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.     

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.     

Dynamic electron polarization created by the radical-triplet pair mechanism: Application to the studies on excited state deactivation processes by free radicals

The radical-triplet pair mechanism for chemically induced dynamic electron polarization (CIDEP) created in the quenching of excited state molecules by free radicals is explained on the basis of recent time-resolved electron spin resonance spectroscopic results and theoretical studies. The CIDEP of 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) and galvinoxyl radicals exhibit various CIDEP patterns of net and mutliplet types and CIDEP phases of absorption and emission. The CIDEP patterns are described by the quartet-doublet state mixings within the radical-triplet encounter pairs. The mixings by the spin-dipolar and the hyperfine interactions are responsible for the net and the multiplet patterns, respectively. The factors controlling the CIDEP phases are the spin multiplicity of the excited state quenched by radicals and the sign of the intermolecular exchange interaction of the radical-triplet encounter pairs. In particular, the intermolecular charge transfer effect on the exchange interaction is discussed much in detail from the viewpoints of CIDEP magnitudes and phases. A CIDEP creation in the O₂(¹Δ_g)-TEMPO system is also introduced and is described by the radical-triplet pair mechanisms. Applications of this CIDEP used as a probe of O₂(¹Δ_g) in condensed phase are mentioned.     

瞬态自由基的二维化学诱导动态电子自旋极化(2D-CIDEP)

在恒定、线性的梯度磁场存在下,用时间分辨ESR对某些典型体系在脉冲激光作用下产生的瞬态自由基的化学诱导动态电子极化信号进行了研究,发现用此方法可以获得它们的空间分辨及空间-时间分辨的2D-CIDEP信号.     

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.     

Multiplet-to-net CIDEP conversion by MW-pulses with adiabatically ramped amplitude

ABSTRACT

A method is proposed to manipulate electron spin order of spin-correlated radical pairs. As radical pairs are often born in a well-defined spin state, e.g. in the singlet state, they acquire Chemically Induced Dynamic Electron Polarisation (CIDEP). In the case of singlet-state preparation CIDEP is of the multiplet (or anti-phase) type resulting in reduction of EPR (electron paramagnetic resonance) signals due to overlap of absorptive and emissive lines in the spectrum. Here we propose to convert the singlet spin order into net magnetisation of the radical pair by applying a microwave field, with its amplitude slowly (adiabatically) reduced to zero. We demonstrate that by properly choosing the microwave frequency one can completely convert the singlet order into net polarisation of the radical pair with significant enhancement of the signal as compared to multiplet CIDEP. Calculations show that the technique is operative for both weakly coupled and strongly coupled spin pairs. Potential applications of the method are discussed.     

激光光解苯甲醛自由基的时间-分辨ESR研究

利用高时间分辨ESR波谱仪,对苯甲醛为光敏分子的各体系进行了CIDEP(化学诱导动态电子极化)研究,揭示了苯甲醛体系的CIDEP信号的产生过程,并进一步确认了其产生过程中的极化特征.     

国内CIDEP研究进展

化学诱导动态电子极化（CIDEP）是检测瞬态顺磁粒子并表征其特征的强有力的手段,对于研究光化学和光物理瞬态过程的微观机理和规律有重要意义. 本文较为详细地总结了4种常见的CIDEP机理,讨论了各种极化谱及相应的极化条件;简要介绍了国内研究小组在CIDEP理论以及在均相溶液和微复相体系中光化学过程的CIDEP研究成果.      

285 nm激光光解对苯醌-乙二醇体系的CIDEP波谱解析

在285 nm紫外激光辐照下,用时间分辨电子自旋共振(TRESR)波谱仪研究了对苯醌/乙二醇(PBQ/EG)体系,得到了发射的苯半醌中性自由基PBQH.和乙二醇碳自由基R.(OH)2的CIDEP谱.在化学诱导动态电子极化(CIDEP)基本原理的基础上,建立合理的计算模型,编写相应的程序,对PBQH.和R.(OH)2的CIDEP形成机理进行了定量的解析.通过对不同延时下该体系各种自由基及不同机理成分的对比,明显看出由RPM形成的PBQH.比由TM形成的PBQH.衰减快.这种分析方法可以推广应用于其它体系的CIDEP实验谱的解析.     

吩噻嗪-1,4萘醌-乙二醇体系光化学过程的时间分辨ESR研究

用时间分辨ＥＳＲ方法研究了吩噻嗪（ＰＴＨ）－１，４－萘醌（ＮＱ）－乙二醇（ＲＨ）体系中的光化学过程．只观察到萘醌的中性半醌自由基ＮＱＨ．＊的全发射极化的ＣＩＤＥＰ（化学诱导动态极化）信号，并未观察到极化的萘醌负离子ＮＱ－·＊的ＣＩＤＥＰ信号，表明ＲＨ与三重态ＮＱ间的质子转移反应远比ＰＴＨ与ＮＱ间的电子转移反应为快．ＮＱＨ．＊的ＣＩＤＥＰ信号随ｐＨ值的变化表明体系中同时有极化自由基ＮＱＨ．＊与ＮＱＨ．＋＊２存在，并且其间有质子交换．     

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.     

