Quenching platinum octaethylporphine phosphorescence in solution by poly(ferrocenylsilane)

We describe experiments that determine the quenching kinetics by poly(ferrocenylsilane) (PFS) for platinum octaethylporphine (PtOEP) phosphorescence in toluene solution. The phosphorescence quenching process was interpreted in terms of diffusion-controlled kinetics. Pulsed-gradient spin-echo nuclear magnetic resonance (PGSE NMR) and dynamic light scattering (DLS) were used to characterize the diffusion behavior of PFS and PtOEP in toluene solution. We found that the ferrocene group present in the repeat unit of polymer backbone is a good quencher for PtOEP phosphorescence. Quenching by the polymer involves the entire PFS polymer chain instead of individual ferrocene groups. The intrinsic quenching ability of PFS was found to be higher than that of a model compound, Bu-FS, that contains a single ferrocene group.     

CHEMILUMINESCENCE QUENCHING TERMS

Abstract— A methodology for the determination of chemiluminescence quenching terms is given. By using the oxidation of tetrakis(dimethylamino)ethylene as an example, it is shown how the nature and magnitude of the quenching terms help to elucidate the reaction mechanism, or how ignoring them can lead to false impressions. It is shown that chemiluminescence quenching effects need not be the same as those for photoluminescence of the same electronically excited species.     

On the Mechanism of Magnetic Quenching of Fluorescence in Gaseous State

The pressure dependence of fluorescence quenching of gaseous glyoxal has been measured in the presence of a magnetic field of 1–8 kG. Below 5 kG both the collision free lifetime and the collisional quenching constant were found to be dependent upon magnetic field strength. Above 5 kG the Collisional quenching constant turned out to be nearly equal to the value without magnetic field and the collision free lifetime took a constant value larger than that without magnetic field. The magnetic enhancement of the intramolecular radiationless transition of gaseous molecules has been studied theoretically. The phenomenon is shown to be explained by considering two mechanisms, mechanisms I and II. Mechanism I is due to the interaction of a primary state with secondary states through the Zeeman hamiltonian. Mechanism II is due to the shift and broadening of appropriate rovibronic levels by the Zeeman effect.     

磁性微球与牛血清白蛋白的相互作用

以纳米级四氧化三铁为磁性载体,以苯乙烯为单体,用微悬浮聚合法制备了聚苯乙烯磁性微球;以牛血清白蛋白(BSA)为模型蛋白,用荧光光谱仪和紫外-可见吸收光谱仪研究了磁性微球与BSA的相互作用.结果表明,磁性微球与BSA结合反应的猝灭机理为静态猝灭.     

FLUIDITY AND OXYGEN PENETRATION OF LIPID VESICLES STUDIED BY FLUORESCENCE PROBES

Abstract. –An analysis of the temperature dependence of trans -stilbene fluorescence yield in dipalmitoyl lecithin vesicles is used to obtain activation energies. The results are interpreted in terms of bilayer fluidity through and above the phase transition. Oxygen quenching of the fluorescence of pyrenebutyric acid (incorporated in dipalmitoyl lecithin and egg lecithin vesicles) is reported as a function of temperature and bulk oxygen concentration. Above the bilayer phase transition, quenching rates (determined by oxygen quenching) decrease with decreasing temperature. A reduction in oxygen quenching is observed through the dipalmitoyl lecithin phase transition.     

Fluorescence quenching of acridinium and 6-methoxyquinolinium ions by Pb2+, Hg2+, Cu2+, Ag+ and hydrogen sulphide

The quenching of the fluorescene of cationic derivatives of 6-methoxyquinoline and acridine by various heavy metal cations and hydrogen sulphide ion have been studied in detail. Stern—Volmer constants were determined and quenching was found to be exclusively dynamic, except for the hydrogen sulphide ion, which acts as both a dynamic and static quencher. The highly efficient static quenching process is interpreted in terms of nucleophilic attack of hydrogen sulphide to the 9-position of acridinium ion, thereby destroying the conjugated system of the fluorophore.     

