Time-resolved Electron Paramagnetic Resonance Spectroscopy Research Articles

Electron Spin Exchange in Linked Phenothiazine–Viologen Charge Transfer Complexes Incorporated in “Through‐Ring” (Rotaxane) α‐Cyclodextrins

A series of covalently bound phenothiazine (PHZ) donor and methylviologen (V) acceptor compounds with polymethylene chain spacers (C8 , C10 , C12 ) were incorporated in a "through-ring" (rotaxane) fashion to α-cyclodextrin (α-CD) hosts such that the alkyl chains were fully extended, with the donor and acceptor on opposite sides of the α-CD cylinder. Photoexcitation of the PHZ unit induces electron transfer from the PHZ first excited triplet state to the V moiety, forming a biradicaloid charge-separated state. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy at the X-band and Q-band microwave frequencies was used to investigate the spin exchange interaction, J, in these biradicaloids. Simulation of the spectra using a "static" model for spin-correlated radical pairs allows extraction of the J values, which are negative in sign and have absolute values range from 2 to 1000 Gauss. Comparison of the PHZn V (n = 8, 10, 12) spectra to those obtained using phenyl ether spacers indicates that π-bonds may assist the electronic coupling. The results are discussed in terms of through-bond vs through-space electronic coupling mechanisms.

Overcoming Coulombic Traps: Geometry and Electronic Characterizations of Light-Induced Separated Spins at the Bulk Heterojunction Interface.

Recent progress is overviewed on experimental elucidations of fundamental molecular functions of the light-energy conversions by the photoactive layers of the organic photovoltalic (OPV) cells by means of the time-resolved electron paramagnetic resonance spectroscopy. Positions and orientations of the unpaired electrons and electronic coupling matrix elements are clarified in photoinduced, primary charge-separated (CS) states. Connections between the molecular geometries and the electronic couplings have been characterized for the initial CS states to elucidate how the structure, orbital delocalization, and molecular libration play roles on exothermic carrier dissociation via a vibrationally relaxed charge-transfer complex with prevention of the energy-wasting charge recombination. Superior functions to biological molecules are presented for the efficient photocurrent generations induced by orbital delocalization and by shallow trap depths at polymer-stacking domains. The above structural and electronic characteristics of the primary electron-hole pairs are essential to evaluations, designs, and developments of the efficient solar cells using organic molecules.

Charge Separation in PCPDTBT:PCBM Blends from an EPR Perspective

Using time-resolved electron paramagnetic resonance (EPR) spectroscopy in conjunction with optical excitation we study charge separation in conjugated polymers blended with [6,6]-phenyl C61-butyric acid methyl ester (PCBM). A direct comparison between samples comprising poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (C-PCPDTBT) and their analogues containing poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,5′-diyl] (Si-PCPDTBT) reveals a remarkable influence of the bridging atom (carbon vs silicon) in the polymer on the EPR spectra. While the EPR signatures of photogenerated positive polarons in C- and Si-bridged PCPDTBT are virtually identical, significant differences are observed with respect to the spin-relaxation behavior. The spin–lattice relaxation time of positive polarons in C-PCPDTBT at low temperature (T = 80 K) is found to be more than two orders or magnitude longer t...

Short-lived phenoxyl radicals formed from green-tea polyphenols and highly reactive oxygen species: an investigation by time-resolved EPR spectroscopy.

Polyphenols are effective antioxidants and their behavior has been studied in depth. However, a structural characterization of the species formed immediately upon hydrogen-atom transfer (HAT), a key reaction of oxidative stress, has not been achieved. The reaction of catechin and green-tea polyphenols with highly reactive O-centered H-abstracting species was studied at the molecular level and in real time by using time-resolved electron paramagnetic resonance (EPR) spectroscopy. This mirrors the reaction of highly reactive oxygen species with polyphenols. The results show that all phenolic OH groups display essentially identical reactivity. Accordingly, there is no site specificity for HAT and initial antioxidative events are demonstrated to be largely ruled by statistical (entropic) factors.

