Dynamic Electron Polarization Research Articles

CIDEP Studies of Fullerene-Derived Radical Adducts

Photolyses of solutions containing organomercury compounds (HgR2) in the presence of C60 fullerene have been investigated by Fourier transform time-resolved EPR (FT TR EPR) and continuous-wave EPR (CW EPR) techniques. By FT TR EPR, both electron-spin-polarized 3C60 (A polarization) and electron-spin-polarized adducts •C60R (E/A + E polarization) are observed. The CW EPR spectra of the •C60R radicals under steady-state irradiation also exhibit some electron-spin polarization. The chemically induced dynamic electron polarization (CIDEP) in the FT TR EPR experiments is explained by the following series of steps. Photolysis initially causes cleavage of the organomercury compounds into radicals that add to C60 to form •C60R. The latter combine to form the dimers, [C60R]2, which are thermally stable and accumulate in the samples. In all of the reported experiments, a certain quantity of dimers is produced by photolysis before the EPR spectra are acquired. In the FT TR EPR experiments, laser excitation produces 3C60 by excitation of C60 and •C60R by photocleavage of the dimers. The observed E/A CIDEP patterns at short (<1 μs) delays after the laser flash are proposed to be a result of the creation of polarization through the radical-pair mechanism (RPM) resulting from the interactions of two •C60R radicals (geminate or free) formed from the photocleavage of [C60R]2 dimers. The additional E polarization observed at later times (>1 μs) is proposed to result from the interaction of 3C60 with •C60R radicals, creating E polarization through the radical-pair-triplet mechanism (RPTM). The polarization observed in the CW EPR experiments is attributed to the maintenance of polarization through the radical lifetime because of the extremely long spin−lattice relaxation of the •C60R radicals. The latter conclusion is consistent with the very small (50 mG) line widths of the adduct radicals. An upper limit for the bond energy of the [C60R]2 dimers of 226 kJ/mol is established by the observation of the CIDEP of •C60R radicals when 532-nm excitation is employed. The role of multiple adducts in the observed FT TR EPR spectra is discussed.

An Analysis of S-T0 Mixing Polarization of CIDEP in Terms of the Lennard-Jones Fluids: 1-Hydroxy-1-methylethyl Radical in 2-Propanol

Abstract Calculations of the magnitudes of the polarization (PRPM) due to singlet-triplet (S-T0) mixing of the radical pair mechanism of chemically induced dynamic electron polarization (CIDEP) are made by numerically solving the stochastic Liouville equations for fluids in which intermolecular interactions are represented by the Lennard-Jones (LJ) potentials. PRPM and the relation between PRPM and the difference in the resonance frequencies of radicals of the pair (2Qab) are examined for the continuum and for various models with different relative strengths of solute-solute, solute-solvent, and solvent-solvent interactions.In the continuum, PRPM reaches the maximum value at small |J0| (J0 is the exchange interaction at a distance of the closest approach),and levels off at large |J0|, whereas in the LJ fluids it increases in the region of large |J0| and reaches a second maximum.At low temperatures, m, which is defined as the exponent in the form of PRPM ∝ Qabm, approaches 0 in the LJ fluids in the region of |J0| = 1011—1012 s-1.The calculated results are compared with the experimental results obtained for 1-hydroxy-1-methylethyl radicals in 2-propanol.The main features of the experimental observations are qualitatively better explained by the present models of the LJ fluids than by the continuum with |J0| = 1011—1012 s-1.These results are ascribed to the effect of the solvent structure which tends to keep the solvent-separated radical pair for a longer time.

