Radical Pair Recombination Research Articles

EPR study of photoinduced charge transfer in a poly(3-hexylthiophene)-fullerene composite

Magnetic, relaxation, and dynamic parameters of radical pairs of positively charged polarons and negatively charged anion radicals of fullerene that are induced by photons with an energy of 1.7–3.4 eV are studied by the methods of photoinduced electron paramagnetic resonance for the poly(3-hexylthiophene)fullerene composite. The above charge carriers show mutual independence, which is provided by a different interaction with their microenvironment. The paramagnetic susceptibility of spin pairs reflects the dipoledipole interaction and activation dynamics of paramagnetic sites in the polymer-fullerene system. The rate of recombination of radical pairs is controlled by the mutual space distribution of carriers of various charges and by the energy of excitation photons. Quasi-one-dimensional diffusion of polarons along polymer chains and rotational diffusion of fullerene molecules about the selected molecular axis are likewise controlled by the energy of photons and can be described in terms of the activation Elliott hopping model. The dependence of the main values of magnetic, relaxation, and dynamic parameters of charge carriers on the energy of photons is explained by the inhomogeneous distribution of molecular clusters in the polymer-fullerene composite. The annealing of this composite leads to an enhanced formation of polymer crystallites and fullerene clusters. Hence, the effective dimension of the system increases and its electron characteristics are improved.

ChemInform Abstract: Radical Pair Recombination Stereoselectivity as a Probe of Magnetic Isotope and Magnetic Field Effects.

Photolysis of meso- or dl-2,4-diphenylpentan-3-one within NaY zeolites, coadsorbed with a chiral inductor, lead to enantioselective radical pair recombinations which are sensitive to 13C isotope effects, but insensitive towards external magnetic field effects.

The chemical zeno effect in exchange-bounded radical pairs: 2. Singlet and random radical pairs

Manifestations of the chemical Zeno effect (slowing down of the quantum evolution of radical pairs as a result of spin-selective recombination of exchange-bounded radical pairs) are considered. In radical pairs created in singlet and random spin states, the chemical kinetics is described by the sum of kinetic evolution (exponential) and oscillatory functions. The kinetic function is a superposition of two exponential functions. Recombination monotonically decreases the amplitude of quantum beats in singlet radical pairs. The dependence of the amplitude of quantum beats in noncorrelated radical pairs on the recombination rate constant is not monotonic.

Cage effect dynamics

The kinetics of recombination/dissociation of photogenerated radical pairs (RPs) is described with a generalized model (GM), which combines exponential models (EMs) and contact models (CMs) of cage effect dynamics. The main assumption of EM is the irreversible dissociation of RP as a first-order reaction. CM takes into account repetitive contacts of radicals in the pair or reversible diffusional separation of radicals. The present GM accounts for both the recombination of RP within the cage of outcome and their diffusional separation in a free space. GM predicts that a short initial time RP decay is described by EM, and RP kinetics at longer times of observation is described by CM. This theory successfully describes available experimental data. Solvent viscosity effect on the cage effect values is also analyzed.

The Role of Key Amino Acids in the Photoactivation Pathway of the Synechocystis Slr1694 BLUF Domain

BLUF (blue light sensing using FAD) domains belong to a novel group of blue light sensing receptor proteins found in microorganisms. We have assessed the role of specific aromatic and polar residues in the Synechocystis Slr1694 BLUF protein by investigating site-directed mutants with substitutions Y8W, W91F, and S28A. The W91F and S28A mutants formed the red-shifted signaling state upon blue light illumination, whereas in the Y8W mutant, signaling state formation was abolished. The W91F mutant shows photoactivation dynamics that involve the successive formation of FAD anionic and neutral semiquinone radicals on a picosecond time scale, followed by radical pair recombination to result in the long-lived signaling state in less than 100 ps. The photoactivation dynamics and quantum yield of signaling state formation were essentially identical to those of wild type, which indicates that only one significant light-driven electron transfer pathway is available in Slr1694, involving electron transfer from Y8 to FAD without notable contribution of W91. In the S28A mutant, the photoactivation dynamics and quantum yield of signaling state formation as well as dark recovery were essentially the same as in wild type. Thus, S28 does not play an essential role in the initial hydrogen bond switching reaction in Slr1694 beyond an influence on the absorption spectrum. In the Y8W mutant, two deactivation branches upon excitation were identified: the first involves a neutral semiquinone FADH(*) that was formed in approximately 1 ps and recombines in 10 ps and is tentatively assigned to a FADH(*)-W8(*) radical pair. The second deactivation branch forms FADH(*) in 8 ps and evolves to FAD(*-) in 200 ps, which recombines to the ground state in about 4 ns. In the latter branch, W8 is tentatively assigned as the FAD redox partner as well. Overall, the results are consistent with a photoactivation mechanism for BLUF domains where signaling state formation proceeds via light-driven electron and proton transfer from Y8 to FAD, followed by a hydrogen bond rearrangement and radical pair recombination.

