Spin dynamics of the radical pair in a low magnetic field studied by the transient absorption detected magnetic field effect on the reaction yield and switched external magnetic field

The spin dynamics of the radical pair generated from the photocleavage reaction of (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (TMDPO) in micellar solutions was studied by the time-resolved magnetic field effect (MFE) on the transient absorption (TA) and by a novel technique, absorption detected switched external magnetic field (AD-SEMF). Thanks to the large hyperfine coupling constant (A = 38 mT), a characteristic negative MFE on the radical yield was observed at a magnetic field lower than 60 mT whereas a positive effect due to the conventional hyperfine (HFM) and relaxation mechanisms (RM) was observed at higher magnetic field. The negative effect can be assigned to the mechanism "so-called" low field effect (LFE) mechanism and has been analyzed thoroughly using a model calculation incorporating a fast spin dephasing process. The time scale of the spin mixing process of LFE studied by AD-SEMF is shorter than the lifetime of the recombination kinetics of the radical pair. These results indicate that the LFE originates from the coherent spin motion. This can be interfered from the fast spin dephasing caused by electron spin interaction fluctuations.     

Real-time observation of the spin-state mixing process of a micellized radical pair in weak magnetic fields by nanosecond fast field switching

The singlet-triplet spin-state mixing process of a singlet-born radical pair confined in a sodium dodecyl sulfate (SDS) micelle was studied by observing the nanosecond switched external magnetic field (SEMF) effect on the transient absorption signals. A long-lived singlet radical pair is generated by the photoinduced bond cleavage reaction of tetraphenylhydrazine in an SDS micelle. Application of off-on type SEMF results in the increase of the free radical yield contrary to the decrease produced by an applied static magnetic field. The S-T mixing process in low magnetic field was observed by means of a delay-shift SEMF experiment. Observed incoherent mixing processes are explained in terms of the interplay between coherent hyperfine interaction and fast dephasing processes caused by the fluctuation of electron-spin interactions. Singlet-triplet and triplet-triplet dephasing rate constants are determined independently to be 2 x 10(8) and 0.2 x 10(8) s(-)1, respectively, by a simulation based on a modified single-site Liouville equation. This is the first direct observation of the incoherent spin-state mixing process at magnetic fields comparable to the hyperfine interactions of the radical pair.     

Electron self-exchange kinetics determined by MARY spectroscopy: theory and experiment

The electron self-exchange between a neutral molecule and its charged radical, which is part of a spin-correlated radical ion pair, gives rise to line width effects in the fluorescence-detected MARY (magnetic field effect on reaction yield) spectrum similar to those observed in EPR spectroscopy. An increasing self-exchange rate (i.e., a higher concentration of the neutral molecule) leads to broadening and subsequent narrowing of the spectrum. Along with a series of MARY spectra recorded for several systems (the fluorophores pyrene, pyrene-d(10) and N-methylcarbazole in combination with 1,2- and 1,4-dicyanobenzene) in various solvents, a theoretical model is developed that describes the spin evolution and the diffusive recombination of the radical pair under the influence of the external magnetic field and electron self-exchange, thereby allowing the simulation of MARY spectra of the systems investigated experimentally. The spin evolution of the radicals in the pair is calculated separately using spin correlation tensors, thereby allowing rigorous quantum mechanical calculations for real spin systems. It is shown that the combination of these simulations with high resolution, low noise experimental spectra makes the MARY technique a novel, quantitative method for the determination of self-exchange rate constants. In comparison to a simple analytical formula which estimates the self-exchange rate constant from the slope of the linear part of a line width vs concentration plot, the simulation method yields more reliable and accurate results. The correctness of the results obtained by the MARY method is proved by a comparison with corresponding data from the well-established EPR line broadening technique. With its less stringent restrictions on radical lifetime and stability, the MARY technique provides an alternative to the classical EPR method, in particular for systems involving short-lived and unstable radicals.     

Direct observation of f-pair magnetic field effects and time-dependence of radical pair composition using rapidly switched magnetic fields and time-resolved infrared methods

A rapidly switched (<10 ns) magnetic field was employed to directly observe magnetic fields from f-pair reactions of radical pairs in homogeneous solution. Geminate radical pairs from the photoabstraction reaction of benzophenone from cyclohexanol were observed directly using a pump-probe pulsed magnetic field method to determine their existence time. No magnetic field effects from geminate pairs were observed at times greater than 100 ns after initial photoexcitation. By measuring magnetic field effects for fields applied continuously only after this initial geminate period, f-pair effects could be directly observed. Measurement of the time-dependence of the field effect for the photolysis of 2-hydroxy-4-(2-hydroxyethoxy)-2-methylpropiophenone in cyclohexanol using time-resolved infrared spectroscopy revealed not only the presence of f-pair magnetic field effects but also the ability of the time dependence of the MARY spectra to observe the changing composition of the randomly encountering pairs throughout the second order reaction period.     

