One-electron Oxidation Potentials Research Articles

Intramolecular Hydrogen Bonding in Bisphenol Developing Agents for Photothermographic Application

The effects of intramolecular hydrogen bonding on the electron transfer properties of a series of bisphenol derivatives, as compared with those of monophenol derivatives in which a hydroxy group of the bisphenols is replaced with a methoxy group, have been investigated in relation to their utility as photothermographic developers. The oxidation of bisphenol derivatives with one-electron oxidants occurs to give the radical cation, followed by deprotonation, to produce the phenoxyl radical. Both the radical cations and phenoxyl radicals have been detected by laser flash photolysis measurements. Rates of hydrogen transfer reactions from a series of bisphenol derivatives to cumylperoxyl radicals have also been determined by monitoring the decay of the ESR signal of cumylperoxyl radical produced by photoirradiation of an oxygen saturated propionitrile solution of cumene and di-t-butylperoxide in the presence of the bisphenol derivatives. Intramolecular hydrogen bonding plays an important role in determining the overall oxidation reactivity in the two-electron oxidation process of bisphenol derivatives by decreasing the one-electron oxidation potentials and also by facilitating the deprotonation step from the bisphenol radical cation to produce a phenoxyl radical.

UV/Vis/NIR spectral properties of triarylamines and their corresponding radical cations

The one-electron oxidation potential of 10 triarylamines 1– 10 with all permutations of chloro-, methoxy- and methyl-substituents in the three para-positions were determined by cyclic voltammetry. The half wave potential E 1/2(I) of the first oxidation wave correlates linearly with the number of chloro- and methoxy-substituents. A high long-term stability of the first oxidation wave for all triarylamines was observed by multi-cycle thin-layer measurements. AM1-CISD derived values of the absorption energies are in good agreement with the experiments but differ strongly for the oscillator strengths as well as for neutral compounds 1– 10 and their corresponding mono radical cations. The small solvent dependence of the experimental UV/Vis spectra in CH 2Cl 2 and MeCN reflects a minor charge transfer (CT) character of the electronic transitions of neutral and cationic compounds.

Theoretical investigation of carbazole derivatives as hole-transporting materials in OLEDs

Carbazole and its derivatives were widely used as the hole-transporting material. In this study, ab initio DFT B3LYP/6-31G* was performed for the carbazole and 1-, 2-, 3-, 4- and 9-mono-substituted as well as 3,6-di-substituted carbazoles with electron-donating and -withdrawing substituents. The total energies, HOMO and LUMO orbital energies of these compounds at neutral, cation and anion states were obtained based on the optimized geometrical structures. The ionization potential ( I p) and electron affinity ( E a) were generated by means of the DFT calculated total energy of neutral, cation and anion states. These theoretical I p and E a were consistent with experimental data. Further investigation on 1-, 2-, 3- and 4-mono-substituted carbazole, we conclude that these carbazole derivatives with electron-donating substituents should decrease their I p and E a values while increasing the HOMO and LUMO energies. On the contrary, the mono-substituted carbazole with electron-withdrawing substituent have the opposite results to those carbazole derivatives with electron-donating substituents. Comparing to carbazole, Δ E decreased in the mono-substituted carbazole derivatives with electron-donating and -withdrawing substituents. The substituent effect in 3,6-di-substituted carbazoles is more obvious than that of 3-substituted carbazole. According to the calculated results, the calculated I p has an excellent agreement with the experimental one-electron oxidation potential ( E p/2). Presumably, this calculation can be employed to predict EL character for other OLED materials.

