Solvent-separated Radical Ion Pair Research Articles

Electron Transfer Photochemistry of Geraniol and (E,E)-Farnesol. A Novel “Tandem”, 1,5-Cyclization, Intramolecular Capture

The electron transfer photoreaction between 9,10-dicyanoanthracene (DCA) and (E)-3,7-dimethylocta-2,6-dien-1-ol (geraniol, (E)-1) in dichloromethane produces mainly cis-2-(2-propenyl)-trans-5-methylcyclopentanemethanol, cis,trans-3, whereas irradiation of acetonitrile solutions of 1,4-dicyanobenzene (DCB) and (E)-1 forms mainly two trans-fused 3-oxabicyclo[3.3.0]octanes (4, R = H; 5, R = p-C6H4CN). Analogous results are observed in the photoreaction of farnesol ((E,E)-2) with DCB in the presence of phenanthrene as cosensitizer. The photoreaction of DCB with 1 or 2 has sufficient driving force (ΔG = −0.7 eV in acetonitrile) for the generation of solvent-separated radical ion pairs (SSRIPs); in contrast, the marginal driving force (ΔG ≈ 0 eV) of the DCA sensitized reaction allows only the formation of contact radical ion pairs (CRIPs). The resulting radical cations react by five-center C−C cyclization, yielding di-tertiary, 2,α-bifunctional methylidenecyclopentyl radical cations, cis- or trans-B•+. Subseque...

Interactions in crystals CX. The structure of 9,10-bis(diisopropylsilyl) anthracene and of its “naked” radical anion

To further explore anthracene derivatives with bulky 9,10-substituents and a bent molecular skeleton due to repulsive interactions with adjacent ring hydrogens, we have synthesized 9,10-bis(diisopropylsilyl)anthracene and determined its molecular structure. Reduction with lithium metal in THF solution under aprotic conditions and argon yields its radical anion, which has been characterized by its ESR and ENDOR signal patterns. Crystallization from the red THF solution following addition of an n-hexane layer yielded the radical anion as blue, air- and moisture-sensitive needles. Its structure, determined at 130 K in an N 2 flow, is discussed by comparison with that of the neutral molecule and those of 9,10-bis(trimethylsilyl)anthracene and its solvent-separated radical ion pair [((H 3C) 3Si) 2C 14H 8 •−]-[Na +(DME) 3], and in the light of literature examples and the results of MNDO calculations.

Photophysical and fluorescence quenching properties of peropyrene in solution

The photophysical properties of peropyrene in toluence and acetonitrile solutions at room temperature were investigated with stationary and time-resolved fluorescence and absorption measurements. Peropyrene is highly fluorescent ( Φ F = 0.93 in toluene) and exhibits several absorption and fluorescence spectroscopic features similar to those of perylene. The absorption spectra of the triplet molecules ( 3PP ∗) and the radical anions ( 2PP −) and cations ( 2PP +) in acetonitrile were recorded. Photoinduced electron transfer reactions between singlet excited peropyrene ( 1PP ∗) and organic electron acceptors were investigated in acetonitrile. The reaction between 1PP ∗ and 1,3-dinitrobenzene (DNB) was studied in more detail. The time-resolved detection of 3PP ∗ and 2PP + as primary products in the fluorescence quenching process allowed the elucidation of the deactivation of the solvent-separated radical ion pair (SSRIP) 1( 2PP +3. 2DNB −). The rate constants for the spin-allowed and the spin-forbidden charge recombination (CR) reactions were determined to be k CR G = (4 ± 2) × 10 10s −1 and k CR T = (6 ± 3) × 10 8s −1. Together with the results from our previous studies of intermolecular electron transfer reactions involving perylene and perylene dyes, these rate constants and their dependences on the corresponding free energy changes are discussed within the framework of non-adiabatic electron transfer theory.

Role of Excited Singlet State in the Photooxidation of Carotenoids: A Time-Resolved Q-Band EPR Study

Spin-polarized 35 GHz time-resolved EPR (TREPR) spectra were obtained for the first time of the cation radicals of β-carotene (I), 15,15‘-didehydro-β-carotene (II), 7‘,7‘-dicyano-7‘-apo-β-carotene (III), and 7‘-cyano-7‘-ethoxycarbonyl-7‘-apo-β-carotene (IV) and the anion radical of the solvent which were formed by 308 nm photoexcitation of the carotenoids in carbon tetrachloride solution. Although the EPR spectra are weak in intensity due to the small dimensions of the 35 GHz quartz flat cell and have very broad line widths, it was possible to positively determine from the polarization pattern that the electron transfer to the solvent occurs from the excited singlet state of the carotenoids. The 35 GHz EPR spectra consist of two resolved EPR lines (one absorption and one emission) from which it has been possible to measure the g factor and ΔHpp of the solvent-separated radical ion pair that was formed; measurements were not possible at 9 GHz.

