Reactive ion pairs from the charge-transfer excitation of electron donor-acceptor complexes

Masnovi, John; Kochi, Jay K.; Hilinski, Ε. F.; Rentzepis, P. M.

doi:10.1021/ja00266a003

Cited by 83 publications

(29 citation statements)

References 2 publications

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“…In methylene chloride solution the anodically generated cation radical absorbs at 440 and 725 nm (20). The longer wavelength in both cases is outside the continuum probed in our experiment; however, we can assign the ca.…”

Section: Quantum Yieldsmentioning

confidence: 49%

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Electron transfer sensitized photolysis of 'onium salts

DeVoe¹,

Sahyun²,

Schmidt³

et al. 1988

Can. J. Chem.

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. Can. J. Chem. 66, 319 (1988). We have studied the anthracene-sensitized photolyses of both diphenyliodonium and triphenylsulphonium salts in solution using both steady-state and laser flash photolysis techniques. Photoproducts, namely, phenylated anthracenes along with iodobenzene or diphenylsulphide, respectively, are obtained from both salts with quantum efficiencies of ca. 0.1 at 375 nm. We infer the intermediacy of diphenyliodo and triphenylsulphur radicals formed by single electron transfer from the singlet-excited anthracene. We have developed a quantitative model of this chemistry, and identify the principal sources of inefficiency as back electron transfer, which occurs at nearly the theoretically limiting rate, intersystem crossing from the initially formed sensitizer-'onium salt encounter complex, and in-cage radical recombination. Faisant appel aux techniques de 1'Ctat stationnaire et de la photolyse laser eclair, on a CtudiC la ptiotolyse sensibiliske par l'anthrackne des sels de diphCnyliodonium et de triphCnylsulfonium en solution. Pour chacun des sels, a 375 nm, les efficacitCs quantiques en photoproduits, soit des anthracknes phtnylCs ainsi que respectivement de l'iodobenzkne et du sulfure de diphenyle, sont d'environ 0 , l . On en dCduit que les radicaux diphCnyliodo-et triphCnylsoufre se forment comme intermCdiaires par le transfert d'un Clectron de l'anthrackne excitC singulet. On a dCvelopp6 un modkle quantitatif de cette chimie et on a identifit que les principales causes de I'inefficacitC sont un transfert en retour d'tlectron qui se produit a une vitesse proche de la limite thkorique, un passage intersystkme se produisant a partir du complexe de rencontre sensibilisateur-sel onium qui se forme initialement et une recombinaison radicalaire se produisant dans la cage.[Traduit par la revue]

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Section: Quantum Yieldsmentioning

confidence: 49%

“…The value of k6 is in the range expected for exergonic, adiabatic electron transfers (13,24) and observed in the radical pair formed from photolysis of the anthracene-TCNE complex (20). The value for kg likewise compares with the rate constant observed for in-cage combination in the anthracene-tetranitromethane system (20).…”

Section: Computer Modellingmentioning

confidence: 54%

Electron transfer sensitized photolysis of 'onium salts

DeVoe¹,

Sahyun²,

Schmidt³

et al. 1988

Can. J. Chem.

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“…These considerations allow us to write the mechanism of scheme [8], which incorporates both the excitation of the C T state of 1 and the intermediacy of phenyl cation. There is precedent for decay of a C T excited state to a donor (or acceptor) localized excited state; for example, the C T excited state of the anthracene-TCNE complex decays to a triplet anthracene containing complex, which then dissociates to 1 liberate triplet excited anthracene (32). Within this scheme we hv…”

Section: Discussionmentioning

confidence: 99%

Transient intermediates in the photolysis of iodonium cations

DeVoe¹,

Sahyun²,

Serpone³

et al. 1987

Can. J. Chem.

