Hydrogen-bonding dynamics in photoinduced electron transfer in a ferrocene-quinone linked dyad with a rigid amide spacer

A newly designed ferrocene-quinone dyad with an amide space (Fc-Q) is employed to examine formation of the hydrogen bonding in the one-electron reduced form (Q*-) and the dynamics in the photoinduced electron-transfer reaction from the ferrocene to the quinone moiety. Photoexcitation of the Q moiety in Fc-Q in deaerated PhCN with 388 nm results in intramolecular electron transfer from Fc to the singlet excited state of Q to produce Fc+-Q*- without changing the conformation (<1 ps), followed by hydrogen bond formation with the amide proton of the spacer (tau = approximately 5 ps). The resulting radical ion pair decays via a back electron transfer to the ground state at a longer time scale with a rate constant of 2.6 x 108 s-1.     

Metal ion-promoted intramolecular electron transfer in a ferrocene-naphthoquinone linked dyad. Continuous change in driving force and reorganization energy with metal ion concentration

Thermal intramolecular electron transfer from the ferrocene (Fc) to naphthoquinone (NQ) moiety occurs efficiently by the addition of metal triflates (M(n)()(+): Sc(OTf)(3), Y(OTf)(3), Eu(OTf)(3)) to an acetonitrile solution of a ferrocene-naphthoquinone (Fc-NQ) linked dyad with a flexible methylene and an amide spacer, although no electron transfer takes place in the absence of M(n)()(+). The resulting semiquinone radical anion (NQ(*)(-)) is stabilized by the strong binding of M(n)()(+) with one carbonyl oxygen of NQ(*)(-)( )()as well as hydrogen bonding between the amide proton and the other carbonyl oxygen of NQ(*)(-). The high stability of the Fc(+)()-NQ(*)(-)/M(n)()(+)() complex allows us to determine the driving force of electron transfer by the conventional electrochemical method. The one-electron reduction potential of the NQ moiety of Fc-NQ is shifted to a positive direction with increasing concentration of M(n)()(+), obeying the Nernst equation, whereas the one-electron oxidation potential of the Fc moiety remains the same. The driving force dependence of the observed rate constant (k(ET)) of M(n)()(+)-promoted intramolecular electron transfer is well evaluated in light of the Marcus theory of electron transfer. The driving force of electron transfer increases with increasing concentration of M(n)()(+) [M(n)()(+)], whereas the reorganization energy of electron transfer decreases with increasing [M(n)()(+)] from a large value which results from the strong binding between NQ(*)(-) and M(n)()(+).     

Thermochromism of metal ion complexes of semiquinone radical anions. Control of equilibria between diamagnetic and paramagnetic species by Lewis acids

Metal ion complexes of semiquinone radical anions exhibit different types of thermochromism depending on metal ions and quinones. Metal ion complexes of 1,10-phenanthroline-5,6-dione radical anion (PTQ(.-)) produced by the electron-transfer reduction of PTQ by 1,1'-dimethylferrocene (Me(2)Fc) in the presence of metal ions (Mg(2+) and Sc(3+)) exhibit the color change depending on temperature, accompanied by the concomitant change in the ESR signal intensity. In the case of Mg(2+), electron transfer from Me(2)Fc to PTQ is in equilibrium, when the concentration of the PTQ(.-)-Mg(2+) complex (lambda(max) = 486 nm) increases with increasing temperature because of the positive enthalpy for the electron-transfer equilibrium. In contrast to the case of Mg(2+), electron transfer from Me(2)Fc to PTQ is complete in the presence of Sc(3+), which is a much stronger Lewis acid than Mg(2+), to produce the PTQ(.-)-Sc(3+) complex (lambda(max) = 631 nm). This complex is in disproportionation equilibrium and the concentration of the PTQ(.-)-Sc(3+) complex increases with decreasing temperature because of the negative enthalpy for the proportionation direction, resulting in the remarkable color change in the visible region. On the other hand, the p-benzosemiquinone radical anion (Q(.-)) forms a 2:2 pi-dimer radical anion complex [Q(.-)-(Sc(3+))(2)-Q] with Q and Sc(3+) ions at 298 K (yellow color), which is converted to a 2:3 pi-dimer radical anion complex [Q(.-)-(Sc(3+))(3)-Q] with a strong absorption band at lambda(max) = 604 nm (blue color) when the temperature is lowered to 203 K. The change in the number of binding Sc(3+) ions depending on temperature also results in the remarkable color change, associated with the change in the ESR spectra.     

