Ionic species in γ-irradiated poly(methyl methacrylate) at 77°K

Ionic species in poly(methyl methacrylate) (PMMA) have been studied by optical absorption and ESR spectrometry. γ-Irradiation of PMMA containing aromatic solutes gave rise to the absorption spectra of corresponding cation radicals. The G value for the formation of cation radicals was determined to be 1.5. Anionic radicals of the solutes were not detected for the aromatic solutes studied. Anionic species of pure PMMA were identified by both absorption spectra and ESR spectra. It has an absorption maximum at 440 nm. A sharp singlet with a line width of about 5 G which was found by ESR spectrometry was tentatively assigned to the trapped electron of PMMA. The results show that a large number of electron traps in PMMA may exist on the ester side chain themselves.     

Ionic and excited species in irradiated poly(vinyl chloride) doped with aromatic admixtures: low temperature studies

The influence of temperature (77–290 K) on the fate of dopant radical ions and respective excited states in irradiated poly(vinyl chloride) (PVC) matrix, doped with pyrene, (Py) and tris(2-ethylhexyl) trimellitate (TOTM) is described. At 77 K dopant radical ions start to recombinevia tunneling charge transfer, leading to weak isothermal luminescence (ITL). The wavelength-selected radiothermoluminescence (WS RTL) broad maxima observed for doped PVC in the temperature range 95–110 K have a similar origin, i.e., recombination of dopant radical ionsvia tunneling. Apart from the peaks representing the absorption of dopant radical ions the absorption maximum at 411 nm found for the PVC−Py system is attributed to Py−Cl adduct generated in Py^•++Cl⁻ reaction. The mechanisms involved in these processes are discussed.     

Effect of doping on thermoluminescence in polymers. II. Polycarbonate doped with triphenyl methane and xanthene dyes

The thermoluminescence (TL) of polycarbonate doped with triphenyl methane and xanthene dyes was measured following irradiation with ultraviolet light at 77 K and the effect exercised by systematically varying excitation wavelength, irradiation dose, and dopant concentration were investigated. The TL glow curves are characterized by two distinct peaks that can be attributed to genuine emissions of the dopant-polymer systems and involve continuous distributions of apparent activation energies as determined by partial heating techniques. The low-temperature peak extends over the temperature interval corresponding approximately to the complex β relaxation of the polymeric units (short-range chain motions), but no obvious correlation between the two phenomena can be evidenced because the TL peak is never structured and the values and type of evolution of apparent activation energies as a function of temperature are clearly different from those observed for the β relaxation. In this temperature range the TL properties are consistent with an ionization of the dye followed by a recombination process involving electron tunneling from trap to luminescence center rather than thermal excitation. The second TL peak appears in a unusually high temperature region (> 250 K) where no intrinsic relaxation of the polymeric matrix is known to occur. This is not to say that the chain motions cannot contribute to some extent to the detrapping processes. Generally speaking, however, the TL results show that interpreting glow curves only in terms of relaxation processes should be done with great caution and that in certain cases an observed emission must be considered as specific to the dopant-polymer system rather than to the polymer itself.     

Photoinduced charge separation in titania nanotubes

The photocatalytic one-electron oxidation reaction of an aromatic compound during UV light irradiation of titania nanotubes and nanoparticles was investigated using time-resolved diffuse reflectance spectroscopy. Remarkably long-lived radical cations of the aromatic compound and trapped electrons were observed for the nanotubes when compared to those for nanoparticles. The influences of the morphology on the one-electron oxidation process of an aromatic compound adsorbed on the surface were discussed in terms of the charge recombination dynamics between the radical cation and electrons in TiO2.     

Ion recombination in PMMA doped with pyrene as observed by wavelength selected radiothermoluminescence

Radioluminescence from electron-irradiated poly(methyl methacrylate), PMMA, pure and doped with pyrene (Py) was investigated in the temperature range of 77-295 K. The spectra of emission were recorded, and temperature dependences of radioluminescence intensities at chosen wavelengths were examined using a novel wavelength-selected radiothermoluminescence (WS RTL) technique. The correlation between the WS RTL peaks and matrix relaxation temperatures were found. The experimental results were explained in terms of solute radical ion recombination leading to the Py monomer and excimer fluorescence. The luminescence results were correlated with the decay of Py ions as observed by spectrophotometric absorption method.     

