表面活性剂TX-100与SDBS胶束中光解苯甲醛自由基的化学诱导动态电子极化

利用时间分辨ESR波谱仪，研究了苯甲醛在乙二醇和表面活性剂SDBS，TX-100 的胶束溶液中的激光光解化学诱导动态电子极化（CIDEP）现象。苯甲醛在激光照 射下可以从体系和自身中得到氢生成α-羟基苄自由基和苯酰自由基，在SDBS胶束 中是自由基对机理RPM极化，而在TX-100胶束中是三重态机理TM极化。计算机模拟 谱图进一步证实了自由基的产生和极化机理。     

Theoretical aspects of chemically induced magnetic polarization

The role of theory in guiding and simplifying interpretation of electron spin resonance experiments on photochemical and other reactions involving free radical intermediates is surveyed. Emphasis is on models which provide a physical picture as well as quantitative estimates for such phenomena as the radical pair mechanism of chemically induced electron spin polarization (CIDEP), the closely related process of spin exchange during radical-radical encounters, and spin lattice relaxation. Some specific topics discussed are: 1) an improved quantitative model of STo CIDEP combining an initial stage of polarization development followed partial loss of this polarization to spin exchange, 2) the relation between the spin exchange and recombination rate constants, and 3) simplification of spin-lattice relaxation in the common case of spin-rotation relaxation. The modification of the polarization processes in two-dimensional and closed three-dimensional systems is also discussed     

Effect of nitroxide radicals on chemically induced dynamic electron polarization of spin-correlated radical pairs in aqueous micellar solutions of sodium dodecyl sulfate

The multispin systems consisting of spin-correlated radical pairs (SCRPs) and stable nitroxide radicals, localized in micelles of sodium dodecyl sulfate (SDS), were studied by ESR and pulse laser photolysis techniques. In all the systems studied, the stable nitroxide radicals exert no effect on the shape of the ESR spectra of the SCRPs (in particular, on the shape of their antiphase structure) and on the decay kinetics of the ESR signal of the SCRPs. In the SDS micelles, the electron spin polarization transfer from the nonequilibrium electron spin states of the molecular triplets (SCRP precursors) is the most efficient mechanism of generation of the electron spin polarization in nitroxide radicals. The experimental data also show that the nitroxide radicals and SCRP radicals are most probably distributed uniformly in the micellar phase. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1390–1401, July, 2008.     

Bimolecular homolytic substitution (S(H)2) reactions with hydrogen atoms. time-resolved electron spin resonance detection in the pulse radiolysis of alpha-(methylthio)acetamide

Pulse radiolysis of aqueous solutions of alpha-(methylthio)acetamide produced unexpectedly large quantities of acetamide radicals that were identified by time-resolved electron spin resonance (TRESR) spectroscopy. The pH dependence of the TRESR-measured radical yields, results from selective scavenging reactions, and density functional theory predictions of the reaction thermochemistry prove that bimolecular homolytic substitution, S(H)2, of the acetamide radical fragment by a H atom is the most likely formation pathway.     

Net and multiplet CIDEP of the observer spin in recombination of radical-biradical pair

Net and multiplet chemically induced dynamic electron polarization (CIDEP) of the observer/catalyst spin formed in recombination of the radical-biradical pair is studied theoretically. We obtained analytical expressions for the observer spin CIDEP in the high magnetic field and for the multiplet polarization in zero magnetic field. Polarization in the vicinity of the so-called J resonance and its magnetic field dependence are investigated numerically. The observer spin methodology can be useful for probing magnetic interactions in the short-lived spin triads.     

Electrostatic control of spin exchange between mobile spin-correlated radical pairs created in micellar solutions

A series of photoinduced H-atom abstraction reactions between anthraquinone-2,6,-disulfonate, disodium salt (AQDS) and differently charged micellar substrates is presented. After a 248 nm excimer laser flash, the first excited triplet state of AQDS is rapidly formed and then quenched by abstraction of a hydrogen atom from the alkyl chain of the micelle surfactant, leading to a spin-correlated radical pair (SCRP). The SCRP is detected 500 ns after the laser flash using time-resolved (direct detection) electron paramagnetic resonance (TREPR) spectroscopy at X-band (9.5 GHz). By changing the charge on the surfactant headgroup from negative (sodium dodecyl sulfate, SDS) to positive (dodecyltrimethylammonium chloride, DTAC), TREPR spectra with different degrees of antiphase structure (APS) in their line shape were observed. The first derivative-like APS line shape is the signature of an SCRP experiencing an electron spin exchange interaction between the radical centers, which was clearly observable in DTAC micelles and absent in SDS micellar solutions. Solutions with surfactant concentrations well below the critical micelle concentration (cmc) or solutions where micellar formation had been disrupted (1:1 v/v CH(3)CN/H(2)O) also showed no APS line shapes in their TREPR spectra. These results support the conclusion that electrostatic forces between the sensitizer (AQDS) charge and the substrate (surfactant) headgroup charge are responsible for the observed effects. The results represent a new example of electrostatic control of a spin exchange interaction in mobile radical pairs.     

