Photocleavage reaction of bromine substituted aromatic acyl compounds studied by CIDEP and transient absorption spectroscopy

The photocleavage of the CBr bond in bromoacetylnaphthalene is investigated by transient absorption and time resolved EPR spectroscopy. In the transient absorption of 2-bromo-2′-acetylnaphthalene, the absorption band observed at λ_max ~440 nm is assigned to the triplet state of the parent molecule. After decay of the triplet absorption, a long lived absorption band is observed at λ_max ~380 nm, which is assigned to naphthoylmethyl radical. The yield of this radical is not dependent on the concentration of oxygen even though the absorption band of the triplet state was quenched by addition of oxygen. Thus we conclude that the spin multiplicity of the precursor molecule is singlet. The CW time resolved EPR spectrum shows a typical E?/A CIDEP pattern of three hyperfine lines of the naphthoylmethyl radical. This result suggests some contribution from triplet precursor molecules. However, a careful analysis of the time profile of the CIDEP intensity observed by FT-EPR revealed that the polarization is generated from the radical pair mechanism (RPM) from the encountered pair of two free naphthoylmethyl radicals and the radical-triplet pair mechanism. RPM polarization by the geminate radical pair, formed by the Br atom and the naphthoylmethyl radical, is not observed. This fact indicates that large spin-orbit coupling (Δg and/or fast spin relaxation by g anisotropy) spoils the RPM polarization. The finding is in contrast to the recent observation of RPM polarization in the Cl cleavage reaction of 1-(chloromethyl)naphthalene.     

FT-EPR study of the pH dependence of the photochemistry of sesamol in aqueous solution

A Fourier transform electron paramagnetic resonance (FT-EPR) study was made of the photochemistry of 3,4-methylenedioxyphenol (sesamol, SEOH)) in aqueous solution. FT-EPR measurements show that in alkaline (pH 11) solution, pulsed-laser excitation of SECT leads to photoionization giving the hydrated electron and SEO^⋅ free radical. Resonance signals from these paramagnetic species develop with instrument-controlled rise time. They exhibit a low-field emission/ high-field absorption (E/A) CIDEP pattern with the transition from emission to absorption occurring at the resonance of the hydrated electron. It is shown that the spin polarization stems from contributions from the ST₀ radical pair mechanism (E/A) and triplet mechanism (A). From this it is concluded that photoionization of sesamol occurs via the triplet excited state. In neutral and acidic (pH 4–7) aqueous solution, photoexcitation generates SEO^⋅ and cyclohexadienyl-type radicals. In this case, radicals grow in over a period of 1–2 μs and FT-EPR spectra display an E/A pattern with the inversion point in the center. The lowering of the pH of the solution apparently is accompanied by a strong reduction in the relative importance of photoionization. From the FT-EPR data it can be deduced that in neutral and acidic solutions the dominant reaction channel is H-atom transfer. In this respect, the photochemistry of sesamol differs from that of phenol andp-cresol. For these phenols the change in pH does not affect the appearance of the FT-EPR spectra. Apparently, the change in electronic structure caused by the methylenedioxy substituent strongly affects the excited state reactivity of sesamol.     

FT-EPR study on the sign of exchange interaction in triplet naphthalene-galvinoxyl encounter pair

Chemically induced dynamic electron polarization (CIDEP) created in the quenching of triplet naphthalene by galvinoxyl were investigated by time-resolved Fourier-transform electron paramagnetic resonance (FT-EPR) measurements with monitoring a free induction decay signal of a pulsed microwave irradiation. Transient FT-EPR spectra of galvinoxyl with CIDEP were observed in various nonpolar solvents with different viscosity. A transient FT-EPR signal phase shows remarkable dependence on the viscosity: FT-EPR signal phases were absorption and emission in the solvents with low and high viscosity, respectively. Time evolutions of the FT-EPR signal of galvinoxyl were well simulated by a model of the radical-triplet pair mechanism (RTPM) for CIDEP. A sign of theJ value in the triplet naphthalene-galvinoxyl system in various solvents were discussed on the basis of the sign rule in the RTPM and the transient FT-EPR signal phase. One of possible explanation for the solvent viscosity dependence of the transient FT-EPR signal phase was pressented on the basis of hypothetical model of theJ value.     

FT-EPR study of photoionization in micellar solutions

Fourier Transform EPR (FT-EPR) was used to study the formation and decay of free radicals produced by photoionization of phenothiazine (PTH) solubilized in aqueous SDS and Triton X-100 micellar solutions in the absence and presence of electron acceptors. CIDEP spectra produced by PTH photoionization in micellar solution differ from those found in homogeneous solution. The effect is attributed to changes in relative importance of single-photon, singlet excited state, and biphotonic, triplet excited state, photoionization. With quinone acceptors present in the bulk aqueous phase, photoionization of PTH in SDS, results in instantaneous formation of quinone anion radicals that carry the spin polarization of the precursor hydrated electrons. If the acceptor is anchored in the micelle, electron capture cannot compete with electron escape into the aqueous phase. Instead, anion radicals are formed primarily by reductive quenching of³PTH*. This process gives rise to a spectrum that is attributed to long-lived spin-correlated radical pairs, [PTH⁺…Q^?].     

