and the magnetic field effect as well as high efficiency of the RP quenching by molecular oxygen ( 3 O 2 ) are discussed with respect to the different behaviour of the RP in several conformers. In opened conformer of RP (with low value of exchange interaction, J , between radical centers) the hyperfine coupling (hfc) and the relaxation mechanism of T-S evolution of the RP are active; in closed conformer of RP where 2J is large enough the spin-orbit coupling dominates. The stimulation of the T-S transitions in closed RP by the exchange interaction of the radicals with 3 O 2 is very efficient.

Kinetics of oxygen induced delayed fluorescence of eosin adsorbed on alumina. The dependence on dye and oxygen concentrations

Abstract The kinetics of oxygen quenching of eosin (E) triplets adsorbed on alumina and concomitant 1 O 2 feedback-induced delayed fluorescence (DF) of E, were measured at different O 2 and E concentrations by the diffuse-reflectance laser flash technique. The global kinetic analysis in terms of 1 O 2 feedback in which the energy transfer from 1 O 2 to 3 E generates 1 E ∗ revealed the dependence of kinetic parameters on oxygen content and sample loading which matches the aggregation of E on the surface. The excitation migration in dye and oxygen manifolds is assumed to be involved resulting in high DF efficiency.

Geminate recombination kinetics of triplet radical ion pairs on silica studied by diffuse reflectance laser flash photolysis

Abstract Geminate recombination kinetics has been studied using the diffuse reflectance laser flash technique of radical ion pairs (RIP) formed by electron transfer from triphenylamine to triplet benzophenone, 9,10-anthraquinone, duroquinone or 2,6-diphenyl- 1,4-benzoquinone both adsorbed onto porous silica. The kinetics can be described by two formalisms: a first-order law with Gaussian distribution on the free energy which gives an average rate constant k ≈ 106 s−1 and a small width of distribution; or by a fractal-like analysis which supports the contribution of a fast motion in the radical ion pair recombination and the heterogeneity of the media. A retardation of RIP geminate recombination up to 25% under an external magnetic field shows evidence of some contribution of the magnetosensitive mechanisms of the RIP spin evolution.

Geminate recombination kinetics of triplet radical ion paris on porous glass: Magnetic field effect

Abstract Geminate recombination kinetics of radical ion pairs formed by electron transfer from triphenylamine or tri(4-bromophenyl)amine to triplet 9,10-anthraquinone adsorbed onto optically transparent porous glass have been studied, using the laser flash technique. The kinetics are described by the sum of two exponentials, ascribed to the existence of two kinds of “supercages” on the surface. Introduction of a heavy Br atom leads to acceleration of the geminate recombination; application of an external magnetic field results in retardation. These effects are due to the contribution of the recombination route through the separated radical ion pair, where the hyperfine coupling mechanism of spin evolution is active, and intersystem backward electron transfer.

Solvent effects on the magnetic field dependent recombination kinetics of the Zn-porphyrin—viologen dyad radical ion pair state

Abstract Magnetic field dependent recombination kinetics of the triplet radical pair state (RIPS) of the semirigid Zn-porphyrin—viologen (PPhVi2+) dyad in which Zn-porphyrin (P) and viologen (Vi2+) linked by the semirigid spacer (Ph) containing a 1,4-phenylene moiety has been studied by nanosecond laser flash photolysis technique in sixteen solvents of widely varying properties. The influence of solvent viscosity, the size of solvent molecules and the specific solvation on the triplet RIPS recombination kinetics in an external magnetic field has been found and discussed in terms of the hyperfine coupling mechanism including an exchange interaction modulated by the stochastic spacer motion in low magnetic fields and the spin—orbit coupling induced intersystem electron transfer in high magnetic fields.

Effect of zeolite properties on ground-state and triplet-triplet absorption, prompt and oxygen induced delayed fluorescence of tetraphenylporphyrin at gas/solid interface.

The ground-state and transient absorption, prompt and delayed fluorescence of tetraphenylporphyrin (TPP) adsorbed onto the external surface of different zeolites was monitored using diffuse-reflectance steady-state and laser flash photolysis. The delayed fluorescence (DF) of TPP detected in the presence of O2 is attributed to the energy transfer from 3TPP to 3O2 to form 1O2 and subsequent energy transfer from 1O2 to some other 3TPP within the organised molecular ensembles on the zeolite surface. The spectroscopic and kinetic parameters, namely the yield of DF (2-20% relative to prompt fluorescence), depend on the zeolite properties: the observed differences were correlated with the acid-base properties of the two zeolite series studied in this work (KA, NaA, CaA) and (NaA, NaX, NaY).

