Spin-dependent recombination of the charge-transfer state in photovoltaic polymer/fullerene blends

ABSTRACT

Q-band electron spin echo (ESE) spectroscopy was applied for studying the spin-dependent recombination of charge transfer (CT) states in the benchmark organic photovoltaics (OPV) blend of poly(3-hexylthiophene-2,5-diyl) and [6,6]-phenyl C61 butyric acid methyl ester (P3HT/PC₆₀BM). Selective microwave excitation and a special protocol for ESE data treatment allowed to suppress the ESE signal of thermalised polarons and weakly coupled CT states and to address CT states with a relatively short distance between positive and negative polarons (1.5 nm < r?<?2.5 nm). Inversion of the in-phase ESE signal with increase of the delay after laser flash was observed for the regioregular P3HT^?+/PC₆₀BM^?? CT state at a temperature of 40 K. This effect is very similar to the inversion of the time resolved (TR) EPR spectrum of the same system obtained previously. Both effects can be explained by spin-dependent recombination of the CT state, with the recombination via the triplet channel proceeding much slower than via the singlet channel. For the regiorandom P3HT^?+/PC₆₀BM^?? CT state no ESE sign inversion was observed in an analogous experiment. The result suggests the importance of CT state formation via a triplet exciton, a process which was not considered previously for the P3HT/PC₆₀BM blend.     

Low-Temperature Dynamical Transition in Lipid Bilayers Detected by Spin-Label ESE Spectroscopy

Data on neutron scattering in biological systems show low-temperature dynamical transition between 170 and 230 K manifesting itself as a drastic increase of the atomic mean-squared displacement, 〈x²〉, detected for hydrogen atoms in the nano- to picosecond time scale. For spin-labeled systems, electron spin echo (ESE) spectroscopy—a pulsed version of electron paramagnetic resonance—is also capable of detection of dynamical transition. A two-pulse ESE decay in frozen matrixes is induced by spin relaxation arising from stochastic molecular librations, and allows to obtain the 〈α²〉τ_c parameter, where 〈α²〉 is a mean-squared angular amplitude of the motion and τ_c is the correlation time lying in the sub- and nanosecond time ranges. In this work, the ESE technique was applied to spin-labeled amphiphilic molecules of three different kinds embedded in bilayers of fully saturated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and mono-unsaturated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipids. Two-pulse ESE data revealed the appearance of stochastic librations above 130 K, with the parameter 〈α²〉τ_c obeying the Arrhenius type of temperature dependence and increasing remarkably above 170–180 K. A comparison with a dry sample suggests that onset of motions is not related with lipid internal motions. Three-pulse ESE experiments (resulting in stimulated echos) in DPPC bilayers showed the appearance of slow molecular rotations above 170–180 K. For D₂O-hydrated bilayers, ESE envelope modulation experiments indicate that isotropic water molecular motions in the nearest hydration shell of the bilayer appear with a rate of ~?10⁵ s^?1 in the narrow temperature range between 175 and 179 K. The similarity of the experimental data found for three different spin-labeled compounds suggests a cooperative character for the ESE-detected molecular motions. The data were interpreted within a model suggesting that dynamical transition is related with overcoming barriers, of 10–20 kJ/mol height, existing in the system for the molecular reorientations.     

掺杂的a-Si1-xCx:H膜的LESR研究

本文系统地研究了77K下掺杂(B或P)的a-Si_1-xC_x:H膜在平衡状态和弱光照下的ESR特性,文中报道了该膜价带尾态中定域化空穴的ESR吸收谱。实验结果还表明:虽然a-Si_1-xC_x:H膜中由于B掺杂会引起光电导的改善,但却不能使悬挂键总浓度减小。 关键词：     

Electron Spin Echo of Photoinduced Spin-Correlated Polaron Pairs in P3HT:PCBM Composite

A variation of the electron spin echo (ESE) signal caused by laser pulse in a blend of [6,6]-phenyl C₆₁ butyric acid methyl ester and poly(3-hexylthiophene) (P3HT:PCBM) was detected. This variation was attributed to light-generated paramagnetic species in P3HT:PCBM blend, with non-equilibrium spin polarization. The echo-detected electron paramagnetic resonance (EPR) spectrum of these species closely resembles the time-resolved EPR spectrum of spin-correlated polaron pair PCBM^?/P3HT⁺ (Behrends et al. in Phys. Rev. B 85:125206, 2012) and was assigned to this pair. The characteristic times for polarization and coherence decay (9 ± 1 and 1.0 ± 0.2 μs, respectively) were measured for the PCBM^?/P3HT⁺ pair at 77 K. These times are long enough, which shows the possibility of the application of the ESE technique for studying spin evolution of light-generated charge transfer intermediates in composites of fullerenes and conductive polymers.     

