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.     

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.     

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.     

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.     

Photoinduced electron transfer and spin dynamics in mixed porphyrin-quinone solutions studied by time-resolved EPR

From time-resolved direct detection cw EPR with pulsed laser excitation, the photoinduced electron transfer and spin dynamics (CIDEP) in mixed zinc-tetraphenylporphyrin (ZnTPP)/benzo-1,4-quinone (BQ) ethanol solutions were determined as functions of temperature and BQ concentration. At lower temperatures the EPR spectra reveal that mixing of the S and T_?1 states in the charge separated radical pair gains in importance relative to the ST₀ mixing. Furthermore, at lower temperatures, the EPR spectra of the spin-correlated radical pairs of ZnTPP⁺ and BQ⁷ could also be observed. From the temperature/viscosity dependence of the electron transfer rates and of the polarization contributions from the triplet and radical pair mechanisms, deviations from a macroscopic diffusion behaviour are inferred at lower temperatures.     

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.     

Spin polarization in triplet states in solution

A method is described for measuring the absolute value of the spin polarization in a triplet state in solution through CIDEP observations of the radicals formed on its reaction, and through it the anisotropy of the rates of the spin-selective intersystem crossing process. A long-accepted equation concerned with triplet mechanism spin polarization is shown to be inadequate to reproduce observed behaviour, and evidence produced to suggest that the observed polarization is affected by radical relaxation. The method also allows determination of the absolute polarization in the radicals. A novel analysis of the relative contributions of TM and RPM processes to observed spectra provides further evidence for the conclusions.     

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.     

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.     

Quantitative time-resolved EPR CIDEP study of the photodecomposition oftrans-azocumene in solution

The cumyl radical system, which is created after laser flash irradiation oftrans-azocumene in benzene solution at room temperature, is investigated using time-resolved EPR spectroscopy. From the quantitative analysis of EPR time-profiles at different microwave powers the spin relaxation timesT ₁=3.5±0.3 μs andT ₂=2.5±0.1 μs are evaluated as well as the magnitude of the chemically induced electron polarization (CIDEP), which is generated by the radical pair mechanism (RPM). The geminate RPM polarization is found to be considerably smaller than the F-pair one, 32±2 and 48±5 in units of the Boltzmann polarization, respectively. This is attributed to an initial radical separation in the geminate pair, caused by the cleavage reaction. Besides cleavage, the photoexcitedtrans-azocumene also decays via isomerization to the thermally unstablecis-isomer, the lifetime of which is found to be 14±3 μs at 293 K in benzene, three times longer than in cyclohexane. The quantum yield of free radicals, escaping from the primary cage, is determined as 0.28±0.06 for the decay of the excitedtrans-azocumene and 0.18±0.04 for the thermal cleavage of thecis-isomer. The self-termination of cumyl radicals proceeds with a rate constant 2k _t=7±1)·10⁸ M^?1s^?1 in benzene at RT.     

Solvent effects on the intrinsic enhancement factors of the triplet exciplex generated by photoinduced electron transfer reaction between eosin Y and duroquinone

The spin dynamics of the duroquinone anion radical (DQ^?-) generated by photoinduced electron transfer reactions from triplet eosin Y (³EY^2-) to DQ have been studied by using transient absorption and pulsed EPR spectroscopy. Unusual net-absorptive electron spin polarization plus net-emissive polarization were observed, suggesting the production of the triplet exciplex or contact radical pair as the reaction intermediate. The kinetic parameters and intrinsic enhancement factors of the electron spin polarization were determined in various alcoholic solvents. The net-absorptive electron spin polarization was also observed in ethanol-water mixed solvents. The solvent effects on the radical yield are analysed on the basis of a stochastic Liouville equation established for the magnetic field effects on the radical yield. The zero-field splitting constants of the triplet exciplex are estimated from the solvent viscosity dependence of the enhancement factors due to spin-orbit coupling induced depopulation of the reaction intermediate.     

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.     

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.     