Kinetics and magnetic field effect in geminate recombination of triplet radical pairs adsorbed onto porous glass studied by laser flash technique

Abstract Geminate recombination kinetics of radical pairs (RP) formed by electron or hydrogen atom transfer from triphenylamine, tri(4-bromophenyl)amine, 4-phenylaniline or 4-phenylphenol to triplet 9,10-anthraquinone, benzophenone or 4-bromobenzophenone both adsorbed onto an optically transparent SiO 2 porous glass have been studied, using the laser flash technique with spectrophotometric registration. The kinetics are adequately described by the sum of two exponentials, ascribed to the existence of different kinds of “supercages” on the surface. At the same time, the simplest approximation by only one exponential is fair in many cases, because the contribution of a “slow” exponential is comparatively low and is masked by the slow component because of the decay of the escaped radicals. Introduction of a heavy Br atom leads to the acceleration of the geminate recombination; application of an external magnetic field results in retardation. The heavy-atom effect displays the contribution of the intersystem backwards electron transfer or intersystem recombination in the contact states of a triplet RP owing to the spin-orbit coupling. The magnetic field effect is the result of the significant contribution of the recombination route through the separated RP, where the hyperfine coupling and relaxation mechanisms of the RP spin evolution are active.

Magnetic field effect on the recombination of the triple radical ion pair state of the Zn-porphyrin—violegen dyad: the influence of the supramolecule microenvironment

Abstract Photochemical behaviour of the semirigid Zn-porphyrin—violegen dyad depends on its microenvironment which affects the rate of the dyad conformational transitions and the radical ion pair (RIP) triple—singlet splitting in zero magnetic field. The heavy-atom environment of RIP enhances the RIP recombination and quenches the magnetic field effect.

Effects of spacer structure and external magnetic fields on the recombination kinetics of radical ion pair states of zinc porphyrin–viologen dyads

Magnetic field effects on the recombination kinetics of the triplet radical ion pair state (RIPS) of the Zn porphyrin viologen dyad (P–Sp–Vi2+) with Zn porphyrin (P) and viologen (Vi2+) linked by a flexible [(CH2)n, n= 3, 6, 10] or semi-rigid (with the 1,4-phenylene and 4,4′-biphenylene moieties) spacer (Sp) have been studied by the nanosecond laser flash photolysis technique in acetonitrile. As the Sp lengthens the rate constant (kr) of the RIPS recombination in zero magnetic field (B= 0) increases from 0.80 × 106 to 5.9 × 106 s–1, while the kr value in a strong magnetic field (B= 0.24 T) is almost invariant at (0.3–0.7)× 106 s–1. The experimental results are discussed in terms of a simple kinetic scheme including only two subensembles of the RIPS conformers (‘closed’ and ‘open’). The rate-determining processes of the RIPS recombination are concluded to be the singlet–triplet conversion stimulated by the hyperfine coupling modulated by the exchange interaction in zero magnetic field and spin–orbit coupling induced intersystem electron transfer in a strong magnetic field. The considerations of the spin dynamics based on the relative yields and the formation kinetics of the triplet RIPS are involved.

RESEARCH NOTE: TRIPLET RADICAL ION PAIR STATE OF THE Zn‐PORPHYRIN‐VIOLOGEN DYAD AS A MAGNETIC FIELD SENSITIVE PROBE OF PHASE TRANSITIONS IN SMALL UNILAMELLAR VESICLES

Abstract— The magnetic field effect on the recombination kinetics of the triplet radical ion pair state (RIPS) of the Zn‐porphyrin‐viologen dyad (P‐Ph‐Vi2+) in the small unilamellar vesicles (SUV) of d, l‐dipalmitoyl‐α‐phosphatidylcholine has been studied by the nanosecond laser flash photolysis technique at 5–60°C. The increase in temperature from 25 to 40°C enhances the rate constant (kr) of the RIPS recombination in zero magnetic field from 0.9 × 106 to 1.6 × 106 s‐1 while k, is temperature insensitive at 5–25 and 40–60°C. The typical break in the kr temperature dependence is observed in the temperature range of the phase transition of the SUV bilayers from the solid to the fluid state. The kr value in a strong magnetic field (B= 0.24 T) is equal to 2.7 × 105 s‐1 it is independent of temperature at 5–60°C. The shape of the magnetic field dependence of kr is unaffected by the phase transition of the SUV bilayers and is characterized by the existence of an initial plateau of kr at B= 0 to 0.5 mT.