Geminate Radical Pair Research Articles

Photocleavage reaction of bromine substituted aromatic acyl compounds studied by CIDEP and transient absorption spectroscopy

The photocleavage of the CBr bond in bromoacetylnaphthalene is investigated by transient absorption and time resolved EPR spectroscopy. In the transient absorption of 2-bromo-2′-acetylnaphthalene, the absorption band observed at λmax ~440 nm is assigned to the triplet state of the parent molecule. After decay of the triplet absorption, a long lived absorption band is observed at λmax ~380 nm, which is assigned to naphthoylmethyl radical. The yield of this radical is not dependent on the concentration of oxygen even though the absorption band of the triplet state was quenched by addition of oxygen. Thus we conclude that the spin multiplicity of the precursor molecule is singlet. The CW time resolved EPR spectrum shows a typical E*/A CIDEP pattern of three hyperfine lines of the naphthoylmethyl radical. This result suggests some contribution from triplet precursor molecules. However, a careful analysis of the time profile of the CIDEP intensity observed by FT-EPR revealed that the polarization is generated from the radical pair mechanism (RPM) from the encountered pair of two free naphthoylmethyl radicals and the radical-triplet pair mechanism. RPM polarization by the geminate radical pair, formed by the Br atom and the naphthoylmethyl radical, is not observed. This fact indicates that large spin-orbit coupling (Δg and/or fast spin relaxation by g anisotropy) spoils the RPM polarization. The finding is in contrast to the recent observation of RPM polarization in the Cl cleavage reaction of 1-(chloromethyl)naphthalene.

Relaxation induced radical pair recombination in micelles

We have derived accurate, analytic expressions for the quantum yield and the decay constants of a geminate radical pair reaction in a micelle. The relative diffusive motion of the radicals, intraradical relaxation (both transverse and longitudinal), and homogeneous scavenging are included. The general expression is exact for the assumed diffusion model, but we have also introduced two approximation schemes, called slow and fast mixing, which cover the complete parameter range. The high accuracy of these approximations is demonstrated by comparison with numerically exact results. A simple expression, derived for the limiting case of diffusion controlled recombination through the singlet channel, is compared with the previously introduced semi-phenomenological exponential, two-positional, and supercage models as well as with approximations based on a level population description. This comparison gives insight into the applicability of the approximate models and the physical meaning of the model parameters. The work is based on the general Green's function results derived in our previous paper [Chem. Phys. 260 (2000) 125].

Excited Donor–Acceptor Complexes in the Polymerization of Vinyl Monomers

Butyl methacrylate was found to affect the composition of radical intermediates formed in the photoreduction of benzophenone with triethylamine. In the presence of the monomer, the yield of free radicals decreased and the yield of complexes of the geminate radical pair increased. This was explained by the formation of excited ternary complexes resulted from the interaction of the excited triplet state of benzophenone with the ground-state complex of butyl methacrylate and triethylamine. The substituent effect in benzophenone on the stability of the radical complex was studied. The reaction rate constant for the decay of the radical complex was correlated with the Hammett σ0c constant that determines the mesomeric effect of the substituent.

Specific features of geminate radical pair recombination in high magnetic field

The specific features of diffusion-assisted geminate radical pair (RP) recombination in high magnetic field is analyzed in detail. It is shown that the dependence of the recombination yield R on the intraradical magnetic interaction Ā (hyperfine interaction, etc.) significantly depends on the parameter ξ∼(Ā/Dα2), in which D is the diffusion coefficient and α−1 is the characteristic length of the spin exchange interaction. Simple formula, accurately describing the dependence R(Ā) over a wide region of parameters of the system, is proposed and thoroughly discussed.