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Can. J. Chem. 65, 2342(1987.Pulsed picosecond laser photolysis of the diphenyliodonium cation indicates that excitation of the allowed transition is followed by rapid (ps) deactivation to a homolytically dissociative state, presumably T I . Steady-state photolysis yields iodobenzeneand biphenyl from the primary radical products. Iodobiphenyl is also observed. We infer in-cage recombination of these radical products, with intervention of a carbocationic intermediate. Laser flash photolysis of diphenyliodonium iodide in its charge transfer band yields an initial transient with a lifetime of ca. 200 ps. A strongly absorbing secondary transient is observed on the nanosecond time scale. Iodobenzene is the exclusive reaction product. We infer that it is not formed directly from the primary photoproduct, but through in-cage recombination of phenyl cation with iodide anion, as in benzenediazonium iodide. R. J. DEVOE, M. R. V. SAHYUN, N. SERPONE et D. K. SHARMA. Can. J. Chem. 65, 2342 (1987) La photolyse laser pulste au picoseconde du cation diphtnyliodonium indique que l'excitation de la transition permise est suivie par une dksactivation rapide (ps) conduisant a un ttat dissocik homolytique, probablement T I . La photolyse A l'ttat dissocit homolytique, probablement T I . La photolyse 5 l'ktat stationnaire conduit a la formation d'iodobenzene et de biphtnyle a partir des produits radicalaires primaires. On observe aussi la formation d'iodobiphtnyle. On en conclut qu'il se produit une recombinaison en cage de ces produits radicalaires, avec une intervention d'un intermkdiare carbocationique. La photolyse laser flash de l'iodure de diphknyliodonium dans sa bande de transfert de charge conduit initialement la formation d'une espece transitoire dont le temps de demi-vie est d'environ 200 ps. On observe aussi la formation d'une espkce transitoire secondaire qui absorbe fortement et dont le temps de demi-vie est de l'ordre de la nanoseconde. L'iodobenzkne est le seul produit dq la rkaction.On en dtduit qu'il ne se forme pas directement a partir du photoproduit primaire, mais plutdt par une recombinaison en cage du cation phtnyle avec un anion iodure, comme dans le cas de l'iodure de benzenediazonium.[Traduit par la revue] Introduction Photolabile 'onium salts, particularly those of aryldiazonium, diaryliodonium, and triarylsulfonium cations, have found application in imaging media and in radiation curable coatings (1). The mechanism of photolysis of diphenyliodoniurn cations was probed by following the absorption spectroscopy of light absorbing transient intermediates on the nanosecond time scale by Klemm et al. (2) and by Pappas et a1. (3). These workers observed iodobenzene radical cation and, in the former instance, phenyl radical, both of which had been formed on a time scale fast compared to the resolution capabilities of their experiments. Accordingly, we decided to probe this chemistry with picosecond time-resolved absorption spectroscopy.

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“…(6) etfectively occurs with the absorption of the excitation photon (hvnl in accord with the Mulliken theory applicable to weak complexes 2 "'. Accordingly, the ion radicals HBt and TCNE-;-are bom from the precursor complex as an intimate contact ionpair (CIP) since the photochemical time scale of < 30 ps obviates significant competition from diffusional processes 291 301 [HB,TCNE] must therefore be included in any mechanistic formulation of CT cycloaddition by taking particular cognizance of how it decays. Most important is the competition from the return to the ground state by back electron transfer (bet) with a first-order rate constant that is estimated to be kbet > 10 10 s _, from the redox potentials 31 • 32 >.…”

Section: Mechanism Of Charge-transfer Cycloadditionmentioning

confidence: 99%

Charge‐Transfer Cycloaddition of Homobenzvalene with Tetracyanoethylene

1990

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The transient yellow color observed in the cycloaddition of homobenzvalene (HB) with tetracyanoethylene (TCNE) is associated with the charge-transfer complex [HB, TCNE). The deliberate photoexcitation of [HB,TCNE) affords a mixture of charge-transfer cycloadducts (1, 2, and 3) that differs from thatThe Diels-Alder cycloadditon of tetracyanoethylene (TCNE) to various anthracenes is visually characterized by transient colorswith the vivid hues covering the entire visible spectrum attendant upon substituent changes on the aromatic donor (Ar) 1 -3 >. Spectrophotometric sturlies establish the colors to derive from weak 1:1 complexes that are initially formed upon the exposure of TCNE to arenes, i.e.with the formation constant K typically less than 10 2 >. As such, the spectral characterization of these transient molecular complexes by their intense electronic absorption in the visible region is in accord with Mulliken's prediction of the charge-transfer (Cn transition from the electron-rich aromatic donor (Ar) to the electron-poor acceptor (TCNE) 4 • 51 • The close relationship of the CT excited state and the transition state of the thermal process can be deduced from the linear free energy corrclation observed between the charge-transfer excitation energy (hvCT) of the [TCNE-Anthracene] complex and the secondorder rate constant (log k2) for the Diels-Alder cycloaddition 6 1. lndeed, a related study 7 1 of the [2 + 4] cycloaddition of TCNE and diphenylbenzocyclobutene (DBC) recently established the Chargetransfer activation to follow the same stereochemical course as that obtained by the thermal valence tautomerization of DBC, as previously delineated by Huisgen, Quinkert, and co-workers 8 • 9 l. However, the attempt to demonstrate the direct involvement of the [fCNE-Arene] complex in the Diels-Alder cycloaddition by Cf activation was beset with experimental difficulties 10 >. Thus, those anthracenes (with electron-withdrawing substituents) which could be examined photochemically afforded thermally labile (i. e. reversible) Diels-Alder adducts, whereas the other analogues yielded stable adducts but at thermal rates that were too fast to provide an unambiguous answer. Accordingly, we have focussed our attention in this study on the facile cycloaddition of TCNE to homobenzvalene 11 >, when we learned of the fleeting yellow color 121 that was suggestive of the CT complexes as transient intermediates of the type described above.

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Reactive ion pairs from the charge-transfer excitation of electron donor-acceptor complexes

Cited by 83 publications

References 2 publications

Electron transfer sensitized photolysis of 'onium salts

Electron transfer sensitized photolysis of 'onium salts

Transient intermediates in the photolysis of iodonium cations

Charge‐Transfer Cycloaddition of Homobenzvalene with Tetracyanoethylene

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