Quencher Molecules Research Articles

Effects of chain stiffness on the quenching of an excited polymer by small quenchers

We present a theory for analyzing the effects of chain stiffness on the diffusion-influenced quenching kinetics of an excited polymer. We model the polymer as an optimized Rouse–Zimm chain and the quencher molecule as a spherical particle. The excitation is considered to be localized at any one monomer, or to move randomly along the chain backbone. In regard to the dependence on the chain stiffness, we found two distinctive kinetic regimes. When the excitation migration rate is small, the quenching rate decreases as the chain becomes stiffer. On the other hand, when the mobility of excitation is large, the opposite trend is observed. We also investigate the dependence of Stern–Volmer coefficient on the length and stiffness of the chain in the fast excitation migration limit.

Chirality-dependent interactions between molecular propeller structures in solution. Chiral recognition and discrimination processes modulated by temperature and incremental changes in structural chirality

Time-resolved chiroptical luminescence (TR-CL) measurements are used to study chirality-dependent intermolecular interactions in dynamic excited-state quenching processes. The measurements are carried out on solution samples that contain a racemic mixture of chiral luminophore molecules (with enantiomeric structures denoted by Λ L and Δ L) and a small, optically resolved concentration of chiral quencher (CQ) molecules. The luminophores are excited with a pulse of linearly polarized laser radiation to produce an initially racemic excited-state population of Λ L* and Δ L* enantiomers, and TR-CL measurements are then used to monitor the differential decay kinetics of the Λ L* and Δ L* subpopulations. Observed differences between the Λ L* and Δ L* decay kinetics reflect differential rate processes and efficiencies for Λ L*–CQ vs. Δ L*–CQ quenching actions, and they are diagnostic of chiral discriminatory interactions between the luminophore and quencher molecules. Twelve different luminophore–quencher systems are examined, in both H 2O and D 2O solutions, and in each case the quenching kinetics are measured over the 273–308 K temperature range. In all of the systems examined here, quenching occurs via electronic energy-transfer processes in transient (Λ L*–CQ) and (Δ L*–CQ) encounter complexes, and the chiral discriminatory rate parameters reflect the relative stabilities and lifetimes of these complexes as well as their structures and internal (electronic and nuclear) dynamics. All of the luminophore and quencher molecules examined in this study have three-bladed propeller-like structures that are very similar in overall shape and size. However, they exhibit small differences in the structural details of their propeller blades, and it is found that these small differences in structure can produce both qualitative and very substantial quantitative differences in their chiral recognition and discrimination properties.

Nanowells on silica particles in water containing long-distance porphyrin heterodimers.

Smooth and nonswelling spherical silica particles with a diameter of 100 nm and an aminopropyl coating are soluble in water at pH 11, coagulate quickly at pH 3, and redissolve at pH 9. Electron microscopy as well as visible spectra of covalently attached porphyrins indicate the aggregation state of the particles. Long-chain alpha,omega-dicarboxylic acids with a terminal oligoethyleneglycol (=OEG)-amide group were attached in a second self-assembly step to the remaining amine groups around the porphyrins. Form-stable 2-nm wells were thus obtained and were characterized by fluorescence quenching experiments using the bottom porphyrin as a target. The one-dimensional diffusion of fitting quencher molecules along the 2-nm pathway took several minutes. Porphyrins with a diameter above 2 nm could not enter the form-stable gaps at all. Added tyrosine stuck irreversibly to the walls of the nanowells and prevented the entrance of quencher molecules, the OEG-headgroups fixated 2,6-diaminoanthraquinone. A ring of methylammonium groups was then fixed at the walls of the wells at a distance of 5 or 10 A with respect to the bottom porphyrin. 2,6-Disulfonatoanthraquinone was attached only loosely to this ring, but the exactly fitting manganese(III) meso-(tetraphenyl-4-sulfonato)porphyrinate (Mn(III) TPPS) was tightly bound. Transient fluorescence experiments showed a fast decay time of 0.2 ns for the bottom porphyrin, when the Mn(III) TPPS was fixated at a distance of 5 A. Two different dyes have thus been immobilized at a defined subnanometer distance in an aqueous medium.