Vector model of electron spin echo envelope modulation due to nuclear hyperfine and zeeman interactions

The transverse electron spin magnetization of a paramagnetic center with effective spinS=1/2 interacting with nonquadrupolar nuclei may be presented as a function of pairs of nuclei magnetization vectors which precess around the effective magnetic field directions. Each vector of the pair starts its precession perpendicular to both effective fields. The free induction decay (FID) signal is proportional to the scalar product of the vectors for nuclear spinI=1/2. The electron spin echo (ESE) signal can be described with two pairs of magnetization vectors. The ESE shape is not equal to two back-to-back FID signals except in the absence of ESE envelope modulation. A recursion relation is obtained which allows calculation of ESE signals for larger nuclear spins in the absence of nuclear quadrupole interaction. This relation can be used to calculate the time course of the ESE signal for arbitrary nuclear spins as a function of the nuclear magnetization vectors. While this formalism allows quantitative calculation of modulation from nuclei, it also provides a qualitative means of visualizing the modulation based on simple magnetization vectors.     

Pulsed ESR and molecular motions

The possibilities of applying three different pulsed ESR techniques have been considered: 1. Phase relaxation measurements by electron spin echo (ESE) affords the estimation of the correlation time of the motion in the region up to 10^?5 s. The results of theoretical and experimental analysis of the effect of methyl group rotation in nitroxide radicals have been proposed. 2. The method of pulsed saturation involving detection of ESE signal allows the range of the measured times to be extended up to the values of about 10^?2 s. The rotation of CH₂ group in the CH₂CO₂ ^? radical and that of the CH₃ group in the CH₃CHCO₂ ^? radical have been investigated. 3. The method of pulsed saturation combined with pulsed scanning of H₀ allows the analysis of the rotationally induced redistribution of the pulsed saturation throughout the ESR spectra of the radicals. This version of pulsed ESR has been used to study the mobility of nitroxide spin labels.     

Nucleoside-copper(II) system in water and on 13X-zeolite studied by continuous-wave and pulsed-wave ESR

In order to study the effects of large-surface-area solids on the formation of biomacromolecules, copper(II)-nucleosides complexes were studied in water at high pH and after contact with the cavity walls of 13X-zeolite. The results were mainly collected by electron spin resonance in continuous (cw-ESR) and pulsed (ESE) wave. Particular attention was dedicated to adenosine as a nucleoside model for the formation of compounds which were fully characterized in alkaline water solution and after adsorption on commercial 13X-zeolite. In aqueous solution, adenosine was coordinated to copper(II) through deprotonated hydroxyl groups in the 2′ and 3′ positions of the ribose unit, with the formation of [Cu(adenosine)₂(H₂O)₂]²⁻. When adsorbed on zeolite, both cw-ESR and ESE showed that a fraction of the adsorbed complexes did not change their structure and showed high mobility in the zeolite faujasite cavity. The remaining fraction directly interacted with²⁷Al nuclei of the zeolite framework. Other copper(II)-nucleosides behaved in the same manner. Surprisingly enough, alkaline solutions of nucleosides were able to remove copper (II) from zeolite in the form of [Cu(adenosine)₂(H₂O)₂]²⁻ which freely moved in the supernatant aqueous liquid.     

Anomalous pulse angle dependence of the single and double quantum echoes in a photoinduced spin-correlated coupled radical pair