Continuous Wave Photolysis Magnetic Field Effect Investigations with Free and Protein-Bound Alkylcobalamins

The activation of the Co-C bond in adenosylcobalamin-dependent enzymes generates a singlet-born Co(II)-adenosyl radical pair. Two of the salient questions regarding this process are: (1) What is the origin of the considerable homolysis rate enhancement achieved by this class of enzyme? (2) Are the reaction dynamics of the resultant radical pair sensitive to the application of external magnetic fields? Here, we present continuous wave photolysis magnetic field effect (MFE) data that reveal the ethanolamine ammonia lyase (EAL) active site to be an ideal microreactor in which to observe enhanced magnetic field sensitivity in the adenosylcobalamin radical pair. The observed field dependence is in excellent agreement with that calculated from published hyperfine couplings for the constituent radicals, and the magnitude of the MFE (<18%) is almost identical to that observed in a solvent containing 67% glycerol. Similar augmentation is not observed, however, in the equivalent experiments with EAL-bound methylcobalamin, where all field sensitivity observed in the free cofactor is washed out completely. Parallels are drawn between the latter case and the loss of field sensitivity in the EAL holoenzyme upon substrate binding (Jones et al. J. Am. Chem. Soc. 2007, 129, 15718-15727). Both are attributed to the rapid removal of the alkyl radical immediately after homolysis, such that there is inadequate radical pair recombination for the observation of field effects. Taken together, these results support the notion that rapid radical quenching, through the coupling of homolysis and hydrogen abstraction steps, and subsequent radical pair stabilization make a contribution to the observed rate acceleration of Co-C bond homolysis in adenosylcobalamin-dependent enzymes.

TR-CIDNP as tool for quantitative analysis of hyperfine couplings in elusive radicals

For many years Chemically Induced DynamicNuclear Polarization (CIDNP) has been successfullyused for investigating the kinetics of fast reactions ofradicals [1]. The origin of CIDNP is the nuclear spinselective recombination of radical pairs. This case isgiven when the electronic singlettriplet conversion isthe bottleneck for the radical pair recombination. As aconsequence, radical pairs have different reactivity indifferent nuclear spin states, with the result that thediamagnetic reaction products have nonthermal populations of their nuclear spin energy levels. CIDNPmanifests itself in anomalous phase and intensity ofthe NMR spectra of the diamagnetic molecules. Thetimeresolved (TR) version of the CIDNP techniquehas a particularly high content of information abouttransient radicals [2] as it can differentiate primary andsecondary reaction steps. First, CIDNP spectrarecorded right after recombination of the geminateradical pairs (geminate CIDNP) are a fingerprint ofthe transient radicals. Second, CIDNP effects can bedetected in liquids at ambient conditions, whereasconventional EPR spectroscopy often allows one toinvestigate the elusive radical species only in frozenmatrix at cryogenic temperatures. Third, the CIDNPtime dependence (kinetics) gives information aboutthe rates of radical termination, intramolecular electron migration and nuclear spin relaxation in transientradicals.In the present work, we will investigate theoretically and experimentally the possibility of extractinghyperfine coupling constants (HFCCs) in radicalsfrom the geminate CIDNP spectrum recorded at high

The effect of reagent structure and the medium on radical pair recombination in polymers

