Contact Radical Ion Pair Research Articles

Exciplex mechanism of fluorescence quenching in polar media

The formation of exciplexes (non-emitting or poorly emitting) is suggested as one of the causes for deviations of experimental data on fluorescence quenching in polar solvents from the classical model of excited-state electron transfer yielding radical ion pairs. Several evidences for the formation of such exciplexes were found for fluorescence quenching of aromatic compounds by weak electron donors and acceptors. For cyano-substituted anthracenes exciplex emission can be observed in the presence of quenchers even in polar solvents. In other systems, indirect evidences of exciplex formation were observed: nonlinear dependence of the inverse value of excited pyrene lifetime on the concentration of the quencher; very small and, in some cases, even negative experimental activation energies of pyrene fluorescence quenching, which are much less than activation energies, calculated from the experimental values of the quenching rate constants etc. The proposed model explains the difference between theoretical and experimental dependencies of logk Q vs. Gibbs energy of electron transfer ‡DGET and other experimental features known for fluorescence quenching by electron donors and acceptors. This model states that the exciplex is in equilibrium with the encounter complex and apparent quenching rate constants are controlled by two main factors — the lifetime of the exciplex and the enthalpy of its formation. Experimentally observed dependence of apparent quenching rate constant on ‡GET is caused by the dependence of the exciplex formation enthalpy on ‡GET, which is quite different from the dependence of electron transfer activation energy on ‡GET predicted by the theoretical models. Simulations of the dependencies of logk Q vs. ‡GET according to the exciplex formation model confirms its agreement with the experimental data. Electronic structure of the exciplex involved may be close to contact radical-ion pair only at ‡GET < 0, when the rate of quenching is limited mainly by the diffusion, but for ‡GET> 0, the structure of the exciplex should be much less polar.

Application of Kramers theory to the diffusional separation of the trans-stilbene/fumaronitrile contact radical ion pair in alkyl nitrile solvents

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTApplication of Kramers theory to the diffusional separation of the trans-stilbene/fumaronitrile contact radical ion pair in alkyl nitrile solventsBulang Li and Kevin S. PetersCite this: J. Phys. Chem. 1993, 97, 29, 7648–7651Publication Date (Print):July 1, 1993Publication History Published online1 May 2002Published inissue 1 July 1993https://doi.org/10.1021/j100131a038RIGHTS & PERMISSIONSArticle Views84Altmetric-Citations10LEARN 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 (474 KB) Get e-Alerts

Concentration-dependent photoinduced electron-transfer reactions: 1:1 and 1:2 radical-ion complexes

For some exciplexes of 9, 10-dicyanoanthracene (DCA) with naphthalenes and phenanthrenes as donors, the fluorescence quantum yields and lifetimes decrease with increasing donor concentration. The rate constants for these self-quenching reactions increase with decreasing redox energy of the pair and with increasing solvent polarity. Both parameters increase the charge-transfer (CT) character of the exciplex. When the CT character is low, the quenching rate constants are too small to measure (<10 7 M -1 s -1 ), but with increasing CT character, to the limit that the exciplex becomes essentially a contact radical-ion pair, the interception rate constant approaches the diffusion-controlled limit. The product of the self-quenching reactions is a 1:2 radical-ion complex

The photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction (Part 4): methanol–olefins, methyl 4-cyanobenzoate

