Solvent effect on the reactivity of monosubstituted phenols towards singlet molecular oxygen (O2(1Δg)) in alkaline media

1993 ◽  
Vol 71 (8) ◽  
pp. 1247-1252 ◽  
Author(s):  
Marta Luiz ◽  
María I. Gutiérrez ◽  
Graciela Bocco ◽  
Norman A. García
Keyword(s):  
Molecular Oxygen ◽  
Rate Constant ◽  
Reaction Pathway ◽  
Aqueous Media ◽  
Solvent Polarity ◽  
Alkaline Media ◽  
Quantum Yields ◽  
Quenching Rate ◽  
Singlet Molecular Oxygen ◽  
Quenching Rate Constant

The influence of solvent polarity on the dye-sensitized photooxidation (singlet molecular oxygen (O2(1Δg)) mediated) of a series of para-substituted phenolates was studied. Kinetic evidence obtained shows that the overall and the pure chemical interactions, phenolate–O2(1Δg), depend on the solvent polarity in a different way. This is clearly reflected in the efficiency of O2(1Δg) photooxidation of the substrates: surprisingly, the photooxidation quantum yield increases as the overall quenching rate constant decreases. The substrate photooxidation quantum yields obtained ranged from 0.05 to 0.15, the upper limit corresponding to a moderately polar medium (a benzene–methanol mixture) and the lower to an aqueous medium. We conclude that a high solvent polarity favours only the obtainment of the encounter complex (O2(1Δg)–phenolate), whereas the reactive step is affected in much the same way as those processes where charges are neutralized along the reaction pathway. The results obtained are discussed in terms of a partly polar excited state complex between O2(1Δg) and the phenolates. The rate constant for the reactive pathway in both organic and aqueous media is correlated with the Hammet-type substituent constant R−.

scholarly journals A kinetic study on singlet molecular oxygen (O2(1Δg)) generation and quenching by dihydroxynaphthalenes

1996 ◽  
Vol 74 (1) ◽  
pp. 49-54 ◽  
Author(s):  
Marta Luiz ◽  
Arnaldo T. Soltermann ◽  
Alicia Biasutti ◽  
Norman A. Garcia
Keyword(s):  
Kinetic Study ◽  
Solvent Polarity ◽  
Point Of View ◽  
Quantum Yields ◽  
Quenching Rate ◽  
Singlet Molecular Oxygen ◽  
Substitution Pattern ◽  
Time Resolved ◽  
Encounter Complex ◽  
Quenching Rate Constant

A kinetic study on the aerobic (O2(1Δg)-mediated) photooxidation of a series of dihydroxynaphthalenes was carried out. Members of this family of compounds include important environmental contaminants. The interaction with O2(1Δg), determined by both time-resolved phosphorescence and static methods, yields a quenching rate constant in the range of 105–109 M−1 s−1, depending on the solvent polarity, pH, and substitution pattern of the dihydroxynaphthalene. According to experimental evidence, the mechanism of the interaction seems to be mediated by an encounter complex with a considerable charge transfer component. From a mechanistic point of view this characteristic of DHN indicates a behaviour very similar to that of simple substituted phenols and dihydroxybenzenes. Quantum yields for DHN photooxidation (Φr) indicate that these substrates are excellent candidates for O2(1Δg)-mediated degradation under environment conditions. Furthermore, these compounds proved to be good self-sensitizers upon irradiation at 337 nm. Quantum yields (ΦΔ) for the production of O2(1Δg) were determined to be on the order of 0.25 in benzene. Key words: dihydroxynaphthalenes, photooxidation, quenching, sensitization.not available

Photophysics of novel 22π porphyrinoids

2012 ◽  
Vol 16 (05n06) ◽  
pp. 499-507 ◽  
Author(s):  
Daniel O. Mártire ◽  
Sigrid Russell ◽  
Hans-Jürgen Dietrich ◽  
Carlos J. Cobos ◽  
Silvia E. Braslavsky
Keyword(s):  
Triplet State ◽  
Molecular Oxygen ◽  
Photophysical Properties ◽  
Intersystem Crossing ◽  
Quenching Rate ◽  
Triplet Energy ◽  
Singlet Molecular Oxygen ◽  
Time Resolved ◽  
Quenching Rate Constant ◽  
New Compounds

The photophysical properties of toluene solutions of two new 22π expanded porphycene compounds were measured using a combination of various steady-state and time-resolved techniques. The determined triplet energy (E T = 109 ± 3) kJ.mol-1, coincident with the calculated E T = (96.0 ± 10) kJ.mol-1, of both red absorbing compounds is higher than the energy required to excite ground state molecular oxygen to singlet molecular oxygen. However, the intersystem crossing yield is very low (ca. 10-2), which makes these compounds poor photosensitizers. The triplet state yield of the two expanded 22π porphyrinoid compounds is much lower than that of the parent porphycene, whereas their fluorescence is as high (ca. 30%) as the value for porphycene. The slower than diffusional quenching rate constant of a porphycene triplet state by the two new compounds reflects a steric hindering factor of the exothermic energy transfer.