Nanosecond time-resolved magnetic field effect on radical recombination in solution

The time dependence of the magnetic field effect on radical recombination in solution has been analyzed experimentally and theoretically. For the geminate recombination of anthracene anions and dimethylaniline cations in a polar solvent, the effect originates from a magnetic field dependent production of triplet states in an initially singlet phased radical pair, induced by hyperfine interaction of the unpaired electrons with the nuclei. The magnetic field dependence of the triplet yield shows a lifetime broadening of the energy levels of the radical pair if a short delay-time between radical production and triplet observation is chosen. The agreement of this delay-time dependent broadening effect with the theoretical results proves directly the coherence of the spin motion in the radical pairs.     

Electron hopping and magnetic field dependent spin dynamics of radical ions in solution

The role of hopping on the geminate recombination of radical ions (N,N-dimethylaniline cation and anthracene anion) in acetonitrile is studied via the nanosecond time-resolved magnetic field effect on the triplet yield and the influence of donor concentration thereon. Increasing donor concentration leads to lifetime broadening of the magnetic field dependence of the triplet yield. Responsible for this effect is the perturbation of the coherent spin motion caused by hopping of the electron spin between donor sites of different nuclear spin configuration. Comparison of experimental results with calculations based on the semiclassical theory of spin motion yields an estimate of the hopping rates. Deuteration of both radicals influences the halfwidth of the magnetic field effect: at long probing times and low donor concentrations the halfwidth measured for protonated radical ions exceeds the one for the deuterated species: at short delay times and large donor concentrations, i.e. high hopping rates, this isotopic effect is reversed.     

Dynamics of intramolecular electron transfer reaction of FAD studied by magnetic field effects on transient absorption spectra

The kinetics of intermediates generated from intramolecular electron-transfer reaction by photo irradiation of the flavin adenine dinucleotide (FAD) molecule was studied by a magnetic field effect (MFE) on transient absorption (TA) spectra. Existence time of MFE and MFE action spectra have a strong dependence on the pH of solutions. The MFE action spectra have indicated the existence of interconversion between the radical pair and the cation form of the triplet excited state of flavin part. All rate constants of the triplet and the radical pair were determined by analysis of the MFE action spectra and decay kinetics of TA. The obtained values for the interconversion indicate that the formation of cation radical promotes the back electron-transfer reaction to the triplet excited state. Further, rate constants of spin relaxation and recombination have been studied by the time profiles of MFE at various pH. The drastic change of those two factors has been obtained and can be explained by SOC (spin-orbit coupling) induced back electron-transfer promoted by the formation of a stacking conformation at pH > 2.5.     

Electron spin density distribution in the special pair triplet of Rhodobacter sphaeroides R26 revealed by magnetic field dependence of the solid-state photo-CIDNP effect

Photo-CIDNP (photochemically induced dynamic nuclear polarization) can be observed in frozen and quinone-blocked photosynthetic reaction centers (RCs) as modification of magic-angle spinning (MAS) NMR signal intensity under illumination. Studying the carotenoidless mutant strain R26 of Rhodobacter sphaeroides, we demonstrate by experiment and theory that contributions to the nuclear spin polarization from the three-spin mixing and differential decay mechanism can be separated from polarization generated by the radical pair mechanism, which is partially maintained due to differential relaxation (DR) in the singlet and triplet branch. At a magnetic field of 1.4 T, the latter contribution leads to dramatic signal enhancement of about 80,000 and dominates over the two other mechanisms. The DR mechanism encodes information on the spin density distribution in the donor triplet state. Relative peak intensities in the photo-CIDNP spectra provide a critical test for triplet spin densities computed for different model chemistries and conformations. The unpaired electrons are distributed almost evenly over the two moieties of the special pair of bacteriochlorophylls, with only slight excess in the L branch.     

