Dynamic Quenching Rate Constants Research Articles

Inhibiting mechanisms of metal ion complexation on photogenerated reactive intermediates derived from dissolved black carbon

Dissolved black carbon (DBC), an important photosensitizer in surface waters, can influence the photodegradation of various organic micropollutants. In natural water systems, DBC often co-occurs with metal ions as DBC–metal ion complexes; however, the influence of metal ion complexation on the photochemical activity of DBC is still unclear. Herein, the effects of metal ion complexation were investigated using common metal ions (Mn2+, Cr3+, Cu2+, Fe3+, Zn2+, Al3+, Ca2+, and Mg2+). Complexation constants (logKM) derived from three-dimensional fluorescence spectra revealed that Mn2+, Cr3+, Cu2+, Fe3+, Zn2+, and Al3+ quenched the fluorescence components of DBC via static quenching. The steady-state radical experiment suggested that in the complex systems of DBC with various metal ions, Mn2+, Cr3+, Cu2+, Fe3+, Zn2+ and Al3+ inhibited the photogeneration of 3DBC* via dynamic quenching, which reduced the yields of 3DBC*-derived 1O2 and O2·-. Moreover, 3DBC* quenching by metal ions was associated with the complexation constant. A strong positive linear relationship existed between logKM and the dynamic quenching rate constant of metal ions. These results indicate that the strong complexation ability of metal ions enabled 3DBC quenching, which highlights the photochemical activity of DBC in natural aquatic environments enriched with metal ions.

Photocatalyst assemblies with two halide ions

A series of ruthenium polypyridyl photocatalysts bearing amide functional groups were designed that successfully promoted halide assembly in CH2Cl2 and CH3CN solution. In CH2Cl2, halide assembly was accompanied by a visible color change, and the spectral changes presented clear evidence for two halide binding events, yielding a 1:2 ruthenium:halide assembly. In the more polar solvent CH3CN, 1:2 assembly structures were also observed with chloride, bromide, and iodide, and large equilibrium constants were measured for association of the first and second halide (K11 = 0.04 – 2 × 106 M−1, K12 = 0.01 – 3 × 105 M−1). Varying the functional groups on the ancillary ligands tuned the excited-state reduction potentials (Ru2+⁎/+), resulting in a photocatalyst capable of performing iodide oxidation. Quenching of the photocatalyst excited state resulted in static and dynamic quenching, and a Stern-Volmer analysis yielded two linear regions at low and high iodide concentrations. The dynamic quenching rate constants (kq = 6.8 and 4.0 × 1010 M−1 s−1) and static quenching constants (KS = 2.4 and 0.13 × 104 M−1) at low and high iodide concentrations, respectively, were consistent with dynamic quenching of Ru2+ and [Ru2+,I−]+, and static quenching of [Ru2+,I−]+and [Ru2+,2I−]. Transient absorption spectroscopy revealed that the quenching reaction yielded a reduced ruthenium (Ru+) as the primary photoproduct and diiodide (I2•−) as a secondary photoproduct. The second-order rate constant for I2•− formation was measured to be 2.5 × 1010 M−1 s−1, a value consistent with the diffusion limited reaction. The transient absorption data indicates that oxidized halide photoproducts only result from the diffusional quenching reactions, and not from static quenching with an associated iodide ion. Fast back-electron transfer rates and low cage-escape yields in the ruthenium:iodide assemblies are invoked to explain why the static quenching pathway does not lead to measurable photoproduct yields.

Dynamic and static quenching of 2-aminopurine fluorescence by the natural DNA nucleotides in solution

2-aminopurine (2AP) is a responsive fluorescent base analogue that is used widely as a probe of the local molecular environment in DNA. The ability of 2AP to report changes in local conformation and base-stacking interactions arises from the efficient quenching of its fluorescence by the natural DNA bases. However, the mechanism of this inter-base quenching remains imperfectly understood. Two previous studies of the collisional quenching of 2AP by the natural bases, in different buffer solutions, showed that dynamic quenching efficiency depends on the identity of the natural base, but disagreed on the relative quenching efficiencies of the bases. We report a comprehensive investigation of inter-base quenching of 2AP by the natural nucleoside monophosphates (NMPs), replicating the buffer conditions used in the previous studies. Using time-resolved fluorescence measurements to distinguish between dynamic and static quenching, we find that the dynamic quenching rate constants of the different bases show a consistent trend across both buffers, and this is in line with a charge-transfer mechanism. Time-resolved measurements also provide insight into static quenching, revealing formation of 2AP-NMP ground-state complexes in which 2AP displays a very short fluorescence lifetime, comparable to that seen in oligonucleotides. In these complexes, the dependence of the rate of quenching on the partner base also supports a charge-transfer mechanism.

