Picosecond excited-state absorption of alkylnitrobenzenes in solution

Yip, R. W.; Sharma, Dinkar; Giasson, R.; Gravel, Denis

doi:10.1021/j150668a004

Cited by 87 publications

(95 citation statements)

References 2 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…The triplet character of the red absorption was verified by transient EPR measurements at 77 K in a toluene glass (SI Appendix) and the intersystem crossing time observed here is comparable to that observed for nπ Ã states in nitrobenzenes (13). The T a band reaches its maximum amplitude within a few ps and then decays, without a change in spectral position on a time scale of 2-3 μs.…”

Section: Resultssupporting

confidence: 81%

Concerted electron-proton transfer in the optical excitation of hydrogen-bonded dyes

Westlake

Brennaman²,

Concepcion³

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

154

View full text Add to dashboard Cite

The simultaneous, concerted transfer of electrons and protonselectron-proton transfer (EPT)-is an important mechanism utilized in chemistry and biology to avoid high energy intermediates. There are many examples of thermally activated EPT in ground-state reactions and in excited states following photoexcitation and thermal relaxation. Here we report application of ultrafast excitation with absorption and Raman monitoring to detect a photochemically driven EPT process (photo-EPT). In this process, both electrons and protons are transferred during the absorption of a photon. Photo-EPT is induced by intramolecular charge-transfer (ICT) excitation of hydrogen-bonded-base adducts with either a coumarin dye or 4-nitro-4′-biphenylphenol. Femtosecond transient absorption spectral measurements following ICT excitation reveal the appearance of two spectroscopically distinct states having different dynamical signatures. One of these states corresponds to a conventional ICT excited state in which the transferring H þ is initially associated with the proton donor. Proton transfer to the base (B) then occurs on the picosecond time scale. The other state is an ICT-EPT photoproduct. Upon excitation it forms initially in the nuclear configuration of the ground state by application of the Franck-Condon principle. However, due to the change in electronic configuration induced by the transition, excitation is accompanied by proton transfer with the protonated base formed with a highly elongated þ H─B bond. Coherent Raman spectroscopy confirms the presence of a vibrational mode corresponding to the protonated base in the optically prepared state.electron transfer | proton-coupled electron transfer P roton-coupled electron transfer (PCET), in which electrons and protons are both transferred, is at the heart of many energy conversion processes in chemistry and biology (1-6). PCET reactions can occur by sequential two-step transfers (e.g., electron transfer followed by proton transfer, ET-PT, or proton transfer followed by electron transfer, PT-ET) or by concerted electron-proton transfer (EPT) (1, 2). EPT pathways are important in avoiding high-energy intermediates, playing an integral role in photosynthesis and respiration, for example.Photo-driven EPT (photo-EPT), with electron and proton transfers occurring simultaneously during the optical excitation process, would appear to be ruled out on fundamental grounds, because electronic excitation occurs rapidly on the time scale for nuclear motions, including proton transfer. Using a combination of femtosecond pump-probe and coherent Raman techniques, we have observed simultaneous electron-proton transfer induced by intramolecular charge transfer (ICT) excitation in two different hydrogen-bonded adducts formed between an organic dye (A─O─H) and an external base (:B). One is formed between a para-nitrophenyl-phenol and an amine base, and the other between a coumarin derivative and an imidazole base (Fig. 1).The shift in electron density away from the hydroxyl group to the intramolecular ...

show abstract

Section: Resultssupporting

confidence: 81%

Concerted electron-proton transfer in the optical excitation of hydrogen-bonded dyes

Westlake

Brennaman²,

Concepcion³

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

154

View full text Add to dashboard Cite

show abstract

“…l), using the third harmonic from a YAG laser system, were assigned to the triplet excited state on the basis of the similarity of their spectra and lifetimes with those that we previously reported for other relatively unreactive simple nitrobenzene compounds (5,9), and by the observed decrease in the transient lifetimes of 6 and 7 on addition of the tri let quencher, 1-hydroxymethyl-1 -methyl-2,4-cycloheradieneq(kq = (1.2 + 0.8) X lo9 M-I s-I and (2.6 t 0.5) X lo9 M-' s-' for 6 and 7, respectively). The two transient absorption bands at 425-450 nrn and a weaker band in the 600-650 nm region are almost identical in terms of the peak position, width, and area with those due to the triplet state for the simple nitrobenzene compounds.…”

Section: Resultsmentioning

confidence: 97%

“…With the OMA system used in our experiment there is only a marginal increase in signal-to-noise ratio of the spectrum with further averaging owing to drift within the system. Characterization of the triplet excited states of 6 and 7 is based on the similarity of their spectra and lifetimes to those that we previously reported for other relatively umeactive nitrobenzene compounds (5,9), and on the observed decrease in the transient lifetimes of 6 and 7 on addition of ca. 1.5 M of the triplet quencher, l-hydroxymethyl-l-methyl-2,4-cyclohexadiene.2…”

Section: Flash Photolysis Experimentsmentioning

confidence: 90%

Photochemistry of the o-nitrobenzyl system in solution: effects of distance and geometrical constraint on the hydrogen transfer mechanism in the excited state

Gravel¹,

Giasson²,

Blanchet³

et al. 1991

Can. J. Chem.