Modeling of Diffusion-Reaction Processes Involving Geminate Radical Pairs

A Markovian theory is developed to study the efficiency of diffusion-reaction processes involving geminate radical pairs moving on the surface of a catalyst (e.g., zeolite) support. Results obtained for the simplest (lattice) system are corroborated by Monte Carlo calculations, and these are extended to study the effect of system size. A kinetic model is developed to account for the results obtained and leads to the general conclusion that the photochemical generation of a triplet pair, followed by the subsequent diffusion and eventual recombination of the radicals R↑ and R↓, lengthens the mean reaction time by a factor of ∼2, relative to the case in which R↑ and R↓ are assumed to be present at the very outset.

Time-Resolved Spectroscopic Studies of B12Coenzymes: Influence of Solvent on the Photolysis of Adenosylcobalamin

Femtosecond-to-nanosecond transient absorption spectroscopy is used to investigate the photolysis of coenzyme B12, 5‘-deoxyadenosylcobalamin, as a function of solvent environment comparing water, ethylene glycol, and mixtures of water and ethylene glycol. Photolysis in ethylene glycol is characterized by the clean formation of a cob(II)alamin species on a time scale ≤ 28 ps. Competition between cage escape and geminate recombination of the initial radical pair leads to a nanosecond photolysis quantum yield of ca. 8%. This is in contrast to the photolysis of adenosylcobalamin in water, where an additional intermediate state is identified, and the net quantum yield for photolysis is three times higher. The additional intermediate observed in aqueous solution may correspond to a base-off alkylcobalamin or to a cob(II)alamin-like state having an enhanced rate for ground-state recovery. The competition between cage escape and geminate recombination for adenosyl and cob(II)alamin radical pairs is investigated by using mixtures of ethylene glycol and water to vary the viscosity systematically, and thereby influence the rate for escape from the initial solvent cage. The intrinsic rate constant for geminate recombination is found to be kR = 1.39 ± 0.06 ns-1, independent of the solvent system. The effective recombination rate is solvent dependent, reflecting competition between recombination (kR), solvent-dependent cage escape (kE = 0.46 ± 0.07 cp ns-1/η, where η is the solvent viscosity), and the formation of a caged radical pair species incapable of direct recombination (kIA = 0.13 ± 0.06 ns-1). The most likely explanation for the inactive caged radical pair is the interconversion between singlet and triplet geminate radical pairs.

Photoinduced hydride transfer reaction between methylene blue and leuco crystal violet

The photoinduced hydride transfer reaction between methylene blue (MB+) and leuco crystal violet (CVH) was investigated spectrophotometrically under conditions of direct excitation of MB+ with steady-illumination of visible light and of photosensitization by benzophenone (BP) and α-nitronaphthalene (NNP) with UV light. It was found that upon the irradiation of the solution of MB+ and CVH with visible light, MB+ disappeared and crystal violet (CV+) formed simultaneously. The ratio of the CV+ concentration formed to that of MB+ consumed was unity. The photosensitized reactions by NNP gave similar results. From the findings that the fluorescence of MB+ was not quenched by CVH and the triplet photosensitizations were observed, it was concluded that the hydride transfer reaction in acetonitrile occurs in the triplet state of MB+. The quantum yield for the disappearance of MB+ in acetonitrile was estimated to be 1.1×10−4 and it was independent of CVH concentration. The electron transfer from CVH to triplet methylene blue (3MB+∗) and the back electron transfer within the geminate radical pair must occur effectively. A spin adduct of a radical formed during the reaction was detected by the ESR method with 2-methyl-2-nitrosopropane (MNP) as a radical trap. This radical was attributed to CV. The results can be explained by a mechanism with stepwise electron–proton–electron transfers. In tert-butyl alcohol, however, it was observed that the fluorescence of MB+ was quenched by CVH and the quantum yield of the disappearance of MB+ depended on CVH concentration. Different mechanism from that in acetonitrile was proposed for the reaction in tert-butyl alcohol.