Photophysics and photochemistry of porphyrin core PAMAM dendrimers. Excited states interaction with quinones

Quenching of excited states of the meso-substituted tetraphenyl porphyrins with PAMAM branches, and one with PAMAM generation 1 peripherally-modified with thiazole groups by p-benzoquinones, were studied in DMF solution. Fluorescence lifetime measurements evidence a static component in the quenching process. The dynamic quenching rate constants were close to the diffusional limit except for the case of duroquinone. Apparent association constants of the porphyrin compounds with the quinones were determined by comparison of static and dynamic measurements of fluorescence quenching. The ground state interaction is very weak with the exception of tetrachlorobenzoquinone. The triplet state decay kinetics and the transient absorption spectra in the presence of the quinones were determined by laser flash photolysis. Triplet quenching rate constants were lower than those of singlet quenching. An electron transfer mechanism was proposed to explain the results. The long-time transient absorption spectra were assigned to the radical cation of the dyes. Relative radical yields were determined and a dependence on the side chains and the quencher was observed.

Mechanistic Insight into the Effect of Metal Ions on Photogeneration of Reactive Species from Dissolved Organic Matter.

The photogeneration of reactive species (RS) from dissolved organic matter (DOM) exhibits a great impact on the attenuation of pollutants in natural waters. However, the effect of metal ions on the photogeneration of excited triplet-state DOM (3DOM*), singlet oxygen (1O2), and hydroxyl radical (•OH) by effluent organic matter (EfOM), fulvic acid (FA), and humic acid (HA) is poorly understood. Here, we provided the first evidence that the quenching of 3DOM* was positively correlated with the complexation capacity of metal ions with DOM. Generally, the paramagnetic metal ions (Cr3+, Mn2+, Fe3+, and Cu2+) exhibited higher conditional stability constants (log KML) with DOM and stronger inhibition for RS than the others (Mg2+, Ca2+, Al3+, and Zn2+). For DOM of different sources, the metal binding capacity increased in the order of EfOM < HA < FA and the humic substances were more susceptible to metal ions. The inhibition was attributed to both static and dynamic quenching of 3DOM* by metal ions. The dynamic quenching rate constants of metal ions for 3DOM* were estimated as ∼109 M-1 s-1, which was positively related to the corresponding log KML. These findings highlight crucial links between metal-DOM complexation and 3DOM* quenching and, consequently, the inhibition of RS.

Photoinduced electron transfer reactions of pyranine with benzoquinone and titanium dioxide.

The fluorescence quenching of pyranine by benzoquinone in acetonitrile medium was studied using steady-state and time-resolved fluorescence techniques. The quenching process was characterized by a Stern-Volmer plot, which displayed a linear aspect. From the linear plot, the bimolecular quenching rate constant was obtained. The obtained rate constants are within diffused controlled limits. The results show that benzoquinone can efficiently quench the fluorescence of pyranine with dynamic quenching rate constants in the order of 1010 M-1 s-1 , suggesting that the pyranine can act as a good electron donor for photoinduced electron transfer in artificial photosynthesis and organic solar cells. In addition, the electron injection dynamics of a pyranine/titanium dioxide semiconductor film was also investigated and electron injection from the excited state pyranine into the conduction band of titanium dioxide is suggested. These preliminary results hold promise for the possibility of using pyranine in dye-sensitized solar cells. Copyright © 2016 John Wiley & Sons, Ltd.

