Reaction Yield Detected Magnetic Resonance Research Articles

Kinetics of Prompt Fluorescence Excited by a Laser Pulse in a Molecular Crystal in a Microwave Field

The pulsed version of the reaction yield detected magnetic resonance (RYDMR) method is theoretically considered for the reaction scheme according to which the annihilation of triplet excitons in a molecular crystal occurs. Analytical expressions are obtained that describe the kinetics of prompt fluorescence decay with time under the conditions of its excitation by an ultrashort laser pulse. The corresponding shape of the decay curves of prompt fluorescence is determined. It is shown that the obtained curves can be used for a more detailed determination of the reaction rate constants than in the case of the stationary version of the RYDMR method.

Theoretical Description of Pulsed RYDMR: Refocusing Zero-Quantum and Single Quantum Coherences

Abstract A theoretical description of pulsed reaction yield detected magnetic resonance (RYDMR) is proposed. In RYDMR, magnetic resonance spectra of radical pairs (RPs) are indirectly detected by monitoring their recombination yield. Such a detection method is significantly more sensitive than conventional electron paramagnetic resonance (EPR), but design of appropriate pulse sequences for RYDMR requires additional effort because of a different observable. In this work various schemes for generating spin-echo like signals and detecting them by RYDMR are treated. Specifically, we consider refocusing of zero-quantum coherences (ZQCs) and single-quantum coherences (SQCs) by selective as well as by non-selective pulses and formulate a general analytical approach to pulsed RYDMR, which makes an efficient use of the product operator formalism. We anticipate that these results are of importance for RYDMR studies of elusive paramagnetic particles, notably, in organic semiconductors.

Theory of pulsed reaction yield detected magnetic resonance

We propose pulse sequences for Reaction Yield Detected Magnetic Resonance (RYDMR), which are based on refocusing the zero-quantum coherences in radical pairs by non-selective microwave pulses and using the population of a radical pair singlet spin state as an observable. The new experiments are analogues of existing EPR experiments such as the primary echo, Carr-Purcell, ESEEM, stimulated echo and Mims ENDOR. All pulse sequences are supported by analytical results and numerical calculations. The pulse sequences can be used for more efficient and highly detailed characterization of intermediates of chemical reactions and charge carriers in organic semiconductors.

Photogeneration of free charge carriers in the donor–acceptor complex TBPDA·(C60)2

The effect of a weak magnetic field with B0 < 10000 G on the photoconductivity of the layered donor–acceptor complex TBPDA·(C60)2 has been studied. The Reaction Yield Detected Magnetic Resonance (RYDMR) spectrum provides evidence of a spin nature of the mechanism of free charge carrier generation in magnetic fields. A model of triplet–triplet annihilation of charge transfer excitons has been suggested. The model interprets the effect of a weak magnetic field on photoconductivity in TBPDA·(C60)2. The photoconductivity spectrum of the complex shows two peaks at 470 and 700 nm. The comparison of this spectrum with the optical absorption one indicates that both intermolecular charge transfer between neighboring C60 molecules and direct charge transfer from the TBPDA to the C60 molecules contribute to free charge carrier generation in TBPDA·(C60)2.

Radiation Chemistry of Polysaccharides: 2. Free-Radical Mechanisms of Formation of Citric Acid and Dicarboxylic Acids

Based on experimental data on the buildup of citric acid and dicarboxylic (succinic, malic, fumaric, tartaric, and malonic) acids in native starches, low-temperature ESR measurement results, and reaction rate- or reaction yield-detected magnetic resonance data (room temperature, cellulose), the sequences of elementary events (mechanisms) of transformation of free radicals in the anhydroglucose ring of glucose high polymers up to the formation of these acids were determined. Time ranges for the generation of radicals in reactions, such as dehydration (Н2О β-elimination), oxygenation, isomerization involving C–C and C–O bond cleavage, and rearrangement of C–H bonds, were estimated at (3–7) × 10–9 to (8–10) × 10–8 s.