Triarylcarbinols, the recombination products of ketyl and aminyl radicals identified by visible absorption spectra as relevant triarylmethane dyes, are formed during irradiation of poly(methyl methacrylate), poly(vinyl chloride), polystyrene and cellulose triacetate films containing substituted benzophenones and aromatic amines. Quantum yields of the reactions are determined. The features of reagents and polymer structures effect on the formation rates and yields of products are discussed. The reaction only proceeds in local zones of relatively large (∼ nm) size, as is found based on the analysis of quantum yield dependence on amine concentration. This stipulates the presence of magnetic-spin effects leading to a significant (by 10–20%) product accumulation rate decrease in poly(vinyl chloride) and polystyrene as external magnetic field is applied. A mechanism of significant photochemical post-effect in films produced from chlorine-containing solvent is suggested and kinetic description is proposed. The formation of dye at irradiated film treatment by HCl is diffusion limited. This allows using the reactions studied in the investigations of both slow and fast diffusion processes in polymers.

Thermoluminescence: theory

Thermoluminescence (TL) probes the emission of luminescence associated with the de-trapping of a radical pair as the temperature is increased. This technique has proved useful for characterizing the energetic arrangement of cofactors in photosynthetic reaction centers. In the original TL theory, stemming from solid-state physics, the radical pair recombination was considered to coincide with the light-emitting process. In photosynthetic systems, however, recombination takes place through various routes among which the radiative pathway generally represents a relatively minor leak, and the theoretical framework must be modified accordingly. The radiative route is the one with the largest activation energy and is thus (still) more disfavored at low temperature, so that during the heating process, the TL peak tends to lag behind the decay of the radical pair. A consequence is that the integrated luminescence emission increases with the heating rate. In this article, we examine how the characteristics of the TL emission depend on the redox potentials of the cofactors, showing good agreement between theory and experimental studies on Photosystem (PS) II mutants. We also analyze the effect on (thermo-) luminescence of the connectivity of the light-harvesting pigment antenna, and show that while this should affect significantly luminescence kinetics at room temperature, the effect on TL is expected to be small.

Investigation of geminate recombination of radical ion pairs generated by dissociation of exciplexes in moderately polar solvents using the photoconductivity technique

A new approach to determination of the recombination rate of radical ion pairs in moderately polar solvents is presented. It is based on an investigation of transient photocurrents caused by dissociation of exciplexes generated in photoinduced electron transfer reactions. It has been shown that the recombination rate of geminate ion pairs can be found from the photocurrent rise time. We have applied such an approach to transient photocurrents observed by Hirata et al. [Y. Hirata, Y. Kanda, N. Mataga, J. Phys. Chem. 87 (1983) 1659] for the pyrene/dicyanobenzene system in solvents of moderate polarity. The increase of the obtained recombination rate of photogenerated ions with increasing polarity of solvent testifies that ions recombine mainly by the backward electron transfer from the dicyanobenzene anions to solvent-separated cations of pyrene.

Excited State, Charge Transfer, and Spin Dynamics in DNA Hairpin Conjugates with Perylenediimide Hairpin Linkers

A series of short DNA hairpins (nG) using perylene-3,4:9,10-bis(dicarboximide) (PDI) as the hairpin linker was synthesized in which the distance between the PDI and a guanine-cytosine (G-C) base pair is systematically varied by changing the number (n - 1) of adenine-thymine (A-T) base pairs between them. Due to the relatively large hydrophobic surface of PDI, the nG hairpins dimerize in buffer solutions. The photophysics and photochemistry of these hairpins were investigated using femtosecond transient absorption and time-resolved electron paramagnetic resonance (TREPR) spectroscopy. Photoexcitation of the self-assembled PDI dimer within each nG hairpin results in subpicosecond formation of its lower exciton state ((1*)PDI(2)) followed by formation of an excimer-like state ((1*X)PDI(2)) with tau = 10-28 ps. Both of these states are lower in energy than (1*)PDI, so that neither can oxidize A, C, and T. Electron transfer from G to (1*)PDI(2) is faster than formation of (1*X)PDI(2) only for 1G. Electron transfer from G to (1*X)PDI(2) for 2G-8G, occurs by the superexchange mechanism and, thus, becomes exponentially less efficient as the G-PDI(2) distance increases. Nevertheless, TREPR studies show that photoexcitation of 2G and 4G produce spin-correlated radical ion pairs having electron spin polarization patterns indicating that a low yield of charge separation proceeds from (1*X)PDI(2) by the radical pair intersystem crossing (RP-ISC) mechanism to initially yield a singlet radical ion pair. The strong spin-polarization of the radical ion pairs makes it possible to observe them, even though their concentration is low. As expected, the hairpin lacking G (0G) and that having the longest G-PDI(2) distance (8G) display no TREPR radical ion pair signals. Hairpins 0G, 2G, 4G, and 8G all exhibit triplet EPR spectra at 85 K. Simulations of the spectra show that (3*)PDI is produced mainly by a spin-orbit-induced intersystem crossing mechanism, while the spectra of 2G and 4G have 5% and 21% contributions, respectively, from (3*)PDI produced by charge recombination of radical ion pairs that originate from RP-ISC. These low percentages of RP-ISC derived (3*)PDI result mainly from the low yield of radical ion pairs in 2G and 4G.