ESR and NMR studies of transients and micromechanisms occurring in initator systems for laser induced polymerization

The paper informs about the possibilities of studying the path-ways of photolytically induced radical formation and of the first steps of radical reactions by methods of magnetic resonance. The investigations were carried out on initiator systems for laser induced polymerization containing anthraquinone. triethylamine and erythrosine in various solvents. The applied methods were time-domain and steady-state cw-ESR, electron spin echo (ESE), ¹H NMR and stimulated nuclear polarization (SNP). The chemically induced polarization of electron spins and nuclear spins (CIDEP and CIDNP) and their relaxation was used as main source of information about the fast laser—induced reactions.     

Electron transfer pathways and protein response to charge separation in photosynthetic reaction centers: time-resolved high-field ENDOR of the spin-correlated radical pair P865(+)QA(-)

Recently we reported the first observation of time-resolved (TR) high-frequency (HF) electron nuclear double resonance (ENDOR) of the transient charge separated state P865(+)Q(-)A in purple photosynthetic bacterial reaction centers (RC) (Poluektov, O. G., et al. J. Am. Chem. Soc. 2004, 126, 1644-1645). The high resolution and orientational selectivity of HF ENDOR allows us to directly probe protein environments by spectrally selecting specific nuclei in isotopically labeled samples. A new phenomenon associated with the spin correlated radical pair (SCRP) nature of P865(+)Q(-)A was observed. The TR-HF ENDOR spectra of protein nuclei (protons) surrounding deuterated QA(-) exhibit a derivative-like, complicated line shape, which differs considerably from the HF ENDOR spectrum of the protein nuclei surrounding thermally equilibrated QA(-). Here, a theoretical analysis of these observations is presented that shows that the positions and amplitudes of ENDOR lines contain information on hyperfine interactions (HFI) of a particular nucleus (a proton of the protein) with both correlated electron spins. Thus, spin density delocalization in the protein environment between the SCRP donor and acceptor molecules can be revealed via HF ENDOR. Novel approaches for acquiring and analyzing SCRP ENDOR that simplify interpretation of the spectra are discussed. Furthermore, we report here that the positions of the ENDOR lines of the SCRP shift with an increase in the time after laser flash, which initiates electron transfer. These shifts provide direct spectroscopic evidence of reorganization of the protein environment to accommodate the donor-acceptor charge-separated state P865(+)QA(-).     

pi-topology and spin alignment utilizing the excited molecular field: observation of the excited high-spin quartet (S = 3/2) and quintet (S = 2) states on purely organic pi-conjugated spin systems.