Low field RYDMR: effects of orthogonal static and oscillating magnetic fields on radical recombination reactions

Reactions involving spin correlated radical pairs as intermediates are known to be sensitive to applied static and/or oscillating magnetic fields. In the reaction yield detected magnetic resonance (RYDMR) technique, an electromagnetic field in resonance with the electron Zeeman splitting produced by a strong static field is used to perturb the singlet ↔ triplet interconversion of the radical pair and so to affect the yield of geminate recombination. New experiments are described in which weak radiofrequency fields (≲ 300μT) in the frequency range 1–80 MHz are applied to radical ion pairs derived from pyrene and 1,3-dicyanobenzene, in the presence of a weak (≲ 3.0 mT) static magnetic field. Such experiments test the viability of RYDMR in low fields, provide insight into the crossover region between the zero-field and high field cases, and may give information on the distribution of radical pair lifetimes.

Application of a liquid chromatography detector to time-resolved RYDMR spectroscopy: a comparison of in situ and ex post facto measurements

A photodiode-array (PDA) UV–VIS detector for liquid chromatography is applied to time-resolved reaction yield detected magnetic resonance (RYDMR) measurements. The results derived from the yields of cage and escape products in the photoreaction of 2-methyl-1, 4-naphtnoquinone in a sodium dodecylsulfate micelle are found to be identical with those derived from the yield of escaping semiquinone radical detected by transient optical absorption. This implies practical linearity between the yields of escaping radicals and escape products. High sensitivity of the PDA detector enables application of escape product yields for kinetic analysis by reducing microwave-induced perturbation.

Validity and possibility of photoconductivity-detected magnetic resonance (PCDMR) method as one of reaction-yield-detected magnetic resonance (RYDMR) methods

Fluorescence-detected magnetic resonance and photoconductivity-detected magnetic resonance are simultaneously applied to the system of a radical-ion pair formed in the photoinduced electron transfer reaction between pyrene and 1,3-dicyanobenzene. The former monitors the exciplex fluorescence via the singlet radical-ion pair, while the latter monitors free ionic radicals escaping from the radical-ion pair. Both spectra show good coincidence, which shows that the population of the singlet state is proportional to the total radical-ion pair one. The data obtained from the photoconductivity-detected magnetic resonance method suggest the faster quenching reaction of the triplet state of the radical-ion pair than that of the singlet state.

Effect of Polymethylene-Chain Dynamics on the Lifetime of a Charge-Separated Biradical Studied by RYDMR Spectroscopy

The novel solvent-viscosity dependence of the time-resolved reaction-yield-detected magnetic resonance (RYDMR) spectra is observed for the polymethylene-linked biradical generated in the photoinduced intramolecular electron-transfer reaction between N,N-dimethylaniline (DMA) and phenanthrene (Ph) in the compound of the type Ph−(CH2)10−O−(CH2)2−DMA. With decreasing the solvent viscosity, the line width and peak intensity of the RYDMR B0 (external magnetic field) spectrum increases and decreases, respectively. This result is interpreted in terms of the decrease of the lifetime of intermediate ionic biradicals at lower viscosity. This explicitly shows that the lifetime of the ionic biradical is governed by the reencounter frequency of the two terminal radicals determined by the polymethylene-chain dynamics. The lifetime of the spin states in the ionic biradical is estimated to be less than 2 ns.

Several RYDMR studies on the radical-ion pair formed in the photolysis of TMPD: Photoconductivity, transient-absorption and fluorescence detection methods