In this work the response of a spin-correlated coupled radical pair to the sequence flash-t-P _ζ-τ-P _2ζ-T is investigated. For the theoretical analysis, the density operator formalism is used. Analytical expressions are derived for the electron spin single (SQ ESE) and double-quantum echoes (DQ ESE) as a function of pulse flip angle and singlet-triplet mixing angle. To illustrate the theoretical results, computer simulations are presented. In the limit of weak coupling, the “out-of-phase” SQ ESE is shown to be of a pure two-spin order having the maximal amplitude for the flip angle of 65.9°. The echo following the Hahn sequence vanishes in the same limit. This confirms the theoretical result already presented in the literature. However, the more general analysis shows that outside the weak coupling approximation the Hahn echo is of purely one-spin order, whereas the echo following the flash-t-P _ζ-τ-P _2ζ-t sequence has its maximal amplitude for the flip angle of 75° and the singlet-triplet mixing angle of 27°. The “in-phase” single- and double-quantum echoes are shown to vanish due to averaging out, within the electron spin resonance spectrometer deadtime, of contributions modulated with the sum and difference of the zero-quantum beat frequency and the frequency due to the spin-spin interaction within the pair. The calculated out-of-phase DQ ESE signal is inverted with respect to the out-of-phase SQ ESE and has only the half of its amplitude. The DQ ESE vanishes for the Hahn sequence. The echo has maximal amplitude in the weak-coupling limit for the flip angle of 65.9°. In contradiction to the analytical result previously published, the out-of-phase DQ ESE does not vanish for long τ and large zero-quantum-beat frequency.     

Peculiarities of free induction and primary spin echo signals for spin-correlated radical pairs

Keeping in mind ion-radical pairs in a photosynthesis reaction centre first of all, we calculated free induction and spin echo (ESE) signals for an ensemble of radical pairs which initially start in a singlet state. It was shown that the intensity of signals should oscillate depending on the time interval τ between the start of a pair and a microwave pulse forming free induction (FI) or between the start of a pair and the first of two microwave pulses forming primary ESE signal. ESE phase of spin-correlated pairs does not coincide with the corresponding ESE phase of radical pairs in thermal equilibrium. One should also note an interesting feature of FI: immediately after the microwave pulse free induction signal equals zero, and non-zero free induction signal appears only due to spin evolution. This behaviour formally resembles the situation occurring when the primary ESE is formed: a light pulse which creates spin-correlated radical pairs acts as the first microwave pulse in conventional spin echo experiments. Analysis of FI and ESE in experiments on pulse photolysis or radiolysis may provide useful information about the contribution of spin-correlated radical pairs.     

ESEEM of disordered systems in frequency domain: Analytical formulae in the case of nuclear spin 1/2 and weak hyperfine interaction

The analytical expressions for the spectral density of the dead time free electron spin echo envelope modulation (ESEEM) signal of disordered system are obtained for a paramagnetic center with nuclear spin 1/2 and weak axially symmetric hyperfine interaction. The spectral density is given by the Fourier transformation of the ESE signal averaged over all orientations. The order of the two linear operations may be changed. Fourier transformation of the nonaveraged ESE signal supplies us with the sum of the Dirac δ -functions. Averaging of such a spectrum is a rather trivial operation leading to the spectral densities in the final form.     

A selective hole burning method applied to determine distances between paramagnetic species in photosystems

A method of selective hole burning in EPR spectra was applied to determine the distances from a radical to the acceptor quinone-iron in bacterial and plant photosystems. A low amplitude hole burning 180° pulse and high amplitude 90° and 90° pulses applied to detect ESE of P870⁺ inRb. Sphaeroides and the distance from the primary electron donor P870⁺ to the acceptor Q _A ^? Fe²⁺ was determined to be 26±2 Å from the dipolar broadening of the burned hole in P870⁺ EPR. This result is consistent with that given by X-ray analysis and susceptibility measurement. In plant photosystem II the same method was applied to the EPR spectrum of tyrosine D⁺, but the effect of crystalline field splitting of Fe²⁺ ion was taken into consideration. The effective spin value for the ferrous iron in PS II was found to be 0.8 and the distance between the radical and the non-heme iron was obtained to be 42±2 Å.     

Pulsed electron double resonance (PELDOR) and its applications in free-radicals research

The papers related to the theoretical background and experimental investigations by pulsed electron double resonance (PELDOR) are reviewed. The main aim of this pulsed ESR application is to study the dipole-dipole spin interaction. In PELDOR the ESR spectrum is excited by two ESE pulses at frequencyω _a and additional pumping pulse atω _b. Decay functionV(T) of the ESE signal, when the time intervalT between the first ESE pulse and pumping pulse is varied, contains the information on dipole-dipole couplings in the spin system. The kinetics ofV(T) decay strongly depends upon distance, mutual orientation inside interacting spin pairs and space distribution of radicals throughout the sample. The distances between spins which were measured or estimated using PELDOR in the papers reviewed are in the range of 15 ÷ 130 Å. This pulsed ESR technique turns now to be a powerful supplement to conventional ESE in studying the free radicals space distribution..     