Excited State Dynamics of Methyl Viologen. Ultrafast Photoreduction in Methanol and Fluorescence in Acetonitrile

The photophysical and photochemical deactivation pathways of electronically excited methyl viologen (1,1‘-dimethyl-4,4‘-bipyridinium, MV2+) were studied in several polar solvents at room temperature using a variety of ultrafast time-resolved and steady-state spectroscopic techniques. The results highlight the very strong electron accepting character of the lowest singlet excited state of MV2+. Transient absorption was measured between 270 and 740 nm as a function of delay time after excitation of the strong π−π* transition of MV2+ by a 150 fs, 265 nm pump pulse. In methanol, the radical cation of methyl viologen (MV•+) appeared within our time resolution, indicating that forward electron transfer from a nearby donor quenches electronically excited MV2+ in < 180 fs. Identical dynamics within experimental uncertainty were observed for the chloride salt of MV2+ and for the salt prepared with tetrafluoroborate counterions. This latter “superhalide” ion has a condensed-phase detachment threshold that is too high to permit oxidation by the excited state of MV2+. Thus, electron transfer does not take place within an associated MV2+-counterion complex in methanol but results instead from oxidation of a solvent molecule. Photoreduction of MV2+ in methanol is a novel example of ultrafast electron-transfer quenching of a photoexcited acceptor in an electron-donor solvent. This is the first demonstration that a hydrogen-bonding solvent can serve as the electron donor in an ultrafast intermolecular ET reaction. Decay of the initial MV•+ population and simultaneous recovery of ground-state MV2+ with a characteristic time constant of 430 ± 40 fs were observed immediately after the pump pulse and assigned to back electron transfer in the geminate radical pair. Despite the high rate of back electron transfer, a significant fraction of the initial radical pairs avoid recombination, and a finite yield (∼12%) of MV•+ ions is observed at delay times > 2 ps. There was no evidence of photoreduction when the solvent was acetonitrile or water. Both of these solvents have high gas-phase ionization potentials that prevent oxidation by excited MV2+. The transient absorption signals indicate, however, that very different excited-state decay channels exist in these two solvents. In aqueous solution, an unknown nonradiative decay process causes decay of excited MV2+ with a time constant of 3.1 ps in H2O and 5.3 ps in D2O. In acetonitrile, on the other hand, the transient absorption decays hundreds of times slower and fluorescence is observed. This is the first report of an efficient radiative decay pathway for MV2+ in fluid solution. The excited-state absorption spectrum (S1→SN spectrum) of MV2+ was measured in acetonitrile and the fluorescence was characterized by time-correlated single-photon counting and steady-state measurements. The fluorescence quantum yield is 0.03 ± 0.01 and the lifetime in acetonitrile at room temperature is 1.00 ± 0.04 ns. The fluorescence is efficiently quenched by electron transfer from added quenchers with gas-phase ionization potentials lower than about 10.8 eV. Using the measured emission spectrum, the excited-state reduction potential is determined to be E° (MV2+*/MV•+) = 3.65 V, confirming the highly oxidizing character of this photoexcited dication.

Molecular dynamics simulation of the geminate radical recombination in solution: effects of the large dipole moment of the radicals

Molecular dynamics simulation of a simple model system of the geminate radical pair in solution has been performed to elucidate the dynamic behavior of radicals. The diffusion process of radicals in the microscopic region was simulated and the effect of the dipole of the radicals was investigated in both the nonpolar and polar solvents. We found that the dipole-dipole interaction stabilized the radical pair with a small separation. The dipole pair can be the precursor of the experimentally observed sandwich radical dimer. The conformation of the dipole pair may not be favorable for the recombination of the radicals, which can be the reason of the high escape probability observed for the p-aminophenylthiyl radical.

Control over the Recombination Channel in Geminate Radical Pairs with a Large HFC Constant

Control over the Recombination Channel in Geminate Radical Pairs with a Large HFC Constant

A comparative study on electron-transfer fluorescence quenching by aliphatic and aromatic amines

The electron-transfer (ET) fluorescence quenching of aromatic compounds by aliphatic amines (DABCO, ABCO, and triethylamine) was studied in acetonitrile. The fluorescence quenching rate constant k q suddenly decreases when the free energy change (Δ G fet ) of full ET in fluorescence quenching increases from −0.6 to −0.4 eV, indicating that the downhill region shifts by ca. −0.5 eV compared with the Rehm–Weller plot, where both fluorescer and quencher are aromatic. The fluorescence quenching distance r q increases with decreasing Δ G fet and the rate k bet of back ET within geminate radical pairs shows a bell-shaped dependence on the free energy change (Δ G bet) of back ET. Both free energy dependence of r q and k bet are the same as those of aromatic electron donor and acceptor system.