Partitioning of unsaturated hydrophilic monomer in microemulsion media monitored by pyrene fluorescence method

AbstractThe extent of intra‐ and interchain associations of (un)charged water‐soluble monomers in the homogeneous and micellar solutions was studied with steady‐state fluorescence spectroscopy. Fluorescence spectroscopic experiments were performed on uncharged (acryl amide) and charged hydrophilic monomers [zwitterionic 3‐dimethyl(methacryloyloxyethyl)ammonium propane sulfonate (DMAPS), etc.] with pyrene as a probe. In both the homogeneous and micellar solutions, linear Stern–Volmer plots were obtained that implied that the quenching process can be considered as totally dynamic. The Stern–Volmer constant (KSV) for DMAPS decreased with an increasing dielectric constant of solvent and the concentration of simple electrolyte. An abrupt decrease in KSV was observed in the presence of a small amount of anionic emulsifier [below the critical micelle concentration (cmc)]. The dependence of KSV on pH for DMAPS was described by a curve with a maximum at about pH = 7. This was interpreted in terms of segregation of DMAPS and the variation of a optimal microenvironment for the probe and quencher with pH. The quenching rate in the micellar solutions strongly increased above the cmc but was lower than that in the homogeneous solutions. In the micellar solutions (above the cmc), the microenvironment for an interaction between the probe and quencher was suggested to be the whole microdroplet. The dependence of KSV on pH for DMAPS is described by a curve with a maximum at about pH = 9.3. The synergistic effect arises from the segregation of charged quencher molecules within the microdroplets. The complex (or strong interaction) between quencher and additive(s) is supposed to increase the dynamic nature of microdroplets that provides an optimal microenvironment for probe and quencher. A good coemulsifier, however, removes quencher from the interface and creates a barrier for entering monomer (quencher) into the core of micelles; therefore, quenching is depressed. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 571–581, 2003

Quenching of an excitation moving on a flexible polymer

We study theoretically the irreversible quenching of an excitation migrating on a polymer by small quencher molecules. Using a simplified superposition approximation, we solve the hierarchical reaction-diffusion equations and obtain the quenching rate and survival probability of the excitation. Depending upon the excitation mobility as well as upon the chain length, the quenching rate shows various behaviors, which is absent in the small molecular quenching reaction. In the diffusion-controlled limit, the quenching rate is found to increase as the excitation mobility increases and converge, for sufficiently large excitation mobility, to a constant value proportional to the radius of gyration of the polymer. This means that fast migration of single excitation, assisted by the dynamics of the polymer, enhance the reaction rate as much as that of the reaction of a static polymer where all the monomers are reactive.

Capillary electrophoresis and fluorescence studies on molecular beacon-based variable length oligonucleotide target discrimination.

Molecular beacons (MBs) are oligonucleotide probes having a compact hairpin structure, with a fluorophore attached to one end and a quencher molecule attached to the other end. In its native state, the fluorophore is quenched by virtue of its proximity to the quencher molecule. Upon hybridization with its complementary oligonucleotide target, fluorescence is elicited due to a conformational change that results in separation of the fluorophore and quencher molecule. The present study describes the hybridization interaction of an MB to various complementary target sequences. The effects of temperature and length of complementary target sequences on hybridization were investigated using capillary electrophoresis and solution-based fluorescence techniques. Hybridization efficiency was dependent on the ability of the target sequences to destabilize the stem region by binding directly to the stem region. Optimal hybridization occurred between 40 and 50 degrees C for all targets tested, with the true target forming a more stable hybrid complex.

Examination of the viability of the Smoluchowski–Collins–Kimball model in fluorescence quenching of S2-xanthione