Long‐lived species generated via irradiation of poly(methyl methacrylate) containing pyrene

The fate of long‐lived species [pyrene (Py) radical cations and Py–cyclohexadienyl‐type radicals] generated by electron‐beam irradiation at room temperature in poly(methyl methacrylate) (PMMA) doped with Py is described. The separation of reacting solute intermediates and the relaxation phenomena seem to be the main factors limiting the reactivity of long‐lived Py species in the PMMA matrix. The temperature‐stimulated recombination of ionic species in PMMA doped with Py results in Py excited‐state formation. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 4110–4118, 2001     

Structural character and energetics of tyrosyl radical formation by electron/proton transfers of a covalently linked histidine-tyrosine: a model for cytochrome C oxidase

The structural, energetic, and electronic and IR spectroscopic properties for a model of the cross-linked histidine-tyrosine (His-Tyr) residues as found in cytochrome c oxidase (CcO) are investigated by ab initio methods. The formation of a His-Tyr radical is studied by two paths: proton release followed by electron release and vice versa. The energetics for the proton/electron releases of the Tyr depend modestly on the cross-linked His substituent and, more sensitively, on the charge of the cation attached to the imino N site of the His residue. Protonation of the imino N site significantly increases the electron ionization potential and decreases the proton dissociation energy, making them competitive processes. A positive charge placed at the imino N site, whose value is scanned from zero to one, shows a continuous increase in ionization potential and a decrease in proton dissociation energy, with the +1 limit agreeing well with the protonated imino N site result, indicating a dominant electrostatic effect. The charge populations and the spin density distributions of the His-Tyr model, the radical cation formed by electron ionization, the anion formed by proton dissociation, and the final His-Tyr radical depend sensitively on the substituents, implying a modulation role on the charge transfer between the phenol and imidazole rings, especially for the charged species. His-Tyr and protonated His-Tyr exhibit differences among their respective structural isomers with consequences on their IR absorptions. Small barriers between their pseudo-cis and pseudo-trans rotamers demonstrate the relative flexibility between the two rings, and these may facilitate proton release and charge transfer. The cation effect demonstrates that the cationized cross-linked His-Tyr should be the best candidate to mimic the covalently ring-linked histidine-tyrosine structure in CcO.     

Thermoluminescence and a new organic light-emitting diode (OLED) based on triplet-triplet fluorescence of the trimethylenemethane (TMM) biradical

The results of an investigation of the thermoluminescence (TL) and electroluminescence (EL) of arylated methylenecyclopropanes 1, systems whose photoinduced electron-transfer (PET) chemistry has been thoroughly studied, are described. In both the TL and EL experiments with 1, electronically excited triplet trimethylenemethane (TMM) biradicals (3)2** are generated by back electron transfer (charge recombination) of a TMM radical cation (hole) 2*+, formed by isomerization of the substrate radical cation (hole, 1*+). The application of this chemistry to the design of new organic light-emitting diodes (OLEDs) is described. The mechanistic features of this reaction system have the potential of overcoming significant problems (e.g., quantum efficiency, difficulty obtaining long wavelength emission, and device durability) normally associated with OLEDs that rely on the use of organic closed-shell hydrocarbons.     

Substituent effects on the ionization reaction of beta-mesylate phenethyl radicals

A series of beta-methanesulfonate phenethyl radicals bearing a range of electron donating and withdrawing aromatic substituents were generated and studied in a variety of solvent mixtures using nanosecond laser flash photolysis. Rate constants for the formation of the corresponding styrene radical cation via heterolytic loss of the beta-mesylate leaving group were measured using time-resolved absorption spectroscopy. The ionization reaction was investigated in a variety of solvents and solvent mixtures including 1,1,1,3,3,3-hexafluoro-2-propanol, 2,2,2-trifluoroethanol, acetonitrile, methanol and water. The influence of substituent electronic effect and solvent polarity on the kinetics of the beta-heterolysis reaction are discussed and assessed using the sigma+ Hammett parameter and Y(OMs) values, respectively. The small magnitude of m calculated for the formation of the 4-methoxystyrene radical cation by ionization of the mesylate group (m = 0.33) in aqueous methanol mixtures is compared to values obtained for the formation of the same radical cation via loss of chloride and bromide where m = 0.56 and m = 0.45, respectively.     

Alkali metals plus complexants: From alkalides and electrides to aromatic anions

Electron transfer from an alkali metal to a suitable complexant for the cation can yield crystalline ionic solids that contain the complexed cation and either an alkali metal anion (alkalide) or a trapped electron (electride). The nature and properties of electrides are emphasized in this paper. When the organic complexant contains aromatic groups, the anionic species is an aromatic radical anion. Preliminary work on the addition of alkali metals to LOGEAs (large organic globular electron acceptors) is described and strategies for the synthesis of mixed electride-anion compounds are considered.     

