Quenching Of Excited States Research Articles

Photophysical and Photochemical Studies of [Pt(binap)2]: Excited State Quenching by Oxygen

Abstract The quenching of the photoluminescence of [Pt(binap)2] by oxygen has been studied. Generation of singlet oxygen was confirmed by NIR spectroscopy. The usefulness of the Pt complex as a photosensitizer was established using a singlet oxygen scavenger, 1,3-diphenylisobenzofuran (DBF). The mechanism for the generation of singlet oxygen is discussed.

Quenching of excited states of chlorophyll molecules in submembrane fractions of Photosystem I by exogenous quinones

The ability of three substituted quinones, 2,5-dibromo-3-methyl-6-isopropyl- p-benzoquinone (DBMIB), 2,6-dichloro- p-benzoquinone (DCBQ), and tetramethyl- p-benzoquinone (duriquinone) to quench the excited states of chlorophyll (Chl) molecules in Photosystem I (PSI) was studied. Chl fluorescence emission measured with isolated PSI submembrane fractions was reduced following the addition of exogenous quinones. This quenching progressively increased with rising concentrations of the exogenous quinones according to the Stern–Volmer law. The values of Stern–Volmer quenching coefficients were found to be 3.28×10 5 M −1 (DBMIB), 1.31×10 4 M −1 (DCBQ), and 3.7×10 3 M −1 (duroquinone). The relative quenching capacities of the various exogenous quinones in PSI thus strictly coincided to those found for the quenching of Fo level of Chl fluorescence in isolated thylakoids, which is emitted largely by Photosystem II (PSII) [Biochim. Biophys. Acta (2003) 1604, 115–123]. Quenching of Chl excited states in PSI submembrane fractions by exogenous quinones slowed down the rate of P700, primary electron donor of PSI, photooxidation measured at limiting actinic light irradiances thus revealing a reduced photochemical capacity of absorbed quanta. The possible involvement of non-photochemical quenching of excited Chl states by oxidized phylloquinones, electron acceptors of PSI, and oxidized plastoquinones, mobile electron carriers between PSII and the cytochrome b 6/ f complex, into the control of photochemical activity of PSI is discussed.

Diastereomeric differentiation in the quenching of excited states by hydrogen donors.

Chiral dyads of (S)-ketoprofen and (S)- or (R)-tetrahydrofurfurylamine show diastereomeric differentiation in photoinduced hydrogen abstractions, which could be directly followed by time-resolved observation of the ketone triplet state. A unimolecular rate constant of kH=3.0×105 s−1 was found for the S,S diastereomer, while the S,R diastereomer reacts four times slower (kH=7.5×104 s−1).

Clusters in Polymer−Surfactant AOT Microemulsions Probed by Excited State Quenching Kinetics

Excited-state quenching kinetics were applied to sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT) reversed micelles with poly(oxy)ethylene (POE) of molecular weight 35 000 Da where clusters were previously detected [Laia,C. A. T.; Brown ,W.; Almgren, M.; Costa, S. M. B. Langmuir 2000, 16, 465]. The data obtained in AOT reversed micelles, with a fixed value of the molar ratio of water to surfactant (w 0 = [H 2 O]/[AOT]) w 0 = 20, were compared in the absence and presence of the polymer. Two reaction time scales were investigated with different probes: 1,3,6,8-pyrenetetrasulfonic acid (PTSA) in the nanosecond range and Cr(bpy) 3 3+ in the microsecond range, using, respectively, time-resolved fluorescence with picosecond resolution and laser flash photolysis techniques. The experimental decays were analyzed assuming two components, one due to polymer-free micelles described by a Poisson distribution model kinetics and another associated with the polymer-induced micelle clusters, analyzed according to a model based on the concept of random walks in regular compact lattices. A step frequency, k w = 10 7 s - 1 , was extracted from the nanosecond results for the quencher's random walk within the cluster. By contrast, the triplet absorption quenching studies did not distinguish between free micelles and clusters and afforded an aggregation number for the former in the range 260 ′ 40, in good agreement with literature data. Decay simulations confirm the validity of the data treatment used in the nanosecond range, but the short time behavior predicted in the microsecond range was not observed within the equipment time resolution.