Photochemical primary process of photo-Fries rearrangement reaction of 1-naphthyl acetate as studied by MFE probe

Photo-Fries rearrangement reactions of 1-naphthyl acetate (NA) in n-hexane and in cyclohexane were studied by the magnetic field effect probe (MFE probe) under magnetic fields (B) of 0 to 7 T. Transient absorptions of the 1-naphthoxyl radical, T-T absorption of NA, and a short-lifetime intermediate (τ = 24 ns) were observed by a nanosecond laser flash photolysis technique. In n-hexane, the yield of escaped 1-naphthoxyl radicals dropped dramatically upon application of a 3 mT field, but then the yield increased with increasing B for 3 mT < B≤ 7 T. These observed MFEs can be explained by the hyperfine coupling and the Δg mechanisms through the singlet radical pair. The fact that MFEs were observed for the present photo-Fries rearrangement reaction indicates the presence of a singlet radical pair intermediate with a lifetime as long as several tens of nanoseconds.     

Optically detected ESR and low magnetic field signals from spin triads: 2-imidazoline-1-oxyl derivatives in X-irradiated alkane liquids as a method to study three-spin systems

This contribution reports the design and synthesis of a series of spin-labeled charge acceptors to produce three-spin systems of "radical ion/biradical ion" type in X-irradiated alkane liquids. This opens the way to study spin triads in experimental conditions, in which short-lived radical ion pairs are conventionally studied, thus offering optically detected techniques such as magneto-resonance OD ESR and level-crossing MARY spectroscopy. The structure of the synthesized 2-imidazoline-1-oxyl derivatives is A-Sp-R, where A is a positive or negative charge acceptor, R is a stable radical, and Sp is a hydrocarbon bridge. The set of 20+ compounds represent a convenient tool to construct experimental three-spin systems with various properties, e.g. with the "third" spin introduced into one or the other partner of the radical ion pair. The degree of exchange coupling between the two paramagnetic fragments in the biradical ion has been demonstrated to strongly depend on the type of the radical fragment R and the structure of the bridge Sp. As a result, a series of acceptors with systematically reduced exchange interaction has been synthesized, and optimal systems for the observation of low magnetic field effect have been found. In the most favorable case, an OD ESR signal from a spin triad living as short as ca. 100 ns has been registered as a single unresolved line. The exchange integral for this biradical anion (9) was estimated from OD ESR and ESR experiments to be ca. 10(3) G by the order of magnitude, which is much greater than the hyperfine couplings in the biradical ion but much smaller than the thermal energy kT.     

Radio frequency magnetic field effects on a radical recombination reaction: a diagnostic test for the radical pair mechanism

The photoinduced electron-transfer reaction of chrysene with isomers of dicyanobenzene is used to demonstrate the sensitivity of a radical recombination reaction to the orientation and frequency (5-50 MHz) of a approximately 300 muT radio frequency magnetic field in the presence of a 0-4 mT static magnetic field. The recombination yield is detected via the fluorescence of the exciplex formed exclusively from the electronic singlet state of the radical ion pair Chr*+/DCB*-. Magnetic field effects are simulated using a modified version of the gamma-COMPUTE algorithm, devised for the simulation of magic angle spinning NMR spectra of powdered samples. The response of a chemical or biological system to simultaneously applied radio frequency and static or extremely low-frequency magnetic fields could form the basis for a diagnostic test for the operation of the radical pair mechanism that would not require prior knowledge of the nature and properties of the radical reaction.     

Two-step laser excitation fluorescence study of the magnetic field effect on the hydrogen abstraction reaction of anthraquinone in SDS micellar solution

The magnetic field effect on the hydrogen abstraction reaction of anthraquinone in sodium dodecyl sulfate (SDS) micellar solution has been studied by two-step laser excitation fluorescence including a variable delay technique. The risetime of the “cage product” is 380 ns at 0 G and 890 ns at 800 G, while its yield decreases by 20% at 800 G. The results are interpreted in terms of the radical pair model of CIDNP.     

Determination of radical re-encounter probability distributions from magnetic field effects on reaction yields

