The absolute kinetics for radical addition and electronic energy transfer to [1.1.1]propellane

1998 ◽  
Vol 76 (10) ◽  
pp. 1474-1489 ◽  
Author(s):  
P F McGarry ◽  
J C Scaiano
Keyword(s):  
Free Radicals ◽  
Rate Constants ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Laser Flash ◽  
Electronic Energy ◽  
Triplet States ◽  
Quenching Rate ◽  
Electronic Energy Transfer ◽  
Diffusion Controlled

Free radicals react more readily with [1.1.1]propellane, 1, than with styrene. For example Et3Si· reacts with 1 and styrene with rate constants of 6 × 108 M-1 s-1 and 2 × 108 M-1 s-1, respectively. Fluorenone, phenanthrene, triphenylene, benzophenone, and pyrene transfer electronic energy to 1 with rate constants well below the diffusion-controlled limit. For example, triplet benzophenone is quenched by 1 with a bimolecular rate constant of 9.9 × 106 M-1 s-1. A linear dependence of the log of the quenching rate constants, log kq, upon the excited-state energy of the donors is found.Key words: propellane, laser flash photolysis, free radicals, triplet states.

Time-resolved nanosecond and picosecond absorption studies of excited-state properties of 1-thiobenzoylnaphthalene

1989 ◽  
Vol 67 (6) ◽  
pp. 967-972 ◽  
Author(s):  
R. Minto ◽  
A. Samanta ◽  
P.K. Das
Keyword(s):  
Rate Constants ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Picosecond Laser ◽  
Laser Flash ◽  
Quantum Yields ◽  
Pulse Excitation ◽  
Nanosecond Laser Pulse ◽  
Nanosecond Laser ◽  
Quenching Rate

1-Thiobenzoylnaphthalene (TBN), known for its pericyclization reaction from the lowest excited singlet state (S1), has been subjected to nanosecond and picosecond laser flash photolysis studies. The two major transients observed in the course of nanosecond laser pulse excitation are (i) the short-lived triplet characterized by two absorption maxima (400–410 and 740–750 nm) and submicrosecond intrinsic lifetimes (80–130 ns) and (ii) a relatively long-lived species (λmax = 520 nm and τ = 220–240 ns). Various triplet-related photophysical data of TBN, including self-quenching and bimolecular quenching rate constants, have been determined. The existence of a photochemical path from S1 manifests itself in low intersystem crossing quantum yields, particularly in the polar/hydrogen-bonding solvent, methanol. From the build-up of the triplet under picosecond excitation into S1 the lifetime of the latter is estimated to be ≤ 50 ps (in benzene). The fast intrinsic decay of TBN triplet is attributable to facile intra- and intermolecular photochemistry. The 520 nm transient species could not be definitively assigned, except that it is neither a triplet nor a triplet-derived product and that it arises via photochemistry from S1. Keywords: laser flash photolysis, triplet, transients, absorption maxima, lifetimes, quenching rate constants, photochemistry, 1-thiobenzoylnaphthalenes.

scholarly journals Dynamics of small molecules within the F127 PEO-PPO-PEO triblock copolymer gel and sol phases studied at the molecular scale

2022 ◽  
Author(s):  
Suma S. Thomas ◽  
Helia Hosseini-Nejad ◽  
Cornelia Bohne
Keyword(s):  
Excited States ◽  
Rate Constants ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Laser Flash ◽  
Supramolecular System ◽  
Quenching Rate ◽  
Naphthalene Derivatives ◽  
Decay Rate Constant ◽  
Triplet Excited States

The dynamics of naphthalene derivatives with different hydrophobicities bound to F127 polyethyleneoxide-polypropyleneoxide-polyethyleneoxide (PEO-PPO-PEO) micelles in the gel and sol phases were studied using a quenching methodology for the triplet excited states of the naphthalenes. Studies with triplet excited states probe a larger reaction volume than the volumes accessible when using fluorescent singlet excited states. The use of triplet excited states enables the determination of the dynamics between different compartments of a supramolecular system, which in the case of F127 micelles are the micellar core, the micellar corona and the aqueous phase. This report includes laser flash photolysis studies for the four naphthalene derivatives in the F127 gel and sol phases. The triplet excited states were quenched using the nitrite anion as the quenchers. The association and dissociation rate constants of the naphthalenes from the micelles and the quenching rate constants for the naphthalenes bound to the micelles were determines from the curved quenching plot (observed decay rate constant vs. nitrite concentration).

