Reinvestigation of vibronic level dependence of fluorescence quantum yields and radiative and nonradiative lifetimes of p‐difluorobenzene in its first excited singlet state (1B2u)

1977 ◽  
Vol 67 (1) ◽  
pp. 236-241 ◽  
Author(s):  
Leonard J. Volk ◽  
Edward K. C. Lee
Keyword(s):  
Singlet State ◽  
Excited Singlet State ◽  
Quantum Yields ◽  
Excited Singlet ◽  
Fluorescence Quantum Yields ◽  
Vibronic Level

Fluoreszenz -Untersuchungen an Distyrylbenzolen

1988 ◽  
Vol 43 (6) ◽  
pp. 583-590 ◽  
Author(s):  
Dieter Oelkrug ◽  
Klaus Rempfer ◽  
Ellen Prass ◽  
Herbert Meier
Keyword(s):  
Singlet State ◽  
Fluorescence Decay ◽  
Excited Singlet State ◽  
Quantum Yields ◽  
Excited Singlet ◽  
Fluorescence Quantum Yields ◽  
Forbidden Transition ◽  
Decay Times ◽  
Fluorescence Decay Times

Abstract The absorption and fluorescence of three isomeric distyrylbenzenes are investigated as function of temperature. From the fluorescence decay times and fluorescence quantum yields two classes of oligostyrylarenes can be distinguished. A decisive criterion for this classification is, whether the first excited singlet state S1 belongs to an allowed or forbidden transition S0→S1.

The photochemistry of ring-substituted cinnamyl acetates

2006 ◽  
Vol 84 (9) ◽  
pp. 1146-1154 ◽  
Author(s):  
S A Fleming ◽  
L Renault ◽  
E C Grundy ◽  
J A Pincock
Keyword(s):  
Triplet State ◽  
Singlet State ◽  
Acetoxy Group ◽  
Excited Singlet State ◽  
Excited Triplet State ◽  
Quantum Yields ◽  
Excited Singlet ◽  
Major Pathway ◽  
Excited Triplet ◽  
Fluorescence Quantum Yields

The photochemistry of the (E)-cinnamyl acetates ((E)-1-aryl-3-propenyl acetates, 8a–8e) with substituents H, 4-CH3O, 3-CH3O, 4-CF3, and 3-CF3, respectively, was examined in both cyclohexane and methanol solvents. Alkene isomerization (E to Z) occurred more efficiently than other reactions and evidence is presented that this process occurs from the excited triplet state. In a slower process, 1,3-migration of the acetoxy group led to the rearranged 3-aryl-3-propenyl acetate isomers (9a–9e) as the major pathway, particularly in cylohexane. In methanol, the isomeric ethers 3-aryl-3-methoxypropene (14) and 1-aryl-3-methoxypropene (15) were formed by reaction of methanol with the photochemically generated cation. The combined yield of 14 and 15 (95% and 5%, respectively) was quantitative for the 4-methoxyphenyl compound (8b). Independent irradiations of the isomers 9a–9c demonstrated that the ethers 14 and 15 were primary photoproducts from 8 and not secondary photoproducts from 9. Fluorescence quantum yields and excited singlet state lifetimes indicated that the reactions, other than the E to Z isomerization, are from the excited singlet state.Key words: cinnamyl acetates, photochemistry, E/Z isomerization, photocleavage, fluorescence.

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

2005 ◽  
Vol 83 (9) ◽  
pp. 1237-1252 ◽  
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 structure–reactivity 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.

Steric effects in the phototransposition reactions of dialkylbenzenes

2006 ◽  
Vol 84 (1) ◽  
pp. 10-20 ◽  
Author(s):  
C M Gonzalez ◽  
J A Pincock
Keyword(s):  
Singlet State ◽  
Activation Barrier ◽  
Photophysical Properties ◽  
Dominant Mode ◽  
Excited Singlet State ◽  
Quantum Yields ◽  
Reactive Intermediate ◽  
Temperature Dependent ◽  
Excited Singlet ◽  
General Observation

The photochemistry, photophysical properties, and temperature dependence (–25 to +65 °C) of fluorescence by quantum yields and excited singlet state lifetimes in acetonitrile have been examined for three sets of dialkylbenzene derivatives: Set 1 — ortho-xylene (10), tetralin (11), and indan (12); Set 2 — 2,3-dimethylbenzonitrile (9-23), 5-cyanotetralin (T-23), and 4-cyanoindan (I-23); and Set 3 — 3,4-dimethylbenzonitrile (9-34), 6-cyanotetralin (T-34), and 5-cyanoindan (I-34). Phototransposition reactions occur for 10, 9-23, 9-34, and T-34. Fitting of the temperature-dependent fluorescence data to an Arrhenius expression gave A and Ea values for all substrates studied except I-23 and I-34. The fluorescence intensity of these two compounds was essentially independent of temperature. For the other compounds, the data revealed that the activation barrier separating the excited singlet state (S1) from the reactive intermediate, a prefulvene biradical, was the important one in determining the reaction efficiency. The dominant mode of decay of the reactive intermediate was internal return to the starting material. Moreover, the general observation was made that nitrile substitution ortho to one of the alkyl groups in these dialkylbenzene derivatives reduced the rate at which they were converted to the reactive intermediate and, therefore, also the efficiency of the phototransposition reactions.Key words: phototranspositions, substituted benzenes, temperature-dependent fluorescence, activation parameters.