Quenching of porphyrin excited states by adjacent or distant porphyrin cation radicals in molecular arrays

The quenching of photoexcited porphyrins by porphyrin cation radicals at adjacent or distant sites in diphenylethyne-linked porphyrin dyads and triads has been studied using static and time-resolved optical spectroscopy. The excited-state quenching is highly efficient in all cases (>99%) and occurs in ⩽11 ps between adjacent sites (by through-bond energy/charge transfer) and in ⩽55 ps between distant sites (likely by superexchange-assisted energy transfer). The rates can be tuned using the porphyrin-linker connection motif. These results should prove useful in the design of arrays that use electro- or photo-chemically generated porphyrin cation radicals for molecular switching and other applications in molecular photonics.

Photodynamics of the bacteriochlorophyll-carotenoid system. 1. Bacteriochlorophyll-photosensitized oxygenation of beta-carotene in acetone.

Carotenoids are well-known physical quenchers of chlorophyll excited states and reactive oxygen species both in vivo and in vitro. They may also be involved in chemical quenching undergoing, e.g. isomerizations or oxidations. We have found that beta-carotene (Car) in aerobic acetone is rapidly oxygenated under strong illumination with red light (lambda exc > or = 630 nm) in the presence of bacteriopheophytin a. At the same time the photosensitizer undergoes only slight (< 10%) photodegradation. By preparative high-performance liquid chromatography as many as seven major products of oxygen attachment to Car have been isolated. Their molecular masses show that Car sequentially accumulates up to six oxygen atoms while its C40-skeleton remains intact.

Ferrocene and ferrocenyl derivatives in luminescent systems

Owing to their fairly high stability under visible irradiation, ferrocene and ferrocenyl derivatives are widely used in luminescent systems. They are classical quenchers of excited states. Both energy and electron transfer may be involved, depending on the nature of the excited species. Inter- or intramolecular quenching are encountered. Applications span from the study of reaction mechanisms to that of organized or biological media. Recently, dyads and polyads designed for their ability to mimic photosynthetic centers or for their photodiode properties have also been obtained. Finally, the incorporation of a ferrocenyl derivative in a luminescent system does not necessarily lead to luminescence quenching. New applications are emerging, in which advantage is taken of the presence of ferrocene acting as a redox center: this gives optically and electrochemically active sensors. The present review encompasses the literature up to November 1999.

Triplet quenching and antioxidant effect of several carbon black grades in the photodegradation of LDPE doped with benzophenone as a photosensitiser

In a previous paper it was demonstrated for the first time, that carbon black pigments have the ability to operate as effective singlet and triplet quenchers. This work was undertaken directly in the polymer using the inherent long-lived luminescent species as triplet donor species. In order to ascertain the effectiveness of the quenching mechanism and its inter-relationship with carbon black parameters, a total of eight different carbon black pigments have been incorporated into low density polyethylene at different concentrations (0.1, 0.2, 0.4% w/w) and their effects determined on the quenching the triplet state of a known triplet photosensitiser, benzophenone (0.05% w/w). Using phosphorescence analysis at 77 K a large variability was observed in the benzophenone triplet lifetime quenching, the effect varying with the pigment type and the concentration of pigment. Photodegradation rates on the films were also performed in order to relate the quenching effect of the carbon black with films exposed to UVA light at 70°C. An unusual synergistic effect between the carbon black and the benzophenone sensitiser was observed compared with films containing only carbon black. The implications of carbon black pigments to operate as “super quenchers”, as well as antioxidants in this study and under practical concentration conditions is discussed. Although not consistent throughout, there is some indication that low surface area carbon black pigments can be the most effective quenchers of active excited states. However, the presence of surface containing oxygen groups can be related to the synergistic effect reported, using experimental design statistical analysis.

Behaviour of carbon black pigments as excited state quenchers in LDPE

A total of eight different carbon black pigments have been incorporated into low density polyethylene at 0.2% w/w concentration and their ability to operate as effective singlet and triplet quenchers examined using luminescence analysis. All pigments effectively reduced the fluorescence and phosphorescence emission intensities. A large variability was observed in the triplet lifetime quenching, the effect varying with the pigment type. Although not consistent throughout, there is some indication that low surface area carbon black pigments can be the most effective quenchers of active excited states. The implications of carbon black pigments to operate as “super quenchers” under practical concentration conditions is discussed.

