Chemically Induced Dynamic Electron Polarization Signals Research Articles

Time-Resolved Electron Paramagnetic Resonance Investigation on the Quenching Reaction of Photo-Induced Excited Triplet Vitamin K3 by Antioxidant Vitamin C in Homogeneous and Micelle Solutions

The reaction mechanism and dynamics of the quenching of the excited triplet of vitamin by vitamin C was studied in both homogenous ethylene glycol-H2O plus hexadecyltrimethylammonium bromide, aerosol OT, sodium dodecyl sulfate, and Triton X-100 micelle solutions by time-resolved electronic paramagnetic resonance. The photolysis of vitamin K3 in EG-H2O solution led to the chemically induced dynamic electron polarization (CIDEP) of the vitamin K3 neutral radical VK3H• and ethylene glycol ketyl radical, which showed that a hydrogen atom abstraction reaction of with the solvent ethylene glycol occurred. The triplet mechanism was the primary mechanism underlying the generation of CIDEP, suggesting that quickly reacted with ethylene glycol before spin-lattice relaxation. During the photolysis of vitamin K3 and vitamin C in ethylene glycol-H2O solution, abstracted hydrogen atoms not only from the solvent ethylene glycol but also from a vitamin C monoanion. The stronger CIDEP signal of the vitamin C monoanion radical indicated that can be rapidly quenched by vitamin C. In hexadecyltrimethylammonium bromide (aerosol OT, sodium dodecyl sulfate)/ethylene glycol-H2O micelle solutions, lipid-soluble needed to diffuse around the micelle surface to react with the water-soluble vitamin C. The spin-lattice relaxation of the parent resulted in weaker CIDEP of the reaction-generated radicals. Furthermore, the attraction between the positive charge layer of the hexadecyltrimethylammonium bromide micelle and vitamin C monoanion made the quenching reaction of by vitamin C rapid, whereas the repulsion between the negative charge layer of aerosol OT (sodium dodecyl sulfate) micelle and vitamin C monoanion made the quenching reaction slow. For Triton X-100 micelle, the coexistence of vitamin K3 and vitamin C in the same polyethylene glycol shell resulted in the fastest quenching reaction and strong CIDEP of the monoanion radical.

Photoinduced reaction of excited triplet 9,10-anthraquinone quenched by antioxidant vitamin C

The quenching of the photoinduced excited triplet of 9,10-anthraquinone ( 3 AQ � ) by Vitamin C (VC) has been studied in ethylene glycol-water (EG-H2O, EG/ H2O=9/1, v/v) homogeneous by using time-resolved electronic paramagnetic reso- nance (TR-EPR) and laser flash photolysis techniques. According to the chemically induced dynamic electron polarization (CIDEP) and transient absorptive spectra, the reaction intermediates and the reaction mechanism have been analyzed. The rate con- stant for the quenching of excited triplet 3 AQby VC has been measured. The results indicate that 3 AQcan capture hydrogen atom not only from ethylene glycol molecule but also from VC. The strong CIDEP signal of VC monoanion radical indicates that the quenching of VC to 3 AQis obvious. The rate for the quenching of 3 AQby VC is

A Time‐Resolved Electron Spin Resonance Study of the Generation Mechanism of the Benzoquinone Anion Radical

Abstract The chemically induced dynamic electron polarization (CIDEP) spectra for the photolysis of the p‐benzoquinone (PBQ)/formamide (FM) system in acid, basic, and micellar environments were obtained with a homemade time‐resolved electron spin resonance (TR‐ESR) spectrometer. When the PBQ/formamide was photolyzed, the CIDEP spectrum consisted of strong CIDEP signals of the benzosemiquinone anion radical PBQ•− and weak signals for the benzosemiquinone neutral radical PBQH•. When the PBQ/formamide was irradiated in basic or micellar solution, the CIDEP signal for PBQ•− only was obtained. When the PBQ/formamide was irradiated in acid solution, only the CIDEP signal of PBQH• existed. These experimental results indicate that the anion radical PBQ•− was formed from the dissociation of the neutral radical PBQH• accompanying polarization transfer.