Effect of charge transfer state on the exchange interaction of radical-triplet encounter pairs

Chemically induced dynamic electron polarization (CIDEP) of galvinoxyl was measured in various excited molecule-galvinoxyl systems prepared by laser photolysis. Most of the systems examined showed net emission CIDEP, which is well explained by the quartet precursor radical-triplet pair mechanism with exchange interaction,J, of negative sign (quartet is higher than doublet). Several systems with molecules such as naphthalene, quinoxaline, biphenyl and triphenylene, however, showed net absorption CIDEP. Time profiles of CIDEP and kinetic analysis of quenching suggest that net absorption CIDEP is generated during the triplet quenching process by the galvinoxyl radical. We conclude that the net absorption CIDEP is produced during the triplet quenching if theJ value of radical-triplet encounter pair is positive. This is the first report of the radical-triplet encounter pairs with positiveJ value. The mechanism for this unusual positive sign ofJ value is discussed on the basis of the spin-selective configuration interaction between the doublet spin correlated states of radical-triplet and charge transfer encounter pairs.

ChemInform Abstract: Applications of Chemically Induced Dynamic Electron Polarization to Mechanistic Photochemistry

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Hydrogen atom transfer and electron transfer reactions in the triplet π,π* state of 1,4-anthraquinone studied by CIDEP techniques and laser flash photolysis

Photochemical hydrogen atom transfer (HT) and electron transfer (ET) reactions of 1,4-anthraquinone (1,4-AQ) have been studied in acetonitrile at 295 K by means of CIDEP (chemically induced dynamic electron polarization) techniques and laser flash photolysis. It was shown on the basis of CIDEP measurements that both HT and ET reactions in the excited state of 1,4-AQ originated from the triplet state. Quantitative investigations on the photochemical reactions of 1,4-AQ were carried out by laser photolysis at 355 nm. The HT reaction from 4-phenylphenol (PhPhOH) to the 3(π,π*) state of 1,4-AQ (31,4-AQ*) proceeded rapidly with a rate constant (kHT) of 5.3×109 dm3 mol−1 s−1, where the efficiencies for HT (ϕHT) and induced quenching (IQ, ϕIQ) were obtained to be 0.57 and 0.43, respectively. Similar HT reactions were also observed for 31,4-AQ* with phenol (kHT=2.3×109 dm3 mol−1 s−1, ϕHT=0.49, ϕIQ=0.51) and 2,6-di-tert-butylphenol (kHT=1.9×109 dm3 mol−1 s−1, ϕHT=0.47, ϕIQ=0.53). The observed rapid HT reactions were shown to be due not to hydrogen atom abstractions but to protic hydrogen atom transfer reactions. The ET reaction from 1,2,4,5-tetramethoxybenzene (TMB) to 31,4-AQ* took place with a rate constant (kET) as high as 8.5×109 dm3 mol−1 s−1, which was close to the diffusion-controlled rate constant with efficiencies of ET (ϕET=0.77) and IQ (ϕIQ=0.23). These fast reactions may proceed via triplet exciplexes between 31,4-AQ* and phenols (or TMB) with charge transfer character.

Chemically Induced Dynamic Electron Polarization in the Photoreduction of Aromatic Ketones with ππ*-Type Triplet State

Chemically induced dynamic electron polarization (CIDEP) spectra of radicals produced in the photoreduction reaction of aromatic ketone/triethylamine systems were measured by time resolved ESR method. Anion radicals with net absorption (Abs) CIDEP were produced in ethanol with a base such as KOH, while ketyl type radicals with net emissive (Em) CIDEP have been formed in cyclohexane and benzene. Laser power dependence measurement indicates that net Em CIDEP of ketyl radicals is generated by a two-photon process while the production of ketyl radical itself is by one-photon process. Therefore, it was concluded that net Em CIDEP is generated through radical-triplet pair mechanism (RTPM) between the triplet state aromatic ketone and the produced ketyl radical. While RTPM is reasonably considered as an origin of net Em CIDEP for ketyl radicals, it is difficult to determine the CIDEP mechanism of the net Abs polarization of anion radicals in the present systems. Possible mechanisms were tentatively proposed for ...