Spectroscopic evidence for a redox-controlled proton gate at tyrosine D in Photosystem II.

Tyrosine D (TyrD) is one of two well-studied redox active tyrosines in Photosystem II. TyrD shows redox kinetics much slower than that of its homologue, TyrZ, and is normally present as a stable deprotonated radical (TyrD(•)). We have used time-resolved continuous wave electron paramagnetic resonance and electron spin echo envelope modulation spectroscopy to show that deuterium exchangeable protons can access TyrD on a time scale that is much faster (50-100 times) than that previously observed. The time of H/D exchange is strongly dependent on the redox state of TyrD. This finding can be related to a change in position of a water molecule close to TyrD.

Photoinduced electron transfer between 2-methylanthraquinone and triethylamine in an ionic liquid: Time-resolved EPR and transient absorption spectroscopy study

Photoinduced electron transfer between 2-methylanthraquinone (MeAQ) and triethylamine (TEA) in a room-temperature ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), was investigated by comparing the time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy and the transient absorption spectroscopy. The results of TR-EPR spectroscopy, in which MeAQ was 8mmolL−1 and TEA was 150mmolL−1, indicated that the transient radical would exist longer time in [bmim][PF6] than in acetonitrile. At the delay time of 8μs after laser excitation, the TR-EPR signal transformed from an emissive peak into an absorptive peak when the experiment was performed in [bmim][PF6]. The results of the transient absorption spectroscopy, in which MeAQ was 0.1mmolL−1 and TEA was 2.2mmolL−1, showed that the efficiency and the rate of the photoinduced electron transfer reaction in [bmim][PF6] were obviously lower than that in acetonitrile. It was concluded that various factors, such as concentration, viscosity and local structural transformation of the solution, have an influence on the process of photoinduced electron transfer in [bmim][PF6].

Radical-triplet pair interactions as probes of long-range polymer motion in solution.

Radical-triplet pair interactions are used to investigate the dynamics of acrylic polymers in dilute solution. Methyl methacrylate was randomly copolymerized with a small amount of an amine-containing monomer to create the polymers. The amine subunits were then oxidized to nitroxide moieties (stable free radicals). Using time-resolved electron paramagnetic resonance (TREPR) spectroscopy on the sub-microsecond time scale, competition is observed between two deactivation processes of the ester side chain photoexcited triplet state: (1) Norrish I α-cleavage, leading to a 21-line main chain polymeric free radical TREPR spectrum with emissive triplet mechanism spin polarization, and (2) spin polarization transfer or quenching by a nearby stable nitroxide radical, which gives a 3-line spectrum exhibiting emissive radical-triplet pair spin polarization. The main chain polymer radical signal dominates the TREPR spectrum if the probability of radical-triplet pair encounters is low. These competing reactions show a strong dependence on nitroxide incorporation (mol %), temperature, solvent, and acrylic polymer ester side chain structure. A comparison of steady state EPR and TREPR signals from inter- vs intramolecular processes clearly demonstrates the influence of polymer chain dynamics on the observed phenomena.

Structure and Reactivity of Radicals Produced by Photocleavage of Oxime Ester Compounds Studied by Time-resolved Electron Paramagnetic Resonance Spectroscopy

Abstract Photodissociation of three oxime ester compounds was investigated using time-resolved electron paramagnetic resonance spectroscopy. Direct evidences for the appearance of methyl and iminyl radicals were obtained, and their reactivity toward the acrylate monomer was studied.

