Chemically Induced Dynamic Electron Spin Polarization Research Articles

Characteristic features of the singlet–triplet mechanism of the electron spin polarization

Characteristic features of net chemically induced dynamic electron spin polarization (CIDEP) P n in triplet–radical (TR) quenching are analyzed in detail within the framework of a general model that enables one to analyze CIDEP both numerically and analytically. This model also makes it possible to accurately describe the nonadiabatic transitions between the terms of the TR-pair spin Hamiltonian that lead to CIDEP generation. The proposed theory yields a simple analytical dependence of P n on the parameters of the model. In particular, it is shown that, within a wide region of parameters, the dependence of P n on the coefficient of relative TR diffusion D r is described by a simple linear relation: $$P_n^{ - 1}\left( {{D_r}} \right) = {Q_0} + \overline {{q_n}} {D_r}$$ (with Q 0 and $${{q_n}}$$ independent of D r ). It is also demonstrated that the numerical and analytical results obtained are very useful in analysis of experimental data, as demonstrated by analyzing the experimental dependence of P n on D r .

Radical-triplet pair mechanism of electron spin polarization. Detailed theoretical treatment.

Specific features of net chemically induced dynamic electron spin polarization (CIDEP) P(n), generated in liquid-phase triplet-radical (TR) quenching, are analyzed in detail within the general model, which allows for fairly simple analysis of CIDEP both numerically and analytically. This model enables one to accurately treat nonadiabatic transitions between the terms of TR-pair spin Hamiltonian, resulting in CIDEP generation. The proposed theory predicts fairly simple analytical dependence of P(n) on parameters of the model. In particular, it is shown that within the wide region of parameters the P(n) dependence on the coefficient of relative TR diffusion D(r) is described by simple linear relation P(n)(-1)(D(r)) ≈ Q0 + q̅(n)D(r) (Q0 and q̅(n) are independent of D(r)). It is also demonstrated that obtained numerical and analytical results are very helpful for the analysis of experimental data, which is demonstrated by analyzing the experimental D(r)-dependence of P(n).

Photoredox Chemistry of AOT: Electron Transfer and Hydrogen Abstraction in Microemulsions Involving the Surfactant

Time-resolved magnetic resonance experiments (TREPR and CIDNP) are used to investigate previously unobserved redox chemistry of the surfactant dioctyl sulfosuccinate ester (AOT) using the photoexcited triplet state of anthraquinone 2,6-disulfonate (3AQDS*). Several different free radicals resulting from two independent oxidation pathways (electron transfer and hydrogen abstraction) are observed. These include the radical ions of AQDS and sulfite from electron-transfer processes, carbon-centered radicals from H-atom abstraction reactions, and an additional carbon-centered radical formed by electron transfer from the AOT sulfonate head group followed by the loss of SO3. The radicals exhibit intense chemically induced dynamic electron spin polarization (CIDEP) in their TREPR spectra. The intensity ratios of the observed TREPR signals for each radical depend on the water pool size and temperature, which in turn affect the predominant CIDEP mechanism. All signal carriers are accounted for by simulation, and CIDNP results provide strong supporting evidence for the assignments.

Photoreductive Self-Assembly from [Mo7O24]6- to Carboxylates-Coordinated {Mo142} Mo-Blue Nanoring in the Presence of Carboxylic Acids

The primary steps of the photoredox reaction between [Mo7O24]6- and carboxylic acid electron (and proton) donors in aqueous solutions are investigated by the chemically induced dynamic electron spin polarization (CIDEP) spectroscopy. The excitation of the O-->Mo ligand-to-metal charge-transfer (LMCT) bands of [Mo7O24]6- in the presence of CH3CO2H induces the emissive electron spin polarization (ESP) of *CH2CO2 and *CH3 radicals with an accompanying formation of the one-electron reduced species [Mo7O23(OH)]6-, which is demonstrated by the triplet mechanism involving the O --> Mo LMCT triplet states. The prolonged photolysis of the solution containing [Mo7O24]6- and CH3CO2H at pH = 3.4 leads to the formation of the acetate/propionate-coordinated {Mo142} Mo-blue nanoring, [MoV28MoV(I)114O429H10(H2O)(49)(CH3)CO2 triple bond Ac5(C2H5CO2 triple bond Pr)]30- (1a) through the formation of the cis-configured dimeric dehydrative condensation to two-electron reduced Mo-blue [(Mo7O23)2]10- ({Mo14}). 1a is isolated as a [NH4]+/[Me3NH]+-mixed salt which is formulated as [NH4]27[Me3NH]3[Mo(V)28Mo(VI)114O429H10(H2O)49(CH3CO2)5(C2H5CO2)].150 +/- 10H2O (1) by results of elementary analysis, single-crystal X-ray analysis, 1H NMR, IR, and UV/Vis measurements, and manganometric redox titration. Based on the building-block sequence of for 1a, the bottom-up processes from [Mo7O24]6- to the {Mo142} ring in the coexistence of beta-[Mo8O26]4- are discussed by (i) the stabilization of the molecular curvature of {Mo14} through both the intramolecular transfer of monomolybdates and the intermolecular transfer of monomolybdates as degradation fragments of beta-[Mo8O26]4-, to yield {Mo21} and {Mo20} building blocks, (ii) the outer-ring formation resulting from seven successive two-electron-photoreductive condensations among {Mo21} and {Mo20}, and (iii) inner-ring formation resulting from eight successive dehydrative condensations between monomolybdate linkers attached to the neighboring head Mo sites.