The fluorescence quenching of S2-xanthione by five different hydrocarbon quenchers in perfluoro-1,3-dimethylcyclohexane has been studied assuming the Smoluchowski–Collins–Kimball (SCK) model. The fluorescence decay curves were measured by means of a picosecond time-resolved, single-photon counting technique. The model used has been shown to satisfactorily describe the process of S2-thioketones fluorescence quenching for six donor–quencher–solvent systems best fitting the model assumption [Krystkowiak and Maciejewski, J. Chem. Phys. 117, 2246 (2002)]. The systems studied were characterized by a short lifetime of an excited donor (equal to 152±2 ps in perfluoro-1,3-dimethylcyclohexane), the quenching process mechanism demanding a direct contact of reacting molecules and at the end the inert solvent. The influence of the shape, size, reactivity anisotropy of quencher molecules on the possibility of the SCK model application, and the values of molecular parameters: R (the sum of the molecular radii), D (the sum of diffusion coefficients), and the specific rate constant of the process (κ) was studied for S2-xanthione–n-hexane, S2-xanthione–n-decane, S2-xanthione–n-hexadecane, and S2-xanthione–longicyclene systems. The system with benzene as a quencher was studied for comparative reasons because of a different mechanism of the S2-xanthione fluorescence quenching process, which does not need a direct contact of reagent molecules. We have confirmed that the fluorescence quenching process of S2-xanthione by hydrocarbons could be described by a Smoluchowski–Collins–Kimball model only when the quencher molecules are characterized by a well-defined shape (not necessarily spherical) and isotropic reactivity and when the excited donor and quencher molecules are in direct contact within the contact pair. A satisfactory fit of the Smoluchowski–Collins–Kimball function the donor fluorescence decay in the presence of the quencher cannot be a sufficient criterion of the correctness of the model application. The parameters R, D, and κ determined for electronically excited donor–quencher pairs should be compared with these obtained from an independent study.

Excited state quenching via "unsuccessful" chemical reactions.

We discuss the results of recent photochemical reaction path computations on 1n,pi* azoalkanes interacting with a single quencher molecule. We provide computational and experimental evidence that there are two basic mechanisms for the true quenching of 1n,pi* states both based on unsuccessful chemical reactions. The first mechanism is based upon an unsuccessful hydrogen atom transfer and may occur through two different (direct and stepwise) routes. The second mechanism is based on an unsuccessful charge transfer reaction that occurs exclusively in a direct fashion. We show that the efficiency of the two quenching mechanisms is substantially due to the existence of two different types of conical intersections between the excited and ground state potential energy surfaces of the reacting bimolecular system.

TOTAL FLUORESCENCE QUANTUM YIELD AND RED SHIFT OF TOTAL FLUORESCENCE MAXIMUM AS PARAMETERS TO INVESTIGATE POLYCYCLIC AROMATIC COMPOUNDS PRESENT IN DIESEL FUEL

A method has been developed to find out total fluorescence quantum yield (ΦF T) of petroleum products like diesel, which provides the overall fluorescence quantum efficiency of a fluorophore at various possible excitation wavelengths. This simple method is suitable for characterization of petroleum products like diesel. The concentration dependent quantitative investigation of PACs present in diesel with respect to variation in ΦF T and red shift of excitation/emission maximum has been studied. The molecular interaction of PACs in fluorophoric mixtures like resonance energy transfer and self-quenching via solvent collision has been explained. Effect of solvent and external quencher molecule on diesel has also been studied. Nitrobenzene is found to be a selective quencher for PACs of diesel.

Quenching of the excited singlet state of acridine and 10-methylacridinium cation by thio-organic compounds in aqueous solution

In this work, the lowest excited singlet states of acridine (Acr), acridinium (AcrH+) and 10-methylacridinium (AcrMe+) are quenched by sulfur-containing amino acids and carboxylic acids in aqueous solution. Both steady-state and time-resolved fluorescence techniques were used to monitor the quenching of fluorescence. Stern–Volmer plots of the fluorescence intensity showed a static component (KS) to the quenching. The experimental KS values were compared to theoretical KS values for outer-sphere complexes based on Debye–Hückel theory and the Fuoss equation. The general agreement between experimental and theoretical KS values indicate that the static quenching can be attributed to non-fluorescing ion pairs associated as simple outer-sphere complexes. The computed values of the interionic distances of the ion pairs are consistent with the ion pairs of the ZAZQ=−1 and −2 cases being solvent-separated ion pairs while those of the ZAZQ=−3 case are contact ion pairs. The effect of the reactants’ charges on the quenching rate constants (dynamic component) was observed for the reactions of AcrMe+ with the anionic forms of the quenchers (having charges ZQ=−1, −2 and −3). The rate constants (extrapolated to ionic strength, μ=0) for the quenching processes were determined to be 0.3–5.3×1010M−1s−1 depending on the ionic charge (ZQ) of the quencher used. These trends in the quenching rate constants are rationalized with a quenching scheme for electron transfer. Analogous quenching rate constants for alanine and glycine were found to be at least an order of magnitude lower. Photoinduced electron transfer from the sulfur atom of the quencher molecule to the acridine excited singlet state is suggested to be the most likely mechanism of the process under discussion.