Spin-dynamics of the spin-correlated radical pair in photosystem I. Pulsed time-resolved EPR at high magnetic field

Spin-dynamics of the spin-correlated radical pair (SCRP) P(700)(+)A(1A)(-) in the photosystem I (PSI) reaction center protein have been investigated with high-frequency (HF), time-resolved EPR spectroscopy. The superior spectral resolution of HF EPR enables spin-dynamics for both the donor and acceptor radicals in the pair to be monitored independently. Decay constants of each spin were measured as a function of temperature and compared to data obtained at X-band EPR. Relaxation times, T(1), and decay rates, k(S), are the same at both X- and D-band magnetic fields. The spin-dynamics within the radical pair were determined from theoretical simulation of experimental time-resolved HF EPR spectra. At low temperatures, T < 60 K, the decay of the SCRP from the singlet state, k(S), is the predominant process, while at high temperatures, T > 130 K, the T(1) relaxation is much faster than k(S). The recombination rate k(S) was observed to decrease as the temperature is increased. These EPR spectral results are in agreement with previously reported optical measurements of P(700)(+)A(1)(-) radical pair recombination.

Effects of low temperature stress on excitation energy partitioning and photoprotection in Zea mays.

Analysis of the partitioning of absorbed light energy within PSII into fractions utilised by PSII photochemistry (ΦPSII), thermally dissipated via ΔpH- and zeaxanthin-dependent energy quenching (ΦNPQ) and constitutive non-photochemical energy losses (Φf,D) was performed in control and cold-stressed maize (Zea mays L.) leaves. The estimated energy partitioning of absorbed light to various pathways indicated that the fraction of ΦPSII was twofold lower, whereas the proportion of thermally dissipated energy through ΦNPQ was only 30% higher, in cold-stressed plants compared with control plants. In contrast, Φf,D, the fraction of absorbed light energy dissipated by additional quenching mechanism(s), was twofold higher in cold-stressed leaves. Thermoluminescence measurements revealed that the changes in energy partitioning were accompanied by narrowing of the temperature gap (ΔTM) between S2/3QB- and S2QA- charge recombinations in cold-stressed leaves to 8°C compared with 14.4°C in control maize plants. These observations suggest an increased probability for an alternative non-radiative P680+QA- radical pair recombination pathway for energy dissipation within the reaction centre of PSII in cold-stressed maize plants. This additional quenching mechanism might play an important role in thermal energy dissipation and photoprotection when the capacity for the primary, photochemical (ΦPSII) and zeaxanthin-dependent non-photochemical quenching (ΦNPQ) pathways are thermodynamically restricted in maize leaves exposed to cold temperatures.

