Geminate Radical Pair Research Articles

Medium influence on the mechanism of the photoreaction of anthraquinone with triethylamine

The influence of polarity and viscosity of medium on the geminate radical pairs formed by photoreaction of anthraquinone with triethylamine has been studied by time-resolved CIDNP and stimulated nuclear polarization (SNP). Radical and radical ion mechanisms have been shown to be the main ways of the reaction proceeding in non-polar and polar media, respectively. In media with intermediate polarity both the mechanisms are realized.

Temperature effect on back electron-transfer reactions within a geminate radical pair: The influence of the solvent on the adiabaticity of the process

A study of the temperature dependence (from 233 to 353 K) of the rate of back electron-transfer reactions within geminate radical pairs by measurement of the free radical yield is reported. The radical pair is generated by photoinduced electron transfer with rhodamine 6G and oxazine 118 cations as electron acceptors and aromatic amines and methoxy-benzene derivatives as electron donors in acetonitrile, methanol and ethanol. In acetonitrile, the back electron transfer is non-adiabatic and apparent negative activation energies are observed for barrierless reactions. In alcohol solvents, an anomalously large temperature dependence is observed, which is attributed to a solvent-controlled adiabatic behaviour.

Magnetic field effects on the decay rates of photogenerated geminate radical pairs in reversed micelles

Laser excitation (351 nm) of zinc tetraphenylporphinate-viologen pairs in reversed micelles (AOT/isooctane or n-hexane) afforded triplet radical pairs in a cage, and the decay was remarkably retarded in external magnetic fields (0-1 T). In the case of the porphyrin-viologen linked compound, the radical decay rate decreased by an order of magnitude to reach a plateau region at above 0.3 T. Exactly the same behavior was also observed with nonlinked porphyrin-viologen pairs, which consisted of amphiphilic or cationic porphyrin and a zwitterionic viologen, in a small water pool (ca. 10 {angstrom} in radius) of the reversed micelles. The large external magnetic field effects on the linked (or pseudolinked) radical pairs were explained on the basis of Zeeman splitting of the triplet in combination with electron spin relaxation from the sublevels (relaxation mechanism), and supporting evidence was provided by the use of paramagnetic lanthanide ions.

Diffusion dependence of absolute chemically induced nuclear polarizations from triplet radical pairs in high magnetic fields

Absolute net nuclear polarizations are determined for the products of geminate radical pairs formed after triplet cleavage of ditert-butyl ketone in four different solvents as functions of temperature. A correlation with the diffusion coefficients of the radicals shows that for high temperatures/low viscosities the polarizations increase as D − 1 2 whereas for low diffusivities a levelling-off is observed. This dependence is analyzed by the high-field radical pair theory of CIDNP and yields information on the initial radical pair distance and on exchange parameters. Disproportionation to combination ratios of the radical pairs and their viscosity dependence are also reported.

Backward electron transfers within geminate radical pairs formed by electron-transfer quenching of phosphorescent states of tris(2,2'-bipyrazine)ruthenium(II) and tris(4-methyl-2-(2'-pyridyl)pyrimidine)ruthenium(II)

Backward electron transfers within geminate radical pairs formed by electron-transfer quenching of phosphorescent states of tris(2,2'-bipyrazine)ruthenium(II) and tris(4-methyl-2-(2'-pyridyl)pyrimidine)ruthenium(II)

Absolute chemically induced nuclear polarizations and yields from geminate radical pair reactions. A test of high-field radical pair theories

Absolute net nuclear polarizations and reaction yields are determined for the products of geminate radical pairs formed after triplet α-cleavage of di-tert-butyl ketone and during the singlet photo-Fries reaction of p-methylphenyl- p-chlorobenzoate. These and other experimental data are compared with values predicted by several variants of the high-field radical pair theory. Full agreement between experiment and theory is found to require proper inclusion of electron exchange effects in the latter, and a previous analytical model is modified to allow for this.

The CIDEP spectrum of a radical pair of the acetone ketyl radical in solution. Effect of deuteration

We report the CIDEP spectrum of a geminate radical pair of the acetone- d 6 ketyl radical in solution. Deuterated acetone shows a radical pair spectrum more clearly than protonated acetone and is found to be suitable for an investigation of the radical pair. It is also found that the radical pair spectrum of acetone- d 6 shows less M 1-dependent line broadenings than protonated acetone.

Modification of photochemical reactivity by cyclodextrin complexation: a remarkable effect on the photobehavior of .alpha.-alkyldibenzyl ketones

The Norrish type I and type II reactions of cyclodextrin-included \alpha-alkyldibenzyl ketones were examd. in aq. soln. and in the solid state. The photolysis of solid cyclodextrin complexes led to a single product, diphenylethane, and that of complexes in aq. soln. gave a product arising from the rearrangement of \alpha -alkyldibenzyl ketones. Conformational and super-cage effects are proposed to be responsible for this photobehavior. The difference in the products from solid and soln. complexes is attributed to the differences in the restriction imposed by the host on the translational motions of the geminate radical pairs.