1D Radical Motion in Protein Pocket: Proton-Coupled Electron Transfer in Human Serum Albumin

Photoinduced, proton-coupled electron transfer (ET) between 9,10-anthraquinone-2,6-disulfonate (ADQS) and an amino acid residue of tryptophan in human serum albumin (HSA) was observed using time-resolved electron paramagnetic resonance (TREPR). The ET reaction reduces the protein binding affinity of the ligand. TREPR chemically induced dynamic electron polarization (CIDEP) spectra establish that photoinduced ET takes place from the tryptophan residue (W214) to the excited triplet state of AQDS2- while bound in subdomain IIA, a protein cleft of HSA. The TREPR CIDEP signals also reveal that the anion radical of the ligand escapes toward the bulk water region by a one-dimensional translation diffusion process within the protein's pocket area. This pilot study of HSA demonstrates how TREPR CIDEP can provide significant means to investigate dynamic characteristics of protein-surface reactions.

Control of ferromagnetic and antiferromagnetic couplings in a galvinoxyl–triplet chrysene encounter complex through solvent polarity

Energy difference, J, between the quartet and doublet spin states of a galvinoxyl radical–triplet chrysene pair as an encounter complex in solution was investigated by measuring chemically induced dynamic electron polarization (CIDEP) of galvinoxyl with a time-resolved ESR (TR-ESR) method. All the TR-ESR spectra of galvinoxyl show net type CIDEP signals due to doublet–triplet magnetic interactions. The CIDEP phase switches from absorption in non-polar solvent to emission in polar solvent. The signals were analyzed on the basis of the radical–triplet pair mechanism (RTPM) for CIDEP creation, the results of which indicate that galvinoxyl–triplet chrysene pairs show ferromagnetic and antiferromagnetic couplings in non-polar and polar solvents, respectively. This solvent polarity-controlled switching of magnetic coupling in the galvinoxyl–triplet chrysene pair was interpreted by introducing the effect of intermolecular charge transfer interaction on the J value.

Energy Separation between Quartet and Doublet Spin States of Radical−Triplet Encounter Pairs; Unusual Ferromagnetic Interaction in a 1,1-Diphenyl-2-picrylhydrazyl and Triplet Coronene Pair

Time-resolved ESR (TR-ESR) measurements were performed for chemically induced dynamic electron polarization (CIDEP) created in various systems of the 1-diphenyl-2-picrylhydrazyl (DPPH) radical and triplet organic molecules. TR-ESR spectra of DPPH show a net emission CIDEP signal in most of the DPPH−triplet systems. The only exception was observed for a DPPH−coronene system in which the TR-ESR spectrum of DPPH presents a net absorption CIDEP signal. The origin of these net emission and absorption CIDEP signals is interpreted in terms of the radical−triplet pair mechanism (RTPM) for a triplet quenching process. To confirm that the CIDEP is created by RTPM, we simulated the time profiles of the TR-ESR signal with modified Bloch and kinetic equations. Stern−Volmer type analyses of the CIDEP intensities also support the assignment for the proposed CIDEP mechanism. According to the signal sign rule in RTPM, we examined the energy difference, J, of the quartet and doublet spin states of the radical−triplet encounter pair (RTP). J is defined by the equation J = E(2RTP) − E(4RTP) where E(2RTP) and E(4RTP) represent the energies of doublet and quartet RTP states, respectively. According to the CIDEP signal, most of the RTPs in DPPH−triplet molecule systems show a normal negative J-value, while a DPPH−triplet coronene pair shows an unusual positive J-value. The mechanism for this unusual antiferromagnetic coupling in the DPPH−triplet coronene pair is explained by introducing an intermolecular charge-transfer interaction.