Interfacial Electron Transfer Induced by Light in Methylene Blue Incorporated in Nafion Membranes. Laser Kinetic Spectroscopy

Quenching of excited states of methylene blue was studied in Nafion−H+, Nafion−Na+ and Nafion−Fe2+ membranes. Protonated, not protonated, and dimer forms of methylene blue in Nafion showed different fluorescence properties. Fe2+ is observed to quench the fluorescence of methylene blue embedded in the Nafion membrane. The excitation of monomer of methylene blue molecules induces the triplet excited state of the dye as revealed by laser spectroscopy. Methylene blue radical generation involved excitation of the dye dimeric species or excimers. Nafion−Fe2+ membrane quenched the excited methylene blue, leading to the formation of Fe3+ and the semiquinone of the methylene blue radical. The back reaction between Fe3+ and the semiquinone of the methylene blue radical occurred in the millisecond range and was seen to proceed by second-order kinetics. The latter process involves a dynamic mechanism of reaction including motion of one of the reactants or both. The mechanism of the photoreduction of MB incorporated i...

Quenching of Excited States of Pyrene by Tetranitromethane on SiO2 Surfaces: Effect of Reaction Temperature, Silica Pore Size, and Surface Silanol Content

Quenchers which adsorb strongly to the silica gel surface show limited quenching of the excited states of coadsorbed donors, and when quenching does occur it is predominantly static quenching. This is observed for several quenchers including nitromethane, tetranitromethane, and methyl viologen. The strong adsorption of tetranitromethane (TNM) to the surface silanol groups is indicated by the significant adsorption from cyclohexane solutions and the lack of any desorption with continuous evacuation of the sample after addition from solution or by vapor transfer. The strong adsorption of TNM is observed in the quenching characteristics of several adsorbed donors including the singlet and triplet excited states of pyrene. The quenching mechanism is partially determined by the binding characteristics of the coadsorbed electron acceptors and partially by the surface structure of the porous silica gel support material. Removing the high density of surface silanol groups changes the adsorption characteristics reducing the adsorption efficiency of both pyrene and TNM. The photophysics and photochemistry between coadsorbed donor and acceptors are also affected by reducing the silanol group concentration. The singlet quenching mechanism changes from predominantly static to increasingly dynamic as the silanol surface concentration is reduced. TNM movement on the surface is indicated by the loss of quenching reaction with decreased sample temperatures. The pyrene triplet excited state is quenched by coadsorbed TNM at lower TNM concentrations than are required for singlet quenching. The dynamic quenching mechanism of the triplet increases significantly using dehydroxylated silica gel. Excited-state quenching occurs by electron transfer on the surface with the formation of the pyrene cation. The strong binding of tetranitromethane to the silica gel surface also changes the photoinduced reaction products. Nitropyrene is formed along with nitroform indicating the reaction mechanism involves charge transfer with pyrene cation formation by a similar mechanism as occurs in acetonitrile.

Effects of ultraviolet absorbers on rates of anaerobic coloration of lignin model compounds

The long-wave (λ≥295 nm) anaerobic photochemistry of two frequently used lignin model compounds, i.e., I (3,4-di-methoxy-α-(2-methoxyphenoxy)-β-hydroxypropiophenone) and II (1-(3,4-dimethoxyphenol)-2-(2-methoxyphenoxy) propan-1,3-diol) was studied in the presence of three UV absorbers (Tinuvin P, Tinuvin 213 and 2,4-dihydroxybenzophenone), the additives being incorporated into thin films of I and II. Reactions were followed by rates of formation of new, long-wave chromophores (coloration). All of the additives reduced not only the rates of increased absorption, but also the extent of coloration. The most effective additive was Tinuvin P. The relationship between rates of inhibited reactions and concentration of additive was nonlinear, but best represented by the Perrin equation, suggesting that these compounds are not acting simply as UV absorbers, but also as quenchers of excited states of I and II.

Quenching of excited states of pyrene derivatives by amphiphilic nitroxide radicals in cationic micellar solutions. Dynamics and location of the guest molecules in the aggregates

Steady-state and nanosecond time-resolved studies have been performed on fluorescence quenching of excited pyrene sulfonic acid and pyrene butyric acid (PBA and PSA, respectively, in the form of sodium salts) by n-doxylstearic methyl esters ( n-DSE, n=5, 10, 12) in aqueous solution of cationic micelles of hexadecyltrimethylammonium chloride (HTAC, 0.1 M). The results are compared with those reported earlier for pyrene (P) as a luminophore (Szajdzinska-Pietek and Wolszczak, Chem. Phys. Lett., 270 (1997) 527). It is shown that PBA/ n-DSE and PSA/ n-DSE luminophore/quencher pairs, unlike P/ n-DSE pairs, are not suitable for determination of the micellar aggregation number, neither by the ‘quasi-static’ method of Turro—Yekta nor by the dynamic method of Infelt—Tachiya. It is suggested that the fluorescence decay curves in the examined systems can be adequately described in terms of dispersive kinetics using the time-dependent rate coefficient for intramicellar quenching process. The average location of the luminophore and the quencher molecules in the aggregates is discussed, considering previous findings of electron spin echo modulation studies of n-doxylstearic acid spin probes in micellar systems.

