Photochemistry of α-chloro- and α-bromoacetophenone. Determination of extinction coefficients for halogen–benzene complexes

1988 ◽  
Vol 66 (6) ◽  
pp. 1474-1478 ◽  
Author(s):  
W. G. McGimpsey ◽  
J. C. Scaiano
Keyword(s):  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Benzene Solution ◽  
Halogen Bond ◽  
Bond Cleavage ◽  
Laser Flash ◽  
Quantum Yields ◽  
Extinction Coefficients ◽  
Transient Data

The photochemical dehalogenation of α-haloacetophenones (Cl, Br) in benzene solution has been examined under conditions of continuous and pulsed laser irradiation. Product and quantum yield studies indicate that carbon-halogen bond cleavage occurs with quantum yields of 0.88 and 0.41 for α-chloro- and α-bromoacetophenone, respectively. These values are based on trapping studies in which the PhC(O)ĊH2 radicals produced photochemically are scavenged by hydrogen donors such as benzenethiol. Laser flash photolysis studies lead to intense transient signals due to benzene–halogen complexes. Combination of transient data and quantum yields lead to extinction coefficients of 1 800 and 23 700 M−1 cm−1 for the chlorine (490 nm) and bromine (550 nm) complexes, respectively.

Laser flash photolysis determination of absolute rate constants for reactions of bromine atoms in solution

1993 ◽  
Vol 115 (18) ◽  
pp. 8340-8344 ◽  
Author(s):  
J. C. Scaiano ◽  
M. Barra ◽  
M. Krzywinski ◽  
R. Sinta ◽  
G. Calabrese
Keyword(s):  
Rate Constants ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Laser Flash ◽  
Absolute Rate

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

1992 ◽  
Vol 70 (6) ◽  
pp. 1784-1794 ◽  
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.

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

2014 ◽  
Vol 68 (8) ◽  
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.

Article

1999 ◽  
Vol 77 (5-6) ◽  
pp. 1066-1076
Author(s):  
Hideo Tomioka ◽  
Junichi Nakajima ◽  
Hidehiko Mizuno ◽  
Eiji Iiba ◽  
Katsuyuki Hirai
Keyword(s):  
Electron Spin Resonance ◽  
Electron Spin ◽  
Room Temperature ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Benzene Solution ◽  
Laser Flash ◽  
Spin Resonance ◽  
Structure Reactivity Relationship ◽  
Resonance Laser

A series of triplet 9-triptycyl(aryl)carbenes, where aryl groups are phenyl, 1- and 2-naphthyl, and 9-anthryl, is generated by photolysis of the corresponding diazomethanes and observed directly by spectroscopic means. Their structures are characterized by electron spin resonance (ESR) spectroscopy in a 2-methyltetrahydrofuran matrix at 77 K, and the reactivities are investigated by laser flash photolysis in degassed benzene solution at room temperature. Comparison of the data with other arylcarbenes bearing a series of substituents, i.e., hydrogen, phenyl, naphthyl, and anthryl groups, revealed an interesting relationship between structures and reactivities of triplet arylcarbenes.Key words: steric protection, stability of triplet carbenes, electron spin resonance, laser flash photolysis, structure-reactivity relationship.

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.

Singlet Mechanism for Trans → Cis Photoisomerization of Quaternary Salts of 4-Substituted 4′-Azastilbenes (R = CN, H, CH3, and OCH3) and their Quinolinium Analogues. VIII

1985 ◽  
Vol 40 (5) ◽  
pp. 525-537 ◽  
Author(s):  
H. Görner ◽  
A. Fojtik ◽  
J. Wróblewski ◽  
L. J. Currell
Keyword(s):  
Triplet State ◽  
Low Temperatures ◽  
Room Temperature ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Ion Pairs ◽  
Laser Flash ◽  
Quantum Yields ◽  
Polar Solvents ◽  
Effects Of Temperature

The trans → cis photoisomerization of a series of substituted stilbazolium salts (At+X− , At+: trans-1-alkyl-4-[4-R-styryl]-pyridinium and -quinolinium, R = CN. H, CH3 and OCH3, X− = I and ClO4−) was studied by laser flash photolysis and steady state irradiation measurements. The quantum yields of cis ⇄ trans photoisomerization (ϕc → t and ϕt → c ) and of fluorescence of the trans isomers (ϕf) were determined in several solvents at room temperature and at low temperatures in mixtures of either 2-methyltetrahydrofuran-dichloromethane or ethanolmethanol (E-M). In polar solvents at room temperature ϕt → c is substantial ( ≧ 0.3) and ϕf is small (10−3-10−2). Competition of fluorescence and an activated step in the trans → cis pathway is indicated by the effects of temperature on ϕf and ϕt → c (activation energy: 2 - 3 kcal/mol). A transient, observed only at low temperatures (lifetime τT > 0.5 ms in E-M below -170°C), is assigned to the lowest triplet state with trans configuration. On the basis of the effects of temperature on ϕf, ϕt → c , and the triplet yield and those of quenchers on ϕf and ϕt → c , involvement of the triplet state in the twisting process at room temperature is excluded. Therefore, a singlet mechanism is suggested for the trans → cis photoisomerization of the stilbazolium salts examined. Significant reduction of ϕt → c for iodides in solvents of moderate polarity, where ion pairs are present, is accounted for by photoinduced electron transfer in competition to trans → cis photoisomerization.

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