Diffusional Fluorescence Quenching of Aromatic Hydrocarbons

2003 ◽  
Vol 57 (5) ◽  
pp. 538-544 ◽  
Author(s):  
Clelia Canuel ◽  
Sophie Badre ◽  
Henning Groenzin ◽  
Markus Berheide ◽  
Oliver C. Mullins
Keyword(s):  
Fluorescence Quenching ◽  
Molecular Oxygen ◽  
Rate Constant ◽  
Aromatic Hydrocarbons ◽  
Diffusion Rate ◽  
Energy Difference ◽  
Quenching Rate ◽  
Quenching Rate Constant ◽  
Diffusion Rate Constant ◽  
Organic Fluorophore

The quenching of the fluorescence of five aromatic hydrocarbons by three halogenated organics and by molecular oxygen has been measured. Both fluorescence intensity and fluorescence lifetime measurements have been employed to validate results and interpretation; linear Stern–Volmer analyses are shown to apply throughout. The fluorescence quenching rate constant of molecular oxygen for the five aromatic hydrocarbons is essentially equivalent to the diffusion rate constant independent of the fluorophore excitation energy. The halogenated organic–fluorophore rate constants vary by a factor of 965 and are shown to correlate roughly with the energy difference between the quencher and fluorophore excited electronic states in accord with a standard model of quantum two-level mixing. The value of the coupling interaction energy is ∼2500 cm−1.

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.

Lifetime of Singlet Oxygen and Quenching by NaN3 in Mixed Solvents

1982 ◽  
Vol 37 (5) ◽  
pp. 649-652 ◽  
Author(s):  
N. Miyoshi ◽  
M. Ueda ◽  
K. Fuke ◽  
Y. Tanimoto ◽  
M. Itoh ◽  
...  
Keyword(s):  
Activation Energy ◽  
Charge Transfer ◽  
Singlet Oxygen ◽  
Mixed Solvents ◽  
Solvent Polarity ◽  
Thermal Lensing ◽  
Partial Charge ◽  
Partial Charge Transfer ◽  
Quenching Rate ◽  
Quenching Rate Constant

Singlet oxygen was generated by the photosensitization of erythrosine. The lifetime of singlet oxygen and the quenching rate constant for singlet oxygen by NaN3 were measured by a thermal lensing method in MeOH-H2O mixed solvents. The reciprocal of the lifetime increased linearly with the increase of the H2O mole fraction. Semi-log plot of the quenching constant against the reciprocal of the solvent polarity exhibited a linear relation. The quenching of the singlet oxygen by NaN3 may proceed through a partial charge-transfer intermediate. The activation energy for the quenching reaction of N3- + 1O2 →[N3·1O2-] increased with the increase of the solvent polarity. The lifetime was also measured in MeOH-ethyleneglycol mixed solvents, and its relation with viscosity was obtained

Temperature dependence of the O+I(P21/2)→O+I(P23/2) quenching rate constant

2009 ◽  
Vol 105 (9) ◽  
pp. 094911 ◽  
Author(s):  
Pavel A. Mikheyev ◽  
David J. Postell ◽  
Michael C. Heaven
Keyword(s):  
Temperature Dependence ◽  
Rate Constant ◽  
Quenching Rate ◽  
Quenching Rate Constant

Quenching of singlet molecular oxygen by carnosine and related antioxidants. Monitoring 1270-nm phosphorescence in aqueous media

IUBMB Life ◽  
1997 ◽  
Vol 41 (4) ◽  
pp. 687-694 ◽  
Author(s):  
Sergei Yu. Egorov ◽  
Ekaterina Kurella ◽  
Alexander Boldyrev ◽  
Alexander Krasnovsky
Keyword(s):  
Molecular Oxygen ◽  
Aqueous Media ◽  
Singlet Molecular Oxygen

1999 R.U. Lemieux Award Lecture Adventures with azo-, azoxy-, and hydrazoarenes: from the Wallach to the benzidine rearrangement. Molecular electronics