Novel Preorganized Hemispherands To Encapsulate Rare Earth Ions: Shielding and Ligand Deuteration for Prolonged Lifetimes of Excited Eu3+ Ions

Quenching of the luminescent excited state of Eu3+ ions by C-H high-vibrational modes was studied by deuteration of the encapsulating ligand and the solvent. Novel polydentate hemispherands providing nine donor atoms, which form overall neutral complexes with rare earth ions, were synthesized in nine steps, allowing the easy incorporation of deuterium atoms (11a-d·Eu3+). The introduction of tert-butyl groups at the aromatic rings of the ligand further increased the solubility of the complexes in organic solvents (29·Eu3+ and 34·Eu3+). Photophysical studies, viz., luminescence spectra and lifetime measurements, revealed that significant quenching of the Eu3+ excited state is induced by nearby C-H vibrational modes. Substitution of these quenching C-H modes for C-D bonds in the azacrown bridge leads to an enhancement of the luminescent lifetime by a factor of 1.5. C-H high-vibrational modes of the pendant arms which are at a larger distance to the Eu3+ ion than the azacrown bridge hydrogen atoms (determined from the MD calculations) are less efficient quenchers. The number of coordinating methanol molecules to 11a·Eu3+, 29·Eu3+, and 34·Eu3+ estimated by the Horrocks equation is 0.9, 1.2, and 1.9, respectively, as was predicted by MD calculations. Moreover, the experimental data show that quenching of the excited state of well-shielded Eu3+ ions by the C-H modes of the ligand is of the same order of magnitude as quenching by one O-H mode.

Reactivity of di- and tri-nuclear complexes of heavy mid-transition elements: a case of oxo-carboxylato-bridged complexes

Abstract

ChemInform Abstract: Photochemistry and Photophysics of Aromatic Sulfoxides. Part 5. Mechanisms of Excited State Quenching by Aromatic Sulfoxides

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Mechanisms of Excited State Quenching by Aromatic Sulfoxides

Aromatic sulfoxides quench singlet excited states of sensitizers whose singlet energies render energy transfer unlikely as a mechanism. Well over 50 rate constants for singlet quenching of various sensitizers by a series sulfoxides have been obtained, as have estimates of the redox potentials for the series of sulfoxides. These data strongly suggest that the mechanism for quenching involves electron transfer and/or exciplex formation. Charge (electron) transfer is from the sensitizer to the sulfoxide.

The role of macroradicals in the decreasing of the plastic scintillators radiation resistance

The radiation-chemical processes investigated in polymers show that the macroradicals are responsible for the plastic scintillator radiation degradation. The reversible character of the radiation damage makes it evident that macroradicals are the excited state quenchers. It is supposed that the macroradical quenching of the electron-exited states has an unlike mechanism caused by the specific structure of paramagnetic particles and by peculiar formation and localization of macroradicals near the dopants. The dopant locations are the defects in which all radiation processes (such as the localization of the absorbed energy, macroradical formation, interaction between the radicals and the dopants) proceed.

Quenching of Excited States of Aromatic Hydrocarbons and their Derivatives by Oxygen

Quenching of Excited States of Aromatic Hydrocarbons and their Derivatives by Oxygen

Chirality-dependent excited-state quenching of Eu(dpa) 33− and Tb(dpa) 3+3 luminophores by diastereomeric structures of Δ-[Co( trans-1,2-cyclohexanediamine) 3] 3+ in aqueous solution

Four diastereomers of Δ-[Co( trans-1,2-cyclohexanediamine) 3] 3+ are examined as enantioselective excited-state quenchers of chiral Eu(dpa) 3 3− and Tb(dpa) 3 3− luminophores (dpadipicolinate dianion) in solution. In their quenching actions, all four diastereomers can distinguish between and interact differentially with the Λ and Δ enantiomers of the luminophores, but their enantiomeric preferences show interesting variations that may be correlated with chelate-ring conformational structure.