Dicyanobenzene-1,4 (1) and -1,2 are known to undergo substitution upon irradiation, in the presence of an olefin, in acetonitrile–methanol (3:1) solution. The products are 1:1:1 (methanol:olefin:aromatic) adducts, substituted on the aromatic ring with loss of a cyano group. This reaction, referred to as the photo-NOCAS (nucleophile–olefin combination, aromatic substitution) reaction, has been shown to be fairly general with regard to the olefin and the nucleophile that can be incorporated. Less is known about the scope of the reaction incorporating other electron-withdrawing substituted aromatic molecules. The purpose of this study was to determine if methyl 4-cyanobenzoate (10) would also take part in this reaction, to form 4-substituted aromatic esters. Irradiation of acetonitrile–methanol solutions of 10 and olefins 2,3-dimethyl-2-butene (2) and 1-methylcyclohexene (5) gave cyclic imine esters, 11 and 13, respectively, instead of photo-NOCAS products. The photo-NOCAS products were obtained when the codonor biphenyl (4) was added to the irradiation mixture. Formation of the cyclic imine ester is attributed to excitation of the charge-transfer complex formed between 10 and the olefin. The addition of biphenyl (4) serves to generate the contact radical ion pair (CRIP) upon irradiation of the charge-transfer complex between 10 and 4. This CRIP can dissociate to the solvent-separated radical ions, the radical cation of 4 can accept an electron from the olefin, and the olefin radical cation can go on to give the photo-NOCAS products. Irradiation of a solution of 10 and 2 in nonpolar solvent (benzene) gave the oxetane, believed to arise from the exciplex. In addition to photo-NOCAS products from 10, 4-cyanophenylketones 17 and 23 are also formed by attack of the β-alkoxyalkyl radical at the carboxyl carbonyl. The differences in behaviour between 1,4-dicyanobenzene (1) and methyl 4-cyanobenzoate (10) under these reaction conditions are described and explained.

Relationship between exciplex fluorescence and electron transfer in radical ion pairs

The relationship between non-radiative (thermal) return electron transfer within radical ion pairs and the fluorescence spectra of exciplexes is discussed. Those exciplexes in which charge transfer from the donor to the acceptor is nearly complete are essentially contact radical ion pairs, and their fluorescence is a return electron transfer process. In particular, the energy and width of the exciplex fluorescence spectrum are directly related to the electron tranfer reorganization parameters. From the emission spectra of the exciplexes formed by several common acceptors with alkylbenzenes as donors it is found that the reorganization energy for electron transfer decreases with increasing molecular size of the acceptor. The spectra also indicate that in the solvents used in this study, the reorganization parameters with dicyanoanthracene, tetracyanoanthracene, dicyanonaphthalene and dicyanobenzene as acceptors are essentially constant for different alkylbenzene donors. With tetracyanobenzene and, most strikingly, pyromellitic dianhydride, however, the reorganization energy is found to decrease with decreasing oxidation potential of the alkylbenzene donor. The relevance of these observations to the interpretation of the rates of return electron transfer in these system as a function of reaction exothermicity is discussed.

Laser photolysis study of chloranil triplet exciplexes with stilbene

Absorption spectra and decay kinetics of the polar triplet exciplexes (contact radical-ion pairs) formed during quenching of the chloranil triplet state by trans- or cis-stilbenes in benzene with added acetonitrile and methanol, have been studied by laser flash photolysis. The exciplexes include cation-radicals of stilbene dimers, which are deactivated by reverse electron transfer within 10–50 nsec. The dynamics of the intercombination electron transfer and the exciplex dissociation into ion-radicals were determined. The isomerization of stilbene via triplet exciplex formation was not observed.

ChemInform Abstract: Oxidative Photofragmentation of α,β‐Amino Alcohols via Single Electron Transfer: Cooperative Reactivity of Donor and Acceptor Ion Radicals in Photogenerated Contact Radical Ion Pairs

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Oxidative photofragmentation of .alpha.,.beta.-amino alcohols via single electron transfer: cooperative reactivity of donor and acceptor ion radicals in photogenerated contact radical ion pairs

The studies presented in this paper show that α,β-amino alcohols undergo a very clean C−C bond cleavage upon SET (single electron transfer) oxidation by photoexcited electron acceptors in a process which generally culminates in two-electron reduction of the acceptors. For a number of different α,β-amino alcohols, the oxidative fragmentation occurs in a high chemical yield (>90%), yet with low to medium quantum efficiencies (0.0001−0.1) which vary strongly depending on the properties of electron donor (D), acceptor (A), and solvent. The net quantum efficiency reflects the competition between back electron transfer and the chemical redox process. Detailed mechanistic studies were carried out to investigate the visible light induced oxidative fragmentation of α,β-amino alcohols in the presence of electron acceptors including thioindigo (TI), 9,10Dicyanoanthracene (DCA), 2,6,9,10-tetracyanoanthracene (TCA), and 1,4-dicyanonaphthalene (DCN)