Direct detection of methylphenylgermylene and 1,2-dimethyl-1,2-diphenyldigermene — Kinetic studies of their reactivities in solution

2006 ◽  
Vol 84 (7) ◽  
pp. 934-948 ◽  
Author(s):  
William J Leigh ◽  
Ileana G Dumbrava ◽  
Farahnaz Lollmahomed
Keyword(s):  
Acetic Acid ◽  
Rate Constants ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Direct Detection ◽  
Kinetic Studies ◽  
Laser Flash ◽  
Absolute Rate ◽  
Hydrocarbon Solvents ◽  
Diffusion Controlled

Photolysis of 1,3,4-trimethyl-1-phenylgermacyclopent-3-ene (5) in hydrocarbon solvents containing isoprene, methanol, or acetic acid affords 2,3-dimethyl-1,3-butadiene (DMB) and the expected trapping products of methyl phenylgermylene (GeMePh) in chemical yields exceeding 90%. The germylene has been detected in hexane solution by laser flash photolysis as a short-lived species (τ ~ 2 µs) exhibiting a UV-vis absorption spectrum centered at λmax = 490 nm. It decays with second-order kinetics and a rate constant close to the diffusion-controlled limit, with the concomitant growth of a second longer-lived transient (λmax = 420 nm) that is assigned to a mixture of (E)- and (Z)-1,2-dimethyl-1,2-diphenyldigermene (4). Absolute rate constants have been determined for the reactions of the germylene with primary and tertiary amines (n-BuNH2 and Et3N, respectively), acetic acid (AcOH), a terminal alkyne and alkene, isoprene, DMB, CCl4, and the group 14 hydrides Et3SiH and Bu3SnH. GeMePh is slightly more reactive than GePh2 towards all the reagents studied in this work; both are significantly less reactive than GeMe2 toward the same substrates. Absolute rate constants for the reactions of 4 have also been measured or assigned upper limits in every case and are compared to previously reported values for tetraphenyl- and tetramethyl-digermene with the same reagents.Key words: germylene, digermene, kinetics, laser flash photolysis, germirane, germirene, vinylgermirane, complex, UV–vis spectrum, insertion, addition.

Reactivity and photophysical behavior of chromone triplet. A laser flash photolysis study

1987 ◽  
Vol 65 (9) ◽  
pp. 2277-2285 ◽  
Author(s):  
S. Rajadurai ◽  
P. K. Das
Keyword(s):  
Rate Constants ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Laser Flash ◽  
Hydrogen Atom Transfer ◽  
Oxidation Potential ◽  
Pulse Excitation ◽  
Quenching Rate ◽  
Diffusion Limited ◽  
Photophysical Behavior

The chromone triplet ([Formula: see text] in acetonitrile) is produced in quantitative yields upon 308- or 337.1-nm laser pulse excitation and is characterized by submicrosecond lifetimes in solutions at room temperature. The short-lived nature of the triplet is attributable to intrinsically fast T1 [Formula: see text] S0 intersystem crossing, nearly diffusion-limited self-quenching, and facile interactions with solvents in the form of charge and hydrogen-atom transfer. The unusually high self-quenching rate constants, (0.9–4.0) × 109 M−1 s−1, are related in a major part to the presence of the ene double bond at which the photocycloaddition of the triplet may occur; this is supported by large bimolecular rate constants [Formula: see text] observed for chromone triplet quenching by various alkene derivatives. Although, based on low-temperature photophysical behaviors, the lowest triplet state of chromone in polar solvents is expected to be of reduced n,π* character, the reactivity of the triplet toward hydrogen donors is very pronounced in acetonitrile (for example, [Formula: see text] for tri-n-butylstannane and 2-propanol, respectively). Carbon tetrachloride and benzene prove to be facile quenchers of chromone triplet; the quenching interactions probably involve charge transfer, the carbonyl triplet acting as a donor and an acceptor, respectively. The electrophilic role of chromone triplet in the quenching by benzene derivatives is supported by a good correlation between [Formula: see text] and quencher oxidation potential.

Mechanism of acid-catalyzed photoaddition of methanol to 3-alkyl2-cyclohexenones

1995 ◽  
Vol 73 (11) ◽  
pp. 2004-2010 ◽  
Author(s):  
David I. Schuster ◽  
Jie-Min Yang ◽  
Jan Woning ◽  
Timothy A. Rhodes ◽  
Anton W. Jensen
Keyword(s):  
Acetic Acid ◽  
Triplet State ◽  
Rate Constant ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Laser Flash ◽  
Product Formation ◽  
Triplet States ◽  
Quenching Rate ◽  
Acid Catalyzed

Contrary to a previous report, it is concluded that formation of methanol adducts to 3-methyl-2-cyclohexenones and of deconjugated enones on irradiation of the enones in acidified solutions proceeds via protonation of the intermediate enone π,π* triplet excited state and not by protonation of a relatively long-lived ground state trans-cyclohexenone. A rate constant for protonation of the triplet state of 3-methyl-2-cyclohexenone by sulfuric acid of 1.7 × 109 M−1 s−1 was determined by laser flash photolysis in ethyl acetate. Based on quantum efficiencies of product formation, a rate constant of ca. 108 M−1 s−1 was estimated for protonation of the enone triplet by acetic acid, which is too small to cause measurable reduction in the triplet state lifetime in the mM concentration range used in the preparative studies. The intermediate carbocation can be trapped by methanol, or revert to starting enone or the exocyclic deconjugated enone by loss of a proton. Since products revert to starting materials in an acid-catalyzed process, there is an acid concentration at which the yields of products are optimal. This concentration is ca. 6 mM for acetic acid, but is only 0.1 mM for p-toluenesulfonic or sulfuric acids. Product formation could be quenched using 1-methylnaphthalene and cyclopentene as triplet quenchers; in the latter case, formation of [2 + 2] photoadducts was observed to compete with formation of methanol adducts. Quenching rate constants were determined by laser flash studies. Keywords: laser flash photolysis, kinetic absorption spectroscopy (KAS), photoacoustic calorimetry (PAC), protonation of triplet states, trans-cyclohexenones.