Solute radical ion yields of γ-irradiated aromatic compounds in poly(methyl methacrylate) at 77°K

Ionic species in γ-irradiated poly(methyl methacrylate) (PMMA) matrices were investigated. γ-Irradiation of several aromatic solutes in PMMA gave rise to the radical cations and anions of the solutes. The limiting yields of radical cation and anion were determined to be 1.0–1.5 and 0.7–0.9, depending on the characteristics of the solutes, respectively. These values were compared with those in low-molecular-weight matrices. The distance of positive charge migration was estimated. The experimental results show that the charge transfer from PMMA to the solutes may be responsible for the formation of the radical cations and anions.     

Production of free radicals and triplets from contact radical pairs and from photochemically generated radical ions

The quantum yields of triplets and free radicals (or radical ions) that escaped recombination in photochemically created primary radical pairs (or radical ion pairs) are calculated. As the products of monomolecular photodissociation, the neutral radicals appear at contact, while the ions are initially distributed over the space due to distant photoionization (bimolecular electron transfer) in the liquid solution. The diffusional dependence of the quantum yields is shown to be different when recombination starts from contact or from separated reactants. The experimental data for recombination of ionized perylene with aromatic amine counterions is well fitted with the noncontact initial distribution provided the recombination is also noncontact and even more distant than ionization.     

Independent pairs and Monte-Carlo simulations of the geminate recombination of solvated electrons in liquid-to-supercritical water

Independent pairs (IP) and Monte Carlo (MC) simulations are employed to model experimental femtosecond time-resolved pump-probe spectroscopic data on the geminate recombination dynamics of solvated electrons in liquid-to-supercritical water. The hydrated electron was created by two-photon ionization of the neat fluid with a total ionization energy of 9.3 eV. In both numerical approaches, the ejection length, , (i.e. the distance from the ionization core, at which the electron is thermally and spatially localized) is used as the primary adjustable fitting parameter that can bring both model simulations into quantitative agreement with the ultrafast kinetic experiment. The influence of the thermodynamic conditions on the ejection length and on the recombination mechanism is discussed. Whereas in the compressed liquid associated with a high dielectric constant (ε ≥ 20), the electron recombines predominantly with the OH radical, the dissociative recombination via charge neutralization with the hydronium cation takes over at small dielectric constants (ε < 20). The importance of charge-dipole interactions for Monte-Carlo simulations of the recombination reactions of the hydrated electrons in the low-permittivity region is stressed.     

Wavelength‐selected radiothermoluminescence of polyethylene doped with pyrene

Radioluminescence from electron‐irradiated polyethylene doped with pyrene was investigated in the temperature range of 77–273 K. The spectra of emitted light were recorded, and temperature dependences of radioluminescence intensities at chosen wavelengths were examined using a wavelength‐selected radiothermoluminescence technique. The experimental results were explained in terms of an electron‐solute radical cation and solute radical ions recombination. Pyrene excimer emission was observed in the glass transition temperature range of the polyethylene matrix. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3378–3382, 2000     

Photoreactions of aromatic compounds—XXXV: Nucleophilic photosubstitution of methoxy substituted aromatic compounds. Monophotonic ionization of the triplet

Methoxy groups exert an activating and ortho/para directing influence in light induced nucleophilic substitution reactions (cyanation, hydroxylation, etc) of aromatic compounds in aqueous media. The first chemical step in these processes is monophotonic ionization of the aromatic compound in its lowest triplet state, followed by reaction of the radical cation with the nucleophile Quantum yields of photocyanation of 4-fluoro- and 4-chloroanisole indicate that in 99% (mole fraction) water virtually all triplets formed undergo electron ejection.This hypothesis is in agreement with the results of charge density calculations for the radical cations. It is directly supported by the similarity of the product composition of these photochemical substitutions with that of anodic substitutions, where the intermediacy of an aromatic cation is generally accepted. The presence of an oxidizing agent (oxygen, or persulphate) is required only when a hydrogen is replaced. The nucleophilic photosubstitution at electron rich aromatic systems in solvents as water can therefore be classified as an S_r⁺n¹(³Ar^*) process.     

ISOTHERMAL LUMINESCENCE AND THERMO-LUMINESCENCE OF NUCLEIC ACID BASES FOLLOWING γ-IRRADIATION

Abstract— The emission spectra of radiation induced isothermal luminescence (ITL) and the thermolu-minescence (TL) of purines and pyrimidines of nucleic acids and analogue 6-azauracil in the form of pellets of dry polycrystalline powders have been studied at 77 K, and compared with their low temperature fluorescence and phosphorescence spectra. The qualitative and quantitative measurements of isothermal luminescence and thermoluminescence show that the two result from the same radiative transitions. The thermoluminescence emission was observed to coincide with the phosphorescence emission of the compounds in all the cases. The thermoluminescence of the pyrimidines and their analogue, however, have shown an additional component corresponding to their fluorescence in the ultraviolet region. An extension of the Weissbluth model based on the location of the electron traps in relation to the excited states of the compounds is proposed to explain their thermoluminescence emission.     

