Steady state and time-resolved spectroscopic studies of the photochemistry of 1-arylsilacyclobutanes and the chemistry of 1-arylsilenes

Direct photolysis of 1-phenylsilacyclobutane and 1-phenyl-, 1-(2-phenylethynyl)-, and 1-(4'-biphenylyl)-1-methylsilacyclobutane in hexane solution leads to the formation of ethylene and the corresponding 1-arylsilenes, which have been trapped by photolysis in the presence of methanol. Quantum yields for photolysis of the three methyl-substituted compounds have been determined to be 0.04, 0.26, and 0.29, respectively, using the photolysis of 1,1-diphenylsilacyclobutane Φsilene = 0.21) as the actinometer. The corresponding silenes have been detected by laser flash photolysis; they have lifetimes of several microseconds, exhibit UV absorption maxima ranging from 315 to 330 nm, and react with methanol with rate constants on the order of (2-5) × 109 M-1 s-1 in hexane. Absolute rate constants for reaction of 1-phenylsilene and 1-methyl-1-phenylsilene with water, methanol, tert-butanol, and acetic acid in acetonitrile solution have been determined, and are compared to those of 1,1-diphenylsilene under the same conditions. With the phenylethynyl- and biphenyl-substituted methylsilacyclobutanes, the triplet states can also be detected by laser flash photolysis, and are shown to not be involved in silene formation on the basis of triplet sensitization and (or) quenching experiments. Fluorescence emission spectra and singlet lifetimes have been determined for the three 1-aryl-1-methylsilacyclobutanes, 1,1-diphenylsilacyclobutane, and a series of acyclic arylmethylsilane model compounds. These data, along with the reaction quantum yields, allow estimates to be made of the rate constants for the excited singlet state reaction responsible for silene formation. 1-Methyl-1-phenylsilacyclobutane undergoes reaction from its lowest excited singlet state with a rate constant 10-80 times lower than those of the other three derivatives. The results are consistent with a stepwise mechanism for silene formation, involving a 1,4-biradicaloid intermediate that partitions between product and starting material.Key words: silene, silacyclobutane, photochemistry, biradical.

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Cyclobutene photochemistry. Substituent effects on the photochemistry of 1-phenylcyclobutene

Canadian Journal of Chemistry ◽

10.1139/v95-028 ◽

1995 ◽

Vol 73 (2) ◽

pp. 191-203 ◽

Cited By ~ 8

Author(s):

William J. Leigh ◽

J. Alberto Postigo

Keyword(s):

Flash Photolysis ◽

Laser Flash Photolysis ◽

Singlet State ◽

Substituent Effects ◽

Methanol Solution ◽

Excited Singlet State ◽

Acetonitrile Solution ◽

Quantum Yields ◽

Nanosecond Laser ◽

Excited Singlet

The photochemistry and photophysics of 1-phenylcyclobutene and five aryl-substituted derivatives have been studied in various solvents at room temperature. All six compounds fluoresce with quantum yields in the 0.2–0.3 range in cyclohexane and acetonitrile solution. 1-Phenylcyclobutene undergoes [2+2]-cycloreversion [Formula: see text] to yield phenylacetylene upon photolysis in either hydrocarbon or acetonitrile solution, and undergoes (Markovnikov) solvent addition upon irradiation in methanol solution [Formula: see text] in addition to cycloreversion. Triplet sensitization and quenching experiments indicate that cycloreversion and methanol addition are both excited singlet state processes. None of the six compounds studied undergo ring opening to the corresponding 2-aryl-1,3-butadiene in detectable yield. Quantum yields for cycloreversion in cyclohexane, acetonitrile, and methanol solution and methanol addition have been determined for the six compounds, along with excited singlet state lifetimes. The quantum yields and rate constants for cycloreversion and methanol addition are both enhanced by substitution with electron-donating groups. The variation in the rate constant for [2+2]-cycloreversion with substituent indicates that there is substantial dipolar character developed in the cyclobutenyl σ-bond framework during the reaction, in almost exact correspondence with that developed in the π system during photoprotonation. No deuterium scrambling is observed in 1-phenylcyclobutene-2,4,4-d3 after photolysis in pentane solution to ca. 80% conversion, indicating that skeletal rearrangements leading to cyclopropyl carbenes do not occur in the direct photolysis of arylcyclobutene derivatives. A pericyclic mechanism for the photocycloreversion reaction is suggested. Triplet–triplet absorption spectra and triplet lifetimes of 1-phenyl-, 1-(para-methylphenyl)-, and 1-(para-trifluoromethylphenyl)cyclobutene in hydrocarbon solution are also reported. Keywords: photochemistry, cyclobutene, fluorescence, [2+2]-cycloreversion, substituent effects, nanosecond laser flash photolysis, lifetime, triplet state, styrene, photoaddition