Magnetic quenching of positronium

The positron-electron bound system in condensed matter (Ps) is a probe of primary importance for the investigation of the microscopic structure of liquids and non-metallic solids. The physical properties of Ps in matter are generally different with respect to Psin vacuo, owing to the interactions with the surrounding electrons. Information on the structure of Ps can be obtained through the Zeeman effect, that induces a mixing of them=0 Ps ground state sublevels with a shortening of the triplet lifetime (magnetic quenching). This method, which can be coupled with any positron annihilation experimental technique, showed its effectiveness to discriminate among competitive reactions between Ps and the surrounding medium, as well as to clarify the origin of lifetime components of uncertain attribution. The discovery of anomalous magnetic effects in different organic liquids and solids has opened new perspectives in the studies of positron-multielectrons bound systems. Magnetic quenching experiments carried out with polarized positron beams display a complex and fascinating phenomenology, whose explanation could shed light on the role of the positron-medium interactions which take place within the early instants after the emission of the positron. In the present paper various aspects of the magnetic quenching method will be examined, with emphasis on recent experimental results.     

A COMPARISON OF FLUORESCENCE QUENCHING AND FLUORESCENCE DECAY STUDIES WITH TRYPTOPHAN

Abstract— The quenching of the fluorescence of aqueous tryptophan solutions has been studied as a function of emission wavelength using acrylamide as a collisional quencher. Our quenching studies are consistent with recent observations of the heterogeneity of tryptophan fluorescence, but they show a slight discrepancy when compared to certain analyses of the decay of tryptophan fluorescence in terms of two components.     

Lifetime Quenching of the Fluorophore: A Signature of Magnetic Field Maximization

The fluorescence lifetime quenching of the fluorophores in the presence of quenchers were measured with the variation of solvent polarity (e) for the two donor-acceptor (D-A) pair, JV-ethyl carbazole-1,4 dicyanobenzene and pyrene-N,N-dimethylaniline. We observed that maximum quenching of lifetimes occurred at the ε where maximum magnetic field effects had been achieved for both the pairs. The importance of the nature of the ex-ciplex and the distance between D and A in magnetic field maximization has also been elucidated.     

Adsorption of dye from wastewater using chitosan-CTAB modified bentonites

Multifunctional magnetic-fluorescent nanohybrids were successfully fabricated by a facile layer-by-layer (LBL) self-assembly of in situ generated Eu, Gd co-doped LaF(3) nanocrystals (LaF(3):Eu:Gd) on the surface of multi-walled carbon nanotubes (MWNTs). Photoluminescence (PL) quenching occurred when LaF(3):Eu:Gd nanoparticles were directly coated on the surface of MWNTs. By growth of a SiO(2) shell spacer between MWNTs and LaF(3):Eu:Gd nanocrystals, we circumvented the PL quenching and achieved the magnetic-fluorescent MWNTs. Moreover, the nanohybrids showed powerful T(1) and T(2)-weighted magnetic resonance imaging (MRI) signal in water and could be used as MRI contrast agents. As a result, the nanohybrids can be expected to act as a promising multimodal MRI/optical imaging probe.     

Layer-by-layer assembled Fe3O4@C@CdTe core/shell microspheres as separable luminescent probe for sensitive sensing of Cu2+ ions

A novel multifunctional microsphere with a fluorescent CdTe quantum dots (QDs) shell and a magnetic core (Fe(3)O(4)) has been successfully developed and prepared by a combination of the hydrothermal method and layer-by-layer (LBL) self-assembly technique. The resulting fluorescent Fe(3)O(4)@C@CdTe core/shell microspheres are utilized as a chemosensor for ultrasensitive Cu(2+) ion detection. The fluorescence of the obtained chemosensor could be quenched effectively by Cu(2+) ions. The quenching mechanism was studied and the results showed the existence of both static and dynamic quenching processes. However, static quenching is the more prominent of the two. The modified Stern-Volmer equation showed a good linear response (R(2) = 0.9957) in the range 1-10 μM with a quenching constant (K(sv)) of 4.9 × 10(4) M(-1). Most importantly, magnetic measurements showed that the Fe(3)O(4)@C@CdTe core/shell microspheres were superparamagnetic and they could be separated and collected easily using a commercial magnet in 10 s. These results obtained not only provide a way to solve the embarrassments in practical sensing applications of QDs, but also enable the fabrication of other multifunctional nanostructure-based hybrid nanomaterials.     