Ionization of aniline and its N-methyl and N-phenyl substituted derivatives by (free) electron transfer to n-butyl chloride parent radical cations

The electron transfer from aniline and its N-methyl as well as N-phenyl substituted derivatives (N-methylaniline, N,N-dimethylaniline, diphenylamine, triphenylamine) to parent solvent radical cations was studied by electron pulse radiolysis in n-butyl chloride solution. The ionization results in the case of aniline (ArNH2) and the secondary aromatic amines (Ar2NH, Ar(Me)NH) in the synchronous and direct formation of amine radical cations, as well as aminyl radicals, in comparable amounts. Subsequently, ArNH2*+ deprotonates in a delayed reaction with the present nucleophile Cl-, and forms further ArNH*. In contrast, tertiary aromatic amines such as triphenylamine and dimethylaniline yield primarily the corresponding amine radical cations Ar3N*+ or Ar(Me2)N*+, only. The persistent Ar3N*+ forms a charge transfer complex (dimer) with the parent amine molecule, whereas Ar(Me2)N*+ deprotonates to carbon-centered radicals Ar(Me)NCH2*.     

Interactions of Amino Acids with Oxidized Guanine in the Gas Phase Associated with the Protection of Damaged DNA

Density functional theory calculations were employed to study the stabilization process of the guanine radical cation through amino acid interactions as well as to understand the protection mechanisms. On the basis of our calculations, several protection mechanisms are proposed in this work subject to the type of the amino acid. Our results indicate that a series of three‐electron bonds can be formed between the amino acids and the guanine radical cation which may serve as relay stations supporting hole transport. In the three‐electron‐bonded, π–π‐stacked, and H‐bonded modes, amino acids can protect guanine from oxidation or radiation damage by sharing the hole, while amino acids with reducing properties can repair the guanine radical cation through proton‐coupled electron transfer or electron transfer. Another important finding is that positively charged amino acids (ArgH⁺, LysH⁺, and HisH⁺) can inhibit ionization of guanine through raising its ionization potential. In this situation, a negative dissociation energy for hydrogen bonds in the hole‐trapped and positively charged amino acid–Guanine dimer is observed, which explains the low hole‐trapping efficiency. We hope that this work provides valuable information on how to protect DNA from oxidation‐ or radiation‐induced damages in biological systems.     

Experimental and theoretical study of 2,6-difluorophenylnitrene, its radical cation, and their rearrangement products in argon matrices.

2,6-Difluorophenylnitrene was reinvestigated both experimentally, in Ar matrices at 10 K, and computationally, by DFT and CASSCF/CASPT2 calculations. Almost-pure samples of both neutral rearrangement products (the bicyclic azirine and the cyclic ketenimine) of a phenylnitrene were prepared and characterized for the first time. These samples were then subjected to X-irradiation in the presence of CH2Cl2 as an electron scavenger, which led to ionization of the neutral intermediates. Thereby, it was shown that only the phenylnitrene and the cyclic ketenimine yield stable radical cations, whereas the bicyclic azirine decays to both of these compounds on ionization. The cyclic ketenimine yields a novel aromatic azatropylium-type radical cation. The electronic structure of the title compound is discussed in detail, and its relation to those of the iso-pi-electronic benzyl radical and phenylcarbene is traced.     

Hole trapping at N6-cyclopropyldeoxyadenosine suggests a direct contribution of adenine bases to hole transport through DNA

Recent studies predict that adenine radical cation (A*+) contributes to the hole-trapping process through long A/T sequences and exists as a real chemical intermediate. However, the experimental evidence for the existence of A*+ has not been observed in the DNA-mediated hole transport reaction. To examine the direct contribution of A*+, we have developed a novel hole-trapping nucleobase N6-cyclopropyldeoxyadenosine (dCPA) which possesses a cyclopropyl group as a radical trapping device. One-electron oxidation of dCPA revealed that dCPA radical cation undergoes a rapid cyclopropane ring opening. With the use of the dCPA-containing DNA, we have demonstrated that the migrating hole was trapped at CPA incorporated into a long A/T bridge between two GG sites. The present results indicate that nucleobases possessing ionization potential higher than that of dG, such as dA, are able to participate directly in the multistep hopping mechanism.