Supramolecular electron transfer by anion binding

Anion binding has emerged as an attractive strategy to construct supramolecular electron donor-acceptor complexes. In recent years, the level of sophistication in the design of these systems has advanced to the point where it is possible to create ensembles that mimic key aspects of the photoinduced electron-transfer events operative in the photosynthetic reaction centre. Although anion binding is a reversible process, kinetic studies on anion binding and dissociation processes, as well as photoinduced electron-transfer and back electron-transfer reactions in supramolecular electron donor-acceptor complexes formed by anion binding, have revealed that photoinduced electron transfer and back electron transfer occur at time scales much faster than those associated with anion binding and dissociation. This difference in rates ensures that the linkage between electron donor and acceptor moieties is maintained over the course of most forward and back electron-transfer processes. A particular example of this principle is illustrated by electron-transfer ensembles based on tetrathiafulvalene calix[4]pyrroles (TTF-C4Ps). In these ensembles, the TTF-C4Ps act as donors, transferring electrons to various electron acceptors after anion binding. Competition with non-redox active substrates is also observed. Anion binding to the pyrrole amine groups of an oxoporphyrinogen unit within various supramolecular complexes formed with fullerenes also results in acceleration of the photoinduced electron-transfer process but deceleration of the back electron transfer; again, this is ascribed to favourable structural and electronic changes. Anion binding also plays a role in stabilizing supramolecular complexes between sulphonated tetraphenylporphyrin anions ([MTPPS](4-): M = H(2) and Zn) and a lithium ion encapsulated C(60) (Li(+)@C(60)); the resulting ensemble produces long-lived charge-separated states upon photoexcitation of the porphyrins.     

Neutral histidine and photoinduced electron transfer in DNA photolyases

The two major UV-induced DNA lesions, the cyclobutane pyrimidine dimers (CPD) and (6-4) pyrimidine-pyrimidone photoproducts, can be repaired by the light-activated enzymes CPD and (6-4) photolyases, respectively. It is a long-standing question how the two classes of photolyases with alike molecular structure are capable of reversing the two chemically different DNA photoproducts. In both photolyases the repair reaction is initiated by photoinduced electron transfer from the hydroquinone-anion part of the flavin adenine dinucleotide (FADH(-)) cofactor to the photoproduct. Here, the state-of-the-art XMCQDPT2-CASSCF approach was employed to compute the excitation spectra of the respective active site models. It is found that protonation of His365 in the presence of the hydroquinone-anion electron donor causes spontaneous, as opposed to photoinduced, coupled proton and electron transfer to the (6-4) photoproduct. The resulting neutralized biradical, containing the neutral semiquinone and the N3'-protonated (6-4) photoproduct neutral radical, corresponds to the lowest energy electronic ground-state minimum. The high electron affinity of the N3'-protonated (6-4) photoproduct underlines this finding. Thus, it is anticipated that the (6-4) photoproduct repair is assisted by His365 in its neutral form, which is in contrast to the repair mechanisms proposed in the literature. The repair via hydroxyl group transfer assisted by neutral His365 is considered. The repair involves the 5'base radical anion of the (6-4) photoproduct which in terms of electronic structure is similar to the CPD radical anion. A unified model of the CPD and (6-4) photoproduct repair is proposed.     

Influence of donor-acceptor distance variation on photoinduced electron and proton transfer in rhenium(i)-phenol dyads

A homologous series of four molecules in which a phenol unit is linked covalently to a rhenium(I) tricarbonyl diimine photooxidant via a variable number of p-xylene spacers (n = 0-3) was synthesized and investigated. The species with a single p-xylene spacer was structurally characterized to get some benchmark distances. Photoexcitation of the metal complex in the shortest dyad (n = 0) triggers release of the phenolic proton to the acetonitrile/water solvent mixture; a H/D kinetic isotope effect (KIE) of 2.0 ± 0.4 is associated with this process. Thus, the shortest dyad basically acts like a photoacid. The next two longer dyads (n = 1, 2) exhibit intramolecular photoinduced phenol-to-rhenium electron transfer in the rate-determining excited-state deactivation step, and there is no significant KIE in this case. For the dyad with n = 1, transient absorption spectroscopy provided evidence for release of the phenolic proton to the solvent upon oxidation of the phenol by intramolecular photoinduced electron transfer. Subsequent thermal charge recombination is associated with a H/D KIE of 3.6 ± 0.4 and therefore is likely to involve proton motion in the rate-determining reaction step. Thus, some of the longer dyads (n = 1, 2) exhibit photoinduced proton-coupled electron transfer (PCET), albeit in a stepwise (electron transfer followed by proton transfer) rather than concerted manner. Our study demonstrates that electronically strongly coupled donor-acceptor systems may exhibit significantly different photoinduced PCET chemistry than electronically weakly coupled donor-bridge-acceptor molecules.     