Deactivation of Triplet-Excited Riboflavin by Purine Derivatives: Important Role of Uric Acid in Light-Induced Oxidation of Milk Sensitized by Riboflavin

The reactivity of purine derivatives (uric acid, xanthine, hypoxanthine, and purine) toward triplet-excited riboflavin in aqueous solution at pH 6.4 is described on the basis of kinetic (laser flash photolysis), electrochemical (square-wave voltammetry), and theoretical data (density functional theory, DFT). Direct deactivation of triplet-excited riboflavin in aqueous solution, pH 6.4 at 24 degrees C, in the presence of uric acid, xanthine, and hypoxanthine strongly suggests a direct electron transfer from the purine to the triplet-excited riboflavin with k = 2.9 x 10(9) M(-1) s(-1) (DeltaH(++) = 14.7 kJ mol(-1), DeltaS(++) = -15.6 J mol(-1) K(-1)), 1.2 x 10(9) M(-1) s(-1) (DeltaH(++) = 34.3 kJ mol(-1), DeltaS(++) = +45.3 J mol(-1) K(-1)), and 1.7 x10(8) M(-1) s(-1) (DeltaH(++) = 122 kJ mol(-1), DeltaS(++) = +319 J mol(-1) K(-1)), respectively. From the respective one-electron oxidation potentials collected in aqueous solution at pH 6.4 for uric acid (E = +0.686 vs normal hydrogen electrode, NHE), xanthine (E = +1.106 vs NHE), and hypoxanthine (E = +1.654 vs NHE), the overall free energy changes for electron transfer from the quencher to the triplet-excited riboflavin are as follows: uric acid (DeltaG(o) = -114 kJ mol(-1)), xanthine (DeltaG(o) = -73.5 kJ mol(-1)), hypoxanthine (DeltaG(o) = -20.6 kJ mol(-1)), and purine (DeltaG(o) > 0). The inertness observed for purine toward triplet-excited riboflavin corroborates with its electrochemical inactivity in the potential range from 0 up to 2 V vs NHE. These data are in agreement with the DFT results, which show that the energy of the purine highest occupied molecular orbital (HOMO) (-0.2685 arbitrary unit) is lower than the energy of the semioccupied molecular orbital (SOMO) (-0.2557 a.u.) of triplet-excited riboflavin, indicating an endergonic process for the electron-transfer process. The rate-determining step for deactivation by purine derivatives can be assigned to an electron transfer from the purine derivative to the SOMO orbital of the triplet-excited riboflavin. The results show that uric acid may compete with oxygen and other antioxidants to deactivate triplet-excited riboflavin in milk serum and other biological fluids leading to a free radical process.

Antioxidants protect the yeast Saccharomyces cerevisiae against hypertonic stress

Yeast (Saccharomyces cerevisiae) mutants lacking CuZnSOD have been reported to be hypersensitive to hypertonic media and to show increased oxidative damage. This study demonstrates that hypertonic medium (containing 0.8 M NaCl) increases the generation of superoxide and other reactive species in yeast cells. Other sequelae of exposure to hypertonic medium include oxidation of cellular low-molecular weight thiols and decrease in total antioxidant capacity of cellular extracts. Δsod1 mutant is more sensitive than a wild-type strain to colony growth inhibition on a hypertonic medium. Anaerobic conditions, ascorbate, glutathione, cysteine and dithiothreitol are able to ameliorate this growth inhibition but a range of other antioxidants does not protect. The protective ability of the antioxidants does not correlate with the rate of their reactions with superoxide but seems to be conditioned by low redox potential for one-electron oxidation of free radicals of the antioxidants. It suggests that repair of low-redox potential targets rather than prevention of their damage by superoxide is important in the antioxidant protection against oxidative stress induced by hypertonic conditions.