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 electron spin polarization in electron-transfer reactions

Chemically induced electron polarization (CIDEP) has been observed for the durosemiquinone radical anion generated in the flash photolysis of solutions of duroquinone in the presence of various amines. The initial polarization has been measured directly by using a fast response time-resolved E.S.R. spectrometer. The magnitude of polarization is shown to depend on amine concentration and identity, and the solvent medium. Conventional nanosecond flash photolysis has been used to measure duroquinone triplet lifetimes under various conditions. The results are discussed in terms of the triplet mechanism and the radical pair mechanism.     

乙二醇均相溶液中苊醌三重激发态淬灭反应的TR-ESR研究

利用时间分辨电子自旋共振（TR-ESR）方法,研究了乙二醇（EG）均相溶液中稳定自由基TEMPO和生物抗氧化剂维生素C（VC）对苊醌（ACQ）激发三重态3ACQ*的淬灭反应。光解ACQ/EG体系,观察到苊醌中性自由基ACQH•和乙二醇烷基自由基的发射/吸收+发射（E/A+E）的CIDEP信号,ACQH•和CH2(OH)C•HOH由3ACQ*从EG上夺氢生成。光解ACQ/TEMPO/EG体系,3ACQ*与TEMPO相互作用将极化转移给TEMPO。光解ACQ/VC/EG体系,3ACQ*除了从EG上夺氢外,还从VC上夺氢生成VC负离子基As•-。较强的As•-的CIDEP信号表明VC对3ACQ*有明显的淬灭作用。     

CIDEP Created by the Quenching of Photo-Excited Tryptophan at Protein Surface: A Challenge to CIDEP Probing of Protein Structural Changes

Quenching of the triplet excited state of molecular tryptophan by nitroxide radical in 1,4-dioxane and water solutions was investigated by means of time-resolved electron paramagnetic resonance (EPR) and Fourier-transform (FT)-EPR. The chemically induced dynamic electron polarization (CIDEP) signals with net emissive phase were recorded at these quenching events and were analyzed through radical-triplet pair mechanism. The CIDEP time profiles were well reproduced by Bloch and kinetic equations, assuming radical-triplet pair mechanism with the appropriate quenching rate constants. From a comparison of the simulation and the experiment, CIDEP enhancement factor in 1,4-dioxane was determined to be −30 × P _eq, where P _eq is the spin polarization of nitroxide at thermal equilibrium. Net emissive CIDEP was also observed by FT-EPR measurements on the nitroxide quenching of the triplet excited state of tryptophan residue in α-lactalbumin. Magnitude of CIDEP created in α-lactalbumin/nitroxide system depends on the pH condition of α-lactalbumin solution, which is related to protein folding dynamics. We argue the CIDEP mechanism at the α-lactalbumin surface and propose a possibility of a novel CIDEP method to probe a protein surface and structural changes.     

Photo-Induced Electron Transfer in P3DDT, P3OT, M3EH-PPV Conjugated Polymers Blended with Maleic Anhydride in THF Solution Under UV Flash Photolysis Studied by Means of CW TR ESR

Free-radical signals of positive polarons in conjugated polymer chains and maleic anhydride (MA) anion radicals were registered in poly(3-octylthiophene) P3OT:MA and (poly[2,5-dimethoxy-1,4-phenylene-1,2-ethenylene-2-methoxy-5-(2-ethylhexyloxy)?C(1,4-phenylene-1,2-ethenylene)]) M3EH-PPV:MA blends in tetrahydrofuran (THF) solutions under ultraviolet flash photolysis (308?nm) by continuous-wave time-resolved electron spin resonance. Their emissive chemically induced dynamic electron polarization (CIDEP) originated mainly from excited triplet states (triplet mechanism of CIDEP) and partly by from the radical pair mechanism due to the singlet?Ctriplet mixing states. The observed M3EH-PPV polaron spectrum (g ₀?=?2.0029) supports the supposition that the previously registered CIDEP spectra in P3DDT:MA blends (g ₀?=?2.0021) can be attributed to the polaron signals instead of the possible solvate electron signal one.     