Energy gap dependence for intersystem crossing within charge transfer triplet exciplexes

The decay kinetics of triplet exciplexes (TE) with complete charge transfer (contact radical ion pair type) formed by electron transfer from aromatic amines to the triplet excited states of quinones in benzene, chloroform and 1,1,2,2-tetrachloroethane was studied by laser photolysis. The TE decay to the ground state of the initial reagents is mainly due to the direct spin inverted backward electron transfer (intersystem crossing from the triplet charge transfer state to the ground state), which occurs with the first-order rate constants ( k et) in the range 10 6–10 8s −1. A small contribution of the radical pair S-T evolution mechanism (due to electron-nuclear magnetic hyperfine coupling) to TE decay is revealed by the measurement of magnetic field effect. This was observed only in the most polar solvent, where partial dissociation of TE to the corresponding radical ions takes place. Kinetics and thermodynamics of TE formation provide a possibility to evaluate the TE free energy - that of backward electron transfer (Δ G et). The relationship between k et and G et is bell shaped and may be satisfactorily explained on the basis of the simplest quantum harmonic model within the statistical limits of the nonadiabatic multiphonon electron transfer (radiationless transition) theory. The reorganization energies obtained were close to the optimal values for analogous singlet systems. Forbiddance in spin in the process leads to a low value for the electron matrix element involved for the given case, which is due to spin-orbit coupling. The role of mutual orientation and separation between the radical ions in spin-orbit coupling within the TE is discussed.

Effect of the medium on the heavy atom effect in triplet exciplexes

1. The reverse electron transfer in triplet exciplexes consisting of contact radicalion pairs is a function of the spin-orbital interaction and is accelerated upon the introduction of heavy atoms into the reagents. 2. The internal heavy atom effect is significantly enhanced upon solvation of the exciplex by methanol molecules or an increase in the polarity of the medium.

Bell-shaped energy gap dependence for electron transfer rate in triplet exciplexes

Decay rates of triplet exciplexes with complete charge transfer (contact radical ion pairs) formed by electron transfer from aromatic amines to the triplet excited states of quinones in benzene and 1,1,2,2-tetrachloroethane were studied by means of laser photolysis. The decay is due to spin-inverted backward electron transfer-intersystem crossing to the ground state of reagents. The “bell-shaped” dependence of the decay rate on exergonicity of the process is satisfactorily described in terms of non-adiabatic multiphonon electron transfer theory with the parameters λ v,λ s and ħω close to those optimal for analogous singlet systems. Spin forbiddence of the process leads to a low value of the electron matrix element | V|.

Kinetics of the quenching of triplet exciplexes containing contact radical-ion pairs by oxygen and radicals

1. Paramagnetic species quench the triplet exciplexes of contact radical-ion pairs with a rate approaching the diffusion-controlled limit taking account of the spin statistical factor of 1/3. 2. The quenching mechanism lies in an acceleration in the transition of the radical pair between states with different multiplicity due to removal of the prohibition in the collision complex.

Benzophenone triplet exciplexes with aromatic amines

Laser photolysis was used for the direct detection of the spectral and kinetic characteristics of the contact radical-ion pair triplet exciplexes generated upon the interaction of triplet benzophenone with tertiary aromatic amines in nonpolar solvents. The exciplexes disappear as a result of reverse electron transfer and proton transfer.

A photochemical diels—alder reaction of N-methyltriazolinedione

Irradiation of N-methyltriazolinedione (MTAD) and naphthalene leads to a new [4 + 2] cycloadduct. Quantum yield data indicate that the reaction can occur via both singlet and triplet excited MTAD and also via charge transfer irradiation of a ground state complex. Naphthalene quenches the fluorescence of 1MTAD at diffusion-controlled rates. A contact radical–ion pair is suggested as a common intermediate in the singlet and charge transfer reactions. Triplet sensitization of the cycloadduct caused di-π-methane rearrangement.

Electron spin resonance observations of photochemically generated contact ammonium ion-pairs of fluoro-substituted ketones

Contact radical ion-pairs of ammonium and fluoro-substituted ketones were generated in photochemical systems and their here-to-fore elusive esr spectra were characterized.