Geminate Recombination Of Radical Pairs Research Articles

Controlling Spin-Correlated Radical Pairs with Donor-Acceptor Dyads: A New Concept to Generate Reduced Metal Complexes for More Efficient Photocatalysis.

One‐electron reduced metal complexes derived from photoactive ruthenium or iridium complexes are important intermediates for substrate activation steps in photoredox catalysis and for the photocatalytic generation of solar fuels. However, owing to the heavy atom effect, direct photochemical pathways to these key intermediates suffer from intrinsic efficiency problems resulting from rapid geminate recombination of radical pairs within the so‐called solvent cage. In this study, we prepared and investigated molecular dyads capable of producing reduced metal complexes via an indirect pathway relying on a sequence of energy and electron transfer processes between a Ru complex and a covalently connected anthracene moiety. Our test reaction to establish the proof‐of‐concept is the photochemical reduction of ruthenium(tris)bipyridine by the ascorbate dianion as sacrificial donor in aqueous solution. The photochemical key step in the Ru‐anthracene dyads is the reduction of a purely organic (anthracene) triplet excited state by the ascorbate dianion, yielding a spin‐correlated radical pair whose (unproductive) recombination is strongly spin‐forbidden. By carrying out detailed laser flash photolysis investigations, we provide clear evidence for the indirect reduced metal complex generation mechanism and show that this pathway can outperform the conventional direct metal complex photoreduction. The further optimization of our approach involving relatively simple molecular dyads might result in novel photocatalysts that convert substrates with unprecedented quantum yields.

Spin-vibrational interactions in geminate radical pairs: Effective spin Hamiltonian including anharmonicity

An equation for the effective spin Hamiltonian including the interaction of radical pair spins with vibrations in both linear and higher approximations was obtained. Even in the linear approximation, the Hamiltonian was found to be non-Heisenberg. The influence of a sound field on the probability of geminate radical pair recombination is considered in the Appendix. The results obtained can be of use for solving certain nanochemistry problems.

A Supramolecular “Ship in Bottle” Strategy for Enantiomeric Selectivity in Geminate Radical Pair Recombination.

A Supramolecular “Ship in Bottle” Strategy for Enantiomeric Selectivity in Geminate Radical Pair Recombination.

A supramolecular "ship in bottle" strategy for enantiomeric selectivity in geminate radical pair recombination.

[reaction: see text] Reactions in which zeolites are modified with chiral inductors to serve as media for chiral induction are often limited by the propensity of both substrate and inductor to occupy the same supercage. Herein, we report a "ship in bottle" strategy utilizing the thermal decomposition of dioxetanes obtained from oxazolidinone-substituted enecarbamates for the enantioselective generation of methyl desoxybenzoin (MDB). Photoexcitation of the supramolecular geminate molecular pair results in enrichment of the opposite enantiomer of MDB.

Triplet energy distribution in photoinitiators containing two dissociable groups.

Suitable probe molecules containing the benzophenone chromophore and one dissociable bond [perester 1 or C-X (4, 5)] or two such bonds (2, 3) have been synthesized and studied to examine intramolecular triplet energy dispersion. Triplet energies and phosphorescence quantum yields as well as quantum efficiencies of bond scissions have been studied, and the flow of triplet energy in such molecules is discussed. Upon irradiation at 350 nm in either benzene or methanol, the target peresters undergo dissociation of both cleavable groups, producing a pair of radicals. The presence of a benzylic chloride function has little influence on the efficiency of perester dissociation. However, the presence of a benzylic bromide function was found to decrease the quantum yield of decomposition of the perester function of 3. This can be explained by taking into account the effect of the heavy atom and the rate of cage geminate radical pair recombination. The nature of the heavy atom perturbation, however, was found to be different in 5 as compared with 3.

Specific features of geminate radical pair recombination in high magnetic field

The specific features of diffusion-assisted geminate radical pair (RP) recombination in high magnetic field is analyzed in detail. It is shown that the dependence of the recombination yield R on the intraradical magnetic interaction Ā (hyperfine interaction, etc.) significantly depends on the parameter ξ∼(Ā/Dα2), in which D is the diffusion coefficient and α−1 is the characteristic length of the spin exchange interaction. Simple formula, accurately describing the dependence R(Ā) over a wide region of parameters of the system, is proposed and thoroughly discussed.

Magnetic field dependent yield of geminate radical pair recombination in micelles. Effect of intraradical spin lattice relaxation

Abstract The combined effect of coherent spin evolution and spin lattice relaxation on the magnetic field dependent yield of radical pair (RP) recombination in micelles is analyzed quantitatively using the exponential model (EM) and its supercage extension. Furthermore we show that the much simplier Johnson-Merrifield approximation agrees with the EM when the coherent transition frequencies are larger than the rate of escape from the micelle; but for nearly degenerate states only the EM is applicable. Relaxation induced by anisotropic hyperfine- and g -tensors is investigated. For large values of the anisotropic interactions and for long residence times of the RP in the micelle (i.e. small escape rates) the effect is substantial. The most pronounced effect is the decrease of the escape yield Y(B) caused by the τg -relaxation at high fields ( B > 1 T). Some hitherto unobserved spikes in the Y ( B ) curve are found at high fields. These are due to accidental degeneracies of the high field states.