Detailed analysis of the mechanism of net electron spin polarization in liquid phase triplet-radical quenching

The kinetics of the generation of the net chemically induced dynamic electron polarization (CIDEP) in triplet-radical quenching (TRQ) in liquids is theoretically analyzed in detail. The method of treatment of non-adiabatic transitions between states of the TR spin hamiltonian which determine the TRQ is proposed. Analytical formulas are derived for the CIDEP-generation probability Pe and rate Ke, as well as for the TRQ probability Pq and rate Kq. It is shown that the dependence of Ke on the relative diffusion coefficient Dr is often non-monotonic, and this dependence cannot be correctly described by the usually applied relation Ke=KqPe.

Optical and Magnetic Resonance Studies of Ground and Excited Spin States on Some Open-Shell Orgnic Radicals

Study of the electronic excited spin states on open-shell organic radicals are an important research field of organic magnetism. We report the optical and the magnetic resonance study of the electronic excited spin states of naphthalene nitroxide radical (1), pyrene nitronyl nitroxide radical (2), pyrene thioaminyl radical (3). The fluorescence and phosphorescence spectra of 1 and 2 in EPA glass are measured. Both compounds give weak phosphorescence which indicates the existence of metastable excited quartet (S=3/2) state. Doublet like time-resolved ESR signals are observed for these stable radicals. The decay time of the dynamic electron polarization in their excited state is ms order. This finding indicates that the observed signals arises from the radical-triplet pair originating the magnetic exchange interaction and the polarization transfer between the triplet excited states of the condensed polynuclear aromatic groups and the dangling stable radicals. The g value and the polarization change of the doublet like signal were also observed for 3, showing a possibility that the signal comes from the middle Kramers doublet of the resulting quartet excited spin state.

Chemically Induced Dynamic Electron Polarization Study on the Mechanism of Exchange Interaction in Radical Ion Pairs Generated by Photoinduced Electron Transfer Reactions

Photoinduced electron transfer reactions were studied by using the continuous wave time-resolved electron paramagnetic resonance and Fourier-transformed electron paramagnetic resonance spectroscopy in polar solvents. The chemically induced dynamic electron polarization was investigated in both singlet and triplet precursor intermolecular electron transfer systems. The signs of the exchange interaction, which are defined by the energy differences between the singlet and triplet radical ion pairs, depended on the free energy changes for the charge recombination processes. The results are interpreted in terms of the mechanism that the spin selective stabilization and destabilization are caused by the perturbation through the electronic coupling from the ground state and the locally excited triplet state of the donor -acceptor pair at the equilibrium nuclear coordinate. In the singlet precursor electron transfer systems, the positive exchange interaction resulted from the selective triplet stabilization in the radical ion pair, when the ion pair state crossed with the locally excited triplet state at the normal region. In the triplet precursor electron transfer systems, the negative exchange interaction resulted from the selective singlet stabilization when the ion pair state crossed with the singlet ground state at the normal region. When the free energy change is larger than about 1.8 eV, the positive exchange interaction resulted from the spin-selective destabilization in the singlet ion pair, since the level crossing occurs at the inverted region.

Electron Spin Polarization by Intramolecular Triplet Quenching of a Nitroxyl Radical Labeled Thioxanthonedioxide

Time-resolved electron spin resonance (TR-EPR) has been used to investigate the chemically induced dynamic electron polarization (CIDEP) generated by the interaction between a stable free radical (TEMPO) and the triplet states of thioxanthonedioxide derivatives. The intensity of CIDEP for intermolecular triplet quenching (thioxanthonedioxide methylester triplets with TEMPO) and intramolecular triplet quenching were compared using a covalently linked TEMPO−thioxanthonedioxide molecule. It is shown that at equivalent concentrations of TEMPO, the CIDEP is much larger in the case of intramolecular triplet quenching than for intermolecular quenching. The CIDEP intensity for intermolecular triplet quenching is limited to a maximum TEMPO concentration of ∼4 mM (at which point spin−spin interactions become dominant) and reaches only 1/8 of the intensity for intramolecular quenching. Laser flash photolysis experiments at room temperature showed that the intramolecular triplet quenching is fast (τt < 20 ns). This q...