Three different RYDMR (reaction-yield detected magnetic resonance) techniques, photoconductivity detected magnetic resonance (PCDMR), transient-absorption detected magnetic resonance (ADMR) and delayed fluorescence detected magnetic resonance (DFDMR), are applied to the investigation of the radical-ion pair formed in the photolysis of TMPD in mixed alcohol solution. Here, PCDMR is a newly developed RYDMR method to study the radical-ion pair. PCDMR and ADMR methods show the acceleration of the quenching reaction from the triplet manifold of the radical-ion pair by microwave radiation under electron spin resonance conditions. These spectra of the radical-ion pair are significantly broadened probably because of the dynamic behavior of the radical-ion pair interacting strongly. The long lifetime of the radical-ion pair is due to the Coulomb interaction and the special role of the mixed solvent containing 2-propanol that stabilizes the radical-ion pair. Furthermore, the simultaneous observation of the PCDMR and the DFDMR shows different spectra. This indicates that the spectrum observed by the DFDMR is due to the weakly interacting RIP formed through the pathway different from those observed by the other two methods.

Reaction-yield-detected magnetic resonance in the intra-and intermolecular electron transfer reactions

The spin dynamics of the intermediate radical ion pairs formed in the intra-and intermolecular electron transfer reactions between N,N-dimethylaniline and phenanthrene is studied by employing the polymethylene-linked and nonlinked systems. Reaction-yield-detected magnetic resonance (RYDMR) experiments are performed by monitoring the exciplex fluorescence. From the theoretical consideration of the data, we conclude that the lifetime of the radical ion pair in the linked system is shorter than that in the nonlinked system. This tendency is opposite to what is generally observed in linked and nonlinked neutral biradical.

The detection and identification of transient free radicals in solution

The various Electron Spin Resonance methods that have been used for studying transient free radicals are described, and their limitations and advantages discussed. Continuous wave and Fourier Transform time-resolved methods are compared and shown to be complementary in studies of radicals produced using flash photolysis. The role of Chemically Induced Dynamic, Electron Polarization (CIDEP) in determining reaction pathways and in linking the photochemistry and photophysics of molecules is described as is the role played by Reaction Yield Detected Magnetic Resonance (RYDMR) and magnetic field effect (MFE or MARY) studies in understanding the reactions of radicals.

S-band magnetron-based accessory for optical detection of ESR spectra of radiation-induced radical-ion pairs

In the development of the technique of reaction yield detected magnetic resonance (RYDMR), a novel simple accessory for optical detection of ESR (OD ESR), based on the S-band continuous wave magnetron generator is presented. Basic technical requirements for the RYDMR accessories are discussed. The successful use of an S-band magnetron as a microwave source for RYDMR experiments is reported for the first time. Along with the use of a simple loop-gap resonator it permits the studies of both polar and non-polar samples with good sensitivity and field resolution. With the help of this accessory, OD ESR spectra of some new acceptors, acridin and fluorescein, were detected and some new solvents, poly(ethylene glycol) and paraffin-based high-voltage insulator oil, were successfully utilized.

Pulse schemes in reaction yield detected magnetic resonance

Experiments are described in which the timing of a microwave pulse used to promote reaction yield detected magnetic resonance (RYDMR) transitions in a flash-generated spin-correlated radical pair is varied with respect to the time of the flash. Different RYDMR B 1 behavior is observed in each case, and it allows direct assessment of the lifetime of radical pairs via a kinetic analysis

Time-resolved reaction yield detected magnetic resonance (RYDMR)

Novel time-resolved reaction yield detected magnetic resonance studies are reported on exciplex systems consisting of pyrene and dicyanobenzene. It is shown that the fluorescence from the systems, observed under the action of a static and a resonant magnetic field, exhibits a marked variation in time, including oscillations, after the creation of the exciplex. This variation is characteristically different between the three isomers of the dicyanobenzene molecule. In a theoretical analysis, the major experimental features are reproduced and it is suggested that oscillations observed in the fluorescence decay curves may correlate with ST 0 mixing quantum beats. This experiment provides a new method for studying the spin evolution inside radical pair systems and for estimating the lifetime of the spin-correlated radical pair.