Energy gap dependence of spin-inverted electron transfer within geminate radical pairs formed by the quenching of phosphorescent states in polar solvents

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTEnergy gap dependence of spin-inverted electron transfer within geminate radical pairs formed by the quenching of phosphorescent states in polar solventsTakeshi. Ohno, Akio. Yoshimura, Hiroshi. Shioyama, and Noboru. MatagaCite this: J. Phys. Chem. 1987, 91, 16, 4365–4370Publication Date (Print):July 1, 1987Publication History Published online1 May 2002Published inissue 1 July 1987https://doi.org/10.1021/j100300a030RIGHTS & PERMISSIONSArticle Views145Altmetric-Citations50LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (755 KB) Get e-Alerts

Electron spin resonance of spin-correlated radical pairs

Shortly after their formation, transient free radicals trapped in micelles exhibit electron spin resonance (ESR) spectra with “antiphase” lineshapes, i.e. lines with their low-field halves in emission and high-field halves in absorption. It is demonstrated that this hitherto unexplained effect is consistent with the detection of ESR transitions in geminate radical pairs before the radicals have had time to diffuse apart.

Magnetic field effects in chemical reactions in viscous liquids: the cidep amplitude

An expression for the chemically induced dynamic electron polarization (CIDEP) in geminate radical pair recombinations is obtained within the recently proposed correct sudden approximation. This expression is valid for both low and high viscosities and reduces the problem to simple matrix operations. In the (ST o) approximation a simple analytical formula for the CIDEP is deduced which accurately reproduces the results of numerical calculations.

Radical Switch Reaction. A Novel Reaction between Two Free Radicals in a Solvent Cage

Abstract Studies on the thermal decomposition of t-butyl phenylazo sulfone suggested the occurrence of a radical switch reaction, a novel type of reaction between two radicals. A primary geminate radical pair, t-butanesulfonyl and phenyl radical pair, exchange sulfur dioxide to form a new radical pair consisting of t-butyl and benzenesulfonyl radicals. The generation of benzenesulfonyl radical was confirmed by its ESR spectrum and the isolation of products containing a benzenesulfonyl group.

Bell-shaped energy gap dependence of backward electron-transfer rate of Geminate radical pairs produced by electron-transfer quenching of ruthenium(II) complexes by aromatic amines

ADVERTISEMENT RETURN TO ISSUEArticleNEXTBell-shaped energy gap dependence of backward electron-transfer rate of Geminate radical pairs produced by electron-transfer quenching of ruthenium(II) complexes by aromatic aminesTakeshi Ohno, Akio Yoshimura, and Noboru MatagaCite this: J. Phys. Chem. 1986, 90, 15, 3295–3297Publication Date (Print):July 1, 1986Publication History Published online1 May 2002Published inissue 1 July 1986https://doi.org/10.1021/j100406a001RIGHTS & PERMISSIONSArticle Views188Altmetric-Citations55LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (348 KB) Get e-Alerts

Magnetic field effect on the geminate and random radical pairs under reversible photooxidation of chlorophyll a in 1,3-propanediol

The effects of an external magnetic field ( H = 2000 G) on photogenerated geminate (G) and random (F) pairs of chlorophyll a radical cation and p-benzoquinone radical anion in viscous 1,3-propanediol have been measured: α G = 8±1%, α F = − 7±2%. The results have been used for calculation of the probability of the triplet pair S-T evolution and subsequent reaction φ ≈ 0.37 and the fraction of triplet pairs unable to react due to application of a magnetic field β ≈ 0.14. The validity of the estimates has been verified through independent experiments in which the cage effects under photochemical generation of these radical ions have been determined. The role of the hfi and relaxation mechanisms in the S-T evolution of radical pairs in viscous liquids and the differences in the magnetic properties of the G- and F-pairs are discussed.

The N,N-dimethylaniline-photosensitized dechlorination of chlorobenzenes

The N,N-dimethylaniline-(DMA)-photosensitized dechlorination of monochlorobenzene, ortho-, meta- and para-dichlorobenzene and 3,4-dichlorotoluene was studied in methanolic solutions by means of continuous photolysis at 313 nm. Excited singlet deactivation of DMA by the chloro compounds was investigated by fluorescence quenching. Rate constants for triplet quenching were determined by laser flash photolysis. The quantum yield of formation for hydrogen chloride was measured as a function of the halobenzene concentration. A mechanism was proposed which takes into account electron transfer from both singlet and triplet excited states of DMA. The efficiency of the geminate radical pair to dissociate into free ions was estimated. A value near unity was found when the triplet state of DMA was the radical pair precursor.