Time-Resolved EPR Studies of Main Chain Radicals from Acrylic Polymers. Structural Characterization at High Temperatures

Main chain radicals from several acrylic polymers are characterized in liquid solution at high temperatures (∼100 °C) using time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The radicals are produced by laser flash photolysis (248 nm) and subsequent loss of the side chain ester functionality by Norrish I α-cleavage. At these temperatures fast-motion spectra with conformationally averaged hyperfine interactions are observed. The spectra are strongly spin polarized from the triplet mechanism (TM) of chemically induced dynamic electron spin polarization (CIDEP). Computer simulation of the TREPR spectra leads to unambiguous structural characterization of both the main chain radical and the side chain oxo-acyl radical. Hyperfine coupling constants, g-factors, and line widths are reported and discussed for radicals produced from five different acrylic polymers. Variation of the side chain on the polymer backbone leads to significant changes in the observed hyperfine coupling constants. For the main chain radical from poly(fluorooctyl methacrylate) (PFOMA), fast motion of the polymer chain is not accessible at these temperatures. The side chain oxo-acyl radical from PFOMA exhibits long-range 19F hyperfine interactions. Additional TREPR experiments on small molecule model compounds and gel permeation chromotography results of the photolyzed polymers support the conclusion that the primary photodegradation mechanism proposed in this paper is general for acrylic polymers.

H-Atom Electron-Spin Polarization in Irradiated Water and Ice Confined in the Nanopores of Vycor Glass

Chemically Induced Dynamic Electron-spin Polarization (CIDEP) in radiolytically generated H atoms in water-saturated Vycor glass is observed at different temperatures (+29 to −94 °C) by time-resolved pulsed EPR spectroscopy. The kinetics of polarization is investigated by means of the numerical solution of the stochastic Liouville equation (SLE) as applied to a two-site kinetic model. The experimental results are quantitatively simulated assuming that the H atom is one of the partners of the random (or triplet geminate) radical pairs and that the Heisenberg spin−spin exchange interaction is the only inter-radical interaction in such pairs. The recombination of H atoms with radical pair partners is a spin-selective reaction with activation parameters close to those of H-atom diffusion. It is shown that the rate of longitudinal spin relaxation of the partner, the rate of encounters, and the rate of the spin-selective recombination of the H atom with a partner are the most important kinetic parameters defining the observed time evolution of CIDEP. The kinetics of the CIDEP is defined mostly by the polarization generated in random pairs. The contribution of CIDEP from the geminate triplet radical pairs is important only at short times.

CIDEP of 2-Chloroxanthone in 2-Propanol: Excitation Wavelength Dependence and Effect of Cross Relaxation

The pattern of a chemically induced dynamic electron spin polarization (CIDEP) spectrum of 2-chloroxanthone (2-ClXn) in 2-propanol depends on the excitation wavelength. To clarify the cause of this intriguing phenomenon, we have investigated the details of the CIDEP and the transient visible absorption of 2-ClXn in 2-propanol produced by excitation at 355 or 308 nm. The CIDEP spectrum is strongly temperature dependent, which is mainly due to the temperature dependence of the net polarization. The net polarization consists of a fast rising emissive polarization and a slow rising absorptive polarization, both at 308 and 355 nm. However, the relative contributions of the two polarizations are different depending on the excitation wavelength, with more emissive polarization at 308 nm. From the analyses of the time profiles of the CIDEP signals and the results of quenching experiments with 2,6-di-tert-butylphenol, it is concluded that the emissive polarization arises from the triplet mechanism (TM) due to the reaction of highly excited 2-ClXn and the absorptive polarization arises from a slow reaction of the triplet molecules in thermal equilibrium. When excited at 355 nm the phase of the CIDEP spectrum of the 2-hydroxy-2-propyl radical due to the singlet−triplet mixing radical pair mechanism (ST0M−RPM) is inverted at later times. It is shown by simulation that large inverted signals can be produced by the electron−nuclear cross relaxation mechanism under proper conditions.