Observation of broad EPR spectra of transient free radicals by FT-EPR

Application of electron spin echo Fourier transform EPR (ESE-FT-EPR) to photo-induced chemical reactions is presented. Main purpose of this study is to observe broad EPR spectra of free radicals having very shortT ₂ ^* by means of the ESE-FT-EPR technique. Details of the experimental procedures are described. In ESE experiments design of the resonator is important to obtain sufficient spectral bandwidth because of use of multiple pulses which decrease the bandwidth. We designed and constructed Loop-Gap-Resonantors (LGR) for light irradiation experiments and their specifications were examined. The phase cycling method is essential to obtain pure ESE signals and proper time resolution by eliminating unwanted FID signals which result from imperfect pulse angles. We applied this technique to observe the photo-induced electron transfer reaction between tetraphenylporphinato zinc(II) (ZnTPP) and duroquinone (DQ) in an ethanol solution, and successfully observed the time resolved EPR spectra of the both Zn(TPP) cation and DQ anion radicals by ESE-FT-EPR of the Hahn echo. The half-height full-width of envelope of EPR spectrum of Zn(TPP)⁺, which is never observed in ordinary FT-EPR, is about 16 MHz. Specificity of spectra and the time resolution are compared among the ESE-FT-, FT- and cw-Time-Resolved-EPR (cw-TREPR) techniques.     

The striking distance of lightning flashes and the early streamer emission (ESE) hypothesis

The attachment of a lightning flash to a lightning conductor (or to any other structure) takes place through a connecting leader that rises from the structure towards the descending stepped leader of a lightning flash. The spatial separation between the tip of the stepped leader and the lightning conductor (or the grounded structure) at the initiation of the connecting leader is known as the striking distance. In this paper the striking distance of stepped leaders is derived as a function of conductor height, conductor radii and the prospective return stroke current. Based on these results the validity of the early streamer emission (ESE) hypothesis is discussed. According to the ESE hypothesis, the striking distance of a lightning conductor can be increased by the artificial initiation of streamers from a lightning conductor. The results cast doubt on the validity of the ESE hypothesis. This in turn calls for more experimental data and field validations before using the ESE hypothesis in standard lightning protection practice.     

Theory of the proximity effect in junctions with unconventional superconductors

We present a general theory of the proximity effect in junctions between diffusive normal metals (DN) and superconductors. Various possible symmetry classes in a superconductor are considered: even-frequency spin-singlet even-parity (ESE) state, even-frequency spin-triplet odd-parity state, odd-frequency spin-triplet even-parity (OTE) state and odd-frequency spin-singlet odd-parity state. It is shown that the pair amplitude in a DN belongs, respectively, to an ESE, OTE, OTE, and ESE pairing state since only the even-parity s-wave pairing is possible due to the impurity scattering.     

Pulsed-ESR Study of Spin Solitons as a Probe of Neutral-to-Ionic Phase Transition of TTF-CA Single Crystal

Ionic phase of charge-transfer crystal composed of tetrathiafulvalene (TTF) and p -chloranil (CA) has been studied by two-pulse electron-spin echo (ESE) at 4 K. By measuring the angular dependence of the field-swept ESE spectrum, at least three kinds of paramagnetic centers have been resolved, which are presumably ascribed to spin solitons formed at domain boundary. With varying the time separation between the microwave pulses, envelope modulations due to anisotropic hyperfine interaction have been detected. By analyzing the modulation, we have firstly obtained ENDOR spectrum of the spin solitons. Response of the ESE intensity for the optical excitation has been also examined.     