A time-resolved ESR study on the photolysis of benzaldehyde in homogeneous solvent, micelles and microemulsions

The time-resolved electron spin resonance (TRESR) spectra and chemically induced dynamic electron polarization (CIDEP) phenomena obtained from laser flash photolysis of benzaldehyde (BA) in homogeneous solvent, micellar solutions and microemulsions were investigated. The α-hydroxybenzyl radical generated from the hydrogen abstraction of the triplet state of BA from ethylene glycol (EG) or surfactant molecules was observed. The radical pair mechanism (RPM) and triplet mechanism (TM) and computer simulation were applied to express the photolysis. The intensity of CIDEP signals, the lifespan of the radicals and the polarization mechanism were different in EG, micelles or microemulsions because of their different microstructures.

First Demonstration of Selective Triplet Quenching in the Radical−Triplet Pair Mechanism (RTPM) of Chemically Induced Dynamic Electron Polarization (CIDEP)

Emissive chemically induced dynamic electron polarization (CIDEP) signals produced through the interaction between the lowest excited triplet phenothiazine (3PTH) and 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) radical were detected by the time-resolved electron spin resonance (TRESR) method. The emissive spin polarization can be explained by the radical−triplet pair mechanism (RTPM). The fluorescence quenching and photoacoustic calorimetric experiments demonstrate directly that the emissive CIDEP signal is generated by the quartet spin states of the radical−triplet pair surviving through spin selective triplet quenching in the doublet spin states of the radical−triplet pair.

Effects of substitution and excited wavelength on the photochemistry of benzaldehydes studied by CIDEP

Chemically induced dynamic electron polarization (CIDEP) spectra of transient radicals were measured upon photolysis of benzaldehydes (BAs) after 266 nm laser excitation. The substituent effects on CIDEP were studied for a series of substituted derivatives (CH3-, F-, Cl-, Br- and CN-) of BA in 2-propanol at room temperature. For F-, Cl-, and CN-substituted BAs, CIDEP spectra were observed that were due to combinations of, firstly, the triplet mechanism (TM) with emissive (E) phase patterns and, secondly, the S-T,0 mechanism (ST0M) of radical pairs with emissiveoabsorptive (E/A) phase patterns. In contrast, for CH3-substituted BAs, CIDEP spectra due only to the ST0M with E/A phase patterns were observed. The contribution of the TM-induced CIDEP shows an increasing trend for BAs with electron-withdrawing substituents (F-, C1-, and CN-) and a decreasing trend for BAs with an electron-donating substituent (CH3-). These substituent effects on the TM-induced CIDEP were found to be parallel to the effects on the reaction rate of hydrogen abstraction of excited BA molecules: both effects are strongly influenced by the (n, π*) and (π, π*) characteristics in the lowest excited triplet states. Furthermore, clear CIDEP signals due to the benzoyl and alpha-hydroxybenzyl radicals, which are produced by self-quenching of triplet BA, were newly obtained upon photolysis of a concentrated acetonitrile solution of BA (5 × 10-2 mo1 dm-3 ) by a 266 nm laser. This result is not consistent with the proposed wavelength effect on the BA photochemistry, but it is consistent with a normal radical reaction mechanism that shows no wavelength effect.

Effects of substitution and excited wavelength on the photochemistry of benzaldehydes studied by CIDEP

Chemically induced dynamic electron polarization (CIDEP) spectra of transient radicals were measured upon photolysis of benzaldehydes (BAs) after 266 nm laser excitation. The substituent effects on CIDEP were studied for a series of substituted derivatives (CH3-, F-, Cl-, Br- and CN-) of BA in 2-propanol at room temperature. For F-, Cl-, and CN-substituted BAs, CIDEP spectra were observed that were due to combinations of, firstly, the triplet mechanism (TM) with emissive (E) phase patterns and, secondly, the S-T,0 mechanism (ST0M) of radical pairs with emissiveoabsorptive (E/A) phase patterns. In contrast, for CH3-substituted BAs, CIDEP spectra due only to the ST0M with E/A phase patterns were observed. The contribution of the TM-induced CIDEP shows an increasing trend for BAs with electron-withdrawing substituents (F-, C1-, and CN-) and a decreasing trend for BAs with an electron-donating substituent (CH3-). These substituent effects on the TM-induced CIDEP were found to be parallel to the effects on the reaction rate of hydrogen abstraction of excited BA molecules: both effects are strongly influenced by the (n, π*) and (π, π*) characteristics in the lowest excited triplet states. Furthermore, clear CIDEP signals due to the benzoyl and alpha-hydroxybenzyl radicals, which are produced by self-quenching of triplet BA, were newly obtained upon photolysis of a concentrated acetonitrile solution of BA (5 × 10-2 mo1 dm-3 ) by a 266 nm laser. This result is not consistent with the proposed wavelength effect on the BA photochemistry, but it is consistent with a normal radical reaction mechanism that shows no wavelength effect.