Fluorescence quenching by excimer formation: quenching constant approximations for excimer formation-dissociation by classical potential models

Fluorescence quenching by excimer formation is studied on the assumption that the excimer formation and dissociation can be modeled as overdamped motion in an attractive potential (classical potential models). An approach to the zeroth-order, concentration-independent quenching constants is proposed which starts from a mean reaction-time ansatz and reduces the calculation essentially to the solution of the eigenvalue problem for the Smoluchowski operator which describes the excimer equilibration. For a square-well potential model it is shown that a quenching constant expansion in terms of relaxation modes, truncated at the kinetic level, gives a satisfactory approximation of the recently obtained exact zeroth-order result under defined conditions. It is demonstrated how this two-mode approach can be applied for a quenching constant estimation if the excimer formation and dissociation are modeled by more realistic interaction potentials, as for instance, Morse- or Gaussian-type ones.     

Proton and electron transfer in the excited state quenching of phenosafranine by aliphatic amines

The quenching of the excited singlet and triplet states of phenosafranine by aliphatic amines was investigated in acetonitrile and methanol. The rate constants for the quenching of the excited singlet state depend on the one-electron redox potential of the amine suggesting a charge transfer process. However, for the triplet state, quenching dependence on the redox potential either is opposite to the expectation or there is not dependence at all. Moreover, in MeOH the first-order rate constant for the decay of the triplet state, k(obs) presents a downward curvature as a function of the amine concentration. This behavior was interpreted in terms of the reversible formation of an intermediate excited complex, and from a kinetic analysis the equilibrium constant K(exc) could be extracted. The log K(exc) shows a linear relationship with the pKb of the amine. On the other hand, for the triplet state quenching in acetonitrile k(obs) varies linearly with the amine concentration. Nevertheless, the quenching rate constants correlate satisfactorily with pKb and not with the redox potential. The results were interpreted in terms of a proton transfer quenching, reversible in the case of MeOH and irreversible in MeCN. This was further confirmed by the transient absorption spectra obtained by laser flash photolysis. The transient absorption immediately after the triplet state quenching could be assigned to the unprotonated form of the dye. At later times the spectrum matches the semireduced form of the dye. The overall process corresponds to a one-electron reduction of the dye mediated by the deprotonated triplet state.     

Fluorescence quenching of anthracene by indole derivatives in phospholipid bilayers

The quenching of anthracene fluorescence by indole, 1,2-dimethylindole (DMI), tryptophan (Trp) and indole 3-acetic acid (IAA) in palmitoyloleoylphosphatidylcholine (POPC) lipid bilayers was investigated. A very efficient quenching of the anthracene fluorescence in the lipid membrane is observed. Stern-Volmer plots are linear for DMI but present a downward curvature for the other quenchers. This was interpreted as an indication of the presence of an inaccessible fraction of anthracene molecules. By a modified Stern-Volmer analysis the fraction accessible to the quenchers and the quenching constant were determined. The changes in the fluorescence emission spectrum of indole and DMI have been used to calculate the partition constants of these probes into the membranes, and bimolecular quenching rate constants were determined in terms of the local concentration of quencher in the lipid bilayer. The rate constants are lower than those in homogeneous solvents, which may be ascribed to a higher viscosity of the bilayer. No changes in the emission spectra of Trp and IAA are observed in the presence of vesicles, indicating that these probes locate preferentially in the aqueous phase, or in close proximity to the vesicular external interface in a medium resembling pure water. In these cases quenching rate constants were determined in terms of the analytical concentration. In the quenching by DMI a new, red shifted, emission band appears; it is similar to that observed in non-polar solvents and it is ascribable to an exciplex emission. The exciplex band is absent in the quenching by IAA and Trp and only very weakly present when the quencher is indole. From the position of the maximum of the exciplex emission, a relatively high local polarity could be estimated for the region of the bilayer where the quenching reaction takes place.     