Electron-Transfer Oxidation Properties of DNA Bases and DNA Oligomers

Kinetics for the thermal and photoinduced electron-transfer oxidation of a series of DNA bases with various oxidants having the known one-electron reduction potentials (E(red)) in an aqueous solution at 298 K were examined, and the resulting electron-transfer rate constants (k(et)) were evaluated in light of the free energy relationship of electron transfer to determine the one-electron oxidation potentials (E(ox)) of DNA bases and the intrinsic barrier of the electron transfer. Although the E(ox) value of GMP at pH 7 is the lowest (1.07 V vs SCE) among the four DNA bases, the highest E(ox) value (CMP) is only 0.19 V higher than that of GMP. The selective oxidation of GMP in the thermal electron-transfer oxidation of GMP results from a significant decrease in the pH dependent oxidation potential due to the deprotonation of GMP*+. The one-electron reduced species of the photosensitizer produced by photoinduced electron transfer are observed as the transient absorption spectra when the free energy change of electron transfer is negative. The rate constants of electron-transfer oxidation of the guanine moieties in DNA oligomers with Fe(bpy)3(3+) and Ru(bpy)3(3+) were also determined using DNA oligomers containing different guanine (G) sequences from 1 to 10 G. The rate constants of electron-transfer oxidation of the guanine moieties in single- and double-stranded DNA oligomers with Fe(bpy)3(2+) and Ru(bpy)3(3+) are dependent on the number of sequential guanine molecules as well as on pH.

Electron-transfer mechanism in radical-scavenging reactions by a vitamin E model in a protic medium

The scavenging reaction of 2,2-diphenyl-1-picrylhydrazyl radical (DPPH.) or galvinoxyl radical (GO.) by a vitamin E model, 2,2,5,7,8-pentamethylchroman-6-ol (1H), was significantly accelerated by the presence of Mg(ClO4)2 in de-aerated methanol (MeOH). Such an acceleration indicates that the radical-scavenging reaction of 1H in MeOH proceeds via an electron transfer from 1H to the radical, followed by a proton transfer, rather than the one-step hydrogen atom transfer which has been observed in acetonitrile (MeCN). A significant negative shift of the one-electron oxidation potential of 1H in MeOH (0.63 V vs. SCE), due to strong solvation as compared to that in MeCN (0.97 V vs. SCE), may result in change of the radical-scavenging mechanisms between protic and aprotic media.

Catalytic oxidative reactions of organic compounds by nitrogen‐containing copper complexes

AbstractThe dimeric copper(II) complex di‐µ‐chloro‐bis[chloro(di‐3,5‐dimethylpyrazole)copper(II)] (A) in the presence of co‐oxidant hydrogen peroxide acts as a catalyst for the oxidation of benzylic alcohols to give the corresponding aldehydes. In the presence of hydrogen peroxide it also catalyses the oxidation reaction of 2,6‐dimethylphenol to 4,4′‐dihydroxy‐3,5,3′,5′‐tetramethylbiphenyl. The oxidative reactions by bis‐pyridinium tetrachlorocopper(II) (B) in the presence of hydrogen peroxide were compared for similar catalytic reactions of A, and it is observed that B can catalyse the oxidation of aromatic diols, 2,6‐dimethylphenol and thiophenol, but is not suitable for oxidation of benzylic alcohols. Bis‐(N‐phenyl‐3,5‐dimethylpyrazole)copper(II) nitrate monohydrate (C) has a suitable redox potential for one‐electron oxidation. It can oxidize ferrocene to the ferricinium cation, and it can liberate bromine from tetra‐alkylammonium bromides. The complex is catalytically effective for the oxidation of different aromatic and aliphatic aldehydes to the corresponding carboxylic acids. The compound is also effective in transforming benzylic amine to benzylalcohol and benzaldehyde. It can also oxidize diphenylmethane to give benzophenone and diphenylmethanol. It is observed that in each of these complexes a quasi‐reversible Cu(I)–Cu(II) species is present and facilitates the single‐electron oxidation process. Copyright © 2004 John Wiley & Sons, Ltd.