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Laser flash photolysis studies of some rhodamine dyes Characterisation of the lowest excited singlet state of Rhodamine 3B, Sulforhodamine B and Sulforhodamine 101

Journal of the Chemical Society Faraday Transactions ◽

10.1039/a705692c ◽

1998 ◽

Vol 94 (2) ◽

pp. 195-199 ◽

Cited By ~ 18

Author(s):

Paul C. Beaumont ◽

David G. Johnson ◽

Barry J. Parsons

Keyword(s):

Flash Photolysis ◽

Laser Flash Photolysis ◽

Singlet State ◽

Laser Flash ◽

Excited Singlet State ◽

Excited Singlet ◽

Sulforhodamine B ◽

Sulforhodamine 101 ◽

Rhodamine Dyes

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Laser flash photolysis studies of the formation and reactivities of phenyl(naphthyl)methyl carbocations generated from phosphonium salt precursors

Canadian Journal of Chemistry ◽

10.1139/v92-223 ◽

1992 ◽

Vol 70 (6) ◽

pp. 1784-1794 ◽

Cited By ~ 20

Author(s):

E. O. Alonso ◽

L. J. Johnston ◽

J. C. Scaiano ◽

V. G. Toscano

Keyword(s):

Rate Constants ◽

Phosphonium Salt ◽

Flash Photolysis ◽

Laser Flash Photolysis ◽

Ion Pairs ◽

Laser Flash ◽

Product Formation ◽

Quantum Yields ◽

Back Reaction ◽

Transient Experiments

The photolysis of several substituted phenyl(naphthyl)methyl triphenylphosphonium chlorides has been examined using a combination of laser flash photolysis experiments and product studies. Both carbocation and radical intermediates have been characterized in the transient experiments, with the relative yields depending strongly on the solvent. For example, in alcohols, acetonitrile, or aqueous solvents cation formation predominates while acetonitrile/dioxane mixtures (5–10%) are required for the observation of radicals. Quantum yields for cation formation vary from 0.79 in methanol to 0.093 in 1:4 acetonitrile/dioxane, as measured by product studies and transient experiments, respectively. The addition of perchlorate salts leads to dramatic enhancements in the cation lifetimes; the effects are particularly pronounced for acetonitrile/dioxane mixtures where the cation yields also increase by factors of 3–4. In this case the effects are attributed primarily to replacement of chloride by perchlorate in the initial ion pairs. The combined data from both solvent and perchlorate salt effects on the cation lifetimes and yields suggest that the excited state of the phosphonium salt cleaves homolytically, followed by electron transfer within the initial radical/triphenylphosphine radical cation pair to generate carbocation, as opposed to direct heterolytic cleavage. The cation yields also indicate that back reaction to regenerate starting material, as well as product formation within the initial geminate cage, occur in some solvents. The effects of solvent and added perchlorate salts on the rate constants for reaction with nucleophiles have been examined. For example, rate constants that vary by an order of magnitude have been measured for quenching by azide ion in various aqueous acetonitrile and trifluoroethanol mixtures.

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Properties of singlet- and triplet-excited states of hemicyanine dyes

Chemical Papers ◽

10.2478/s11696-014-0547-5 ◽

2014 ◽

Vol 68 (8) ◽

Cited By ~ 3

Author(s):

Daniel Mártire ◽

Walter Massad ◽

Hernán Montejano ◽

Mónica Gonzalez ◽

Paula Caregnato ◽

...