Self Cite

View full text Add to dashboard Cite

. Can. J. Chem. 69, 1193Chem. 69, (1991. Two rigid nitrobenzene derivatives, 5-nitro-1,2,3,4-tetrahydro-1,4-methanonaphthalene (6), and 5-nitro-] ,2,3,4-tetrahydro-1,4-ethanonaphthalene (7), have been synthesized and studied by picosecond flash absorption spectroscopy, steady-state irradiation, and MMX molecular mechanics calculations. Transients with lifetimes of 770 and 410 ps have been detected and assigned to the excited triplet states of 6 and 7, respectively. Consistent with prediction, the o-quinonoid intermediate is not detected in the time domain between the laser excitation and the end of decay of the triplet state. Intramolecular abstraction of the bridgehead benzylic hydrogen by the cited triplet state proceeds with relative rates of 1:125 for 6 and 7, respectively. Since structural constraints prohibit the formation of an orthoquinonoid intermediate in these systems, formation of the usual nitrosoalcohol 13 represents the first substantiation of the biradical route to product (3' + 4, Scheme 1). Absorption of the intermediate triplet biradical resulting from hydrogen abstraction by the triplet excited state was not observed, and is therefore believed to be either very weak in the 400-700 nm region, or located outside the detection region. The kinetic results together with structural parameters determined by molecular mechanics show good correlation between structure and reactivity. Hydrogen atom transfer can proceed at up to 53" deviation from linearity of the C-H-0 nuclei, with transfer rates decreasing sharply as the 0. -.H distance increases from 1.3 to 1.6 A. It is suggested that the conditions of the reaction from the excited triplet state meet those required for a hydrogen tunneling mechanism.Key words: o-nitrotoluene photorearrangement, mechanism of; o-nitrobenzyl systems, mechanism of photorearrangement; 0. . .H distance, influence of, in hydrogen abstraction mechanism; picosecond flash absorption spectroscopy study of o-nitrobenzyl photorearrangement; radical pathway to product in o-nitrotoluene photorearrangement.

show abstract

“…This result gives kinetic support to the proposed mechanism presented in Scheme 1. Simple nitrobenzenes are known to have characteristically short triplet lifetimes of -1 ns (3, 16,17), which has been attributed to rapid radiationless decay to So. Assuming that 7; = 1 X lop9 s, then we calculate 7 : = 0.42 x l o p 9 s and koH = 1 X lo9 M -I s-l.…”

Section: Discussionmentioning

confidence: 99%

“…Simple nitrobenzenes, without strongly electron-donating groups, are believed to have n,n* lowest triplets (1-4, 16,17) in the commonly employed organic solvents. This is consistent with the observation that simple nitrobenzenes react via characteristic n , n * triplet state reactions, in close analogy with many carbonyl n , n * triplcts.…”

Section: -mentioning

confidence: 99%

Photoredox chemistry of nitrobenzyl alcohols in aqueous solution. Acid and base catalysis of reaction

Wan¹,

Yates²

1986

Can. J. Chem.

View full text Add to dashboard Cite

PETER WAN and KEITH YATES. Can. J. Chem. 64, 2076Chem. 64, (1986. The photochemistry of several m-and p-nitrobenzyl a!cohols (1-5) has been studied in aqueous solution. These compounds react via an intramolecular photoredox pathway to give reduced and oxidized moieties of the substituent groups. The reaction is an example of a new type of photoreaction of nitro-substituted aromatic derivatives that is not observed in organic solvents, the presence of water being essential. This effect is exemplified by measuring the quantum efficiency as a function of mol% water in aqueous acetonitrile, methanol, and formamide: the reaction efficiency decreases rapidly as water is depleted in the mixture. Catalytic effects due to the hydronium and hydroxide ions were studied: the para derivatives exhibited hydroxide ion catalysis; the meta derivatives exhibited hydronium ion catalysis. Quantum yields, solvent isotope effects, and a-deuterium isotope effects are reported for the parent derivatives.PETER WAN et KEITH YATES. Can. J. Chem. 64, 2076 (1986).On a CtudiC la photochimie de plusieurs alcools m-et p-nitro benzyliques (1-5) en solution aqueuse. Ces composCs rkagissent selon un processus photoredox intermediaire pour donner des fragments de groupes substituCs oxydCs et reduits. C'est un exemple d'un nouveau type de rCactions photochimiques des dCrivCs aromatiques nitrCs que l'on n'observe pas dans des solvants organiques. L'eau est donc indispensable. On a mis en evidence le r61e de I'eau en mesurant le rendement quantique de la rCaction en fonction du pourcentage molaire d'eau dans des solutions aqueuses d'acetonitrile, de mCthanol et de formamide. Le rendement de la rCaction diminue rapidement lorsque la teneur en eau du mClange diminue. On a Ctudie les effets catalytiques dus aux ions hydronium et hydroxydes. Les rkactions des dCrivCs para sont catalyses par les ions hydroxydes tandis que celles des derives substituks en me'ta sont catalysees par les ions hydroniums. On rapporte les rendements quantiques, les effets isotopiques du solvant et les effets isotopiques du deutCrium en a des derives apparent&.[Traduit par la revue]

show abstract

Picosecond excited-state absorption of alkylnitrobenzenes in solution

Cited by 87 publications

References 2 publications

Concerted electron-proton transfer in the optical excitation of hydrogen-bonded dyes

Concerted electron-proton transfer in the optical excitation of hydrogen-bonded dyes

Photochemistry of the o-nitrobenzyl system in solution: effects of distance and geometrical constraint on the hydrogen transfer mechanism in the excited state

Photoredox chemistry of nitrobenzyl alcohols in aqueous solution. Acid and base catalysis of reaction

Contact Info

Product

Resources

About