Measurements are reported of the effects of 0-23 mT applied magnetic fields on the spin-selective recombination of Py*- and DMA*+ radicals formed in the photochemical reaction of pyrene and N,N-dimethylaniline. Singlet <--> triplet interconversion in [Py*- DMA*+] radical pairs is probed by investigating combinations of fully protonated and fully deuterated reaction partners. Qualitatively, the experimental B1/2 values for the four isotopomeric radical pairs agree with predictions based on the Weller equation using known hyperfine coupling constants. The amplitude of the "low field effect" (LFE) correlates well with the ratio of effective hyperfine couplings, aDMA/aPy. An efficient method is introduced for calculating the spin evolution of [Py*- DMA*+] radical pairs containing a total of 18 spin-1/2 and spin-1 magnetic nuclei. Quantitative analysis of the magnetic field effects to obtain the radical re-encounter probability distribution f (t )-a highly ill-posed and underdetermined problem-is achieved by means of Tikhonov and maximum entropy regularization methods. The resulting f (t ) functions are very similar for the four isotopomeric radical pairs and have significant amplitude between 2 and 10 ns after the creation of the geminate radical pair. This interval reflects the time scale of re-encounters that are crucial for generating the magnetic field effect. Computer simulations of generalized radical pairs containing six spin-1/2 nuclei show that Weller's equation holds approximately only when the radical pair recombination rate is comparable to the two effective hyperfine couplings and that a substantial LFE requires, but is not guaranteed by, the condition that the two effective hyperfine couplings differ by more than a factor of 5. In contrast, for very slow recombination, essentially any radical pair should show a significant LFE.     

Modulation of radical ion pair lifetimes by the presence of a third spin in rodlike donor-acceptor triads

It is well known that the molecular structure of an electron donor-acceptor system can be changed to optimize the electronic coupling between photogenerated radical ion pairs (PRPs), resulting in favorable charge separation (CS) and charge recombination (CR) rates. It would be far more convenient to avoid extensive synthetic modifications to the structure to achieve the same ends by perturbing the electronic properties of the PRP. We present here results on PRPs within rodlike donor-acceptor molecules having a covalently attached stable 2,2,6,6-tetramethylpiperidinoxyl radical (T*). The distances and orientations between all three radicals are highly restricted by the intervening molecular structure, making it possible to directly measure both the CR dynamics and the spin-spin exchange interaction, 2JPRP, between the radicals within the PRPs. The molecular triads studied are MeOAn-6ANI-PI-T* and MeOAn-6ANI-NI-T*, where MeOAn = p-methoxyaniline, 6ANI = 4-(N-piperidinyl)naphthalene-1,8-dicarboximide, NI = naphthalene-1,8:4,5-bis(dicarboximide), and PI = pyromellitimide. These molecules have been characterized using femtosecond and nanosecond transient absorption spectroscopy as well as measurements of 2JPRP using magnetic field effects on the triplet state yield resulting from CR. We find that T* enhances radical pair intersystem crossing (EISC), resulting in an increase or decrease in the PRP lifetime depending on the relative ordering of the energy levels of the PRP and the local neutral triplet states. This is especially pronounced when the PRP is nearly isoenergetic with the neutral triplet state, as is the case for MeOAn-6ANI-NI-T*. The dependence of the 3*NI and 3*6ANI yield on an applied external magnetic field shows a distinct resonance at 2JPRP, the magnitude of which is not perturbed by the presence of the third spin. The sensitivity of this system to changes in spin state may offer ways to externally control the radical ion pair dynamics using pulsed microwaves.     

Theory of magnetic field modulation of radical recombination reactions. II. Short time behavior

The change of spin multiplicity in a radical pair, due to hyperfine interaction and depending on an external magnetic field, is treated by time-dependent perturbation theory. Analytic expressions, valid at short times, but at arbitrary field strengths, are derived which apply to radicals with any given hyperfine structure. The short time region deserves special interest, since here isotope effects in radical reactions, induced by differences in the nuclear magnetic moments rather than in masses, are shown to be much stronger than at longer times.     

Theory of magnetic field modulation of radical recombination reactions. III. Time dependent solution

Weak magnetic fields can have an influence on radical combination reactions if a change of the spin multiplicity of radical pairs is induced by the hyperfine interaction. A theory of such effects is given which allows the direct calculation of the reaction rates and product yields observable in time resolved experiments as functions of the external magnetic field and of the time.     

The Origin of Magnetic Field Dependent Recombination in Alkylcobalamin Radical Pairs