Recombination yield of geminate radical pairs in high magnetic fields: general results and application to free diffusion

We have derived a general and exact expression for the quantum yield of geminate radical pairs (RPs) in high magnetic fields. We consider the dynamical S–T 0 mixing caused by differences in Zeeman and hyperfine interactions, intraradical ( T 1 and T 2) relaxation, and a first-order scavenging process. The recombination process and the exchange interaction are assumed to be isotropic and of the contact type. In order to solve the stochastic Liouville equation, we introduce effective methods to minimize the dimensionality of the problem and to calculate the matrix elements of the superoperators in an arbitrary coupled basis. Our general expression is expressed in terms of the classical Green's function for the relative motion of the radicals, and it is therefore valid for any kind of relative motion. Various limiting cases of the general expression, which are of particular interest, are derived. Explicit analytical formulas for freely diffusing RPs are given, and the high accuracy of the expressions is illustrated by comparison with accurate numerical results.

Heavy cation effect on intersystem crossing between triplet and singlet phenylacyl and benzyl geminate radical pairs within zeolites

The difference in product selectivity observed between dibenzyl ketones and naphthyl esters in zeolites is due to the difference in spin of the radical pairs formed from these precursors. Heavy cations present in zeolites can enhance intersystem crossing between triplet and singlet geminate radical pairs.

Cage-Escape of Geminate Radical Pairs Can Produce Peroxynitrate from Peroxynitrite under a Wide Variety of Experimental Conditions1

The spontaneous and CO2-catalyzed decomposition of peroxynitrite yields HO• and -CO3• radicals, respectively, together with •NO2. The geminate HO•/•NO2 and -CO3•/•NO2 pairs undergo competitive in-cage collapse to nitrate and diffusive separation. Free HO• and -CO3• radicals react with H2O2 and, in the presence of O2, suitable alcohols or formate to produce superoxide, which is trapped by the •NO2 to form peroxynitrate. The formation of peroxynitrate may influence the rate of change in optical density at 302 nm, the wavelength normally employed to monitor peroxynitrite decay, leading to misleading kinetic traces. Tetranitromethane (TNM) was used as a colorimetric probe for superoxide to quantify the yield of free HO• (27−28%) and free -CO3• (32−33%). The yields of both of these free radicals are in excellent agreement with other recent estimates. Superoxide was also detected in some oxygenated aldehyde-catalyzed peroxynitrite decompositions both by peroxynitrate formation and by its reaction with TNM. Superoxide yields, measured with TNM, were aldehyde (RCHO) dependent (R = -O2CC6H4, CH3, CH3CH2, (CH3)3C and HOCH2CHOH; yields were 15, 9, 0.8, 0, and 30%, respectively).

Effects of Molecular Charge on Photoinduced Electron-Transfer Reactions

Fluorescence quenching due to full and partial electron transfer (ET) and back ET within geminate radical pairs are studied (i) in acetonitrile using N-methylacridinium ion (MeAc+) as a monocationic fluorescer and aromatic compounds as neutral quenchers and (ii) in methanol using aromatic compounds as neutral fluorescers and methyl and benzyl viologens as dicationic quenchers. By comparison with the results previously obtained for the system of neutral fluorescer and quencher it is demonstrated that the free energy dependence of the rates of fluorescence quenching and back ET is not affected by molecular charge, but the rate of geminate radical separation into free radicals is. In the system of aromatic fluorescer and quencher, in general, fluorescence quenching is induced by a full ET at long distance (i.e., a long-distance ET) when the free energy change ΔGfet in full ET is more negative than −0.5 eV and by a partial ET at contact distance (i.e., an exciplex formation) when ΔGfet is more positive than −...