The magnetic field dependence of the electron spin polarization in consecutive spin correlated radical pairs in type I photosynthetic reaction centres

The magnetic field/microwave frequency dependence of the spin polarized EPR spectra of the sequential spin correlated radical pairs P⁺A^? ₁ and P⁺F^? _x in type I photosynthetic reaction centres is investigated. Experimental data are presented for photosystem (PS) I and reaction centres of heliobacteria at × band (9.7 GHz) and K band (24 GHz). In photosystem I at ambient temperatures the lifetime of A ^? ₁ is ~290 ns and both states are observable by transient EPR. In heliobacteria, electron transfer to F_x occurs within ~600 ps and only the state P⁺F^? _x is observed. The experimental data show a net polarization of P⁺ in the state P⁺F^? _x, which displays a clear dependence on the strength of the external field. The net polarization generated in sequential radical pairs is expected to pass through a maximum as a function of the Zeeman energy when the characteristic time of singlet-triplet mixing is comparable with the lifetime of the precursor. In PS I, the precursor lifetime (290ns) is much longer than the characteristic time of singlet-triplet mixing at × band (9 GHz, 3 kG) and K band (24 GHz, 8 kG). As a result, the observable net polarization decreases with the field strength in this region. In contrast, in heliobacteria, the precursor lifetime (600 ps) is much shorter than the characteristic time of singlet-triplet mixing, and the net polarization increases in the same range of Zeeman energies. The polarization patterns in these two systems can be described using the specific limiting cases of a short lived and long lived precursor radical pair and written as a sum of several contributions. The spectra are simulated on this basis using parameters derived entirely from independent experimental data, and good agreement between the experimental polarization patterns is obtained. The calculated polarization patterns are sensitive to spin dynamics on a timescale much shorter than the spectrometer response time, and the expected influence of a 10 ns component in the electron transfer, as observed optically in some PS I, preparations is discussed. No significant influence from such a component is found in the spin polarization patterns of PS I from the cyanobacterium Synechocystis 6803.     

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.     

Photo-Induced Electron Spin Polarization in Chemical and Biological Reactions: Probing Structure and Dynamics of Transient Intermediates by Multifrequency EPR Spectroscopy

In this minireview, modern multifrequency electron paramagnetic resonance (EPR) spectroscopy, in particular, at high magnetic fields, is shown to provide detailed information about structure, motional dynamics and spin chemistry of transient radicals and radical pairs occurring in photochemical reactions. Examples discussed comprise spin-polarized radicals and radical pairs in disordered systems, such as ultraviolet-irradiated quinone and ketone compounds in fluid alcohol solutions, green-light initiated electron transfer in biomimetic porphyrin?Cquinone donor?Cacceptor model systems in frozen solution, aiming at artificial photosynthesis, and red-light initiated electron transfer in natural photosynthetic reaction-center protein complexes. The transient paramagnetic states exhibit characteristic electron polarization (CIDEP) effects originating from a triplet mechanism, a radical-pair mechanism or a correlated coupled radical-pair mechanism. They contain valuable information about structure and dynamics of the short-lived reaction intermediates. Moreover, the CIDEP effects can be exploited for signal enhancement. Continuous-wave and pulsed versions of time-resolved high-field EPR spectroscopy, such as transient EPR and electron spin-echo experiments, are compared with respect to their advantages and limitations for the specific photoreaction under study. Furthermore, orientation resolving W-band pulsed electron-electron double resonance (PELDOR) experiments on the spin-correlated coupled radical pair $ {\text{P}}_{865}^{ \cdot + } $ $ {\text{Q}}_{\text{A}}^{ \cdot - } $ in frozen solution reaction centers from the purple photosynthetic bacterium Rb. sphaeroides reveal details of distance and orientation of the pair partners in their charge-separated transient state. The results are compared with those of the ground-state P₈₆₅Q_A. In conjunction with Q-band proton electron-nuclear double resonance (ENDOR) experiments the W-band PELDOR results provide decisive evidence that the local structure of the Q_A binding site does not change under photoreduction of the quinone??in agreement with earlier FTIR studies. The examples given demonstrate that multifrequency EPR experiments on disordered systems add heavily to the capabilities of ??classical?? spectroscopic and diffraction techniques for determining structure?Cdynamics?Cfunction relations of biochemical processes, since short-lived intermediates can be observed in real time while staying in their working states at biologically relevant time scales.     