Photolysis of endoperoxides in the presence of nitroxides: a laser flash photolysis study with optical and ESR detection

Time-resolved electron paramagnetic resonance spectroscopy, transient absorption, and phosphorescence spectroscopy were used to investigate the spin polarization of a nitroxide free radical induced by interaction with singlet oxygen ((1)O2). The latter was generated by photolysis of endoperoxides of two anthracene derivatives. Although both anthracene endoperoxides are structurally similar, opposite spin polarization of the nitroxide was observed. Photolysis of one endoperoxide leads to absorptive nitroxide spin polarization due to interaction with the generated (1)O2. Photolysis of the other endoperoxide generated emissive nitroxide spin polarization, probably due to interaction of the endoperoxide triplet states with nitroxides.

有機太陽電池界面の長距離電子的相互作用：時間分解電子スピン共鳴法による電子状態解析

Time-resolved electron paramagnetic resonance (TREPR) spectroscopy has been utilized at T = 77 K to characterize geometries and electronic couplings (VCR) of transient charge-separated (CS) states in photoactive layers in organic thin film solar cells fabricated by spin-coating of mixed solutions of polyalkylthiophenes (RR-P3AT) and [6, 6]-C61-butyric acid methyl ester (PCBM). Electron-hole distance of the interfacial transient CS states has been revealed to be modulated by alkyl side-chain number in P3AT. This result is explained by a coupling of a hole dissociation to polymer librations by the side-chains. From an exponential decay of VCR with respect to the CS distance, an attenuation factor (βe) of the decay has been determined to be βe＝0.2 Å−1. Such a long-range tunneling feature is explained by generations of shallowly trapped, delocalized electron-hole pairs by the dissociations of the hole toward π-stacking directions at the organic photovoltaic interface.

Time-Resolved EPR Study of Electron-Hole Dissociations Influenced by Alkyl Side Chains at the Photovoltaic Polyalkylthiophene:PCBM Interface.

Nanosecond time-resolved electron paramagnetic resonance (TREPR) spectroscopy has been utilized at T = 77 K to characterize alkyl side-chain effects on geometries and on the electronic couplings (VCR) of transient charge-separated (CS) states in the photoactive layers fabricated by the spin-coating of mixed solutions of regioregular polyalkylthiophenes (RR-P3AT) and [6,6]-C61-butyric acid methyl ester (PCBM). By increasing the alkyl side-chain number from 6 to 12 in P3AT, a highly distant and long-lived CS state has been obtained. This result is explained by a coupling of the hole dissociation to the polymer librations by the side-chains. From an exponential decay of VCR with respect to the CS distance, the attenuation factor (βe) has been determined to be βe = 0.2 Å(-1). Such a long-range tunneling feature is explained by the generations of the shallowly trapped, delocalized electron-hole pairs by the dissociation of the hole toward π-stacking directions at the organic photovoltaic interface.

Time-resolved electron paramagnetic resonance of radical pair intermediates in cryptochromes

Electron transfer plays a key role in many biological systems, including core complexes of photosynthesis and respiration. As this involves unpaired electron spins, electron paramagnetic resonance (EPR) is the method of choice to investigate such processes. Systems that show photo-induced charge separation and electron transfer are of particular interest, as here the processes can easily be synchronised to the experiment and therefore followed directly over its time course. One particular class of proteins, the cryptochromes, showing charge separation and in turn spin-correlated radical pairs upon excitation with blue light, have been investigated by time-resolved EPR spectroscopy in great detail and the results obtained so far are summarised in this contribution. Highlights include the first observation of spin-correlated radical pairs in these proteins, a fact with great impact on the proposed role as key part of a magnetic compass of migratory birds, as well as the assignment of the radical-pair partners and the unravelling of alternative and unexpected electron transfer pathways in these proteins, giving new insights into aspects of biological electron transfer itself.