Magnetic field dependent yield of geminate radical pair recombination in micelles

Abstract The magnetic field dependence of the radical pair recombination yield in micelles is investigated by an accurate numerical solution of the stochastic Liouville equation and by the Johnson-Merrifield approximation. Despite the simplicity of the approximation which is based on simple balance equations for the populations of the states it is found that useful results (~ 10% accuracy) are obtained in the expected region of applicability which includes typical experimental situations.

Theory of geminate recombination of radical pairs with instantaneously changing spin Hamiltonian. IV. Stimulated polarization of nuclei in switched external magnetic field

The present paper reports a theory of stimulated polarization of nuclei (SNP) in a switched external magnetic field formulated for the first time. The Laplace transform of the SNP signal with respect to the delay time t o of the field switching is calculated for arbitrary molecular motion of radicals in solution. The result is expressed in terms of the quadratures of the Green functions specifying the character of the motion. Within the framework of diffusion and phenomenological two-positional models of the relative motion, the analysis of the influence of the delay time of external field switching on the spectral line intensity and shape has been conducted. High sensitivity of the line intensity to the character of the relative molecular motion has been predicted.

Magnetic field effects in adenosylcob(III)alamin photolysis: relevance to B12 enzymes

The magnetic field dependence of geminate radical pair recombination following the photolysis of adenosylcob-(III)alamin (AdoCbl III ) has been studied by continuous-wave photolysis and picosecond laser flash photolysis technique. The net quantum yield (O) for continuous-wave 514-nm photolysis of AdoCbl III is decreased 2-fold at 80 mT in aqueous 75% glycerol and 1.8-fold at 50 mT in aqueous 20% Ficoll-400. No magnetic field dependence is observed in H 2 O. Glycerol is a microviscosigen that enhance geminate radical pair recombination by decreasing diffusion, whereas Ficoll-400 is a macroviscosigen that enhances geminate radical pair recombination by forming «cage» structure in solution

Theory of geminate recombination of radical pairs with instantaneously changing spin Hamiltonian. III. Radical recombination in switched high magnetic field

The theory of the recombination of radical pairs (RPs) in a switched high magnetic field is proposed for the first time. The RP recombination probability is calculated for a stochastic motion of reagents in solution. Within the framework of a phenomenological two-positional model the analysis of the external field switching effect upon the recombination probability and CIDNP effects in the reaction products is carried out. It is shown that the switched magnetic field experiments contain information on the RP magnetic-resonance parameters as well as on kinetic ones. The above experiments are compared with the time-resolved ones, and the “effectiveness” of this kind of experiments is discussed.

Theory of geminate recombination of radical pairs with instantaneously changing spin-Hamiltonian. I. General theory and kinematic approximation

A general theory of geminate recombination of radical pairs with instantaneously changing spin-Hamiltonian has been constructed on the basis of the formalism of Green functions which characterize the molecular motion of reagents. The kinematic approximation has allowed us to reduce the calculation of the values determining the spin state of the products to matrix operations. The operator matrices, involved in the resulting expressions, are determined by the “convolution” of spin dynamics with a function describing molecular motion. The form of such a function has been established for a well-known phenomenological two-positional model of radical pair.

Theory of geminate recombination of radical pairs with instantaneously changing spin-Hamiltonian. II. Method of summation of re-encounter contributions

Two approaches using different methods to describe the molecular motion of reagents in solution, namely the description using the statistics of re-encounters and the description in terms of integrals of the Green function, have been compared for a wide range of problems on pair recombination, such as the recombination of radical pairs whose Hamiltonian instantaneously changes in time. It has been shown that the method of the summation of re-encounter contributions and the kinematic approach are totally identic for the Poisson time-distribution of reagent escape from contact. Both merits and demerits of these approaches are discussed.

Net CIDNP in geminate radical pair recombination in viscous liquids. Comparison of theory and experiment for high magnetic fields

By accurate analytical solution of the stochastic Liouville equation the simple and accurate formula for the geminate recombination probability of radical pair in moderately viscous liquids in the high magnetic field limit is derived. This formula takes into account the spin lattice relaxation of radicals. The formula derived is applied to calculation of chemically induced dynamic nuclear polarization (CIDNP) arising in recombination of the radicals C(CH 3) 3 and OCC(CH 3) 3. The comparison of the calculated and recently measured net CIDNP dependences on relative diffusion coefficient made it possible to obtain some interesting information about the interaction of radicals in liquids.