New insights into the mechanisms of the thermal Fenton reactions occurring using different iron(II)-complexes

Although the Fenton reagent (a mixture of hydrogen peroxide and an iron(II) salt) has been known for more than a century, the manifold mechanisms occurring during the thermal Fenton reaction are still under discussion. Indeed, this discussion served as a powerful driving force for the steadily increasing insight into the field of inorganic radical and electron transfer chemistry. In this work, an experimental approach towards the elucidation of the first steps taking place in the reaction between several iron(II)-complexes and hydrogen peroxide (H2O2) in water at pH = 3.0 is presented. 2,4-xylidine (2,4-dimethylaniline) reacts differently with reactive intermediates via the addition or hydrogen abstraction by the hydroxyl radical (HO*) or electron transfer reactions to higher valent iron-species, such as a hydrated ferryl-complex (Fe(IV)). The chemical reactivity of the employed iron(II)-complexes with H2O2 differed strongly depending on their ground-state one-electron oxidation potentials. The results are interpreted in accordance with the paradigm originally developed by Goldstein et al. which is based on the evidence obtained from the Marcus theory that outer-sphere electron transfer reactions between metal complexes are not likely to occur because they are too slow. Therefore, most of the "Fenton-reagents" form transient metal complexes, which can be described as [LnFe-H2O2]m+. They form, depending on the reaction conditions, either the hydroxyl radical or higher-valent iron complex species.

Effects of metal ions on photoinduced electron transfer in zinc porphyrin-naphthalenediimide linked systems.

Zinc porphyrin-naphthalenediimide (ZnP-NIm) dyads and zinc porphyrin-pyromellitdiimide-naphthalenediimide (ZnP-Im-NIm) triad have been employed to examine the effects of metal ions on photoinduced charge-separation (CS) and charge-recombination (CR) processes in the presence of metal ions (scandium triflate (Sc(OTf)(3)) or lutetium triflate (Lu(OTf)(3)), both of which can bind with the radical anion of NIm). Formation of the charge-separated states in the absence and in the presence of Sc(3+) was confirmed by the appearance of absorption bands due to ZnP(.) (+) and NIm(.) (-) in the absence of metal ions and of those due to ZnP(.) (+) and the NIm(.) (-)/Sc(3+) complex in the presence of Sc(3+) in the time-resolved transient absorption spectra of dyads and triad. The lifetimes of the charge-separated states in the presence of 1.0 x 10(-3) M Sc(3+) (14 micros for ZnP-NIm, 8.3 micros for ZnP-Im-NIm) are more than ten times longer than those in the absence of metal ions (1.3 micros for ZnP-NIm, 0.33 micros for ZnP-Im-NIm). In contrast, the rate constants of the CS step determined by the fluorescence lifetime measurements are the same, irrespective of the presence or absence of metal ions. This indicates that photoinduced electron transfer from (1)ZnP(*) to NIm in the presence of Sc(3+) occurs without involvement of the metal ion to produce ZnP(.) (+)-NIm(.) (-), followed by complexation with Sc(3+) to afford the ZnP(.) (+)-NIm(.) (-)/Sc(3+) complex. The one-electron reduction potential (E(red)) of the NIm moiety in the presence of a metal ion is shifted in a positive direction with increasing metal ion concentration, obeying the Nernst equation, whereas the one-electron oxidation potential of the ZnP moiety remains the same. The driving force dependence of the observed rate constants (k(ET)) of CS and CR processes in the absence and in the presence of metal ions is well evaluated in terms of the Marcus theory of electron transfer. In the presence of metal ions, the driving force of the CS process is the same as that in the absence of metal ions, whereas the driving force of the CR process decreases with increasing metal ion concentration. The reorganization energy of the CR process also decreases with increasing metal ion concentration, when the CR rate constant becomes independent of the metal ion concentration.