Intramolecular charge transfer and exciplex formation in anthracene bichromophoric compounds

A binary compound containing strong electron donor and acceptor groups, ( N,N-dimethylanilino)-CH 2-9-cyanothrace ne ( II), was synthesized, and its photophysical behaviour was studied. Intramolecular exciplex emission was observed in non-polar and moderately polar solvents, but not in polar solvents. Exciplex formation in non-polar and moderately polar solvents accounts for the exothermic electron transfer reaction and/or the efficient coupling of the amine lone pair with 9-cyanoanthracene π orbitals; in polar solvents, the molecular structure corresponds to a solvent-separated radical ion pair formed by intermolecular electron transfer, where the energy is dissipated via back electron transfer (BET) without fluorescence emission. A new, blue-shifted emission at 510 nm appears in the region of the local emission band for compound II at 77 K, which may originate from a ground state, charge transfer (CT) conformer. The dependence of exciplex formation on the solvent and temperature is discussed by comparison with the compound ( N,N-dimethylanilino)-CH 2-anthracene ( I), and a simplified energy diagram showing intramolecular charge transfer and exciplex formation in compounds I and II is described. energy diagram showing intramlecular charge transfer and exciplex formation in compounds I and II is described.

Dynamics of Interconversion of Contact and Solvent-Separated Radical-Ion Pairs

The important intermediates in photoinduced bimolecular electron transfer reactions in solution are the contact radicalion pair (CRIP, A'-D'+) and the solvent-separated radical-ion pair (SSRIP, A'-(S)D+).lg2 Their role in the photochemistry of ground state charge-transfer (CT) acceptor (A)/donor (D) complexes, as an example, is illustrated in Scheme 1. Excitation of the AD complex gives a CRIP that may undergo solvation to form a SSRIP (ksOlv) or return electron transfer to the ground state (k-et)q. Within the SSRIP, return electron transfer (k-& further separation (ksep) to form free (fully separated) radical ions (FRI, A'+ D'+), and feedback (desolvation) to the CRIP (k-solv) may occur. Previous studies of radical-ion-pair dynamics have relied mainly on time-resolved absorption ~pectroscopy.~ However, the CRIP, SSRIP and FRI can all potentially contribute to the transient absorption signals? and the complete dynamics of the interconverting radical-ion pairs cannot be determi~~ed.~g.'~ Here we describe an approach that allows all of the rate constants shown in Scheme 1 to be determined, including the rates of interconversion of the CRIP and SSRIP (ksolv, k-solv), thus establishing their relative energies. Excitation of a CT complex usually results in a transient absorption decay due to reactions of the CRIP and the SSRIP and a relatively long-lived residual absorption due to the FRI? The absorption decay may be resolved into two exponential components, depending upon the relative magnitudes of the appropriate rate constants (Scheme 1). In principle, four independent experimental parameters are available from such experiments, Le., two time constants (11 and 12), the ratio of their preexponential factors (Rabs). and the quantum yield for

Cyanoanthracene-sensitized photooxidations of N,N-dibenzylhydroxylamine and its derivatives: free-energy dependence of back electron-transfer rates within geminate radical ion pairs

The fluorescence of 9-cyanoanthracene (CA) and 9,10-dicyanoanthracene (DCA) is quenched by the title hydroxylamine 1 in acetonitrile according to the Stern–Volmer equation. An analysis of the Rehm–Weller plot of the bimolecular quenching rate constant against the free-energy change involved in the electron-transfer (ET) process indicated the operation of an ET mechanism for the fluorescence quenching of CA and DCA by the hydroxylamine quencher. However, solvent deuterium isotope effects on the quantum yields for the CA- and DCA-sensitized oxidations of 1 in acetonitrile established that there is a substantial contribution of a singlet oxygen mechanism to the overall CA-sensitized reaction in the low concentration ranges of 1 but its contribution markedly decreases with an increase in the concentration of 1, thus enabling us to use the limiting quantum yield (Φlim) for the estimation of the back electron-transfer (BET) rate within the initially-formed geminate radical ion pair (GRIP), which serves as a key intermediate for a superoxide mechanism. In addition to protic solvent effects on the Φlim value, the free-energy dependence of the rate of BET giving the starting 1 and the sensitizer demonstrated that BET within the GRIP obtained by ET from 1 to excited DCA proceeds through a solvent-separated radical ion pair (SSRIP) in the Marcus ‘normal region’ whereas BET within the GRIP formed by ET between 1 and excited CA takes place from a contact radical ion pair (CRIP) in the Marcus ‘inverted region’, except in the CA–N,N-bis(p-methylbenzyl)hydroxylamine system. The BET in this system is suggested to occur not from a CRIP but from a SSRIP in the ‘inverted region’ resulting in a much higher sensitized-oxidation efficiency than expected.