Study of photoinduced electron transfer between [60]fullerene and proton-sponge by laser flash photolysis: addition effects of organic acid

Photoinduced electron-transfer processes between fullerene (C60) and 1,8-bis(dimethylamino)naphthalene, which is called a proton-sponge (PS), have been investigated by means of laser flash photolysis in the presence and absence of CF3CO2H. For a mixture of C60 and PS, the transient absorption spectra showed the rise of the C60 radical anion with concomitant decay of the C60 triplet (3C60), suggesting that photoinduced intermolecular electron transfer occurs via 3C60 in high efficiency in polar solvent. For a covalently bonded C60-PS dyad, photoinduced intramolecular charge-separation process takes place via the excited singlet state of the C60 moiety, although charge recombination occurs within 10 ns. For both systems, electron-transfer rates were largely decelerated by addition of a small amount of CF3CO2H, leaving the long-lived 3C60. These observations indicate that the energy levels for charge-separated states of the protonated PS and C60 become higher than the energy level of the 3C60 moiety, showing low donor ability of the protonated PS. Thus, intermolecular electron-transfer process via 3C60 for C60-PS mixture and intramolecular charge-separation process via 1C60-PS for C60-PS dyad were successfully controlled by the combination of the light irradiation with a small amount of acid.     

Intermolecular electron transfer from excited benzophenone ketyl radical

The electron transfer from the benzophenone ketyl radical in the excited state (BPH(.-)(D(1))) to several quenchers (Qs) was investigated using nanosecond/picosecond two-color two-laser flash photolysis and nanosecond/nanosecond two-color two-laser flash photolysis. The electron transfer from BPH(.-)(D(1)) to Qs was confirmed by the transient absorption and fluorescence quenching measurements. The intermolecular electron-transfer rate constants were determined using the Stern-Volmer analysis. The driving force dependence of the electron-transfer rate was revealed.     

Stepwise charge separation and charge recombination in ferrocene-meso,meso-linked porphyrin dimer-fullerene triad

A meso,meso-linked porphyrin dimer [(ZnP)(2)] as a light-harvesting chromophore has been incorporated into a photosynthetic multistep electron-transfer model for the first time, including ferrocene (Fc), as an electron donor and fullerene (C(60)) as an electron acceptor to construct the ferrocene-meso,meso-linked porphyrin dimer-fullerene system (Fc-(ZnP)(2)-C(60)). Photoirradiation of Fc-(ZnP)(2)-C(60) results in photoinduced electron transfer from the singlet excited state of the porphyrin dimer [(1)(ZnP)(2)] to the C(60) moiety to produce the porphyrin dimer radical cation-C(60) radical anion pair, Fc-(ZnP)(2)(*+)-C(60)(*-). In competition with the back electron transfer from C(60)(*-) to (ZnP)(2)(*+) to the ground state, an electron transfer from Fc to (ZnP)(2)(*+) occurs to give the final charge-separated (CS) state, that is, Fc(+)-(ZnP)(2)-C(60)(*-), which is detected as the transient absorption spectra by the laser flash photolysis. The quantum yield of formation of the final CS state is determined as 0.80 in benzonitrile. The final CS state decays obeying first-order kinetics with a lifetime of 19 micros in benzonitrile at 295 K. The activation energy for the charge recombination (CR) process is determined as 0.15 eV in benzonitrile, which is much larger than the value expected from the direct CR process to the ground state. This value is rather comparable to the energy difference between the initial CS state (Fc-(ZnP)(2)(*+)-C(60)(*-)) and the final CS state (Fc(+)-(ZnP)(2)-C(60)(*-)). This indicates that the back electron transfer to the ground state occurs via the reversed stepwise processes,that is, a rate-limiting electron transfer from (ZnP)(2) to Fc(+) to give the initial CS state (Fc-(ZnP)(2)(*+)-C(60)(*-)), followed by a fast electron transfer from C(60)(*-) to (ZnP)(2)(*+) to regenerate the ground state, Fc-(ZnP)(2)-C(60). This is in sharp contrast with the extremely slow direct CR process of bacteriochlorophyll dimer radical cation-quinone radical anion pair in bacterial reaction centers.     

