scholarly journals SUBSTITUENT EFFECTS ON THE PHOTOCURRENT QUANTUM YIELDS OF PARA-SUBSTITUTED TETRAPHENYLPORPHYRINS

1980 ◽  
Vol 9 (6) ◽  
pp. 627-628 ◽  
Author(s):  
Kazuo Yamashita ◽  
Keiko Miyoshi ◽  
Atsuhiro Osuka ◽  
Hitomi Suzuki
Keyword(s):  
Substituent Effects ◽  
Quantum Yields

Solvent and substituent effects on rearrangement of 4-(4-X-phenyl)-2,7-dioxa-3-azabicyclo[3,3,0]oct-3-enes

1985 ◽  
Vol 50 (9) ◽  
pp. 1971-1981 ◽  
Author(s):  
Lubor Fišera ◽  
Marta Konopíková ◽  
Ladislav Štibrányi ◽  
Hans-Joachim Timpe
Keyword(s):  
Substituent Effects ◽  
Quantum Yields ◽  
Polar Solvents ◽  
Aromatic Residue ◽  
Product Ratio ◽  
Oxazoline Derivatives ◽  
Oxazine Derivatives

Preparation of the title compounds V is described. They give, on irradiation, the 2,3-dihydro-6H-1,3-oxazine derivatives VI as the main products besides the tetrahydrofuro[3,4-d]oxazoline derivatives VII. The VI to VII product ratio depends on the substituent bound to the aromatic residue. Polar solvents favour formation of the VI derivatives in the order Cl >H > CH3. In non-polar solvents the proportion of VII is increased. The quantum yields of the photoreaction vary within the limits from 0.006 to 0.04 (H >F > Cl > CH3 > OCH3).

Substituent Effects on the Luminescence of 2-Substituted 3-Methylquinoxalines in Poly (Vinyl Alcohol) Films*

1991 ◽  
Vol 46 (4) ◽  
pp. 304-306 ◽  
Author(s):  
Z. Gryczyński ◽  
A. Kawski
Keyword(s):  
Polyvinyl Alcohol ◽  
Temperature Dependence ◽  
Vinyl Alcohol ◽  
Substituent Effects ◽  
Quantum Yields ◽  
Poly Vinyl Alcohol ◽  
Band Position ◽  
Fluorescence And Phosphorescence ◽  
Polyvinyl Alcohol Films

Abstract The effect of 2-substitutions (NH2 . O, CH3O, CI. Br) in 3-methylquinoxalines on the fluorescence and phosphorescence band position and intensity at 293 K, and the temperature dependence of their fluorescence and phosphorescence quantum yields were investigated in polyvinyl alcohol) films

Substituenteneffekte im spektroskopischen Verhalten von Maleinsäurederivaten und 4-Cyclopenten-1.3-dionen / Substituent Effects on the Spectroscopic Behavior of Maleic Acid Derivatives and 4-Cyclopentene-1,3-diones

1973 ◽  
Vol 28 (9-10) ◽  
pp. 662-681 ◽  
Author(s):  
Hans-Dieter Scharf ◽  
Hans Leismann
Keyword(s):  
Excited States ◽  
Maleic Acid ◽  
Singlet State ◽  
Substituent Effects ◽  
Quantum Yields ◽  
Mo Calculations ◽  
Excited Singlet ◽  
Inductive Effects ◽  
Phosphorescence Emission ◽  
Spectroscopic Behavior

The lowest excited states of maleic acid- and cyclopentendionderivatives have been investigated by UV-absorption and phosphorescence emission spectroscopy. While the first excited singlet-state S1 is of 1(nπ*)-type in all compounds, the lowest triplet-state T1 was found to be a mixture of 3(nπ*) and 3(ππ*)-configurations. For the emitting derivatives (Ζ ≠ Η) phosphorescence-lifetimes and -quantum yields as well as the probability for intersystem crossing have been determined.Considering the inductive effects of all substituents on the S1 (nπ*) -energy, Coulombpotentials for sulfur and iodine in terms of Huckel-parameters: hs = 0,2 and hj = —0,5 have been derived.The results of Hückel-MO-calculations can be correlated to the measured values of all compounds with the exception of the iodine containing derivatives.