Viscosity Effect Study on Quenching of Photoinduced Excited Triplet Duroquinone by TEMPO

ABSTRACT

Quenching dynamics of excited quinone molecules are given much attention in photochemistry and biochemistry. In order to study the viscosity effect on the quenching of triplet excited state of duroquinone (³DQ?) by stable radical, 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO), this study measured chemically induced dynamic electron polarization (CIDEP) spectra and transient absorptive spectra in various solvents. The solvents used were ethylene glycol, 1,2-propanol and their mixtures with different ratio in volume. The Stern-Volmer plot was obtained form CIDEP spectra of photolysis of DQ with different TEMPO concentrations. Combining the slope of the Stern-Volmer plot with lifetime of ³DQ?, determined from the ³DQ? transient absorbance decay curve, the quenching rate constants of ³DQ? by TEMPO were calculated in each solvent. The results indicate that the quenching rate constant is viscosity-dependent, and that it decreases linearly with the increase in solvent viscosity in the range used in our experiment.     

杜醌自由基光化学路径的CIDEP研究

用自制的高时间分辨电子自旋共振波谱仪,测量得到了酸性、碱性和胶束环境下的光解杜醌/乙二醇(DQ/EG)溶液瞬态自由基的化学诱导动态电子自旋极化(CIDEP)谱.在光解均相DQ/EG溶液时,观察到了以增强发射的的中性杜半醌自由基(DQH.)的CIDEP信号;在碱性环境下(pH=9)和TX-100胶束环境下光解DQ/EG体系时得到杜醌负离子自由基(DQ-.)的CIDEP信号;在酸性环境下(pH=2.5)光解DQ/EG体系时,出现的又是中性杜半醌自由基(DQH.)的CIDEP信号.实验结果显示,DQH.由3DQ*与EG之间的氢原子转移反应生成,DQ.-由DQH.的去质子化反应生成,反应中伴随着极化转移.     

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.     

Magnetic field and spin effects from sequential p-type and d-type triplet mechanisms

CIDEP signals of semireduced thionine radicals produced by reacting thionine triplets with aniline and halogenated anilines were measured by time resolved CW and pulsed FT EPR. For aniline as quencher, the polarization was emissive while for 4-Br- and 3-I-aniline a time dependent change in polarization from emissive to enhanced absorption was observed. For 4-I-aniline the signals were in enhanced absorption for all delay times. The time and concentration dependence of the signals was analysed in terms of a sequential double triplet mechanism: polarization of the thionine triplet due to selective population of the molecular triplet substates (classical ‘p-type’ triplet mechanism) and modification of this polarization by substate selective, heavy atom induced depopulation of triplet exciplexes (triplet contact radical pairs) formed as intermediates in the triplet quenching by electron transfer (‘d-type’ triplet mechanism). A quantitative theoretical treatment that combines the time-integrated solution of the stochastic Liouville equations for precursor triplet and triplet exciplex with the kinetic rate equation of the bimolecular quenching process is presented. The equations derived allow the extraction of two polarization enhancement factors, V _d for the pure d-type and V _pd for the combined p- and d-type triplet mechanism from the concentration dependence of the time dependent CIDEP signals. The CIDEP curves and the previously observed magnetic field and heavy atom effects on the free radical yield can be quantitatively simulated with a consistent set of kinetic parameters.     

Triplet state and phenoxyl radical formation of 3, 4‐methylenedioxy phenol: a combined laser flash photolysis and pulse radiolysis study

Formations of triplet state, molecular cation radical, and phenoxyl radical of 3,4‐methylenedioxy phenol (sesamol, SOH) in organic solvents have been investigated by laser photolysis as well as pulse radiolysis techniques. Photolysis of SOH in cyclohexane has been found to produce both triplet state (λ_max ～ 480 nm) and phenoxyl radical (425–430 nm) of SOH by mono‐photonic processes. However, radical cation (λ_max = 450 nm) and phenoxyl radical of SOH have been observed on radiolysis in cyclohexane. Further, radiolysis of SOH in benzene has been found to produce phenoxyl radical only. Mechanism of phenoxyl radical formation by photo‐excitation of SOH has been studied and triplet energy level of SOH is estimated to lie between 1.85 and 2.64 eV. Copyright © 2007 John Wiley & Sons, Ltd.     