Metal ion-coupled and decoupled electron transfer

Effects of metal ions on thermal and photoinduced electron-transfer reactions from electron donors (D) to electron acceptors (A) are reviewed in terms of metal ion-coupled electron transfer (MCET) vs. metal ion-decoupled electron transfer (MDET). When electron transfer from D to A is coupled with binding of metal ions to A^?, such an electron transfer is defined as MCET in which metal ions accelerate the rates of electron transfer. A number of examples of electron-transfer reactions from D to A, which are energetically impossible to occur, are made possible by strong binding of metal ions to A^? in MCET. The structures of metal ion complexes with A^? are also discussed in relation with the MCET reactivity. The MCET reactivity of metal ions is shown to be enhanced with an increase in the Lewis acidity of metal ions. In contrast to MCET, strong binding of metal ions to A^? results in deceleration of back electron transfer from metal ion complexes of A^? to D⁺ in the radical ion pair, which is produced by photoinduced electron transfer from D to A in the presence of metal ions, as compared with back electron transfer without metal ions. The deceleration of back electron transfer in the presence of metal ions results from no binding of metal ions to A. This type of electron transfer is defined as metal ion-decoupled electron transfer (MDET). The lifetimes of CS state (D⁺–A^?) produced by photoinduced electron transfer from D to A in the D–A linked systems are also elongated by adding metal ions to the D–A systems because of the stabilization of the CS states by strong binding of metal ions to A^? and the resulting slow MDET processes.     

New substituted tetrathiafulvalene-quinone dyads: the influences of electron accepting abilities of quinone units on the metal ion-promoted electron-transfer processes

The metal ion-promoted electron transfer occurs to all new dyads 1, 2, 3, and 4, even one of them, dyad 4, which has a rather weak electron acceptor unit. The results also indicate that the metal ion-promoted electron transfer within the dyads is influenced by the electron accepting abilities of quinone units; dyad 2 with the strongest electron acceptor among the four dyads shows the strongest absorption and ESR signals attributed to TTF.+ in the presence of metal ions.     

Simulating electron transfer attachment to a positively charged model peptide

Ab initio electronic structure methods, including stabilization method tools for handling electronically metastable states, are used to treat a model system designed to probe the electron-transfer event characterizing electron-transfer dissociation (ETD) mass spectroscopic studies of peptides. The model system consists of a cation H(3)C-(C=O)NH-CH(2)-CH(2)-NH(3)(+), containing a protonated amine site and an amide site, that undergoes collisions with a CH(3)(-) anion. Cross-sections for electron transfer from CH(3)(-) to the protonated amine site are shown to exceed those for transfer to the Coulomb-stabilized amide site by 2 orders of magnitude. Moreover, it is shown that the fates of the amine-attached and amide-attached species are similar in that both eventually lead to the same carbon-centered radical species H(3)C-((*)C-OH)NH-CH(2)-CH(2)-NH(2), although the reaction pathways by which the two species produce this radical are somewhat different. The implications for understanding peptide fragmentation patterns under ETD conditions are also discussed in light of this work's findings.     

Long-lived charge-separated state generated in a ferrocene-meso,meso-linked porphyrin trimer-fullerene pentad with a high quantum yield

A meso,meso-linked porphyrin trimer, (ZnP)3, as a light-harvesting chromophore, has been incorporated for the first time into a photosynthetic multistep electron-transfer model including ferrocene (Fc) as an electron donor and fullerene (C60) as an electron acceptor, to construct the ferrocene-meso,meso-linked porphyrin trimer-fullerene system Fc-(ZnP)3-C60. Photoirradiation of Fc-(ZnP)3-C60 results in photoinduced electron transfer from both the singlet and triplet excited states of the porphyrin trimer, 1(ZnP)3* and 3(ZnP)3*, to the C60 moiety to produce the porphyrin trimer radical cation-C60 radical anion pair, Fc-(ZnP)3*+-C60*-. Subsequent formation of the final charge-separated state Fc+-(ZnP)3-C60*- was confirmed by the transient absorption spectra observed by pico- and nanosecond time-resolved laser flash photolysis. The final charge-separated state decays, obeying first-order kinetics, with a long lifetime (0.53 s in DMF at 163 K) that is comparable with that of the natural bacterial photosynthetic reaction center. More importantly, the quantum yield of formation of the final charge-separated state (0.83 in benzonitrile) remains high, despite the large separation distance between the Fc+ and C60*- moieties. Such a high quantum yield results from efficient charge separation through the porphyrin trimer, whereas a slow charge recombination is associated with the localized porphyrin radical cation in the porphyrin trimer. The light-harvesting efficiency in the visible region has also been much improved in Fc-(ZnP)3-C60 because of exciton coupling in the porphyrin trimer as well as an increase in the number of porphyrins.     