Driving Force Dependence of Photoinduced Electron Transfer Dynamics of Intercalated Molecules in DNA

A series of acridinium, quinolinium, and phenanthridinium ions (9-substituted-10-methylacridinium (AcrR+, R = H, Pri, and CH2Ph), 3-substituted-1-methylquinolinium (RQuH+, R = CN and Br), and 5-methylphenanthridinium (5-MePhen+) perchlorate salts) are shown to be intercalated into the DNA double helix from calf thymus. The one-electron reduction potentials (E0red) of these intercalators have been determined in the absence and presence of DNA by both cyclic voltammetry and second harmonic ac voltammetry. The E0red values of intercalators are shifted in a positive direction by intercalation into the DNA double helix. The one-electron oxidation potential (E0ox) of ethidium bromide, which is known to be intercalated into DNA, is also shifted in a positive direction by the intercalation. The wide range of E0red values of intercalators thus determined in the presence of DNA allows us to examine the exact driving force dependence of the rates of photoinduced electron transfer from the singlet excited state of et...

Oxidation mechanism of phenols by dicopper-dioxygen (Cu(2)/O(2)) complexes.

The first systematic studies on the oxidation of neutral phenols (ArOH) by the mu-eta(2):eta(2)-peroxo)dicopper(II) complex (A) and the bis(mu-oxo)dicopper(III) complex (B) supported by the 2-(2-pyridyl)ethylamine tridentate and didentate ligands L(Py2) and L(Py1), respectively, have been carried out in order to get insight into the phenolic O-H bond activation mechanism by metal-oxo species. In both cases (A and B), the C-C coupling dimer was obtained as a solely isolable product in approximately 50% yield base on the dicopper-dioxygen (Cu(2)/O(2)) complexes, suggesting that both A and B act as electron-transfer oxidants for the phenol oxidation. The rate-dependence in the oxidation of phenols by the Cu(2)/O(2) complexes on the one-electron oxidation potentials of the phenol substrates as well as the kinetic deuterium isotope effects obtained using ArOD have indicated that the reaction involves a proton-coupled electron transfer (PCET) mechanism. The reactivity of phenols for net hydrogen atom transfer reactions to cumylperoxyl radical (C) has also been investigated to demonstrate that the rate-dependence of the reaction on the one-electron oxidation potentials of the phenols is significantly smaller than that of the reaction with the Cu(2)O(2) complexes, indicative of the direct hydrogen atom transfer mechanism (HAT). Thus, the results unambiguously confirmed that the oxidation of phenols by the Cu(2)O(2) complex proceeds via the PCET mechanism rather than the HAT mechanism involved in the cumylperoxyl radical system. The reactivity difference between A and B has also been discussed by taking account of the existed fast equilibrium between A and B.

Diels−Alder Reactions of Anthracenes with Dienophiles via Photoinduced Electron Transfer

Photochemical Diels−Alder reaction of anthracene and its derivatives with dienophiles (p-benzoquinone and fumaronitrile) occurs in competition with the dimerization of anthracenes in chloroform at 298 K. The dependence of the quantum yields on the concentrations of dienophiles has revealed that the photochemical Diels−Alder reaction proceeds via electron transfer from the singlet excited states of anthracenes to dienophiles. The rates of photoinduced electron transfer are diffusion limited, agreeing with the largely negative free energy change of electron-transfer judging from the more negative one-electron oxidation potentials of the singlet excited states of anthracenes than the one-electron reduction potentials of dienophiles. The radical ion pair produced in the photoinduced Diels−Alder from the singlet excited states of anthracenes to p-benzoquinone and fumaronitrile has been detected as the transient absorption spectrum at 298 K with use of laser flash photolysis. The diradical intermediates prior t...

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)()(+).

Pivotal role of electrophilicity in glutathione S-transferase induction by tert-butylhydroquinone.