Investigation of collisional quenching of CCl2(X̃ 1B1 and ã 3B1) by some inorganic molecules

CCl2 free radicals were produced by a pulsed dc discharge of CCl4 in Ar. The radicals were electronically excited from the ground electronic state to the A 1B1 (0,4,0) state with Nd:YAG laser pumped dye laser at 541.52 nm in the rR subband. Experimental quenching data of CCl2(A 1B1) and CCl2(a 3B1) by O2, N2, NO, NH3, H2O, CO2, CS2, SO2, and SF6 molecules were monitored by observing the time-resolved total fluorescence signals of the excited CCl2 in a cell where the total pressure was 200 Pa. The observed profiles showed a superposition of two exponential decay components in the presence of the quencher. The quenching rate constants kA of CCl2(A) and ka of CCl2(a) were derived by analyzing the experimental data according to a proposed three-level-model to deal with the CCl2( 1A1, A 1B1, a 3B1) system. The order of magnitude for the rate constant of the quenching of the A state (kA) is 10−10 cm3 s−1, while for the quenching rate constant of the a state (ka) is 30–50% of kA. By changing the excitation wavelength, the radicals of the ground electronic state CCl2() were electronically excited to a different vibronic state, i.e. (0,3,0), (0,4,0), (1,3,0), (0,6,0), (1,4,0) and (2,2,0), of CCl2(A 1B1). We studied the kinetics of the collisional quenching by H2O and NH3 molecules of CCl2(A 1B1) in different vibronic states. It is found that the total quenching rate constant kA is little dependent on the vibronic state. It is possible that the reaction between the quencher molecule and CCl2(A 1B1) undergoes a process without a barrier. An explanation for the measured quenching data using the complex model based on attractive forces between the collision partners is presented.

Synthesis, Characterization, and Luminescence Spectroscopic Accessibility Studies of tris(2,2′-Bipyridine)Ruthenium(II)-Labeled Inorganic-Organic Hybrid Polymers

The modified tris(2,2′-bipyridine)ruthenium(II) complex 2 was anchored via a sol-gel process to different polysiloxane matrices to give a series of novel inorganic–organic hybrid polymers. One of the bipyridine ligands of 2 was provided with a long-chain spacer carrying a triethoxysilyl function (T group) at the end, which enables almost free mobility of the transition metal center. The polymers were swollen in several organic solvents of different polarity to investigate the luminescence behavior in the presence of quencher molecules. The luminescence of 2 was quenched using anthracene and atmospheric oxygen in appropriate concentrations of 5 · 10−4 to 5 · 10−3 and 3 · 10−3 M, respectively. The luminescence behavior of 2 was determined by steady-state and time-resolved luminescence experiments. Translational mobilities of molecular species dissolved in the liquid phase were investigated by the kinetics of luminescence quenching after laser excitation. Both the translational mobility of anthracene and atmospheric oxygen, specified with the rate constant k2 and the percentage of accessible luminescent centers were determined. Translational mobilities and the accessibility for anthracene and atmospheric oxygen in hybrid materials are significantly higher than in conventional Q type polysiloxanes.

Optically active BINOL core-based phenyleneethynylene dendrimers for the enantioselective fluorescent recognition of amino alcohols.

A dramatic enhancement in fluorescence intensity from 1,1'-bi-2-naphthol (BINOL) to dendritic phenyleneethynylenes containing the BINOL core was observed. The strong fluorescence of the dendrimers allows a very small amount of the chiral materials to be used for sensing. The light harvesting antennas of the dendrimer funnel energy to the center BINOL unit, whose hydroxyl groups upon interaction with a quencher molecule lead to fluorescence quenching. This mechanism makes the dendrimers have much more sensitive fluorescence responses than corresponding small molecule sensors. The fluorescence of these dendrimers can be enantioselectively quenched by chiral amino alcohols. It is observed that the fluorescence lifetime of the generation two dendrimer does not change in the presence of various concentrations of 2-amino-3-phenyl-1-propanol. This demonstrates that the fluorescence quenching is entirely due to static quenching. Thus, formation of nonfluorescent ground-state hydrogen-bond complexes between the dendrimers and amino alcohols is proposed to account for the fluorescent quenching. A linear relationship has been established between the Stern-Völmer constant of the generation two dendrimer and the enantiomeric composition of 2-amino-3-phenyl-1-propanol. Such enantioselective fluorescent sensors may allow a rapid determination of the enantiomeric composition of chiral molecules and are potentially useful in the combinatorial search of asymmetric catalysts and reagents.