Electron Spin Polarization Transfer from Photogenerated Spin-Correlated Radical Pairs to a Stable Radical Observer Spin

A series of donor-chromophore-acceptor-stable radical (D-C-A-R(•)) molecules having well-defined molecular structures were synthesized to study the factors affecting electron spin polarization transfer from the photogenerated D(+•)-C-A(-•) spin-correlated radical pair (RP) to the stable radical R(•). Theory suggests that the magnitude of this transfer depends on the spin-spin exchange interaction (2JDA) of D(+•)-C-A(-•). Yet, the generality of this prediction has never been demonstrated. In the D-C-A-R(•) molecules described herein, D is 4-methoxyaniline (MeOAn), 2,3-dihydro-1,4-benzodioxin-6-amine (DioxAn), or benzobisdioxole aniline (BDXAn), C is 4-aminonaphthalene-1,8-dicarboximide, and A is naphthalene-1,8:4,5-bis(dicarboximide) (1A,B-3A,B) or pyromellitimide (4A,B-6A,B). The terminal imide of the acceptors is functionalized with either a hydrocarbon (1A-6A) or a 2,2,6,6-tetramethyl-1-piperidinyloxyl radical (R(•)) (1B-6B). Photoexcitation of C with 416-nm laser pulses results in two-step charge separation to yield D(+•)-C-A(-•)-(R(•)). Time-resolved electron paramagnetic resonance (TREPR) spectroscopy using continuous-wave (CW) microwaves at both 295 and 85 K and pulsed microwaves at 85 K (electron spin-echo detection) was used to probe the initial formation of the spin-polarized RP and the subsequent polarization of the attached R(•) radical. The TREPR spectra show that |2JDA| for D(+•)-C-A(-•) decreases in the order MeOAn(+•) > DioxAn(+•) > BDXAn(+•) as a result of their spin density distributions, whereas the spin-spin dipolar interaction (dDA) remains nearly constant. Given this systematic variation in |2JDA|, electron spin-echo-detected EPR spectra of 1B-6B at 85 K show that the magnitude of the spin polarization transferred from the RP to R(•) depends on |2JDA|.

Variable Electron Transfer Pathways in an Amphibian Cryptochrome

Electron transfer reactions play vital roles in many biological processes. Very often the transfer of charge(s) proceeds stepwise over large distances involving several amino acid residues. By using time-resolved electron paramagnetic resonance and optical spectroscopy, we have studied the mechanism of light-induced reduction of the FAD cofactor of cryptochrome/photolyase family proteins. In this study, we demonstrate that electron abstraction from a nearby amino acid by the excited FAD triggers further electron transfer steps even if the conserved chain of three tryptophans, known to be an effective electron transfer pathway in these proteins, is blocked. Furthermore, we were able to characterize this secondary electron transfer pathway and identify the amino acid partner of the resulting flavin-amino acid radical pair as a tyrosine located at the protein surface. This alternative electron transfer pathway could explain why interrupting the conserved tryptophan triad does not necessarily alter photoreactions of cryptochromes in vivo. Taken together, our results demonstrate that light-induced electron transfer is a robust property of cryptochromes and more intricate than commonly anticipated.

Charge Dynamics in A Donor–Acceptor Covalent Organic Framework with Periodically Ordered Bicontinuous Heterojunctions

Light works: Mechanistic insights into the photochemical events and charge dynamics of a donor–acceptor covalent organic framework were given by time-resolved transient absorption spectroscopy and time-resolved electron spin resonance spectroscopy (see picture). The organic framework triggers ultrafast electron transfer and enables long-distance charge delocalization and exceptional long-term charge separation.

Time-Resolved Electron Paramagnetic Resonance and Phosphorescence Studies of the Lowest Excited Triplet States of Rh(III) Corrole Complexes

The lowest excited triplet (T(1)) ππ* states of gallium (Ga) and various rhodium (Rh) 5,10,15-trispentafluorophenyl corroles (Cors) were studied in the liquid crystal (LC) E-7 and in rigid glasses by time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy. The triplet sublevel energies were experimentally determined by the alignment of the molecules in the LC and by magnetophotoselection in the glass. The sublevel scheme of GaCor was determined by calculating the zero field splitting (ZFS) parameters. Axial ligand effects and quantum chemical calculations were used for the sublevel assignment of RhCors. The anisotropic EPR parameters were used to determine the important higher excited states and the magnitudes of their spin-orbit coupling (SOC) contributions were evaluated. On the basis of these results and analyses, the EPR parameters and triplet lifetime were discussed for each RhCor complex.