Red fluorescent monobenzo-BODIPY dyes: Solvent effects on spectra and efficient fluorescence quenching by quinones and phenols

The fluorescence properties of five monobenzo-BODIPY compounds were measured in nonpolar and polar solvents by UV–vis absorption, steady-state fluorescence and time-correlated single-photon counting methods. These monobenzo-BODIPY dyes emit bright red fluorescence (wavelength >605 nm) with high fluorescence quantum yields (Φf ≥ 0.60) which are slightly affected by solvent polarity. The fluorescence lifetimes of these BODIPY compounds are also long (τf ∼ 5 ns) and not affected remarkably by solvent polarity. These fluorescence properties make them good fluorescent markers/probes in the red visible region. Quinones can efficiently quench the fluorescence of monobenzo-BODIPYs with the dynamic quenching rate constants in the order of 1010 L mol−1 s−1, suggesting that the BODIPY compounds can act as good electron donors for photoinduced electron transfer in artificial photosynthesis and organic solar cells.

Unichromophoric Platinum-Acetylides That Contain Pentiptycene Scaffolds: Torsion-Induced Dual Emission and Steric Shielding of Dynamic Quenching

The effect of the rigid bulky pentiptycene scaffolds on the photoluminescence, redox properties, and oxygen sensing behavior of unichromophoric Pt-acetylides is reported. When the pentiptycene groups are near the Pt(PBu3)2 center, the Pt-acetylides display both blue fluorescence and green phosphorescence with long phosphorescence lifetimes (90-202 μs) in THF. Their phosphorescence intensity is highly sensitive to molecular oxygen, and the emission color depends on the concentration of not only oxygen but also the complexes, which allows a feasible determination of oxygen in the range of 1-5% air volume. The dynamic quenching rate constants decrease linearly with increasing the number of pentiptycene groups, revealing the steric shielding effect of the peripheral rings of pentiptycene. A dependence of oxidation potential on the number of pentiptycene groups also revealed the steric shielding effect on the electron transfer between the complexes and the electrode. In a PMMA matrix, the dual emissive properties are diminished due to increased phosphorescence and decreased fluorescence intensity, and the phosphorescence lifetimes are significantly increased (up to ∼700 μs), leading to an "on-off" optical response to oxygen concentration. Both the dual emissive properties and long-lived triplet excitons are attributed to diminished spin-orbit couplings caused by twisting and steric shielding of the π-conjugated backbone around the Pt center.

A mechanistic study on photocatalysis by thiapyrylium salts. Photodegradation of dimethoate, alachlor and pyrimethanil under simulated sunlight

Photodegradation of dimethoate, alachlor, and pyrimethanil using a thiapyrilium salt (TPTP+) as photocatalyst was performed under simulated sunlight. The photocatalyst removes preferentially pyrimethanil (45% after 60min of irradiation), followed by dimethoate (20%) and alachlor (13%). In the three cases, an important to moderate decrease of the fluorescence intensity upon addition of the pollutants was observed. The active participation of the singlet excited state in the photodegradation of the pollutants was demonstrated; the corresponding dynamic quenching rate constants were found to be 1.9×1010M−1s−1 (pyrimethanil), 6.7×109M−1s−1 (dimethoate) and 9.1×109M−1s−1 (alachlor). Moreover, a decrease in the signal corresponding to the triplet excited state was observed immediately after the laser pulse; again a good correlation was observed between the missing fraction of triplets and the photodegradation rates under solar light. Detection of TPTP+, the reduced species derived from the photocatalyst, as well as Rehm–Weller calculations on the free energy changes, strongly support an electron transfer mechanism occurring from the singlet excited state.

Triiodide Quenching of Ruthenium MLCT Excited State in Solution and on TiO2 Surfaces: An Alternate Pathway for Charge Recombination

The excited states of [Ru(bpy)2(deeb)](PF6)2, where bpy is 2,2-bipyridine and deeb is 4,4'-(CO2CH2CH3)2-2,2'-bipyridine, were found to be efficiently quenched by triiodide (I3-) in acetonitrile and dichloromethane. In dichloromethane, I3- was found to quench the excited states by static and dynamic mechanisms; Stern-Volmer analysis of the time-resolved and steady-state photoluminescence data produced self-consistent estimates for the I3- + Ru(bpy)2(deeb)2+ <==> [Ru(II)(bpy)2(deeb)2+,(I3-)]+ equilibrium, K = 51,000 M(-1), and the bimolecular quenching rate constant, kq = 4.0 x 10(10) M(-1) s(-1). In acetonitrile, there was no evidence for ion pairing and a dynamic quenching rate constant of k(q) = 4.7 x 10(10) M(-1) s(-1) was calculated. Comparative studies with Ru(bpy)2(deeb)2+ anchored to mesoporous nanocrystalline TiO2 thin films also showed efficient excited-state dynamic quenching by I3- in both acetonitrile and dichloromethane, kq = 1.8 x 10(9) and 3.6 x 10(10) M(-1) s(-1), respectively. No reaction products for the excited-state quenching processes were observed by nanosecond transient absorption measurements from 350 to 800 nm under any experimental conditions. X-ray crystallographic, IR, and Raman data gave evidence for interactions between I3- and the bpy and deeb ligands in the solid state.