Determination of Quenching Inhibition Factor and Selective Fluorescence Quenching Study of Perylene, Pyrene and Fluoranthene in Micelle by Cetyl Pyridinium Chloride as a Hydrophobic Quencher Molecule

The fluorescence quenching study of perylene, pyrene and fluoranthene by cetyl pyridinium chloride (CPC) a hydrophobic quencher molecule has been carried out using synchronous fluorescence scan. A quenching inhibition factor (QIF) has been defined for added quencher in inert micellar medium. The other quenching parameters calculated are K Q and E Q . The obtained quenching parameter QIF well compares with these values. The quenching mechanism is found to be explainable by a quencher sphere of action model. In micellar medium the fluorescence quenching efficiency is in the order: pyrene > perylene > fluoranthene. A correlation between alternate and non-alternate PAHs and aromatic ring size with QIF has been reported for CPC in micelle.

Structural changes of α-chymotrypsin in reverse micelles of AOT studied by steady state and transient state fluorescence spectroscopy

Time-resolved fluorescence of α-chymotrypsin (α-chym) solubilised in sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles was studied as a function of the amount of encapsulated water and initial pH. In reverse micelles the anionic interface leads to some conformational rearrangements at “pHext”=10 due to electrostatic interactions with the ionised groups of α-chym.The data point to the protein's location in a bulk water environment, which accounts for its high stability in these reverse micelles and allows the distinction of three emitting classes of tryptophan residues within the protein's matrix. Fluorescence quenching results show that these residues are differently accessed to the quencher molecules used (acrylamide, succinimide and iodide) and are consistent with a mechanism of penetration for the interaction with the protein.

Enantioselective Quenching of Luminescence: Molecular Recognition of Chiral Lanthanide Complexes by Biomolecules in Solution

Principles, experimental techniques, and applications of enantioselective quenching of lanthanide luminescence are addressed. Upon the addition of chiral, enantiomerically resolved quencher molecules to an aqueous, racemic solution of the tris (pyridine-2,6-dicarboxylate) chelate of Eu(III) or Tb(III), strong quenching of the lanthanide luminescence is observed, with a quenching rate that depends on the chirality of the lanthanide species. Quenching by c-type cytochromes and vitamin B12 derivatives is discussed in detail. For the latter quenchers, the energy transfer reaction held responsible for the quenching proceeds via formation of an encounter complex between donor and quencher in which the actual energy transfer takes place. A structural model for this transient pair has been constructed on the basis of data on complexation between B12 and ground-state lanthanide chelate obtained by measurements of lanthanide induced shifts and relaxation of the NMR signals of protons of B12 and from the induced cir...

Concentration dependent red shift: qualitative and quantitative investigation of motor oils by synchronous fluorescence scan

Synchronous fluorescence scan (SFS) has been described as a successful technique to characterize Motor oils like diesel, petrol, kerosene, 2T oil and Mobil. The concentration dependent investigation of Motor oils shows a red shift in λ SFS max. Using red shift of λ SFS max, a method has been developed to quantify Motor oil in the concentration range 5–100% v/v. The concentration dependent overall rate of energy transfer of Motor oil gives a unique behavioral change according to the oil type and SFS is a simpler spectroscopic method to qualitatively differentiate between heavy and light oil. The molecular interaction of polycyclic aromatic compounds (PACs) in fluorophoric mixtures like resonance energy transfer and self-quenching via solvent collision has been clearly explained by SFS method. Effect of solvent and external quencher molecule on Motor oils has also been studied. Nitrobenzene is found to be a selective quencher for PACs of Motor oils.