Substituent effects in aromatic photochemistry. UV irradiation of 3,4-dimethoxybenzonitrile and 3,4-dimethoxyacetophenone in the presence of inorganic anions

1987 ◽  
Vol 52 (4) ◽  
pp. 980-988 ◽  
Author(s):  
Petr Kuzmič ◽  
Milan Souček
Keyword(s):  
Substituent Effects ◽  
Quantum Yields ◽  
Substituted Benzenes ◽  
Cyanide Anion ◽  
Nucleophilic Displacement ◽  
Relative Quantum ◽  
Ultraviolet Photolysis ◽  
Substitution Ratio ◽  
Volmer Plot ◽  
Methoxy Substituent

Ultraviolet photolysis of 3,4-dimethoxybenzonitrile (Ia) and 3,4-dimethoxyacetophenone (IIa) in the presence of the hydroxide or cyanide anion leads to nucleophilic displacement of either the para or the meta methoxy substituent. The ratio of isomeric photoproducts is dependent upon the nature of the nucleophile. Photoreactions with the OH- anion lead exclusively to the substitution at C-3. On the other hand, both isomers are formed when acetophenone IIa is irradiated in the presence of CN-, with the C-3/C-4 substitution ratio 3:2. In the case of the photocyanation of Ia, the C-4 substituted product dominates, the C-3/C-4 ratio ranging from 1:2 to 1:6 in dependence on the nucleophile concentration. These differences between the OH- and CN- anions are related to the results of a fluorescence quenching study which has revealed that only the latter nucleophile interacts with singlet-excited substrates. A non-linear Stern-Volmer plot was observed for the quenching of the second excited state of Ia by the cyanide anion. This indicates several distinct quenching modes, in relation to the concentration dependence of regioselectivity. The activating power of –H, -CN, -COCH3, and -NO2 substituents, as measured by relative quantum yields of disappearance for 3,4-dimethoxy-R-substituted benzenes, is 1:2.5:5:580 and 1:1.5:6:1000 in their photoreactions with OH- and CN- anions, respectively.

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

Substituent effects, Arrhenius activation parameters, and rate constants for the photo–Claisen rearrangement of allyl aryl ethers

2008 ◽  
Vol 86 (7) ◽  
pp. 686-690 ◽  
Author(s):  
Carlos M Gonzalez ◽  
James A Pincock
Keyword(s):  
Rate Constants ◽  
Claisen Rearrangement ◽  
Ion Pairs ◽  
Substituent Effects ◽  
Activation Parameters ◽  
Quantum Yields ◽  
Intermediate Radical ◽  
Fluorescence Quantum Yields ◽  
Aryl Ethers ◽  
Electron Donating Groups

The temperature-dependence of fluorescence quantum yields in both methanol and methylcyclohexane has been used to obtain the rate constants of reaction for the activated process that converts the singlet excited state S1 of a set of ring-substituted aryl allyl ethers to an intermediate radical pair in the photo–Claisen rearrangement. These rate constants are correlated with the O–H bond dissociation energy of the corresponding ring-substituted phenols; that is, electron-donating groups (CH3, OCH3) accelerate the reaction relative to electron-withdrawing groups (CF3, CN). The rate constants obtained span two orders of magnitude, from 5.4 × 107 s–1 for X = 3–CN to 800 × 107 s–1 for X = 4–OCH3, in methylcylcohexane. Moreover, the rate constants are similar in the two solvents, methanol and methcyclohexane, indicating that radical pairs, not ion pairs, are the reactive intermediates, as expected on the basis of previous mechanistic proposals for the photo–Claisen rearrangement. Finally, the rate constants obtained by this temperature-dependent method are in good agreement with those previously reported from a method using the corresponding unreactive anisoles as a model.Key words: allyl aryl ethers, photo–Claisen rearrangement, substituent effects, activation parameters.

Substituent effects and solvents

1992 ◽  
Vol 89 ◽  
pp. 1567-1571
Author(s):  
O Pytela ◽  
M Ludwig
Keyword(s):  
Substituent Effects
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