Photophysical properties of the photosensitizer [Ru(bpy) 2(5-CNphen)] 2+ and intramolecular quenching by complexation of Cu(II)

The lifetime of the 3MLCT emitting state of [Ru(bpy) 2(5-CNphen)] 2+ (bpy = 2,2′-bipyridine, 5-CNphen = 5-cyano-1,10-phenanthroline) has been determined in CH 3CN by flash photolysis and TCSPC techniques. The obtained value, τ = 2.2 μs, points to its potential use as a photosensitizer in molecular devices. The transient spectrum shows that this excited state can be formulated as a metal-to-ligand charge transfer state. Static and dynamic quenching of the complex luminescence by Cu 2+ ions is observed. The static and dynamic components were evaluated by measuring emission intensities and lifetimes at different [Cu 2+]. The combined quenching results point to a strong ground state complexation of Cu 2+ ( K eq = 7500 M −1 for 1:1 complex) with the nitrile group of 5-CNphen. At high [Cu 2+], a short-lived component of the decay has been detected, with k et = 4.0 × 10 9 s −1. This value is compared to the electron transfer rate constant in the encounter complex of the bimolecular quenching, which was obtained from the dynamic quenching rate constant.

Photoinduced Electron Transfer between Fluorescent Dyes and Guanosine Residues in DNA-Hairpins

Nucleobase-specific quenching interactions of fluorescent dyes can be used in singly labeled hairpin-shaped oligonucleotides to detect hybridization to specific target DNA sequences. In these DNA-hairpins, the dye is attached at the 5‘-end and quenched by guanosine residues in the complementary stem. Upon hybridization, a conformational reorganization occurs reflected in an increase in fluorescence intensity. To gain a better insight into the underlying quenching mechanism, we have performed intermolecular quenching experiments with different dyes (rhodamine and oxazine derivatives) and DNA nucleotides. The bimolecular dynamic quenching rate constants kq,dyn of ∼1.0−2.0 × 109 M-1 s-1 are relatively small for all dyes investigated though the measured decrease in fluorescence intensity indicates strong static fluorescence quenching. The data give evidence for the formation of weak or nonfluorescent (fluorescence lifetime, τ < 40 ps) ground-state complexes between the fluorophores and guanosine residues. Onl...

Photoinduced Reactions of Pyrene with Carbon Tetrachloride on Porous Silica Gel

Polyaromatic compounds such as pyrene are adsorbed to the porous silica gel surfaces, and the photoinduced reactions of these species on these surfaces are studied. The pyrene singlet excited state and triplet state are quenched on the surface with coadsorbed carbon tetrachloride (CCl4). Temperature studies with evacuated samples indicate that quenching occurs by CCl4 adsorbed on the surface and not by bombardment from the gas phase. The dynamic quenching rate constant of the pyrene singlet excited state increases after high-temperature pretreatment of the silica. It is suggested that this results from an increase in the rate of diffusion of carbon tetrachloride on the surface after the high-temperature removal of surface silanol groups. The dynamic quenching rate constant of the singlet excited state increases with the silica gel pore size, and the highest rate constant is observed on the 150 A silica surface, which has been pretreated at 600 °C. A large pore size (smaller surface area) and a dehydroxyla...