Electron Spin Resonance of Alkali Metal Contact Ion Pairs of 2,6-di-tert-Butylbenzoquinone in Non Polar Solvent

The question whether excited triplet states of quinones react with a number of substrates such as alcohols, phenols, and amines, via electron transfer mechanism has attracted much attention in recent years. The existence of some triplet exciplex was postulated by Kobashi et al¹ in their study of hydrogen atom abstraction by p-chloranil using laser flash spectrocopic detection. In some recent e.s.r. and CIDEP studies^2,3, however, there is no compelling evidence that the photoreduction of quinones and benzophenones undergoes an initial electron transfer mechanism. It should be noted that most of the e.s.r. studies of quinone radical anions had been carried out in polar solvents. In flash photolysis studies some evidence has indeed been obtained^1,4 in which the efficiency of hydrogen abstraction by excited triplet quinones increases with solvent polarity and therefore it is possible that the initial primary process involves electron transfer followed immediately by proton transfer. On the other hand, we     

CIDEP in radical-singlet molecular oxygen system

Net absorptive CIDEP generation has been demonstrated on singlet molecular oxygen and 4-oxo-2,2,6,6-tetramethyl-1-piperidinyloxyl (OTEMPO) radical system in benzene. CIDEP generation was reasonably explained in terms of the radical-triplet pair mechanism of triplet molecular oxygen-OTEMPO pair with doublet precursor. Several excited molecule-OTEMPO systems have been investigated if this CIDEP generation contributes to their CIDEP spectra. Surprisingly strong CIDEP was produced even in the presence of trace amount of dissolved oxygen, which suggests the importance of complete degassing for CIDEP studies in general systems.     

Effect of halogen atom on electron spin polarization studied by CIDEP,using halogen substituted aromatic acyl compounds

The reaction and spin dynamics of the photocleavage reaction of 2-chloro-2′-acetylnaphthalene were studied by time-resolved FT-EPR and transient absorption (TA) spectroscopy. The photocleavage reaction from both singlet and triplet states was observed by TA and EPR experiments, although the radical cleavage reaction in the excited triplet state is energetically unfavourable. This feature has been explained by the ionic cleavage reaction due to the electro-negativity of the chlorine atoms. The time-resolved FT-EPR spectra were similar to those observed in the bromine substituted compound, 2-BAN, reported in a previous paper. The origin of the electron spin polarization was assigned to the radical triplet pair mechanism (RTPM) and free radical pair mechanism (F-pair RPM) from analysis of the time profiles of the spin polarization.     

Chemically Induced Dynamic Magnetic Polarization. Part X. The Photoreduction of 2-Methyl-p-benzoquinone

While the photoreduction of quinones has been initially used by us as a model system to probe the role of excited triplet states in CIDEP¹ and CIDNP² phenomena, our continuing interest in the quinone photochemical systems is partly due to their photobiological importance. Alkyl substituted quinones have in fact been shown to be associated with chlorophyll³ and to be involved in photosynthesis⁴. The 2-methyl-p-benzoquinone (MQ) is the simplest unsymmetrical alkyl substituted p-quinone which, during photolysis, may lead to two isomeric semiquinone radicals. We are particularly interested in applying the CIDEP and CIDNP techniques to investigate the structure and reactivity of the intermediate semiquinone radicals as they may give some insight to the photochemical properties of the parent 2-methyl-p-quinone.     

Experimental Observation of Free Induction Decay of Phenoxyl-Type Radical

The free induction decay (FID) of the polycrystalline sample of the phenoxyl radical derived from α-(3,5-di-tert-butyl-4-hydroxyphenyl)-N-tert-butylnitrone, which is a novel spin-trapping reagent, in a corresponding diamagnetic matrix was successfully observed after one pulse even at room temperature, while FIDs of phenoxyl radicals usually are inaccessible due to their short relaxation times and the dead time of the pulse EPR spectrometer. This may be the first experimental observation in phenoxyl-type radicals. This study thus showed the unique character of the phenoxyl radical, that is, the distribution of the electron density relative to the usefulness for the spin-trapping techniques. Authors' address: Toshiki Yamaji, Materials Characterization Division, National Metrology Institute, National Institute of Advanced Industrial Science and Technology, Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan     

A time-resolved EPR study of the electron-spin-polarization pathways of p-benzosemiquinone

Benzoquinone (BQ), deuterobenzoquinone (d4-BQ), and hydroquinone (BQH2) are investigated in ethylene glycol by means of direct detection fast time-resolved EPR spectroscopy after laser flash photolysis. The development of the magnetization as a function of time and magnetic field is obtained and analyzed in terms of the Bloch equations and hyperfine parameters. The signals are attributed to the semiquinones BQH(*) and d4-BQH(*). The presence of 1,2-dihydroxyethyl radicals during the photolysis of BQ and d(4)-BQ is verified. No alkyl radicals are observed in solutions of BQ with excess BQH2. Detailed analysis of the chemically induced dynamic electron polarization spectra with respect to their development in time shows that polarization patterns of the semiquinones can be traced back to a superposition of triplet mechanism and radical pair mechanism, the latter arising from geminate T-pairs. Hence, two independent pathways for polarization are assumed: reaction of triplet benzoquinone with ethylene glycol leads to the semiquinone and dihydroxyethyl radicals with all signals in emission, whereas the reaction of triplet BQ and BQH2 yields two semiquinones exhibiting both net emissive and multiplet emissive/absorptive intensity distributions.