Light induced radical pair intermediates in photosynthetic reaction centres in contact with an observer spin label: spin dynamics and effects on transient EPR spectra

A novel strategy is discussed using site directed spin labelling to study the electron transfer process in photosynthetic reaction centres. An algorithm is presented for numerical simulations of the time resolved EPR spectra of radical pair states in the presence of an observer spin label. This algorithm accounts for spin dynamics, charge recombination and relaxation processes. It is shown that satisfactory agreement between experimental and simulated EPR spectra of the first stabilized radical pair state in photosystem I is achieved for various microwave frequencies. Transient EPR spectra for the radical pair state P^?+Q^?- in photosystem I were simulated for various distances and positions of the observer spin label with respect to the acceptor quinone molecule. It is shown that distances up to more than 20 Å give rise to observable changes in the transient EPR spectra. Both the additional spin-spin coupling between the quinone radical and the label and the polarization transfer processes contribute to the changes. Furthermore, the shape and intensity of the EPR spectrum of the spin label is altered by the coupling with the radical pair spins for distances up to 25 Å. Experiments on site directed spin labelled photosystem I are thus expected to provide valuable information on the dynamics of electron transfer in photosystem I.     

Time-resolved EPR study of electron spin polarization and spin exchange in mixed solutions of porphyrin stable free radicals

The time-resolved electron paramagnetic resonance (EPR) spectra are studied in the temperature range of 110–300 K for two mixed solutions of porphyrins, ZnTPP and H₂TPP, in toluene and the stable free radical 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO). The EPR spectra and their kinetic behavior were studied for concentrations of TEMPO varied in the interval from 0.51 to 7.68 mM, while the porphyrin concentration was fixed as 1 mM. The EPR spectra of triplet-state porphyrins and free radicals manifest the chemically induced spin polarization. For the relatively short-lived radical-triplet pairs, with the perturbation theory up to the fourth order, the theoretical expressions are obtained for the triplet and radical spin polarization induced by the enhanced intersystem crossing (ISC) due to the interaction of excited singlet-state porphyrins with free radicals and by the triplet quenching by free radicals. The time-dependent EPR spectra of the triplets are simulated taking into account the spin-lattice relaxation. It is shown that the variation of the triplet EPR spectra shape, when the time of observation increases, arises from the spin-lattice relaxation kinetics. The kinetic behavior of the TEMPO EPR spectrum was simulated on the basis of the kinetic scheme suggested earlier in the literature. The triplet spin-lattice relaxation time, the rate of the ISC and the lifetime of the excited singlet state were estimated by fitting the kinetic curves for the triplet EPR spectra intensity. For the mixed porphyrin-TEMPO solutions, a possible set of the rate constants of important bimolecular processes were determined. For this set of parameters, it turns out that the spin polarization transfer has a smaller rate constant than the rate constant of the diffusion collisions of the triplet and radical. It appears that the rate constant of the ISC catalyzed by radicals is relatively high in the solutions close to the melting point of the solvent and in the soft-glassy state. In the triplet porphyrins the initial spin polarization induced by the spin-selective ISC was found to exceed the equilibrium spin polarization by up to two orders of magnitude.     

Electron spin polarization in consecutive spin-correlated radical Pairs: Application to short-lived and long-lived precursors in type 1 photosynthetic reaction centres

An analytical treatment of the spin dynamics in sequential photoinduced correlated coupled radical pairs is presented and applied to the spectra of the states P⁺A ₁ ^? and P⁺F _x ^? in type 1 photo-synthetic reaction centres. Expressions for the spin polarized spectra are derived for the specific limiting cases of a very short-lived and very long-lived primary radical pair which correspond to the situation found in heliobacteria and photosystem I (PSI), respectively. The inhomogeneous line-broadening due to the unresolved hyperfine couplings is taken explicitly into account. It is shown that the density matrix of the secondary pair ρ₂ can be written as the sum of two terms corresponding to (i) the part which is independent of the spin dynamics in the precursor, (ii) the additional spin polarization which is generated during the lifetime of the precursor and transferred to the secondary pair. The latter term contains two contributions which arise from the difference of the Zeeman interactions of the radicals in the primary pair and from the inhomogeneous line broadening. The predicted polarization patterns are compared to those established for chemically induced dynamic electron polarization (CIDEP) when uncoupled radicals are generated from a radical pair precursor. The expressions are then used to simulate the experimental spectra of the consecutive pairs P⁺A ₁ ^? and P⁺F _x ^? in PSI using parameters derived entirely from independent experimental data. Excellent agreement with the experimental results is obtained. The spectra of P⁺F _x ^? in heliobacteria at X- and K-band are also simulated and it is shown that the observed polarization patterns can be reproduced assuming direct electron transfer from A₀ to F_x with a time constant ofτ = 600 ps.     

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.