Diffusion-model analysis of effective CIDEP distance in solvent-separated radical-ion pair

The radical pair mechanism (RPM) of chemically induced dynamic electron polarization (CIDEP) is theoretically analyzed to determine what intermolecular separations (r _eff) effectively contribute to the CIDEP generated from diffusive, separated radical-ion pairs (RIP) in terms of the chargetransfer interaction in the singlet-triplet energy splitting (J) by taking into account the distance-dependent electronic coupling and reorganization energy. The diffusion-model analysis reveals that the hyperfine-dependent RPM polarization (P _RPM) phase is varied with the driving force (?ΔG _CR) for the charge-recombination (CR) process and that the boundary ?ΔG _CR between the opposite phases coincides well with the total reorganization energy around the diffusible separation distance,r _eff=1.2 nm, between the ion radicals. For the first time, ther _eff is well described by the exponent parameter (β) in the distance-dependent electronic coupling, suggesting that the RPM CIDEP detection can be applied to characterize the electronic coupling in individual solvent-separated RIP systems. It has been concluded that, in contrast to the spin exchange interaction of the neutral radical pairs, the characteristic long-range charge-transfer interaction enables us to utilize the simple diffusion-model analysis to successfully evaluate ther _eff and theP _RPM in homogeneous liquid polar solvents.     

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.     

吩噻嗪-1,4萘醌-乙二醇体系光化学过程的时间分辨ESR研究

用时间分辨ESR方法研究了吩噻嗪(PTH)-1,4-萘醌(NQ)-乙二醇(RH)体系中的光化学过程.只观察到萘醌的中性半醌自由基NQH^*的全发射极化的CIDEP(化学诱导动态极化)信号,并未观察到极化的萘醌负离子NQ·^-*的CIDEP信号,表明RH与三重态NQ间的质子转移反应远比PTH与NQ间的电子转移反应为快.NQH·^*的CIDEP信号随pH值的变化表明体系中同时有极化自由基NQH·^*与NQH·₂^+*存在,并且其间有质子交换.     

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.     

化学诱导动态电子极化自由基-三重态对理论中选择三重态淬灭的证明

用时间分辨ESR测出的吩噻嗪/2,2,6,6-四甲基哌啶氮氧自由基(TEMPO)光诱导产生的三重态发射CIDEP信号可用Blätter提出的自由基-三重态对机理来解释,由荧光光谱及脉冲激光光声微量量热实验结果直接证明了Bläter理论关于自由基只选择淬灭双重自旋态的自由基/三重态对中的三重态的假设.     

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.     

The fast and slow rotation approximations for the description of the kinetics of triplet radical quenching and electron spin polarization

The specific features of the mechanisms and kinetics of generation of net chemically induced dynamic electron polarization (CIDEP) in triplet radical quenching (TRQ) in liquids is analysed in detail. The problem reduces to the analysis of fairly strong non-adiabatic transitions between states of the triplet radical spin Hamiltonian which are known to determine CIDEP generation in TRQ. The analysis is performed in two limits of fast and slow rotation of the triplet molecule using the previously developed method of treatment for non-adiabatic transitions. The method made it possible to derive analytical formulas for the CIDEP generation probability P _e and rate K _e, and for the TRQ probability P _q and rate K _q in the case of relatively strong quenching. It is shown that the dependence of K _e on the relative diffusion coefficient D _r is of bell shape and cannot be described correctly by the usually applied relation K _e = K _q P _e.     

光解杜醌/氢给体均相体系的CIDEP研究

以杜醌为光敏剂,用高时间分辨电子自旋共振 (TRESR)波谱仪研究了杜醌/乙二醇体系、杜醌/氮氧自由基/乙二醇体系的化学诱导动态电子自旋极化(CIDEP).实验表明,紫外激光照射下,在杜醌/乙二醇溶液中得到以三重态机理极化为主的中性杜半醌自由基DQH*和以碳为中心的乙二醇碳自由基R*(OH)2的极化信号,而加入氮氧自由基 TEMPO后,则只观测到极化的TEMPO的E+E/A极化谱,其产生机理属于以四重态为先驱的自由基-三重态对极化机理(QP-RTPM),结合极化强度的理论计算对该体系的极化形成过程进行了分析.     

Time-Resolved ESR Study on Complex Radical Pairs Formed in the Photolysis of Methylene Blue Included in Water-Soluble Sulfonated Calixarenes

The photochemical reactions of methylene blue (MB) included in water-soluble sulfonated calix[n]arenes (n = 4, 6, 8) are studied using a time-resolved electron spin resonance method. The chemically induced dynamic electron polarization (CIDEP) spectra show the formation of the complex radical pair composed of the MB monocation radical and calixarene (phenoxyl-type) radical. The lifetime and broadened spectral shape are dependent on the size of the calixarene and are due to the longitudinal and transverse relaxation mainly induced by the tumbling motion of the radical pair with the spin dipole–dipole interaction. The pair dissociates in a few hundreds of nanoseconds in cases of n = 6 and 8.