Hydrogen bonds not only provide a structural scaffold to assemble donor and acceptor moieties of zinc porphyrin-quinone dyads but also control the photoinduced electron transfer to afford the long-lived charge-separated states

A series of zinc porphyrin-quinone linked dyads [ZnP-CONH-Q, ZnP-NHCO-Q, and ZnP-n-Q (n = 3, 6, 10)] were designed and synthesized to investigate the effects of hydrogen bonds which can not only provide a structural scaffold to assemble donor and acceptor moieties but also control the photoinduced electron-transfer process. In the case of ZnP-CONH-Q and ZnP-NHCO-Q, the hydrogen bond between the N-H proton and the carbonyl oxygen of Q results in the change in the reduction potential of Q. The strong hydrogen bond between the N-H proton and the carbonyl oxygen of Q*- in ZnP-CONH-Q*-,ZnP-NHCO-Q*-, and ZnP-n-Q*- (n = 3, 6, 10) generated by the chemical reduction has been confirmed by the ESR spectra, which exhibit hyperfine coupling constants in agreement those predicted by the density functional calculations. In the case of ZnP-n-Q (n = 3, 6, 10), on the other hand, the hydrogen bond between two amide groups provides a structural scaffold to assemble the donor (ZnP) and the acceptor (Q) moiety together with the hydrogen bond between the N-H proton and the carbonyl oxygen of Q, leading to attainment of the charge-separated state with a long lifetime up to a microsecond.     

Theoretical studies of electron and hydrogen transfer reactions between semiquinone radicals and oxygen

 To explore the interactions between ubiquinones and oxygen in living organisms, the thermodynamics of a series of electron and hydrogen transfer reactions between semiquinone radicals, as well as their corresponding protonated forms, and oxygen, singlet or triplet, were studied using the hybrid Hartree–Fock–density functional theory method Becke's three parameter hybrid method with the Lee, Yang, and Parr correlation functional. Effects of the solvent and of the isoprenyl tail on the electron and hydrogen transfer reactions were also investigated. It is found that semiquinone radicals (semiquinone anion radicals or protonated semiquinone radicals) cannot react with triplet oxygen to form the superoxide anion radical O₂ ⁻. In contrast, neutral quinones can scavenge O₂ ⁻ efficiently. In the gas phase, only protonated semiquinone radicals can react spontaneously with singlet oxygen to produce peroxyl radical (HO₂). However, both semiquinone anion radicals and protonated semiquinone radicals can react with singlet oxygen to produce harmful oxygen radicals (O₂ ^{− a l l b u l l} and HO₂, respectively) in aqueous and protein environments. The free-energy changes of the corresponding reactions obtained for different ubiquinone systems are very similar. It clearly shows that the isoprenyl tail does not influence the electron and hydrogen transfer reactions between semiquinone radicals and oxygen significantly. Results of electron affinities, vertical ionization potentials, and proton affinities also show that the isoprenyl tail has no substantial effect on the electronic properties of ubiquinones. Received: 3 July 2000 / Accepted: 6 September 2000 / Published online: 21 December 2000     

Intramolecular electron transfer within the substituted tetrathiafulvalene-quinone dyads: facilitated by metal ion and photomodulation in the presence of spiropyran

Intramolecular electron transfer is observed for two new substituted tetrathiafulvalene (TTF)-quinone dyads 1 and 2 in the presence of metal ions. On the basis of the electrochemical studies of reference compound 5 and the comparative studies with dyad 3, it was proposed that the synergic coordination of the radical anion of quinone and the oligoethylene glycol chain with metal ions may be responsible for stabilizing the charge-separation state and thus facilitating the electron-transfer process. Most interestingly, the intramolecular electron-transfer processes within these two dyads can be modulated by UV-vis light irradiation in the presence of spiropyran, by taking advantage of its unique properties.     