X- and W-Band Time-Resolved Electron Paramagnetic Resonance Studies on Radical-Excited Triplet Pairs between Metalloporphyrins and Axial-Ligating Nitroxide Radicals

A chemically induced dynamic electron spin polarization (CIDEP) study has been accomplished on radical-excited triplet pairs (RTP) in systems of metalloporphyrins, MgTPP, ZnTPP, and ZnOEP, and pyridine-substituted nitronyl nitroxide radicals, nit-R (R = o-py, m-py, and p-py), by X- (9.5 GHz) and W-band (95 GHz) time-resolved electron paramagnetic resonance (TREPR) in solution. Axial-ligations between the porphyrins and the radicals were ascertained from visible absorption spectra except for the nit-o-py system. The TREPR spectra were composed of two signals, which were assigned to those of the ground (D0) state of the radical and the excited quartet (Q1) state of the RTP. These components showed two kinds of CIDEPs in different time regions. The polarizations of the Q1 state were attributed to radical−triplet pair mechanisms (RTPMs) with singlet and triplet precursors. In the nit-p-py and the nit-m-py systems, the polarizations of the radical were generated via an electron spin polarization transfer (ESPT...

Time‐Resolved (CW) Electron Paramagnetic Resonance Spectroscopy: An Overview of the Technique and Its Use in Organic Photochemistry

Abstract— Steady‐state and time‐resolved electron paramagnetic resonance (TREPR) experiments are described. Comparison of the TREPR continuous wave method to other time domain EPR techniques such as Fourier transform EPR (FT‐EPR) is made, and the advantages and disadvantages of each are presented. The role played by several mechanisms of chemically induced dynamic electron spin polarization (CIDEP) in the appearance of the spectra is explained. The advantages of using higher frequency spectrometers than the standard X‐band (9.5 GHz) are presented and discussed. Examples are presented that are relevant to organic photochemistry and electron donor‐acceptor chemistry. The use of TREPR to study polymer photodegradation, polymer chain dynamics, free radical initiator chemistry and biradical spin exchange interactions is described. Emphasis is placed on magnetic field effects studied by multiple frequency TREPR in these systems. Finally, several future directions in the field are discussed in terms of new developments in microwave and magnetic field technology.

Pressure effects on the electron spin relaxation of several radicals in solution

The pressure effect on the decay rate of chemically induced dynamic electron spin polarization (CIDEP) was investigated on several free-radical intermediates in photolysis, and the spin-lattice relaxation times for these radicals were estimated from the decay rates of CIDEP signals at various pressures. The spin-lattice relaxation rates were retarded by increasing external pressure. From the pressure dependence of the spin-lattice relaxation rates the activation volume was estimated. The activation volumes of these radicals divide into two groups; ≈30 cm 3 mol −1 for negative ions and ≈10 cm 3 mol −1 for neutral radicals.

CIDEP and kinetic studies on photolytically generated transient radicals of maleic anhydride by time-domain ESR

The Chemically Induced Dynamic Electron Spin Polarization (CIDEP) of transient free radicals, as well as reaction orders and radical lifetimes, were studied with the aim to investigate the pathways and elementary mechanisms of the generation and disappearance of radicals in different liquid solutions of maleic anhydride (MA). Most measurements were carried through in tetrahydrofuran and 1,4-dioxane. The time-resolved measurements were performed by means of a modified ERS 220 ESR spectrometer at light pulse lengths in the μs-region. The two electrically neutral radicals formed by proton or by solvent addition to MA disappear primarily by second order reactions and have mean lifetimes between 30 μs and 10 ms, depending on solvent nature, concentration and temperature. The experimental facts point to the coexistence of both the photo-triplet and the radical pair mechanism in these systems with the radical pair mechanism being the major contributor to the observed CIDEP.

Electron Spin Polarization in Photosynthetic Models: Chlorophyll a Mediated Photoreduction of 1,4‐Benzoquinone

AbstractThe observation of Chemically Induced Dynamic Electron Spin Polarization (CIDEP) is reported for the chlorophyll a cation radical and 1,4‐benzosemiquinone anion generated by laser flash photolysis of ethanolic solutions of chlorophyll a and benzoquinone. The signs of the polarizations exhibited by the radicals are consistent with the predominant operation of the Triplet Mechanism for CIDEP. Chemical decay for the photoinduced radicals occurs by recombination via reverse electron transfer (Kt ∼ 2.9×0.8×109 M−1s−1).