Processus primaire de la photolyse et de la photo-oxydation de l'o-phényl-phénol

1980 ◽  
Vol 58 (21) ◽  
pp. 2230-2235 ◽  
Author(s):  
Louise-Marie Coulangeon ◽  
Gilles Perbet ◽  
Pierre Boule ◽  
Jacques Lemaire
Keyword(s):  
Quantum Yield ◽  
Singlet State ◽  
Excited Singlet State ◽  
Quantum Yields ◽  
Oxygen Effect ◽  
Dual Fluorescence ◽  
Excited Singlet ◽  
Photochemical Process ◽  
Phenoxy Radicals ◽  
Effect Of Oxygen

The dual fluorescence observed in aqueous solution of o-phenyl-phenol is attributed to a deprotonation equilibrium in the excited singlet state. The effect of oxygen on the primary photochemical process of formation of phenoxy radicals can be explained in a parallel study of the variations of fluorescence and photo-oxidation quantum yields of this phenolic compound at different pH. It is shown that photoionisation occurs only from the excited singlet state of the phenolate form. Oxygen inhibits recombination of the solvated electron and phenoxy radical. Increase of quantum yield for the disappearance of irradiated o-phenyl-phenol in degased solution also is observed in the presence of electron scavengers like Cd2+ or NO3− ions. Even when the phenoxy radicals are not able to react with oxygen, quantum yield for the disappearance of the phenolic compounds is higher in an aerated than in a degassed solution; the oxygen effect on the primary photochemical step may explain this difference.

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

1995 ◽  
Vol 73 (2) ◽  
pp. 191-203 ◽  
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

Der Substituenteneinfluß auf die Fluoreszenz des 1.3-Diphenyl-2-pyrazolins / Substituent Effects on the Fluorescence of 1,3-Diphenyl-2-pyrazoline

1976 ◽  
Vol 31 (9) ◽  
pp. 1248-1255 ◽  
Author(s):  
H. Strähle ◽  
W. Seitz ◽  
H. Güsten
Keyword(s):  
Absorption Spectra ◽  
Ground State ◽  
Phenyl Ring ◽  
Fluorescence Spectra ◽  
Singlet State ◽  
Fluorescence Decay ◽  
Quantum Yields ◽  
Excited Singlet ◽  
Fluorescence Quantum Yields ◽  
Absolute Fluorescence

The electronic absorption spectra, absolute fluorescence spectra and fluorescence quantum yields as well as the fluorescence decay times of nine 1,3-diphenyl-2-pyrazolines substituted in the para-position of the 1-phenyl ring and of 19 substituted in the paraposition of the 3-phenyl ring were measured in degassed benzene at room temperature. The fluorescence quantum yields are about 0.90 with the exception of the NO2- and J-substituted 1,3-diphenyl-2-pyrazolines. The fluorescence decay time ranges from 2 to 4 nsec. The natural fluorescence lifetimes determined experimentally agree well with those calculated from the absorption spectra according to STRICKLER and BERG. A plot of the Hammett σp-values versus the energies of the absolute fluorescence maxima of the psubstituted 1- and 3-phenyl ring of the 2-pyrazoline system shows a statistically significant linear correlation with moderate precision. The linearity between ground state substituent constants and fluorescence energies reveals that the relative importance of the inductive and the resonance effects of the substituents remains constant during the transition from the ground state to the relaxed excited singlet state. In the excited singlet state of 1,3-diphenyl-2-pyrazoline the p-substituents provide more room for a larger charge displacement in the intramolecular charge-transfer. The structural bands in the fluorescence spectra resulting from strong electron accepting substituents in the para-position of the 1-phenyl ring are considered as the result of the ring breathing vibration due to a local excitation of the 1-phenyl ring.

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

1999 ◽  
Vol 77 (5-6) ◽  
pp. 1136-1147 ◽  
Author(s):  
William J Leigh ◽  
Rabah Boukherroub ◽  
Christine J Bradaric ◽  
Christine C Cserti ◽  
Jennifer M Schmeisser
Keyword(s):  
Rate Constants ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Singlet State ◽  
Fluorescence Emission ◽  
Laser Flash ◽  
Excited Singlet State ◽  
Acetonitrile Solution ◽  
Quantum Yields ◽  
Excited Singlet

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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