As a model system for the photoinduced/photoswitched spin alignment in a purely organic pi-conjugated spin system, 9-[4-(4,4,5,5-tetramethyl-1-yloxyimidazolin-2-yl)phenyl]anthracene (1a), 9-[3-(4,4,5,5-tetramethyl-1-yloxyimidazolin-2-yl)phenyl]anthracene (1b), 9,10-bis[4-(4,4,5,5-tetramethyl-1-yloxyimidazolin-2-yl)phenyl]anthracene (2a), and 9,10-bis[3-(4,4,5,5-tetramethyl-1-yloxyimidazolin-2-yl)phenyl]anthracene (2b) were designed and synthesized. In these spin systems, 9-phenylanthracene and 9,10-diphenylanthracene were chosen as photo spin couplers and iminonitroxide was chosen as a dangling stable radical. Time-resolved electron spin resonance (TRESR) spectra of the first excited states with resolved fine-structure splittings were observed for 1a and 2a in an EPA or a 2-MTHF rigid glass matrix. Using the spectral simulation based on the eigenfield method, the observed TRESR spectra for 1a and 2a were unambiguously assigned as an excited quartet (S = 3/2) spin state (Q) and an excited quintet (S = 2) spin state (Qu), respectively. The g value and fine-structure splitting for the quartet state of 1a were determined to be g(Q) = 2.0043, D(Q) = 0.0235 cm(-1), and E(Q) = 0.0 cm(-1). The relative populations (polarization) of each M(S)() sublevel in Q were determined to be P(+1/2') = P(-1/2') = 0.5 and P(+3/2') = P(-3/2') = 0.0 with an increasing order of energy in zero magnetic field. The spin Hamiltonian parameters for Qu are g = 2.0043, D = 0.0130 cm(-1), and E = 0.0 cm(-1), and the relative populations in Qu were determined to be P(0') = 0.30, P(-1') = P(+1') = 0.35 and P(-2') = P(+2') = 0.0. These are the first observations of a photoexcited quartet and a quintet high-spin state in pi-conjugated triplet-radical pair systems. In contrast high-spin excited states were not observed for 1b and 2b, the pi-topological isomers of 1a and 2a, showing the role of pi-topology in the spin alignment of the excited states. Since a weak antiferromagnetic exchange interaction was observed in the ground state of 2a, the clear detection of the excited quintet high-spin state shows that the effective exchange coupling between the two dangling radicals through the diphenylanthracene spin coupler has been changed from antiferromagnetic to ferromagnetic upon photoexcitation. Thus, a photoinduced spin alignment utilizing the excited triplet molecular field was realized for the first time in the purely organic pi-conjugated spin system. Furthermore, the mechanism for the generation of dynamic electron spin polarization was investigated for the observed quartet and quintet states, and a plausible mechanism of the enhanced selective intersystem crossing was proposed. Ab initio molecular orbital calculations based on density functional theory were carried out to determine the electronic structures of the excited high-spin states and to understand the mechanism of the spin alignment utilizing the excited molecular field. The role of the spin delocalization and the spin polarization mechanisms were revealed on the photoexcited state.     

CIDEP of micellized radical pairs in low magnetic fields

We report the first experimental study of chemically induced electron spin polarization (CIDEP) processes in low magnetic fields for spin-correlated radical pairs (SCRPs) in micellar environments. Photoexcitation of (2,4,6-trimethylbenzoyl) diphenylphosphine oxide (TMBDPO) leads to the radical pair comprised of acyl radical 1 and phosphonyl radical 2. The spin polarization, which is very strong in free solution even at zero field, was detected using L-band time-resolved electron paramagnetic resonance (TREPR) spectroscopy with specially modified resonators. The mechanism of formation and decay of low field CIDEP in SCRPs is presented and discussed. The prominent difference between low and high field spectra in micelles is the absence of anti-phase structure for radical 2 with HFI a > B0. This feature is consistent with the proposed polarization mechanism and theoretical predictions.     

W-band time-resolved electron paramagnetic resonance spectroscopy on transient organic radicals in solution

Time-resolved electron paramagnetic resonance (TREPR) spectra of spin-polarized transient radicals in liquid solution, generated in a continuous flow-system of a W-band (95 GHz) high-field (3.4 T) EPR spectrometer, are reported. The organic free radicals are created by laser flash photolysis of ω,ω-dimethoxy-ω-phenylacetophenone (DMPA) and diphenyl-2,4,6-trimethylbenzoil phosphine oxide (TMDPO) inside the microwave cavity, and are observed at 10 ns to 20 μs delay times after the laser pulse. The analysis of the positions of the well-separated EPR signals at W-band yields the g-values of the observable transients with high accuracy. The chemically induced dynamic electron polarization (CIDEP) patterns are different from those in conventional X-band (9.5 GHz) EPR. This is due to different spin relaxation times at different magnetic fields, to field-dependent CIDEP mechanisms operating in the studied systems, and to the increased Boltzmann polarization at high fields.     

Cidep study of radical-ion pair systems: Photooxidation reactions of carbazoles by maleic anhydride in alcohol solution

Photooxidation (charge transfer) reactions of carbazole derivatives by maleic anhydride (MA) in alcoholic media are studied by a time-resolved cw-ESR (TRESR) and Fourier transform ESR (FTESR) techniques. The CIDEP spectra observed in the systems (methyl-, ethyl-, and phenyl-substituted carbazoles with MA) remarkably depend on the MA concentration. Under the high concentration conditions, an emissive TM (triplet mechanism) polarization with a slight A/E (absorption/emission) pattern of the RPM (radical pair mechanism) is observed. In the low concentration of MA, a TM-like absorptive polarization is superimposed on the A/E pattern of the RPM. Since this reaction takes place exclusively through the triplet state, this spectral phase reveals that the singlet state of the radical-ion pair (RIP) has an energy higher than that of the triplet state, in other words, the sign of the apparent exchange interaction of the present RIP systems is positive. The total absorptive polarization observed in the low concentration of MA is tentatively explained by the polarization transfer from the excited triplet state of carbazoles in thermal equilibrium.     