Study of solid-surface states by RYDMR spectroscopy

A new approach for studying solid-surface states is proposed. This approach is based on the reaction-yield-detected-magnetic-resonance (RYDMR) phenomenon and on detection of magnetic resonance spectra of spin-correlated pairs of particles on solid surfaces. The approach was verified for surface particles arising as follows: (1) From the fracture of AgBr crystals and cellulose samples; (2) under high pressure on the ruby crystal surface. The depth of the surface analysis is about 0.5–1 nm.

Theoretical calculations of RYDMR effects in photosynthetic bacteria

A detailed quantitative analysis of low-field RYDMR (reaction yield detected magnetic resonance) spectra of the photosynthetic bacterium Rhodobacter sphaeroides is presented. Reaction centres, in which the first stable electron acceptor (a quinone) had been pre-reduced, show pronounced temperature dependence around 280 K. This is interpreted in terms of a change in the separation of the constituents of the primary radical pair formed by photoinduced electron transfer.

Magnetic resonance of ultrafast chemical reactions

Using e.s.e. (electron spin-echo), RYDMR (reaction-yield-detected magnetic resonance) and e.s.r. (electron spin resonance) in protein single crystals, we have investigated the cation, the triplet and the initial radical pair associated with the process of photoinduced charge separation in photosynthesis. The initial charge separation of bacterial photosynthesis occurs in a few picoseconds within in a reaction-centre protein containing eight electron-transfer components, namely four bacterial chlorophylls, two bacteriopheophytins and two quinones. Two bacteriochlorophylls, BChl2, form the primary donor and one bacteriopheophytin serves as the primary acceptor. E.s.e. determination of the anisotropic 15N hyperfine interactions have shown that the primary donor cation resides symmetrically in two of the four BChls in the form of a special pair of bacterial chlorophylls, BChl+2. In direct contrast, our e.s.r. studies on the triplet state of the primary donor in single crystals of reaction centres suggest that the triplet state resides asymmetrically in BChl2, where the triplet is highly asymmetrical in R. viridis but is much more symmetrical (C2) in R. sphaeroides. RYDMR studies of the initial radical pair formed indicate that the extra bacteriochlorophyll molecule, BChlB, that is between the special pair donor cation and the primary acceptor bacteriopheophytin anion is not involved in a discrete electron-transfer step in bacterial reaction centres. By combining the results of our magnetic-resonance experiments with X-ray structural information, the following description emerges: (1) the ground-state, primary donor is a supermolecule dimer with approximately C2 symmetry; (2) the bridging BChlB molecule probably functions as a superexchange site for rapid transfer of electrons from the primary donor to the primary acceptor but with negligible back reaction; (3) the special pair, BChl2, is lower in energy than the bridging molecule, BChlB, such that the initial radical pair is formed via super-exchange between the distant (10 A edge-to-edge) special pair BChl2 and the bacteriopheophytin.

Recombination of radicals in solution. A theoretical study on the influence of two- and many-site hopping on the microwave field effect

Unpaired electron spins on radicals in solution can hop between several molecules of the same species. Two types of hopping process are considered: (1) intermolecular hopping, where the unpaired spin can jump to many molecules and (2) intramolecular hopping, where it jumps between the two equivalent sites of a connected dimer. In both cases the hopping process leads to characteristic changes of the magnetic field dependence of the triplet recombination yield observed by reaction yield detected magnetic resonance (RYDMR) experiments. This allows one to discriminate between many- and two-site hopping and to determine the value of the hopping rate.

Theoretical calculations of microwave effects on the triplet yield in photosynthetic reaction centers

Calculations of the effects of a microwave field on the triplet quantum yield and the RYDMR (reaction yield detected magnetic resonance) spectrum of P F are presented. It is found that the RYDMR spectrum is most intense for D < 10 J when the microwave field strength is about twice the strength of the electron exchange interaction. In addition, the linewidth of the RYDMR spectrum at low power is caused mainly by lifetime broadening of the short-lived triplet. Measurements of the RYDMR spectrum should provide one width a more accurate and direct way of determining the lifetime of the P F state and the strength of exchange and dipolar interactions as compared to the study of triplet quantum yield at low static magnetic field.