Elongation of lifetime of the charge-separated state of ferrocene-naphthalenediimide-[60]fullerene triad via stepwise electron transfer

Photoinduced electron-transfer processes of a newly synthesized rodlike covalently linked ferrocene-naphthalenediimide-[60]fullerene (Fc-NDI-C(60)) triad in which Fc is an electron donor and NDI and C(60) are electron acceptors with similar first one-electron reduction potentials have been studied in benzonitrile. In the examined Fc-NDI-C(60) triad, NDI with high molar absorptivity is considered to be the chromophore unit for photoexcitation. Although the free-energy calculations verify that photoinduced charge-separation processes via singlet- and triplet-excited states of NDI are feasible, transient absorption spectra observed upon femtosecond laser excitation of NDI at 390 nm revealed fast and efficient electron transfer from Fc to the singlet-excited state of NDI ((1)NDI*) to produce Fc(+)-NDI(?-)-C(60). Interestingly, this initial charge-separated state is followed by a stepwise electron transfer yielding Fc(+)-NDI-C(60)(?-). As a result of this sequential electron-transfer process, the lifetime of the charge-separated state (τ(CS)) is elongated to 935 ps, while Fc(+)-NDI(?-) has a lifetime of only 11 ps.     

Stepwise vs. concerted pathways in scandium ion-coupled electron transfer from superoxide ion to p-benzoquinone derivatives

Superoxide ion (O2˙-) forms a stable 1 : 1 complex with scandium hexamethylphosphoric triamide complex [Sc(HMPA)(3)(3+)], which can be detected in solution by ESR spectroscopy. Electron transfer from O2˙- -Sc(HMPA)(3)(3+) complex to a series of p-benzoquinone derivatives occurs, accompanied by binding of Sc(HMPA)(3)(3+) to the corresponding semiquinone radical anion complex to produce the semiquinone radical anion-Sc(HMPA)(3)(3+) complexes. The 1 : 1 and 1 : 2 complexes between semiquinone radical anions and Sc(HMPA)(3)(3+) depending on the type of semiquinone radical anions were detected by ESR measurements. This is defined as Sc(HMPA)(3)(3+)-coupled electron transfer. There are two reaction pathways in the Sc(HMPA)(3)(3+)-coupled electron transfer. One is a stepwise pathway in which the binding of Sc(HMPA)(3)(3+) to semiquinone radical anions occurs after the electron transfer, when the rate of electron transfer remains constant with the change in concentration of Sc(HMPA)(3)(3+). The other is a concerted pathway in which electron transfer and the binding of Sc(HMPA)(3)(3+) occurs in a concerted manner, when the rates of electron transfer exhibit first-order and second-order dependence on the concentration of Sc(HMPA)(3)(3+) depending the number of Sc(HMPA)(3)(3+) (one and two) bound to semiquinone radical anions. The contribution of two pathways changes depending on the substituents on p-benzoquinone derivatives. The present study provides the first example to clarify the kinetics and mechanism of metal ion-coupled electron-transfer reactions of the superoxide ion.     

Remarkable effects of counter ions on scandium ion-promoted electron transfer reactions

Scandium ion-promoted electron transfer reactions of p-benzoquinone are remarkably accelerated when tetrakis(pentafluorophenyl)borate anion is used instead of trifluoromethanesulfonate anion as the counter anion of scandium ion. Only a catalytic amount of scandium borate salt (Sc[B(C6F5)4]3) accelerates significantly the Diels-Alder reaction of 9,10-dimethylanthracene with p-benzoquinone, which proceeds via Sc(3+)-promoted electron transfer from the anthracene to p-benzoquinone.     

Kinetics of the interaction of ketyl and semiquinone radicals with dioxygen. Concerted electron and proton transfer involving ketyl and semiquinone radicals

Kinetics of the interaction of ketyl and neutral semiquinone radicals with dioxygen was studied by the flash photolysis technique. The reactivity of neutral semiquinone radicals in the transfer of a hydrogen atom to O₂ is lower than that of ketyl radicals and increases as the reduction ability of the radicals increases, which give evidence for the charge transfer from the radicals to O₂ in the transition state of the reaction. The deuterium kinetic isotope effect of the reaction (up to 2.6) suggests considerable weakening of the O−H bond of the seminquinone radical in the transition state. A cyclic structure of the transition state similar to that in the reactions of ketyl radicals with hydrogen atom acceptors is proposed. In aprotic volvents, solvation has essentially no effect on the reactivity of neutral anthrasemiquinone radicals up to solvent nucleophilicityB≈240. In solvents with higher nucleophilicity and in protic solvents, their reactivity drops sharply. Hydrogen atom transfer reactions involving ketyl and neutral semiquinone radicals are shown to involve concerted electron and proton transfers, and to have transition states in which the partial transfer of an electron and a proton from the ketyl or semiquinone radical to an acceptor occurs. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1131–1137, June, 1997.