Magnetic field effect studies of alkylcobalamin photolysis provide evidence for the formation of a reactive radical pair that is born in the singlet spin state. The radical pair recombination process that is responsible for the magnetic field dependence of the continuous-wave (CW) quantum yield is limited to the diffusive radical pair. Although the geminate radical pair of adenosylcob(III)alamin also undergoes magnetic field dependent recombination (A. M. Chagovetz and C. B. Grissom, J. Am. Chem. Soc. 115, 12152–12157, 1993), this process does not account for the magnetic field dependence of the CW quantum yield that is only observed in viscous solvents. Glycerol and ethylene glycol increase the microviscosity of the solution and thereby increase the lifetime of the spin-correlated diffusive radical pair. This enables magnetic field dependent recombination among spin-correlated diffusive radical pairs in the solvent cage. Magnetic field dependent recombination is not observed in the presence of nonviscosigenic alcohols such as isopropanol, thereby indicating the importance of the increased microviscosity of the medium. Paramagnetic radical scavengers that trap alkyl radicals that escape the solvent cage do not diminish the magnetic field effect on the CW quantum yield, thereby ruling out radical pair recombination among randomly diffusing radical pairs, as well as excluding the involvement of solvent-derived radicals. Magnetic field dependent recombination among alkylcobalamin radical pairs has been simulated by a semiclassical model of radical pair dynamics and recombination. These calculations support the existence of a singlet radical pair precursor.     

Magnetic field effects on the triplet exciplex dynamics in the duroquinone-N,N-dimethylaniline derivative systems

Magnetic field effects (MFEs) on the radical yield in the photoinduced electron transfer reaction from the p-halogen derivatives (4XDMA) of N,N-dimethylaniline to the excited triplet state of duroquinone (DQ) have been investigated in alcoholic solutions at room temperature. In 1-propanol and 1-butanol solutions, the radical yields decreased as the magnetic field increased and became nearly constant at 1-1.8 T in the DQ-4BrDMA and DQ-4IDMA systems, suggesting that the spin-orbit coupling interaction due to the heavy atoms governs the radical yield. On the other hand, in the methanol solution MFE due to a radical pair mechanism was observed. We concluded that the key intermediate to determine the radical yield is the triplet exciplex or contact radical ion pair in the 1-propanol and 1-butanol solutions, while it is the solvent-separated radical ion pair in the methanol solution.     

Revisiting magnetic field effects in homogeneous medium and bio-mimicking environments with emphasis on acridine derivatives

A weak external magnetic field, very close to the hyperfine interactions of the system, can acts as a tool to monitor spin dynamics and assess distance between the components of the spin-correlated transient radical pair or radical ion pair (RIP). The present review focuses on the magnetic field effect (MFE) on the photo-induced electron-transfer (PET) reactions among acridine derivatives and classical as well as biological electron acceptor or donor moieties, which produce spin-correlated RIPs, in homogeneous solvents, heterogeneous micellar media and in biological nanocavities of proteins. Although a confined medium is preferred to observe prominent MFE, yet unanticipated MFE on PET between acridine derivatives [Acridone (AD) and Acridine Yellow (AY)] and classical electron donors is obtained even in homogeneous medium when it consists of impurities like water molecules. In a comparative study of interaction of another acridine derivative, Proflavin (PF⁺) with two electron donors which are amines of aromatic nature, MFE on PET reveal that the bulk and the structure of the electron donor govern the mechanism as well as the spin dynamics of PET. While studying interaction of PF⁺ with a different amine which is aliphatic in nature, MFE on PET implies that it is the nature of the solvent matrix which determines the spin dynamics of PET. The cause of discrepancy in the experimental and calculated values of B_1/2 for 9-amino acridine – methyl viologen system has been delineated. Apart from micellar medium, prominent MFE on PET is also observed while studying the interaction of PF⁺, AY and AD with tryptophan residues present in the nanocavities of serum albumins since the inter-radical distance within primary geminate RIP is enough to make exchange interaction negligible.     

Spin relaxation of a short-lived radical in zero magnetic field

A short-lived radical containing only one I = 1/2 nucleus, the muoniated 1,2-dicarboxyvinyl radical dianion, was produced in an aqueous solution by the reaction of muonium with the dicarboxyacetylene dianion. The identity of the radical was confirmed by measuring the muon hyperfine coupling constant (hfcc) by transverse field muon spin rotation spectroscopy and comparing this value with the hfcc obtained from DFT calculations. The muon spin relaxation rate of this radical was measured as a function of temperature in zero magnetic field by the zero field muon spin relaxation technique. The results have been interpreted using the theoretical model of Fedin et al. (J. Chem. Phys., 2003, 118, 192). The muon spin polarization decreases exponentially with time after muon implantation and the temperature dependence of the spin relaxation rate indicates that the dominant relaxation mechanism is the modulation of the anisotropic hyperfine interaction due to molecular rotation. The effective radius of the radical in solution was determined to be 1.12 ± 0.04 nm from the dependence of the muon spin relaxation rate on the temperature and viscosity of the solution, and is approximately 3.6 times larger than the value obtained from DFT calculations.