Laser flash photolysis studies of 2,4,6-trialkylphenyl ketones

1989 ◽  
Vol 67 (3) ◽  
pp. 473-480 ◽  
Author(s):  
B. Guerin ◽  
L. J. Johnston
Keyword(s):  
Energy Transfer ◽  
Rate Constants ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Laser Flash ◽  
Hydrogen Donor ◽  
Quenching Rate ◽  
Phenyl Ketone

2,4,6-Triisopropylbenzophenone (3), 2,4,6–trimethylbenzophenone (4), and 2,4,6-trimethylacetophenone (5) have been examined by laser flash photolysis. Relatively long-lived triplets compared to similar mono-alkyl substituted ketones have been observed for each (280 ns, >2 μs and 220 ns in acetronitrile for 3, 4 and 5, respectively). Photoenol intermediates with λmax 360 nm and 380 nm were also observed from ketones 3 and 5, respectively. Ketone 3 yielded a mixture of approximately equal amounts of Z and E enols in hexane. The Z enol had a lifetime of ~4 μs while the E enol did not decay appreciably during 100 μs. Biradical intermediates were not observed from any of the three ketones. Quenching rate constants for several energy transfer and hydrogen donor substrates are reported. For example, ketones 3 and 4 react with 1,4-cyclohexadiene with rate constants of 2.8 × 107 and 3.7 × 107 M−1 s−1, respectively, to yield ketyl radicals with λmax ~480 nm. Keywords: photoenolization, trialkyl phenyl ketone, flash photolysis, triplets.

Singlet Methylene Removal Rate Constants from the (0,1,0) Vibrational Level: Enhancement via Complex-Mediated Vibrational Relaxation

2001 ◽  
Vol 215 (6) ◽  
Author(s):  
M.A. Buntine ◽  
G.J. Gutsche ◽  
W.S. Staker ◽  
M.W. Heaven ◽  
K.D. King ◽  
...  
Keyword(s):  
Vibrational Level ◽  
Rate Constants ◽  
Vibrational Relaxation ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Removal Rate ◽  
Laser Flash ◽  
Laser Absorption ◽  
Photolysis Laser ◽  
Singlet Methylene

The technique of laser flash photolysis/laser absorption has been used to obtain absolute removal rate constants for singlet methylene,

Mechanistic studies of aromatic ketone-sensitized photoreduction of bis(acetylacetonato)copper(II)

1983 ◽  
Vol 61 (5) ◽  
pp. 801-808 ◽  
Author(s):  
Yuan L. Chow ◽  
Gonzalo E. Buono-Core ◽  
Bronislaw Marciniak ◽  
Carol Beddard
Keyword(s):  
Rate Constant ◽  
Rate Constants ◽  
Flash Photolysis ◽  
Decay Rates ◽  
Polynuclear Aromatic Hydrocarbons ◽  
Aromatic Ketone ◽  
Quantum Yields ◽  
Triplet States ◽  
Quenching Rate ◽  
Triplet Energy

Bis(acetylacetonato)copper(II), Cu(acac)2, quenches triplet excited states of ketones and polynuclear aromatic hydrocarbons efficiently, but only aromatic ketones with high triplet energy successfully sensitize photoreduction of Cu(acac)2 in alcohols under nitrogen to give derivatives of aeetylacetonatocopper(I), Cu(acac). For the triplet state benzophenone-sensitized photoreduction of Cu(acac)2, the quantum yields of photoreduction (ΦC) and those of benzophenone disappearance (ΦB) were determined in methanol with various concentrations of Cu(acac)2. The values of the quenching rate constant, kq, determined from these two types of monitors on the basis of the proposed mechanism were in good agreement (6.89 ~ 7.35 × 109 M−1 s−1). This value was higher, by a factor of about two, than that obtained from the monitor of the benzophenone triplet decay rates generated by flash photolysis in the presence of Cu(acac)2. The quenching rate constants of various aromatic ketone and hydrocarbon triplet states by Cu(acac)2 were determined by flash photolysis to be in the order of the diffusion rate constant and the quantum yields of these photoreductions were found to be far from unity. Paramagnetic quenching, with contributions of electron exchange and charge transfer, was proposed as a possible quenching mechanism. For a series of aromatic ketone sensitizers with higher triplet energy, this mechanism was used to rationalize the observed high quenching rate constants in contrast to the low quantum yields of photoreduction.

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