Direct Evidence for 1-Methylthymine Radical Cations and Their Transformation to Successor Radicals by Fourier Transform Electron Paramagnetic Resonance in the Nanosecond Time Scale

Using Fourier transform electron paramagnetic resonance (FT EPR), the radicals generated by the oxidation of 1-methylthymine were detected in the nanosecond time scale. The radicals were generated via triplet-sensitized electron transfer to anthraquinone-2,6-disulfonic acid in aqueous solution with laser photolysis at 308 nm. Both radicals show strong chemically induced dynamic electron polarization (CIDEP) by the triplet mechanism. This strong enhancement of the FT EPR intensity enabled the radical cation of 1-methylthymine to be detected in the nanosecond time scale at a low pH. The transformation of the radical cation to the N3-deprotonated successor radical was directly observed in the microsecond time scale. After a few microseconds, additional successor radicals of the primary radical cation were measured depending on the pH, and the radicals formed by the addition of OH- and/or PO43- at C6 of the radical cation were detected.

A CIDEP study of photolysis benzophenone radicals in the microheterogeneous phase systems

The chemically induced dynamic electron polarization (CIDEP) of photolyzed benzophenone radicals in the microheterogeneous phase systems has been made by using the time-resolved electron spin resonance (TRESR) spectrometer. In the AEO9/isoproanol/H2O/C8H18 liquid crystal system, the TRESR sptrtrum of Ph2COH shows an unusual CIDEP of spin correlation radical pair (SCRP). The SCRP formation in these systems resulting from the character microenvironment and the related physical chemistry property are discussed.

Time‐Resolved Electron Paramagnetic Resonance Study of Photoionization of Tyrosine Anion in Aqueous Solution

Abstract— Photoionization of the amino acid tyrosine in basic water was studied by time‐resolved electron paramagnetic resonance (TREPR) at X‐band (9.5 GHz). Photoionization of deprotonated tyrosine leads to a spin‐polarized emissive/absorptive chemically induced dynamic electron polarization (CIDEP) spectrum produced by the radical pair mechanism, with the tyrosyl radical in emission and the solvated electron in absorption, which implies a triplet precursor. The exchange interaction, J, is found to be negative for this radical pair. The triplet photoionization channel is determined to be monophotonic. The singlet channel of photoionization of deprotonated tyrosine is seen only upon addition of the electron acceptor 2‐bro‐mo‐2‐methylpropionic acid (BMPA) to the sample. The singlet channel is isolated by performing TREPR on a sample containing tyrosine, BMPA and a triplet quencher (2,4‐hexadienoic acid). This channel is also found to be monophotonic.

Time-Resolved Electron Paramagnetic Resonance Study of Photoionization of Tyrosine Anion in Aqueous Solution

Abstract— Photoionization of the amino acid tyrosine in basic water was studied by time-resolved electron paramagnetic resonance (TREPR) at X-band (9.5 GHz). Photoionization of deprotonated tyrosine leads to a spin-polarized emissive/absorptive chemically induced dynamic electron polarization (CIDEP) spectrum produced by the radical pair mechanism, with the tyrosyl radical in emission and the solvated electron in absorption, which implies a triplet precursor. The exchange interaction, J, is found to be negative for this radical pair. The triplet photoionization channel is determined to be monophotonic. The singlet channel of photoionization of deprotonated tyrosine is seen only upon addition of the electron acceptor 2-bro-mo-2-methylpropionic acid (BMPA) to the sample. The singlet channel is isolated by performing TREPR on a sample containing tyrosine, BMPA and a triplet quencher (2,4-hexadienoic acid). This channel is also found to be monophotonic.