Effects of lattice and local dynamics in EPR spectra and electron spin relaxation of vibronic Cu(imidazole)6 complexes in Zn(imidazole)6Cl2·4H2O crystals

Cu(im)₆ complexes in Zn(im)₆Cl₂·4H₂O exhibit a strong Jahn-Teller effect which is static below 100 K and the complex in localized in the two low-energy potential wells. We have reinvestigated electron paramagnetic resonance (EPR) spectra in the temperature range 4.2-300 K and determined the deformation directions produced by the Jahn-Teller effect, energy difference 11 cm⁻¹ between the wells and energy 300 cm⁻¹ of the third potential well. The electron spin relaxation was measured by electron spin echo (ESE) method in the temperature range of 4.2-45 K for single crystal and powder samples. The spin-lattice relaxation is dominated by a local mode of vibration with energy 11 cm⁻¹ at low temperatures. We suppose that this mode is due to reorientations (jumps) of the Cu(im)₆ complex between the two lowest energy potential wells. At intermediate temperatures (15-35 K), the T₁ relaxation is determined by the two-phonon Raman processes in acoustic phonon spectrum with Debye temperature Θ_D=167 K, whereas at higher temperatures the relaxation is governed by the optical phonon of energy 266 cm⁻¹. The ESE dephasing is produced by an instantaneous diffusion below 15 K with the temperature-independent phase memory time , then it grows exponentially with temperature with an activation energy of 97 cm⁻¹. This is the energy of the first excited vibronic level. The thermal population of this level leads to a transition from anisotropic to isotropic EPR spectrum observed around 90 K. FT-ESE gives ESEEM spectrum dominated by quadrupole peaks from non-coordinating ¹⁴N atom of the imidazole rings and the peak from double quantum transition ν_dq. We show that the amplitude of the ν_dq transition can be used to determine the number of non-coordinating nitrogen atoms.     

Selection of dipolar interaction by the “2 + 1” pulse train ESE

The new kind of electron spin echo (ESE), the “2 + 1” pulse train, is described. This method allows the measurement of dipole-dipole interactions between paramagnetic centers which are substantially weaker than those that can be measured by the ordinary two-pulse train. The dead time of ESE spectrometer response in this method is decreased to the duration of the first two pulses. The theory of dipole-dipole interaction in the ESE signal decay in the “2 + 1” pulse train is developed for different Flip rates and for different cases of spin spatial distribution. The theoretical data fit the experiment, carried out with model systems of H and D atoms, randomly distributed in the frozen solutions of sulfuric acid, and of biradicals. New data concerning the spatial distribution of radical clusters, resulting from y irradiation of the methanol, are given.     

Electron spin echo at S-band: Nuclear modulations and decay in C60 powder

At S-band (≈ 3 GHz) the modulation amplitude of the Electron Spin Echo patterns is increased with respect to the amplitude at X-band: as a consequence, it is possible to reveal the presence of nuclei not detectable at X-band. In this paper the results obtained by running ESE experiments at S-band on a C₆₀ powder sample containing radicals are shown and discussed. The two- and three-pulse Electron Spin Echo patterns exhibit both¹³C modulation and proton modulation not detected at X-band. The experimental data are quantitatively analyzed by simulating the time-domain patterns and their Fourier transforms. It results that a noticeable amount of proton nuclei surrounds the intrinsic paramagnetic centers. On the basis of the analysis, a possible explanation of their existence is given.     

Molecular Motion in Frozen Phospholipid Bilayers in the Presence of Sucrose and Sorbitol Studied by the Spin-Echo EPR of Spin Labels

Electron spin echo (ESE) spectroscopy, a pulsed version of electron paramagnetic resonance (EPR), was applied to spin-labeled stearic acids in phospholipid bilayers hydrated in the presence of sucrose and sorbitol, which are known for their cryoprotective action on biological membranes. The phospholipids were 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). Stearic acids were labeled by nitroxide 4,4-dimethyl-oxazolidine-1-oxyl (DOXYL) attached rigidly at either the 5th or 16th specific carbon positions. ESE detects fast stochastic small-angle restricted molecular rotations (stochastic molecular librations) with correlation times on the nanosecond timescale. These motions are believed to have the same nature as the anharmonic motions of hydrogen atoms in biological substances detected by neutron scattering and Mössbauer spectroscopy, which become active above 200 K. To ensure that the echo decays indeed originate from fast stochastic molecular librations, a three-pulse stimulated spin echo was employed. It was found that the presence of sucrose or sorbitol suppresses the observed molecular motions. The observed effect was nearly the same for both label positions, indicating that the motions are similarly suppressed near the bilayer surface and in the bilayer interior. This finding suggests non-specific interactions of sugars with bilayer surface, which are likely to influence only the bulk physical properties of hydrated membranes. The results obtained show the usefulness of spin-echo EPR of spin labels when applied to investigate the molecular mechanisms of action of cryoprotective agents on biological systems.