Photolysis of Adenosylcobalamin and Radical Pair Recombination in Ethanolamine Ammonia-Lyase Probed on the Micro- to Millisecond Time Scale by Using Time-Resolved Optical Absorption Spectroscopy

The quantum yield and kinetics of decay of cob(II)alamin formed by pulsed-laser photolysis of adenosylcobalamin (AdoCbl; coenzyme B(12)) in AdoCbl-dependent ethanolamine ammonia-lyase (EAL) from Salmonella typhimurium have been studied on the 10(-7)-10(-1) s time scale at 295 K by using transient ultraviolet-visible absorption spectroscopy. The aim is to probe the mechanism of formation and stabilization of the cob(II)alamin-5'-deoxyadenosyl radical pair, which is a key intermediate in EAL catalysis, and the influence of substrate binding on this process. Substrate binding is required for cobalt-carbon bond cleavage in the native system. Photolysis of AdoCbl in EAL leads to a quantum yield at 10(-7) s for cob(II)alamin of 0.08 +/- 0.01, which is 3-fold smaller than for AdoCbl in aqueous solution (0.23 +/- 0.01). The protein binding site therefore suppresses photoproduct radical pair formation. Three photoproduct states, P(f), P(s), and P(c), are identified in holo-EAL by the different cob(II)alamin decay kinetics (subscripts denote fast, slow, and constant, respectively). These states have the following first-order decay rate constants and quantum yields: 2.2 x 10(3) s(-1) and 0.02 for P(f), 4.2 x 10(2) s(-1) and 0.01 for P(s), and constant amplitude, with no recombination, and 0.05 for P(c), respectively. Binding of the substrate analogue (S)-1-amino-2-propanol to EAL eliminates the P(f) state and lowers the quantum yield of P(c) (0.03) relative to that of P(s) (0.01) but does not significantly change the quantum yield or decay rate constant of P(s), relative to those of holo-EAL. The substrate analogue thus influences the quantum yield at 10(-7) s by changing the cage escape rate from the geminate cob(II)alamin-5'-deoxyadenosyl radical pair state. However, the predicted substrate analogue binding-induced increase in the quantum yield is not observed. It is proposed that the substrate analogue does not induce the radical pair stabilizing changes in the protein that are characteristic of true substrates.

Mechanism of heminal recombination of ketyl and aminyl radicals in solid poly(methyl methacrylate)

The mechanism of formation and recombination of radical pairs upon photoreduction of benzophenone by diphenylamine in solid poly(methyl methacrylate) has been studied. By analyzing the concentration dependences of the yield of recombination products (triarylmethane dye or triarylcarbinol), it has been found that the reaction occurs only in relatively large local sites with an effective radius of ∼1.4 nm. In contrast, the quenching of ketone phosphorescence by amine, resulting in the generation of primary radical pairs, is carried out in nearly every molecular site with an approximate radius of 1.0 nm containing the initial reagents. In films prepared from chlorinated solvents, the degradation of poly(methyl methacrylate) accompanied by the formation of end macroradicals and dehydrochlorination are important side processes. These processes are mainly responsible for significant long-term photochemical posteffect. The absence of marked magnetic effects indicates that the molecular dynamics of reacting particles and their local environment, rather than spin dynamics, controls the evolution of radical pairs.

On the Signaling Mechanism and the Absence of Photoreversibility in the AppA BLUF Domain

The flavoprotein AppA from Rhodobacter sphaeroides contains an N-terminal, FAD-binding BLUF photoreceptor domain. Upon illumination, the AppA BLUF domain forms a signaling state that is characterized by red-shifted absorbance by 10nm, a state known as AppARED. We have applied ultrafast spectroscopy on the photoaccumulated AppARED state to investigate the photoreversible properties of the AppA BLUF domain. On light absorption by AppARED, the FAD singlet excited state FADRED∗ decays monoexponentially in 7ps to form the neutral semiquinone radical FADH•, which subsequently decays to the original AppARED molecular ground state in 60ps. Thus, FADRED∗ is deactivated rapidly via electron and proton transfer, probably from the conserved tyrosine Tyr-21 to FAD, followed by radical-pair recombination. We conclude that, in contrast to many other photoreceptors, the AppA BLUF domain is not photoreversible and does not enter alternative reaction pathways upon absorption of a second photon. To explain these properties, we propose that a molecular configuration is formed upon excitation of AppARED that corresponds to a forward reaction intermediate previously identified for the dark-state BLUF photoreaction. Upon excitation of AppARED, the BLUF domain therefore enters its forward reaction coordinate, readily re-forming the AppARED ground state and suppressing reverse or side reactions. The monoexponential decay of FAD* indicates that the FAD-binding pocket in AppARED is significantly more rigid than in dark-state AppA. Steady-state fluorescence experiments on wild-type, W104F, and W64F mutant BLUF domains show tryptophan fluorescence maxima that correspond with a buried conformation of Trp-104 in dark and light states. We conclude that Trp-104 does not become exposed to solvent during the BLUF photocycle.