Magnetic field effects in radical pair recombination. II. Spin exchange relaxation and CIDN(E)P in bulk recombination

The method of analytical solution of the stochastic Liouville equation developed in the previous paper is applied to derive the Bloch equations describing spin exchange relaxation (SER) and magnetic field effects such as CIDNP and CIDEP in bulk recombination of radicals. Simple analytical expressions for transversal and longitudinal SER rates are obtained which are applicable in the limit |Q|/ Dα 2 ⪡ 1, where Q is the characteristic intraradical magnetic interaction (hfi etc.), D is the relative diffusion coefficient and α −1 is the characteristic size of the exchange interaction potential. These expressions show that in the low viscosity limit | Q| d 2/ D ⪡ 1, where d is the distance of closest approach, the transversal SER rate is equal to the longitudinal one. However in the opposite limit of high viscosity transversal SER rate becomes two times larger than the longitudinal one. These formulae also predict the appearance of significant phase shifts of hfs lines (comparable with the SER line widths) due to SER in the high viscosity limit. The rigorous relation between polarization enhancement in bulk radical recombination and the value of chemically induced dynamic polarization arising in geminate recombination of radical pairs is found. Some experimental investigations of SER and CIDEP in bulk recombination are analyzed by the theory developed. It is shown that such an analysis gives valuable information on the interaction and relative motion of radicals in liquids.

Magnetic field effects in radical pair recombination. I. CIDNP and CIDEP in geminate recombination

The method of analytic solution of the stochastic Liouville equation (SLE) describing geminate radical pair recombination in a magnetic field is developed. The method permits taking into account the exchange interaction J(r) and diffusive relative motion of the radicals. It is based on the sudden perturbation approximation and is valid when ξ=( Q/Dα 2) 1 2 <1, where Q is the characteristic magnetic interaction (hfi, etc.), D is the relative diffusion coefficient and α −1 is the characteristic length factor in the exponential dependence J(r). The solution of the SLE is found as an expansion in ξ up to terms ∼E 2. This solution is applied to derive simple matrix expressions for magnetic field effects (MFEs) such as chemically induced dynamic nuclei (electron) polarization (CIDN(E)P). In the high magnetic field limit in ST 0 approximation the simple analytical formulae for CIDN(E)P are obtained. The formulae derived are quite accurate in a wide range of values of the parameters available in the theory. The high accuracy of these formulae is demonstrated by comparison with the numerical calculations. A number of experimental investigations are analyzed by the formulae of the theory. The analysis shows that in general the SLE formalism makes it possible to describe quantitatively the experiments on MFEs in geminate recombination of radicals.

Theoretical treatment of differential recombination probabilities of radical pairs in high magnetic fields

The geminate recombination of radical pairs (RPs) and the closely related effect of CIDNP are considered in their time evolution during the lifetime of RPs. The recombination is modulated by singlet-triplet transitions of the unpaired electron spins of the RP caused by Δ g and hfs mechanisms. The influence of recombination, consecutive contacts, dephasing caused by recombinationand exchange inter phenomenological models. The results enable the results of measurements of quantum beats in recombination fluorescence and also of time-resolved CIDNP to be interpreted more profoundly.

Magnetic field effects on geminate radical-pair recombination in superviscous liquids: The near-adiabatic limit

The theory of magnetic field effects (MFE) on geminate radical-pair recombination in superviscous liquids is developed. The theory is valid in the near-adiabatic limit defined by the inequality ω/α 2 D>1, where ω is a characteristic intraradical magnetic interaction, α −1 is the range of the exchange interaction potential and D is a relative diffusion coefficient. In the high magnetic field limit in the ST 0 approximation, simple analytical expressions for MFE amplitudes are derived which smoothly match those obtained in the sudden approximation when ω/α 2 D<1.

Recombination of radical pairs in high magnetic fields: A path integral–Monte Carlo treatment

The magnetic field dependence of photoinduced geminate hydrogen (electron) transfer processes originates from a Brownian motion of radical pairs, a hyperfine coupling, and Zeeman-induced precession of the unpaired electron spins and a spin-selective recombination. In this paper we introduce a Wiener path integral description for these processes. This approach entails an integration over all possible diffusion trajectories of two functionals, one that weights the diffusion paths according to the laws of Brownian motion, and a second that accounts for the spin motion and recombination of the radical pairs. This formulation maintains a close connection with the actual physical processes and is suitable for microscopic radical reaction processes under quite general circumstances. For the path integration we employ a Monte Carlo method, and for the electron spin precession we adopt a semiclassical approximation. The theory is then applied to the magnetic field dependence of a photoinduced hydrogen transfer process involving a ketyl–hexyl radical pair recently observed with nanosecond time resolution at fields of a few kG. It is argued that the magnetic field dependence observed cannot be reconciled with a simple hyperfine-coupled Zeeman-induced spin motion. Finally, a simple analytical expression for the magnetic field dependence of geminate radical pair recombination at high magnetic fields is presented.

CIDNP study of the photoreduction of benzophenone with secondary alcohols

CIDNP has been studied during the photoreduction of benzophenone in the presence of isopropanol and benzhydrol. Since the hydrogen transfer reaction of the dimethyl hydroxy methyl radical with benzophenone competes with geminate recombination of radical pairs, the rate of this reaction step could be determined from CIDNP experiments. A discussion is given about the unexpected net CIDNP effects observed during the photoreduction in the presence of benzhydrol. There is no evidence that this polarization arises by a triplet mechanism.