2000 ◽  
Vol 78 (10) ◽  
pp. 1251-1271 ◽  
Author(s):  
Erwin Buncel
Keyword(s):  
Molecular Electronics ◽  
Rate Constant ◽  
Reaction Pathway ◽  
Solvent Polarity ◽  
Molecular Switch ◽  
Reaction Pathways ◽  
Optical Data ◽  
Constant Ratio ◽  
Rate Constant Ratio ◽  
Acid Catalyzed

The author's studies with aromatic azo-, azoxy-, and hydrazo-dye molecules, comprising kinetic and equilibrium investigations, as well as synthesis of novel molecules having photogenic properties, are described under the following highlights: A. Wallach rearrangement and cognate studies with azoxyarenes — (1) Elucidation of the mechanism of the Wallach rearrangement of azoxybenzene through the kinetic observation of a two-proton process which, together with a pKa study, was interpreted on the basis of formation of a deoxygenated, dicationic, symmetrical species as a key, short-lived reaction intermediate. (2) The proposal of a general acid-catalyzed pathway in concentrated sulfuric acid (catalysis by H2SO4 and H3SO+4. (3) Elucidation of the consecutive sulfonations of reaction products of azoxybenzene in the 100% H2SO4 region, and the diprotonation equilibria for p-hydroxyazobenzene, thus shedding light on past reaction pathway and product studies. (4) The observation of a novel reaction pathway for 2,4,6,2',4',6'-hexamethy lazoxybenzene. (5) The observation of a dichotomy of reaction pathways for α- and β-2-phenylazoxynaphthalenes: reaction via the dicationic intermediate and via quinoid intermediate species; comprising two isomeric compounds reacting by different pathways to give the same product. (6) Identification and structure proof of α- and β-isomers observed for the first time in the peracid oxidation of phenylazopyridine. (7) Observation of a rate constant ratio of 22 000 in the rearrangement of these α- and β-isomers, and the proposal of differential barriers for transition states leading to a tricationic intermediate. B. Benzidine rearrangement and cognate studies — (8) Observation of the acid-catalyzed hydroxylation of phenylazopyridine to p-hydroxyphenylazopyridine and the proposal of an SNAr mechanism with formation of an intermediate hydrazo species in the reaction. (9) First study of benzidine type rearrangement-disproportionation of phenylhydrazopyridine in acid media. (10) Proposal of a A [Formula: see text] B [Formula: see text] C [Formula: see text] D type reaction profile for the consecutive hydroxylation[Formula: see text]disproportionation processes of phenylazopyridine in aq H2SO4. (11) Proposal of 10-π and 14π-electron electrocyclic processes in the benzidine type rearrangement-disproportionation of phenylhydrazopyridine. (12) Identification and structural elucidation of a dimer formed from phenylazopyridine as a minor product and proposal of a reaction mechanism. C. Facile acid-catalyzed demethylation via SNAr/A-SE2 mechanisms and studies of tautomerism — (13) Observation of an abnormally facile acid-catalyzed cleavage (demethylation) of 4-methoxyphenylazopyridine via an SNAr mechanism. (14) Observation of two reaction pathways, SNAr and A-SE2, for the consecutive demethylations of 3,4-dimethoxyphenylazopyridine, with rate constant ratio of 7 000:1 favoring the SNAr process. (15) Quantitation of the tautomeric and protonation equilibria of 4-hydroxyphenylazopyridine, produced in (13). D. A new solvent polarity scale, molecular switches, and molecular electronics — (16) Establishment of a π*azo solvent polarity scale based on solvatochromism of a series of azomerocyanine molecules ("Buncel's dye"). (17) Some glimpses are presented of current forays into molecular electronics, as emanating from the above studies: (a) spiropyran (SP) <—> merocyanine (MC) thermo- and photochromic "molecular switch" systems; (b) synthesis and characterization of azo-functionalized star-burst dendrimers with photoswitchable properties and potential applications in optical data storage systems, holographic gratings, and drug delivery systems as host molecules.Key words: Wallach rearrangement, benzidine disproportionation, azoarenes, azoxyarenes, dendrimers, hydrazoarenes, dendrimers, solvatochromism, photochromism, thermochromism, spiropyran-merocyanine molecular switch.

Quenching rate constant for O(1S)+NO

1972 ◽  
Vol 1 (4) ◽  
pp. 341-344 ◽  
Author(s):  
R. Atkinson ◽  
K.H. Welge
Keyword(s):  
Rate Constant ◽  
Quenching Rate ◽  
Quenching Rate Constant
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