Although the induction of glutathione S-transferase (GST) activity by tert-butylhydroquinone (tBHQ) has been well-documented in several cell culture systems and rodent experiments, the exact mechanism responsible for its inducibility is still not thoroughly understood. To more precisely define the molecular mechanism of GST induction by tBHQ, we examined the one-electron oxidation and glutathione (GSH) reaction potentials of tBHQ as compared to its analogue, 2,5-di-tert-butylhydroquinone (DtBHQ). tBHQ and DtBHQ showed similar one-electron oxidation potentials, including free radical quenching (antioxidant), oxidative conversion of both compounds to a benzoquinone form, and Cu(2+)-dependent superoxide generation. On the other hand, the reduced GSH level was observed by the addition of tBHQ, but not DtBHQ, suggesting that tBHQ acts as an electrophile while DtBHQ does not. The data were consistent with the observation that tBHQ more potently induced the GSTP1 gene expression in RL34 cells than DtBHQ did. Moreover, we indeed detected the GSH-tBHQ conjugates in the cells exposed to tBHQ using an electrochemical detector-high-performance liquid chromatography technique. Thus, we conclude that an electrophilic quinone oxidation product that reacts with intracellular nucleophiles including protein thiol or GSH plays a major role in the GSTP1 gene expression.

Driving force dependence of intermolecular electron-transfer reactions of fullerenes.

Pulse-radiolytic studies were performed to determine the rate constants of intermolecular electron transfer (k(et)) from fullerenes (C(60), C(76), and C(78)) to a series of arene radical cations in dichloromethane. The one-electron oxidation potentials of the employed arenes-corresponding to the one-electron reduction potentials of arene pi-radical cations-were determined in dichloromethane to evaluate the driving forces of electron-transfer oxidation of fullerenes with arene pi-radical cations. The driving force dependence of log k(et) shows a pronounced decrease towards the highly exothermic region, representing the first definitive confirmation of the existence of the Marcus inverted region in a truly intermolecular electron transfer. Electron-transfer reduction of fullerenes with anthracene radical anion was also examined by laser flash photolysis in benzonitrile. The anthracene radical anion was produced by photoinduced electron transfer from 10,10'-dimethyl-9,9',10,10'-tetrahydro-9,9'-biacridine [(AcrH)(2)] to the singlet excited state of anthracene in benzonitrile. The rate constants of electron transfer (k(et)) from anthracene radical anion to C(60), C(70), and a C(60) derivative were determined from the decay of anthracene radical anion in the presence of various concentrations of the fullerene. Importantly, a significant decrease in the k(et) value was observed at large driving forces (1.50 eV) as compared to the diffusion-limited value seen at smaller driving forces (0.96 eV). In conclusion, our study presents clear evidence for the Marcus inverted region in both the electron-transfer reduction and oxidation of fullerenes.

Electron Transfer Properties of Bisphenol Derivatives in Relation to Their Developing Properties in Silver Salt Photothermographic Systems

Electron transfer properties of a series of bisphenol derivatives were examined in relation to their developing properties in silver salt photothermography. A t-butyl-substituted bisphenol derivative exhibited the highest developing reactivity among bisphenol derivatives investigated in this study. The substituent effects of bisphenol derivatives on the one-electron oxidation potentials and the rates of electron transfer with one-electron oxidants have revealed that the deprotonation step from the bisphenol radical cation, to produce a phenoxyl radical, plays an important role in determining the overall oxidation reactivity in the two-electron oxidation process. The ESR spectra of phenoxyl radicals derived from bisphenol derivatives indicate formation of an intramolecular hydrogen bond between oxygen and OH group of phenoxyl radicals. The reactivity of the bisphenol derivatives during the oxidation process is controlled by the hydrogen bond formation. The high oxidation reactivity of the t-butyl-substituted bisphenol derivative is ascribed to its low ionization potential and low deprotonation energy, as compared with the other alkyl-substituted bisphenol derivatives.

Radiation chemical studies on thermosensitive N-isopropylacrylamide and its polymer in aqueous solutions.