Chemically Induced Dynamic Electron Polarization Studies of Photochemical Reactions: Acetone in 2-Propanol and Triethylamine

Abstract The results of Chemically Induced Dynamic Electron Polarization (CIDEP) studies of acetone in 2-propanol and triethylamine are reviewed to show how CIDEP studies can reveal interesting details of the initial processes of photochemical reactions. After a brief description of various mechanisms that give rise to CIDEP, the following topics are discussed: a) spectral pattern and the main reacting state, b) CIDEP mechanisms and the temperature and frequency dependences of the spectra, c) direct EPR observation of the radical pair spectra in solution, d) time-developments of the CIDEP signals and spin and reaction dynamics of the initial processes of photochemical reactions, e) quantitative analyses of the magnitudes of CIDEP in terms of a stochastic Liouville equation, and f) some related applications. An overview of the remaining problems and the future outlook is also presented.

Photoinduced hydrogen abstraction reaction of 2,6-dichloro-p-benzoquinone studied by time-resolved electron spin resonance

Steady-state and time-resolved ESR spectra of the dichloro-p-benzosemi-quinone (DCSQ) radicals were measured in the photochemical hydrogen abstraction reaction of 2,6-dichloro-p-benzoquinone (2,6-DCBQ) in chloroform at room temperature. The 2,6-DCSQ radical was observed by steady-state ESR and the magnetic parameters (hyperfine coupling constants and g values) were obtained from its high resolution steady-state ESR spectrum and a computer simulation. The chemically induced dynamic electron polarization (CIDEP) signals of the 2,6- and 3,5-DCSQ radicals were obtained in sub-microsecond time regions after laser excitation of the chloroform solution of 2,6-DCBQ, and were explained by a combination of the radical pair mechanism and the triplet mechanism of CIDEP. The CIDEP signals of the 3,5- and 2,6-DCSQ radicals disappeared with different decay rate constants of (1·57 ± 0·2) × 106 s-1 and (1·10 ± 0·1) × 106 s-1, respectively. These results show that two types of radical (2,6- and 3,5-DCSQ) are produced indiv...

Time-resolved electron spin resonance spectra of linked radical pairs on laser excitation of zinc tetraphenylporphyrin-viologen systems

Photoinduced electron transfer from the zinc tetraphenylporphyrin moiety to the viologen unit in linked compounds in reversed micelles generated radical pairs, which afforded chemically induced dynamic electron polarization (CIDEP) spectra at room temperature. The CIDEP spectra varied with the number of methylene groups (n) in the alkyl spacer of the linked compounds. In the case of longer spacers (n = 6 and 8), common E/A/E/A patterns were observed throughout the measurement (0.7-5 [mu]s). The signal pattern could be simulated by a formula for the well-known spin-correlated radical pair mechanism, and the corresponding exchange coupling (J) was evaluated ([minus]0.4 mT). In the case of ZP4V, the CIDEP signals rapidly changed with elapsed time (0.5-3 [mu]s) from the absorptive to emissive mode. The observation was explained on the basis of a strong contribution of the triplet mechanism in the photogeneration of the radical pair in the early period of measurement. 15 refs., 2 figs.

Unusual observation of chemically induced dynamic electron polarization (CIDEP) in systems containing the propan-2-olyl radical: Involvement of a metastable product

Enhanced absorptive CIDEP signals in some photochemical systems containing propan-2-olyl radicals are shown to result from the secondary photolysis of a metastable product in the reactions. On the basis of the known chemistry of the species and the reported CIDEP observations, the metastable product is proposed to be the enol form of the acetone.