Quenching of photoexcited Ru(bpy)32+by a hydrazone derivative in a poly(ethylene oxide) film

The quenching of photoexcited Ru(bpy)32+ by 2-methyl-4-(N,N′-dibenzylamino)benzaldehyde-N-2-benzothiazolyl-N-n-butylhydrazone (BBTH) in a poly(ethylene oxide) film has been studied. To determine which quenching model can be applied to the present system, the emission quenching and emission decay under various BBTH concentrations were measured. Plots based on relative emission quantum yield showed a downward deviating curve and could not be analyzed by the 13 quenching models reported hitherto. To analyze this quenching reaction, a new model involving static and dynamic mechanisms was proposed which takes into account the dependence of the static quenching rate on the quencher concentration incorporated into a quenching sphere specific for a solid system. The best fit was obtained when the static quenching rate was proportional to the one-third power of the quencher concentration present in the quenching sphere. As a result of this treatment, the quenching distance was found to be 1.7 nm, the static quenching rate constant (kq1) 1.3 × 105 s−1 and the dynamic quenching rate constant (kq2) 8.6 × 106 d mol−1 s−1.

Oxygen fluorescence quenching studies with single tryptophan-containing proteins

The work of Lakowicz and Weber [Biochemistry 12, 4161 (1973)] demonstrated that molecular oxygen is a powerful quencher of tryptophan fluorescence in proteins. Here we report studies of the oxygen quenching of several proteins that have a single, internal tryptophan residue. Among these are apoazurin (Pseudomonas aeruginosa), asparaginase (Escherichia coli), ribonuclease T1 (Aspergillus oryzae), and cod parvalbumin. Both fluorescence intensity and phase lifetime quenching data are reported. By comparison of these data we find that there is a significant degree of apparent static quenching in these proteins. The dynamic quenching rate constants,k q, that we find are low compared to those for tryptophan residues in other proteins. For example, for apoazurin we find an apparentk q of 0.59×10(9) M (-1) s(-1) at 25°C. This value is the lowest that has been reported for the oxygen quenching of tryptophan fluorescence.

Quenching of excited tris(2,2'-bipyridine)ruthenium(II) by metal 1,2-dithiolene complexes

The quenching of excited Ru(bpy)/sub 3//sup 2 +/ by MS/sub 4/C/sub 4/(C/sub 6/H/sub 5/)/sub 4/ and MS/sub 4/C/sub 4/(CN)/sub 4//sup 2 -/ (M = Ni, Pt) has been studied in CH/sub 3/CN and DMF. The dynamic quenching rate constants, which are nearly diffusion controlled, have been analyzed in terms of diffusional parameters and unimolecular quenching constants. The unimolecular quenching constants for the MS/sub 4/C/sub 4/(CN)/sub 4//sup 2 -/ quenchers are lower by 2 orders of magnitude than those for the MS/sub 4/C/sub 4/(C/sub 6/H/sub 5/)/sub 4/ quenchers, and this effect has been attributed to a different degree of adiabaticity. The quenching mechanism has been investigated by using sacrificial scavengers for oxidized or reduced (Ru(bpy)/sub 3//sup 2 +/. Evidence for oxidative quenching by the MS/sub 4/C/sub 4/(C/sub 6/H/sub 5/)/sub 4/ complexes was obtained. For the MS/sub 4/C/sub 4/(CN)/sub 4//sup 2 -/ quenchers, either energy transfer or reductive electron transfer followed by very efficient cage recombination is a possible quenching mechanism. 48 references, 4 figures, 2 tables.

Fluorescence quenching of the buried tryptophan residue of cod parvalbumin.

Cod parvalbumin (isotype III) is a single tryptophan-containing protein. The fluorescence characteristics of this tryptophan residue (lambda em approximately 315 nm) suggest that it is buried from solvent and that it is located in an apolar core of the protein. Solute quenching studies of the tryptophan fluorescence of parvalbumin reveal dynamic quenching rate constants, kq, of 1.1 X 10(8) and 2.3 X 10(9) M-1 s-1 (at 25 degrees C) with acrylamide and oxygen, respectively, as quenchers. From temperature dependence studies, activation energies of 6.5 +/- 1.5 and 6.0 +/- 0.5 kcal/mol are found for acrylamide and oxygen quenching. The kq for acrylamide quenching is found to be relatively unchanged (+/- 10%) by an 8-fold increase in the bulk viscosity (glycerol/water mixture). These temperature and viscosity studies argue that the acrylamide quenching process involves a dynamic penetration of the quencher, facilitated by fluctuations in the protein's structure.