The pulse radiolysis technique has been employed to determine the initiation and propagation rates of different transient species involved in the polymerization of N-isopropylacrylamide (NIPA) in aqueous solutions. Polymerization by anionic mechanism has been observed to be faster than by the free-radical mechanism. The kinetic, spectroscopic and redox properties of the transient species formed upon reaction of primary radiolytic species of water radiolysis with NIPA have been evaluated. The one-electron oxidation potential for the formation of a radical cation is quite high (>2 V), but the one-electron reduction potential is low (in the range of -0.3 to -0.7 V). The radical anion of NIPA is able to undergo an electron-transfer reaction with MV(2+), and has a pK(a) value of 3.2. The tert-butyl alcohol radical was also able to initiate polymerization. Gamma radiation-induced polymerization studies showed that the reaction of H(.)/(.)OH/e(aq)(-)/tert-butyl alcohol radicals with NIPA results in a nearly equal yield of the gel fraction. The hydrogel is observed to have very little swelling below pH 3 and above pH 10. The linear polymer of NIPA formed by irradiating dilute aqueous solution is found to be a thermosensitive polymer with lower a critical solution temperature (LCST) of ~33 degrees C. The diameters of polymer molecules were 290 and 20 nm at temperature below and above LCST, respectively.

Kinetic study of the electron-transfer oxidation of the phenolate anion of a vitamin E model by molecular oxygen generating superoxide anion in an aprotic medium.

Electron-transfer reduction of molecular oxygen (O2) by the phenolate anion (1-) of a vitamin E model, 2,2,5,7,8-pentamethylchroman-6-ol (1H), occurred to produce superoxide anion, which could be directly detected by a low-temperature EPR measurement. The rate of electron transfer from 1- to O2 was relatively slow, since this process is energetically unfavourable. The one-electron oxidation potential of 1- determined by cyclic voltammetric measurements is sufficiently negative to reduce 2,2-bis(4-tert-octylphenyl)-1-picrylhydrazyl radical (DOPPH*) to the corresponding one-electron reduced anion, DOPPH-, suggesting that 1- can also act as an efficient radical scavenger.

Synthesis and electronic properties of regioisomerically pure oxochlorins.

We describe a two-step conversion of C-alkylated zinc chlorins to zinc oxochlorins wherein the keto group is located in the reduced ring (17-position) of the macrocycle. The transformation proceeds by hydroxylation upon exposure to alumina followed by dehydrogenation with DDQ. The reactions are compatible with ethyne, iodo, ester, trimethylsilyl, and pentafluorophenyl groups. A route to a spirohexyl-substituted chlorin/oxochlorin has also been developed. Representative chlorins and oxochlorins were characterized by static and time-resolved absorption spectroscopy and fluorescence spectroscopy, resonance Raman spectroscopy, and electrochemistry. The fluorescence quantum yields of the zinc oxochlorins (Phi(f) = 0.030-0.047) or free base (Fb) oxochlorins (Phi(f) = 0.13-0.16) are comparable to those of zinc tetraphenylporphyrin (ZnTPP) or free base tetraphenylporphyrin (FbTPP), respectively. The excited-state lifetimes of the zinc oxochlorins (tau = 0.5-0.7 ns) are on average 4-fold lower than that of ZnTPP, and the lifetimes of the Fb oxochlorins (tau = 7.4-8.9 ns) are approximately 40% shorter than that of FbTPP. Time-resolved absorption spectroscopy of a zinc oxochlorin indicates the yield of intersystem crossing is >70%. Resonance Raman spectroscopy of copper oxochlorins show strong resonance enhancement of the keto group upon Soret excitation but not with Q(y)()-band excitation, which is attributed to the location of the keto group in the reduced ring (rather than in the isocyclic ring as occurs in chlorophylls). The one-electron oxidation potential of the zinc oxochlorins is shifted to more positive potentials by approximately 240 mV compared with that of the zinc chlorin. Collectively, the fluorescence yields, excited-state lifetimes, oxidation potentials, and various spectral characteristics of the chlorin and oxochlorin building blocks provide the foundation for studies of photochemical processes in larger architectures based on these chromophores.