The lack of LHCII proteins modulates excitation energy partitioning and PSII charge recombination in Chlorina F2 mutant of barley

Analysis of the partitioning of absorbed light energy within PSII into fractions utilized by PSII photochemistry (ØPSII), thermally dissipated via ΔpH-and zeaxanthin-dependent energy quenching (ØNPQ) and constitutive non-photochemical energy losses (ØNO) was performed in wild type and F2 mutant of barley. The estimated energy partitioning of absorbed light to various pathways indicated that the fraction of ØPSII was slightly higher, while the proportion of thermally dissipated energy through ØNPQ was 38% lower in F2 mutant than in WT. In contrast, ØNO, i.e. the fraction of absorbed light energy dissipated by additional quenching mechanism(s) was 34% higher in F2 mutant. The increased proportion of ØNO correlated with narrowing the temperature gap (ΔT M) between S2/3QB- and S2QA- charge recombinations in F2 mutant as revealed by thermoluminescence measurements. We suggest that this would result in increased probability for an alternative non-radiative P680+QA- radical pair recombination pathway for energy dissipation within the reaction centre of PSII (reaction center quenching) and that this additional quenching mechanism might play an important role in photoprotection when the capacity for the primary, zeaxanthin-dependent non-photochemical quenching (ØNPQ) and state transitions pathways are restricted in the absence of LHCII polypeptides in F2 mutant.

Activation Parameters for the Recombination Reaction of Intramolecular Radical Pairs Generated from the Radical Diffusion-Inhibited HABI Derivative

Activation parameters were determined for the recombination of radical pairs arising from newly designed, photochromic, radical diffusion-restricted hexaarylbiimidazole (HABI) derivative. We have developed a new type of radical diffusion-inhibited HABI derivative, which contains two equivalent HABI units and yields a tetraradical with four equivalent 2,4,5-triphenylimidazolyl radical (TPIR) units by photoirradiation. This radical dimerization proceeds by a successive first-order reaction from the tetraradical to the parent molecule via a diradical. The rate constants of each reaction were determined from the decay profile of EPR signal intensities. The entropies of activation (DeltaS(double dagger)) for the first and the successive dimerization steps were estimated to be -178.5 and -205.5 J K(-1) mol(-1), respectively. Within the experimental temperature range, the radical dimerizations are entropy-controlled (-TDeltaS(double dagger) > DeltaH(double dagger)). The large negative DeltaS(double dagger) values imply a highly ordered transition state, indicating that the radical dimerizations occur when the TPIR units interact at a specific orientation. The present study demonstrates the availability of radical diffusion-inhibited HABI for the kinetic study of radical-radical reaction.

Spin-vibrational interactions in geminate radical pairs: Effective spin Hamiltonian including anharmonicity

An equation for the effective spin Hamiltonian including the interaction of radical pair spins with vibrations in both linear and higher approximations was obtained. Even in the linear approximation, the Hamiltonian was found to be non-Heisenberg. The influence of a sound field on the probability of geminate radical pair recombination is considered in the Appendix. The results obtained can be of use for solving certain nanochemistry problems.

Biradicals by triplet recombination of radical ion pairs

Radical ion pairs generated by photoinduced electron transfer may undergo return electron transfer (RET) in pairs of singlet or triplet multiplicity. RET efficiencies are determined by the free energy of RET and the topologies of the potential surfaces of parent molecule, radical ion and triplet state. If radical ion geometries are different from the corresponding triplet states, RET occurs either with cleavage ("dissociative" RET; 1,2-diphenylcyclopropane radical cations) or formation of C-C bonds ("associative" RET; norbornadiene radical cation). Radical ions of some strained ring compounds spontaneously undergo ring-opening; RET to such species form ring-opened triplets without major geometry changes. CIDNP spectroscopy offers unique insights into triplet RET.