Applications of Chemically Induced Dynamic Electron Polarization To Mechanistic Photochemistry

Applications of Chemically Induced Dynamic Electron Polarization To Mechanistic Photochemistry

ERRATUM Chemically induced dynamic electron polarization (CIDEP) in moderately fast-relaxing systems

ERRATUM Chemically induced dynamic electron polarization (CIDEP) in moderately fast-relaxing systems

Electron spin polarization in consecutive spin-correlated radical Pairs: Application to short-lived and long-lived precursors in type 1 photosynthetic reaction centres

An analytical treatment of the spin dynamics in sequential photoinduced correlated coupled radical pairs is presented and applied to the spectra of the states P+A 1 − and P+F x − in type 1 photo-synthetic reaction centres. Expressions for the spin polarized spectra are derived for the specific limiting cases of a very short-lived and very long-lived primary radical pair which correspond to the situation found in heliobacteria and photosystem I (PSI), respectively. The inhomogeneous line-broadening due to the unresolved hyperfine couplings is taken explicitly into account. It is shown that the density matrix of the secondary pair ρ2 can be written as the sum of two terms corresponding to (i) the part which is independent of the spin dynamics in the precursor, (ii) the additional spin polarization which is generated during the lifetime of the precursor and transferred to the secondary pair. The latter term contains two contributions which arise from the difference of the Zeeman interactions of the radicals in the primary pair and from the inhomogeneous line broadening. The predicted polarization patterns are compared to those established for chemically induced dynamic electron polarization (CIDEP) when uncoupled radicals are generated from a radical pair precursor. The expressions are then used to simulate the experimental spectra of the consecutive pairs P+A 1 − and P+F x − in PSI using parameters derived entirely from independent experimental data. Excellent agreement with the experimental results is obtained. The spectra of P+F x − in heliobacteria at X- and K-band are also simulated and it is shown that the observed polarization patterns can be reproduced assuming direct electron transfer from A0 to Fx with a time constant ofτ = 600 ps.

FT-EPR study of alkyl radicals formed in the photochemical reaction of Re(alkyl)(α-diimine) and Ru(alkyl)(α-diimine) complexes

A Fourier transform EPR (FT-EPR) study was made of the photochemistry of [Re(R)(CO)3 (α-diimine)] and [Ru(E)(R)(CO)2(α-diimine)] complexes, where R = alkyl or benzyl, E = I or SnPh3, and α-diimine = 4,4′-dimethyl-2,2′-bipyridine (DMB) orN,N′-diisopropyl-1,4-diazabutadiene (iPr-DAB). Photoexcitation of these complexes leads to homolysis of the metal-alkyl (benzyl) bonds as evident from the detection of the spectra of the alkyl (benzyl) radicals. FT-EPR spectra display strong spin polarization effects attributed to Triplet Mechanism (TM) and Radical Pair Mechanism (RPM) Chemically Induced Dynamic Electron Polarization (CIDEP). CIDEP patterns point to bond dissociation via a triplet state precursor. For a number of complexes, spin polarization was found to exhibit unusually large solvent effects, whereas for one complex the CIDEP pattern proved to be sensitive to the wavelength of laser light used to initiate bond dissociation. These effects reflect the strong dependence of CIDEP on the character of the excited states involved in the photochemical reactions and contribute to the understanding of the reaction mechanism.

Chemically induced dynamic electron polarization (CIDEP) in moderately fast-relaxing systems

Chemically induced dynamic electron polarization (CIDEP) in moderately fast-relaxing systems

Chemically induced dynamic electron polarization (CIDEP) in moderately fast-relaxing systems

The relative intensities of individual hyperfine lines in the radical pair mechanism spin-polarized electron spin resonance spectrum of a radical are imperfectly calculated using standard theory, especially when the counter-radical undergoes fast relaxation. Here an approximate treatment valid in moderately fast relaxing systems is given in which the origin of the effect is treated as a Lorentzian distribution of spin mixing frequencies. This corresponds broadly to considering them to be determined by the lineshape of the electron spin resonance (ESR) spectrum of the counter-radical. The approach is shown to provide an adequate basis for simulation of the spin polarized spectrum of the hydroxymethyl radical produced, along with a fast relaxing acyl radical, on photolysis of hydroxypropanone. In principle it provides a measure of the relaxation time of the acyl through observations on hydroxymethyl. With ESR lines of normal linewidth any correction to existing theory is usually negligible within experimental accuracy.