Electron-transfer-induced photo-oxygenation reactions and the special salt effect: determination of efficiencies of free radical ion formation from solvent-separated radical ion pairs in oxygen-saturated acetonitrile solutions

The special salt effect exerted by LiClO 4 (LP) on the rate of dicyanoanthracene (DCA)-photo-induced electron transfer oxygenation of tetraarylcyclopropane ( 1) and 1,1-diarylethylenes ( 3a–d) in acetonitrile, was used to determine several important parameters of the solvent-separated radical ion pairs (SSIP): (1) the efficiency α o of dissociation into solvated free radical ions, (2) the ratio of rate constants for the separation of SSIP and the back electron transfer within the SSIP ( k sep/ k back) and (3) the ratio of rate constants for scavenging DCA − from SSIP by LP (or Li +) and the back electron transfer ( k L/ k back). The efficiencies α o turned out to be relatively large, i.e. 0.66 for 1, and, increasing with increasing oxidation potentials, between 0.38 for 3a and 0.8 for 3d. Based on the assumption that k L equals the diffusion-controlled rate constant k diff in MeCN, k sep = 1.7 × 10 7 s −1 and k back = 8 × 10 6 s −1 were estimated for 1. Within the limits of error, k sep = (8±2) × 10 7 s −1 was obtained for substrates 3a–d, whereas k back was found to decrease from 1.64 × 10 8 s −1 for 3a to 2 × 10 7 s −1 for 3d, indicating that back electron transfer, associated with Δ G back values of −2.2 to −2.8 eV, increases with decreasing energy content of the SSIP. This result represents another example for back electron transfer occurring in the Marcus ‘inverted region’. However, by assuming k L ⪕ k diff, the absolute values of k back are about one to two orders of magnitude smaller than those obtained by using the generally accepted value of 5 × 10 8 s −1 for k sep. To explain the lower values of k sep and k back, it is proposed that the encounter distance for electron transfer with the electron donors used in this study may be somewhat larger (about 7–10 Å) than the distance (of about 6.5–7.5 Å) usually assumed for these processes with less bulky electron donors.

Picosecond dynamics of stilbene-olefin contact and solvent-separated radical ion pairs: role of electron transfer and radical ion pair diffusion

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTPicosecond dynamics of stilbene-olefin contact and solvent-separated radical ion pairs: role of electron transfer and radical ion pair diffusionKevin S. Peters and Joseph LeeCite this: J. Am. Chem. Soc. 1993, 115, 9, 3643–3646Publication Date (Print):May 1, 1993Publication History Published online1 May 2002Published inissue 1 May 1993https://doi.org/10.1021/ja00062a031RIGHTS & PERMISSIONSArticle Views106Altmetric-Citations26LEARN 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 (526 KB) Get e-Alerts Get e-Alerts

Role of contact and solvent-separated radical ion pairs in the diffusional quenching of trans-stilbene excited singlet state by fumaronitrile

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTRole of contact and solvent-separated radical ion pairs in the diffusional quenching of trans-stilbene excited singlet state by fumaronitrileKevin S. Peters and Joseph LeeCite this: J. Phys. Chem. 1992, 96, 22, 8941–8945Publication Date (Print):October 1, 1992Publication History Published online1 May 2002Published inissue 1 October 1992https://doi.org/10.1021/j100201a045RIGHTS & PERMISSIONSArticle Views89Altmetric-Citations44LEARN 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 (634 KB) Get e-Alerts

Mechanism of the 9,10-dicyanoanthracene-sensitized photooxidation of N,N-dibenzylhydroxylamine in polar and non-polar solvents

The 9,10-dicyanoanthracene (DCA)-sensitized photooxidation of N,N-dibenzylhydroxylamine (DBH) gives α-phenyl-N-benzylnitrone (PBN) and hydrogen peroxide in quantitative yields at low conversions of the starting DBH both in oxygen-saturated polar solvents, acetonitrile and methanol, and in an oxygen-purged non-polar solvent, carbon tetrachloride. Thermodynamic and kinetic analyses of this sensitized oxidation in polar solvents indicate the involvement of superoxide (O2˙–) as the major oxidizing species in the process. Solvent deuterium isotope effects on the quantum yield for the appearance of PBN (ΦPBN) provide supporting evidence supporting evidence for the preferential O2˙– mechanism by which the sensitized reaction in acetonitrile and methanol proceeds. In addition, a higher sensitized-oxidation efficiency is obtained in methanol (limiting quantum yield, ΦPBN, lim= 0.37) than in acetonitrile (ΦPBN, lim= 0.21). This finding is explained in terms of the much slower rate of back electron transfer within the initially-formed solvent-separated radical ion pair in methanol as compared to that in acetonitrile. On the other hand, a comparison of the DBH concentration dependence of ΦPBN for the DCA- and tetraphenylporphine-sensitized photooxidations in carbon tetrachloride establishes the exclusive operation of a singlet oxygen (1O2) mechanism in this non-polar solvent. The chemical deactivation of 1O2 assisted by the hydroxylic hydrogen in DBH, taking place via a charge-transfer complex, is proposed to account for the efficient formation of PBN and hydrogen peroxide in carbon tetrachloride.