Keyword(s):

Flash Photolysis ◽

Laser Flash Photolysis ◽

Fluorescence Emission ◽

Emission Spectra ◽

Laser Flash ◽

Quantum Yields ◽

Triplet States ◽

Fluorescence Emission Spectra ◽

Fluorescence Quantum Yields ◽

Hemicyanine Dyes

AbstractThe fluorescence emission spectra and fluorescence quantum yields of hemicyanine dyes LDS 698, LDS 722, and LDS 730 were measured in different media. No transient species was detected in the laser flash-photolysis experiments performed with Ar-saturated solutions of the dyes in methanol. However, in the presence of 0.08 M potassium iodide, the absorption of the triplet states was clearly observed. Oxygen consumption measurements in the absence and presence of a chemical trap (furfuryl alcohol) in MeOH: H2O (φ r = 1: 1) solutions of the dyes containing KI confirmed the generation of singlet molecular oxygen.

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

Canadian Journal of Chemistry ◽

10.1139/v89-148 ◽

1989 ◽

Vol 67 (6) ◽

pp. 967-972 ◽

Cited By ~ 3

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.

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The structure, photochemical reactivity, and photophysical properties of adamantyl X-substituted aryl ethers and a comparison with the alkyl groups, methyl, tert-butyl, and allyl

Canadian Journal of Chemistry ◽

10.1139/v05-117 ◽

2005 ◽

Vol 83 (9) ◽

pp. 1237-1252 ◽

Cited By ~ 2

Author(s):

A L Pincock ◽

J A Pincock

Keyword(s):

Excited State ◽

Rate Constants ◽

Singlet State ◽

Bond Cleavage ◽

Photophysical Properties ◽

Excited Singlet State ◽

Quantum Yields ◽

Aryl Ether ◽

Excited Singlet ◽

Aryl Ethers

The structure, photophysical properties, and photochemistry of the adamantyl aryl ethers 1 in both methanol and cyclohexane have been examined. UV absorption spectra, 13C NMR chemical shifts, X-ray structures, and Gaussian calculations (B3LYP/6-31G(d)) indicate that these ethers adopt a 90° conformer in the ground state. In contrast, fluorescence spectra, excited singlet state lifetimes, and calculations (TDDFT) indicated a 0° conformer is preferred in the first excited singlet state S1. Irradiation in either solvent results in the formation of adamantane and the corresponding phenol as the major products, both derived from radical intermediates generated by homolytic cleavage of the ether bond. The 4-cyano substituted ether 1j was the only one to form the ion-derived product, 1-methoxyadamantane (16% yield), on irradiation in methanol. Rate constants of bond cleavage for these ethers from S1 were estimated by two different methods by comparison with the unreactive anisoles 2, but the effect of substituents was too small to determine structurereactivity correlations. The temperature dependence of the quantum yields of the fluorescence of the unsub stituted, 4-methoxy and 4-cyano derivatives of 1 and 2 were also determined. These results indicated that the activated process for 1 was mainly bond cleavage for the 4-cyano substrate whereas for 2, it was internal conversion and intersystem crossing. Key words: aryl ether photochemistry, fluorescence, excited-state rate constants, excited-state temperature effects.

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Photodegradation of the lignin model α-guaiacoxyacetoveratrone, unusual effects of solvent, oxygen, and singlet state participation

Canadian Journal of Chemistry ◽

10.1139/v91-016 ◽

1991 ◽

Vol 69 (1) ◽

pp. 104-107 ◽

Cited By ~ 36

Author(s):

John A. Schmidt ◽

Alain B. Berinstain ◽

Francis de Rege ◽

Cyril Heitner ◽

Linda J. Johnston ◽

...

Keyword(s):

Model Compound ◽

Flash Photolysis ◽

Laser Flash Photolysis ◽

Singlet State ◽

Laser Flash ◽

Quantum Yields ◽

Aprotic Solvents ◽

Lignin Model Compound ◽

State Participation ◽

Lignin Model

α-Guaiacoxyacetoveratrone, a lignin model compound, has been studied using a combination of laser flash photolysis and steady state quantum yields. Triplet lifetimes range from 175 to 600 ns while the quantum yields of photodegradation vary from 0.06 to 0.6, depending on the solvent. Singlet processes (Φ ~ 0.1), previously unobserved for aromatic ketones, are implicated in the mechanism, and account for most or all of the degradation in aprotic solvents. Key words: photodegradation, α-guaiacoxyacetoveratrone.

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