Viscosity effect on the CIDEP of laser photolysis of duroquinone in hydrogen-donor solvents

Chemically induced dynamic electron polarization (CIDEP) is one of the important research fields of dynamic spin chemistry. In the present work, with ethylene glycol, 1,2-propanol and their mixtures with different ratios in volume as solvents and duroquinone as photosensitive molecule, the influence of solvent viscosity on the CIDEP of photolyzed duroquinone radicals has been studied experimentally. In each solvent, duroquinone neutral radicals are observed. Duroquinone neutral radicals are generated through hydrogen transfer reaction from hydrogen-donor solvent molecules to excited duroquinone triplet. The CIDEP mechanism is mainly triplet mechanism. The CIDEP intensity of duroquinone neutral radicals decreases with the increase in the solvent viscosity, and the variation was rapid in low-viscosity range and slow in high-viscosity range.     

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 cm³ mol⁻¹ for negative ions and ≈10 cm³ mol⁻¹ for neutral radicals.     

Study on emission quenching by 2,2,6,6-tetramethyl-1-piperidinyloxy free radical

The emission quenching mechanism of four emissive compounds by 2,2,6,6-tetramethyl-1-piperidinyloxy free radical (TEMPO) was investigated using steady state and transient state spectroscopy. The quenching mechanism was considered to be the radical triplet pair mechanism (RTPM) from the experimental results of both the emission quenching study and the steady state and time resolved electron spin resonance (SSESR and TRESR) studies.     

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.     

Magnetic field effects due to spin—orbit coupling in transient intermediates

The effects of anisotropic spin—orbit coupling on the radical yield and CIDEP in chemical reactions of short-lived triplet intermediate are treated in the density matrix formalism. Analytical expressions for the magnetic field effect (MFE) on the lifetime of the triplet precursor and electron spin polarization are obtained in terms of the molecular parameters, reorientational correlation time, and magnetic field strength.     

ENDOR of spin-correlated radical pairs in photosynthesis at high magnetic field: a tool for mapping electron transfer pathways

A new phenomenon has been detected in the time-resolved electron-nuclear double resonance (ENDOR) spectra of the spin-correlated radical pairs in photosynthetic reaction center proteins. The observed effects result from both increased resolution and orientational selectivity provided by high magnetic field EPR and are manifest as specific, derivative-type lines in the ENDOR spectrum. Importantly, the positions and amplitudes of these lines contain information on the interaction of a particular nucleus with both correlated electron spins. Thus, spin density delocalization in the protein environment between the donor and acceptor in the SCRP can be revealed via SCRP ENDOR, providing a unique opportunity to probe the electron-transfer pathways in natural and artificial photosynthetic assemblies.     

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.     

Multifrequency time-resolved electron paramagnetic resonance investigations after photolysis of phosphine oxide photoinitiators. Dependence of triplet mechanism chemically induced dynamic electron polarization on microwave frequency

Phosphinoyl radicals were produced in benzene solution by photolysis of three acylphosphine oxide photoinitiators, diphenyl-2,4,6-trimethylbenzoyl phosphine oxide (I), bis(2,6-dimethoxybenzoyl)-(2,4,4-trimethylpentyl) phosphine oxide (II), and bis(2,4,6-trimethylbenzoyl) phenylphospine oxide (III). The chemically induced dynamic electron polarization (CIDEP) of the radicals was measured by time-resolved electron paramagnetic resonance spectroscopy at different microwave frequencies/magnetic fields, in S- (2.8 GHz, 0.1 T), X- (9.7 GHz, 0.34 T), Q- (34.8 GHz, 1.2 T), and W-bands (95 GHz, 3.4 T). The CIDEP was found to be due to a triplet mechanism (TM) superimposed by a radical pair mechanism comprising ST(0) as well as ST(-) mixing. Contributions of the different CIDEP mechanisms were separated, and the dependence of the TM polarization on microwave frequency was determined. It agrees well with the numerical solution of the relevant stochastic Liouville equation, which proves the TM theory quantitatively. The applicability of previous approximate analytical formulas for the TM polarization is discussed. Parameters of the excited triplet state of III were estimated from the dependence of the TM polarization on microwave frequency. They are zero-field splitting constant 0.169 cm(-1) 相似文献