Effects of weak magnetic fields on free radical recombination reactions

The radical pair mechanism is used to elucidate how applied magnetic fields that are weaker in strength than typical hyperfine interactions can influence the yields and kinetics of recombination reactions of free radicals in solution. The so-called low field effect is shown to arise from coherent superpositions of degenerate electron-nuclear spin states in a spin-correlated radical pair in zero field. A weak applied magnetic field causes these (zero-quantum) coherences to oscillate, leading to coherent interconversion of singlet and triplet electronic states of the radical pair and hence changes in the yields of recombination products and of the free radicals that escape into solution. For singlet geminate radical pairs, the low field effect leads to a boost in the concentration of free radicals, which may be relevant in the context of in vivo biological effects of electromagnetic fields. Using analytical approaches in limiting cases, the maximum possible low field effects are calculated for a variety of radical pairs. Sizeable changes in reaction yields (∽20%) are found for almost any radical pair provided the spin-correlation persists long enough for significant evolution of the electron spins under the influence of the weak applied field. The conditions necessary for observing effects as large as ∽20% are discussed.

Time-Resolved Spectroscopic Studies of B12 Coenzymes: The Photolysis and Geminate Recombination of Adenosylcobalamin

Femtosecond transient absorption spectroscopy has been used to investigate the photolysis of coenzyme B12 (5‘-deoxyadenosylcobalamin). Transient kinetic measurements obtained at wavelengths between 400 and 633 nm were analyzed globally to obtain time constants. Transient spectral data obtained for time delays between 5 ps and 9 ns were analyzed by matrix decomposition to identify distinct spectral components present in the data. Photoexcitation results in homolysis of the carbon−cobalt bond forming a singlet radical pair on a picosecond time scale. The subsequent spectral changes probe conformational relaxation and geminate recombination. Analysis of the spectral data suggests that 76 ± 4% of the geminate radical pairs recombine, resulting in a quantum yield of 0.24 ± 0.04 for the formation of solvent separated radicals, in good agreement with literature values of 0.20 ± 0.03 and 0.23 ± 0.04 [Chen, E.; Chance, M. R. Biochemistry 1993, 32, 1480−1487]. The geminate recombination of the adenosyl radical with...

A novel representation in the CIDEP theory

We propose a new approach for desribing the CIDEP spectra of geminate radical pairs. It allows one to formulate a simple vector model of the CIDEP effect formation, to establish an exact relationship between electron polarization kinetics and microwave field power absorption kinetics, to separate the CIDEP spectrum structure and to introduce a rigid definition of antiphase structure.

Electron spin echo detection of the microwave-induced recombination of transient radical pairs produced in photochemical reactions

The effects of the geminate recombination induced by the pulsed microwave irradiation on the electron paramagnetic resonance spectra of radicals generated in the photoinitiated hydrogen abstraction reaction of 2-methyl-1,4-naphthoquinone in a micellar sodium dodecylsulphate solution have been studied using an inversion recovery method of pulsed electron paramagnetic resonance. It was found that the stimulated nuclear polarization resulting from the microwave-induced recombination of the geminate radical pairs at one (or a limited group) of nuclear spin configurations affects the polarization and intensity of the electron paramagnetic resonance spectra of escaped radicals via the electron-nuclear cross-relaxation mechanism. The long lifetimes of the radical pairs in a micellar system facilitate the observation of the effects. Similar effects, though of much smaller scale, were found to be detectable in certain conditions also in homogeneous solutions.

Supramolecular control of photochemical enantiomeric induction and radical pair recombination in zeolites

Geminate radical pairs are formed from the α-photocleavage of aryl ketones within the supercage of zeolites which has been modified with chiral guest molecules. Herein, we report the supramolecular structure and dynamic control of both enantnantiomeric selectivity and probability of the recombination of the radical pairs.