Quenching Process of Radioluminescence in Anthracene Crystals

Temperature dependences of β-ray-excited radioluminescence and uv-light-excited photoluminescence were measured in anthracene crystals. They depend on crystals, but their ratio shows the same characteristic in all of the crystals. Large temperature dependence of the ratio cannot be explained by the three relaxation processes of highly excited states, i.e. internal conversion and vibrational relaxation, autoionization (preionization) and exciton fission. It is expected by analyzing the data that there must exist a radiationless decay with an activation energy ∼0.048 eV in the relaxation manifolds. Predissociation to a geminate radical pair, which is followed by radiationless geminate recombination to an anthracene molecule or fission to isolated radicals, is considered as a competing process with the above three physical processes.

Photolysis of dibenzyl ketones adsorbed on zeolite molecular sieves. Correlation of observed cage effects with carbonyl carbon-13 enrichment efficiencies

The photolysis of 3-(4-methylphenyl)- 1-phenyl-2-propanone (1) (4-MeDBK) and 1 ,3-diphenyl-2-propanone-13C2 (2') (DBK-I3C) has been carried out on several commonly available zeolites. Dramatic changes in product distributions for 4-MeDBK (1) photolysis were observed on the several zeolites. Percent cage effects were calculated and rationalized in terms of molecular mobility of photogenerated benzyl radicals in the void volumes of zeolites. Percent cage effects correlate well with C-enrichment efficiencies for DBK-C photolysis on these zeolites and, with the yield of a DBK isomer (PMAP), the para-coupling product of the primary geminate radical pair (Figure 3) in the photolysis of DBK-C. The past several years have witnessed an explosive increase in photochemical studies in ordered systems.'-' Although photo- chemical reactions have been performed in micelles, micro- emulsions, liquid crystals, in the solid state, and on silica gel,'-' only a small number of studies have dealt with photochemistry on zeolites, the majority of these being concerned primarily with the water-splitting process.s-12 Recently, however, reports on the use of zeolites for organic photophysical and photochemical studies have appeared.I3-I5 Zeolites are crystalline aluminosilicates of usually well-defined structure.16 Within the zeolite framework are a system of channels and/or supercages of varying dimensions (2-1 3 A).16 The exact nature of these channels or supercages depends on the identity of the particular zeolite. Of importance to the organic chemist is that channel or pore diameters of 26 A can allow the adsorption of benzene and other molecules of similar molecular size.I6 Thus, the possibility that the internal spaces (or void volumes) of zeolites can exert topological control on organic photochemical reactions is an area that warrants investigation, since it is well-known that zeolites display shape-selective catalytic and absorptive properties in industrial chemical processe~.~~,~~ We report herein our study of the photochemical behavior of dibenzyl ketones adsorbed in a number of commonly available zeolites. The results indicate that significant changes in reaction product ratios can be effected by carrying out photochemical reactions in zeolites, thus opening the way for employing these substances as a medium for organic photochemical studies. Results

Magnetic-field-dependent recombination kinetics of geminate radical pairs in reversed micelles of variable size

The magnetic-field-dependent kinetics of geminate radical pairs, created by electron transfer from aneline to thionine triplet in reversed micelles has been studied by nanosecond laser flash spectroscopy. Depending on the water concentration, the rate constant of intramicellar recombination is reduced by a factor between 2.4 and 3.2 in a field of 1 T. The B 1/2 values increase from about 15 to 48 mT when the water-pool radius is increased by a factor of 4. The experimental results demonstrate that triplet-singlet transitions cannot be rate determining in zero field. However, T ∓ → T o and T ∓ → S radical pair spin relaxation must become rate determining as the magnetic field is increased above 10 mT.

Spin exchange effects in radical pair CIDEP ag

An explanation of some unexpected enchancements in the chemically induced dynamic electron polarization (CIDEP) spectra of ketyl radicals is suggested. It requires an electron exchange interaction that is negative for proximate radicals but positive for more distant ones and that geminate radical pairs are formed at a separation smaller than that at which random encounter pairs of the same radicals are able to react.

CIDNP Effects of Sensitized Photochemical Dediazoniation of Arene Diazonium Salts Manipulating CIDNP Intensities by the Experimental Conditions

13C and 15N photo-CIDNP effects were determined for the reversible electron transfer from pyrene to arene diazonium salts on excitation of the charge transfer band at 360 nm. The diazonium salts being the products of back electron transfer (“cage products”) show enhanced absorption for 13 C(1) and the 15N-enriched diazonium group, whereas the escape products, ArH or 15N2, respectively, yield emission signals. It was shown that the intensities of the CIDNP effects depend on the rates of intersystem crossing kisc within the geminate radical pair, i.e. on the magnetic nucleus used as a probe of the CIDNP effect. Using 1H, 13C or 15N the time domain of observation can be manipulated in the ranges of 90-100 ns, 15-20 ns and 3-5 ns, respectively. Furthermore, the CIDNP intensities depend on the proper balance of the rate of electron back transfer, k-e, and the rate kp of formation of the escape product. Since k-e increases with increasing energy of the geminate radical pair, this balance and therefore the CIDNP intensities vary according to the substituent present and the electron donor used