Structure and Dynamics of Photogenerated Triplet Radical Ion Pairs in DNA Hairpin Conjugates with Anthraquinone End Caps

A series of DNA hairpins (AqGn) possessing a tethered anthraquinone (Aq) end-capping group were synthesized in which the distance between the Aq and a guanine-cytosine (G-C) base pair was systematically varied by changing the number (n - 1) of adenine-thymine (A-T) base pairs between them. The photophysics and photochemistry of these hairpins were investigated using nanosecond transient absorption and time-resolved electron paramagnetic resonance (TREPR) spectroscopy. Upon photoexcitation, (1*)Aq undergoes rapid intersystem crossing to yield (3*)Aq, which is capable of oxidizing purine nucleobases resulting in the formation of (3)(Aq(-•)Gn(+•)). All (3)(Aq(-•)Gn(+•)) radical ion pairs exhibit asymmetric TREPR spectra with an electron spin polarization phase pattern of absorption and enhanced emission (A/E) due to their different triplet spin sublevel populations, which are derived from the corresponding non-Boltzmann spin sublevel populations of the (3*)Aq precursor. The TREPR spectra of the (3)(Aq(-•)Gn(+•)) radical ion pairs depend strongly on their spin-spin dipolar interaction and weakly on their spin-spin exchange coupling. The anisotropy of (3)(Aq(-•)Gn(+•)) makes it possible to determine that the π systems of Aq(-•) and G(+•) within the radical ion pair are parallel to one another. Charge recombination of the long-lived (3)(Aq(-•)Gn(+•)) radical ion pair displays an unusual bimodal distance dependence that results from a change in the rate-determining step for charge recombination from radical pair intersystem crossing for n < 4 to coherent superexchange for n > 4.

Role of Bridge Energetics on the Preference for Hole or Electron Transfer Leading to Charge Recombination in Donor-Bridge-Acceptor Molecules

The impact of donor-acceptor electronic coupling and bridge energetics on the preference for hole or electron transfer leading to charge recombination in a series of donor-bridge-acceptor (D-B-A) molecules was examined. In these systems, the donor is 3,5-dimethyl-4-(9-anthracenyl)-julolidine (DMJ-An) and acceptor is naphthalene-1,8:4,5-bis(dicarboximide) (NI), while the bridges are either oligo(p-phenyleneethynylene) (PE(n)P, where n = 1-3) 1-3 or oligo(2,7-fluorenone) (FN(n), where n = 1-3) 4-6. Photoexcitation of 1-3 and 4-6 produces DMJ(+•)-An-PE(n)P-NI(-•) and DMJ(+•)-An-FN(n)-NI(-•), respectively, which undergo radical pair intersystem crossing followed by charge recombination to yield both (3*)An and (3*)NI, which are observed by time-resolved electron paramagnetic resonance (TREPR) spectroscopy. (3*)NI is produced by hole transfer from DMJ(+•) to NI(-•), while (3*)An is produced by electron transfer from NI(-•) to DMJ(+•), using the agency of the bridge HOMOs and LUMOs, respectively. By monitoring the initial population of (3*)NI and (3*)An in 1-6, the data show that charge recombination occurs preferentially by selective hole transfer when the bridge is PE(n)P, while it occurs by preferential electron transfer when the bridge is FN(n). Over time, the initial population of (3*)NI decreases, while that of (3*)An increases, indicating that triplet-triplet energy transfer (TEnT) occurs. The observed distance dependence of TEnT from (3*)NI to An is weakly exponential with a decay parameter β = 0.08 Å(-1) for the PE(n)P series and β = 0.03 Å(-1) for the FN(n) series. In the PE(n)P series, this weak distance dependence is attributed to a transition from the superexchange regime to hopping transport as the energy gap for triplet energy injection onto the bridge becomes significantly smaller as n increases, while in the FN(n) series the corresponding energy gap is small for all n resulting in triplet energy transport by the hopping mechanism.