High-throughput reactor for simulating the flame photometric detector

Quenching of luminescing species by co-eluting hydrocarbons has been widely reported in the flame photometric detector (FPD). This paper describes a novel method of investigating the chemical behavior of both analyte and quencher molecules in the FPD. The method is designed to reproduce the FPD’s behaviour on a large scale by using a custom-built reactor. The high-throughput reactor’s multi-capillary burner, situated inside a glass housing, is well suited to approximate the low-temperature, fuel-rich conditions of the conventional FPD, and also allows the study of various other flame phenomena. Wide regions of gas composition can be accessed by both diffusion- and premixed-type flames, and products can be easily sampled. Effluent collection demonstrates that 2 to 82% of various organic compounds may survive passage through the diffusion flame and be recovered intact. The recovery of several (unchanged) model hydrocarbons was found to decrease with increasing carbon number. Hetero-atoms such as sulfur, nitrogen, or oxygen greatly decrease the recovery of molecules relative to their pure hydrocarbon analogues. Compared to a diffusion flame, the recoveries of n-alkanes from a premixed flame are much lower and largely independent of carbon number or volatility.

Oxidation of Trimethylsilyl Radicals by N2O: A Mechanistic Study

The reaction of Me3Si (Me⋮CH3) radicals with N2O has been studied by analysis of the end products of the mercury-sensitized photolysis of N2O with Me3SiH. The main products found were N2, Me3SiSiMe3, Me3SiOSiMe3, and Me3SiOH. The influence on both major and minor products of reactant pressure, different scavenger and quencher molecules, temperature, and the reactor surface permits the derivation of a reaction mechanism and identification of the intermediates involved. The most important steps comprising the mechanism are oxygen abstraction from N2O by Me3Si with formation of Me3SiO and a chain propagation step, the reaction of Me3SiO and Me3SiH with formation of Me3SiOH and Me3Si. Although the precise nature of the last step is unknown, it is a multistep reaction. Me3SiO radicals also undergo combination with Me3Si radicals. Self-combination under formation of a peroxide does not occur. Vibrationally excited species emerge in Si−O bond-forming processes, and a number of minor products stem from unimolecular decomposition of these excited species. The rate constant of O atom abstraction from N2O (reaction 5) relative to Me3Si radical combination (reaction 4) is given by k5/k41/2 = (6.5 ± 1.3) × 10-12 cm3/2 s-1/2 at room temperature. An activation energy of EA(5) = 23 ± 1 kJ mol-1 is obtained from experiments in the temperature range 295−520 K.

Kinetic Study of the External Heavy-Atom Quenching of Fluorescence in Liquid Solution under High Pressure

The rate constants, kq, for the fluorescence quenching of pyrene by six heavy-atom quenchers, carbon tetrabromide (CBr4), pentabromoethane (PENTBE), 1,2-tetrabromoethane (TETRBE), 1,1-dibromo-2-bromoethane (TRIBE), 1,2-dibromoethane (DIBE), and bromoethane (BE), were measured in methylcyclohexane (MCH) at pressures of up to 650 MPa and 25 °C. The plots of ln kq against pressure show a monotonical decrease for CBr4 and PENTBE, a maximum for TETRBE and TRIBE, and a monotonical increase for DIBE and BE. The activation volumes were estimated as 18.9, 12.0, −10.2, −8.0, −7.3, and −7.1 cm3/mol for CBr4, PENTBE, TETRBE, TRIBE, DIBE, and BE, respectively. For CBr4 and PENTBE, the plots of 1/kq against η, where η is solvent viscosity, were approximately linear with positive intercepts, whereas for TETRBE, TRIBE, DIBE, and BE, they deviated significantly from linearity. These results are interpreted to occur by a quenching mechanism via an encounter complex formed between the singlet state of pyrene and heavy-atom quencher molecules, which is followed by exciplex formation in the solvent cage. It was shown that the ratio kdiff/k-diff, of the rate constant for the formation (kdiff) to that for the dissociation (k-diff), of the encounter complex involved in the kinetic model increases with increasing pressure, and also that the pressure dependence of kdiff/k-diff is described satisfactorily by the radial distribution function at the closest approach distance between the solute molecules. From these facts, together with the finding that kdiff is approximately inversely proportional to the solvent viscosity, the pressure dependence of kq observed in this work was attributed to the competing rate processes of the bimolecular quenching constant, kbim, with kdiff.