Intersystem Crossing Involving Strongly Spin Exchange-Coupled Radical Ion Pairs in Donor–bridge–Acceptor Molecules

Intersystem crossing involving photogenerated strongly spin exchange-coupled radical ion pairs in a series of donor-bridge-acceptor molecules was examined. These molecules have a 3,5-dimethyl-4-(9-anthracenyl)-julolidine (DMJ-An) donor either connected directly or connected by a phenyl bridge (Ph), to pyromellitimide (PI), 1 and 2, respectively, or naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptors, 3 and 4, respectively. Femtosecond transient optical absorption spectroscopy shows that photodriven charge separation produces DMJ(+•)-PI(-•) or DMJ(+•)-NI(-•) quantitatively in 1-4 (τ(CS) ≤ 10 ps), and that charge recombination occurs with τ(CR) = 268 and 158 ps for 1 and 3, respectively, and with τ(CR) = 2.6 and 10 ns for 2 and 4, respectively. Magnetic field effects (MFEs) on the neutral triplet state yield produced by charge recombination were used to measure the exchange coupling (2J) between DMJ(+•) and PI(-•) or NI(-•), giving 2J > 600 mT for 1-3 and 2J = 170 mT for 4. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy revealed that the formation of (3)*An upon charge recombination occurs by spin-orbit charge transfer intersystem crossing (SOCT-ISC) and/or radical-pair intersystem crossing (RP-ISC) mechanisms with the magnitude of 2J determining which triplet formation mechanism dominates. SOCT-ISC is the exclusive triplet formation mechanism in 1-3, whereas both RP-ISC and SOCT-ISC are active for 4. The triplet sublevels populated by SOCT-ISC in 1-4 depend on the donor-acceptor geometry in the charge separated state. This is consistent with the fact that the SOCT-ISC mechanism requires the relevant donor and acceptor orbitals to be nearly perpendicular, so that electron transfer results in a large orbital angular momentum change that must be compensated by a fast spin flip to conserve overall system angular momentum.

Photoinduced Processes in Some Mechanically Interlocked Supramolecules as Studied by Time-Resolved Electron Paramagnetic Resonance

Time-resolved electron paramagnetic resonance (TREPR) spectroscopy was used for studying photoinduced intramolecular electron transfer (ET) and energy transfer (EnT) processes in three mechanically interlocked molecules in which zinc(II)porphyrin (ZnP) and C60 fullerene moieties are arrayed around a central Cu(I) bisphenanthroline core used to assemble these donor–acceptor (D–A) systems. The specific molecules studied include a “long” (ZnP)2–Cu(I)(phen)2–C60 rotaxane as well as ZnP–Cu(I)(phen)2 and ZnP–Cu(I)(phen)2–C60 catenanes, embedded in different phases of nematic liquid crystal and frozen isotropic solvents. It was demonstrated that the routes and rates of the transfer processes in these supramolecules strongly depend on the characteristics of their microenvironment and the molecular entity which was selectively photoexcited. This is reflected by formation of distinct long-lived charge-separated species such as the radical ion pair (ZnP)2•+–Cu(I)(phen)2•– and the metal-to-ligand charge-separated state Cu(II)–(phen)2•– under the various experimental conditions. The results are discussed in terms of the correlation between the chemical structure, conformational mobility, and relaxation pathways of the photoexcited states in these mechanically interlocked systems. Results are compared with previously reported TREPR data on related interlocked D–A porphyrin/fullerene systems.