Influence of Jahn-Teller coupling on the magnetic properties of transition metal complexes with orbital triplet ground terms: magnetization and electronic Raman studies of the titanium(III) hexa-aqua cation

Magnetization and electronic Raman data are presented for salts of the type Cs[Ga:Ti](SO(4))(2) x 12H(2)O, which enable a very precise definition of the electronic structure of the [Ti(OH(2))(6)](3+) cation. The magnetization data exhibit a spectacular deviation from Brillouin behavior, with the magnetic moment highly dependent on the strength of the applied field at a given ratio of B/T. This arises from unprecedented higher-order contributions to the magnetization, and these measurements afford the determination of the ground-state Zeeman coefficients to third-order. The anomalous magnetic behavior is a manifestation of Jahn-Teller coupling, giving rise to low-lying vibronic states, which mix into the ground state through the magnetic field. Electronic Raman measurements of the 1%-titanium(III)-doped sample identify the first vibronic excitation at approximately 18 cm(-1), which betokens a substantial quenching of spin-orbit coupling by the vibronic interaction. The ground-state Zeeman coefficients are strongly dependent on the concentration of titanium(III) in the crystals, and this can be modeled as a function of one parameter, representing the degree of strain induced by the cooperative Jahn-Teller effect. This study clearly demonstrates the importance that the Jahn-Teller effect can have in governing the magnetic properties of transition metal complexes with orbital triplet ground terms.     

INCLUSION AND FLUORESCENE QUENCHING OF 2, 3-DIMETHYLNAPHTHALENE IN β-CYCLODEXTRIN CAVITIES

Abstract— The fluorescence quenching of 2,3-dimethylnaphthalene (DMN) incorporated to β-cyclodextrin (β-CD) cavities by different olefins (fumaronitrile, acrylonitrile, acrylamide and 2-hydroxyethylmethacrylate) has been measured as a function of the β-CD concentration. The quenching efficiency decreases when the β-CD concentration increases, but extrapolation of the data to infinite cavities concentration does not indicate complete protection. These results are interpreted in terms of two quenching processes, one of them taking place between 2,3-dimethylnaphthalene associated to a β-CD cavity and free quencher, and the other between the DMN and the quencher molecule, both associated with a different cavity. The rate constants of both quenching processes and the β-CD quencher association constant are obtained from the dependence of the quenching efficiency with β-CD concentration.     

Magnetic quenching of fluorescence

The direct and indirect mechanisms for the magnetic enhancement of radiationless decay processes are described, and used to account for the experimental observations of the magnetic quenching of fluorescence from carbon disulphide, glyoxal, and nitrogen dioxide.     

Magnetic quenching of positronium in organic solutions

The magnetic quenching of ortho-positronium (o-Ps) in some pure nonpolar liquids (n-hexane, cyclohexane and benzene) and solutions (nitrobenzene in n-hexane, cyclohexane and benzene; and carbon tetrachloride and biphenyl in n-hexane) is examined for steady magnetic fields up to 14 kG by the positron annihilation lifetime technique. The long lifetime of o-Ps is very sensitive to the strength of the external magnetic field, decreasing as the field strength increases. This effect follows from a well-known principle of atomic physics, the quadratic Zeeman effect. With one exception, all the liquids studied here appear to be normal in this regard. The exception is nitrobenzene in n-hexane, which shows a very marked enhancement of quenching beyond the Zeeman effect at low fields.     

Fluorescence quenching of coumarins by halide ions

Fluorescence quenching of 4-methyl-7-methoxy coumarin (1) and 4-methyl-5-ethoxy-7-methoxy coumarin (2) in aqueous solutions have been studied at different concentrations of halide ions (Cl-, Br-, I-), at room temperature approximately 20 degrees C. It is observed that the fluorescence intensity of both the coumarin compounds (1 and 2) decrease with increase in the concentration of Br- and I- ions but remains almost constant in case of Cl- ion. It is observed that the quenching due to halide ions proceeds via both a diffusional and static quenching processes. The rate constants for diffusional as well as static component of quenching process have been calculated using modified Stern-Volmer relation. It is further observed, that I- ion has very high quenching ability than Br- ion and Cl) ion almost behaves as a non-quencher. It is explained in terms of the sphere of action model by showing that the value of radius of sphere of action of the halide quencher is greater than the sum of the radii of the respective coumarin and quencher. Further, pattern of the quenching ability of the halide quenchers is found to be as I- > Br- > Cl- and interestingly this is in the same order as of their ionization energy. Finally, the present quenching process has been attributed to the electron transfer resulting between colliding species.     

Iron Oxide Nanoparticles Labeled with an Excited‐State Intramolecular Proton Transfer Dye

Excited‐state intramolecular proton transfer (ESIPT) is a particularly well known reaction that has been very little studied in magnetic environments. In this work, we report on the photophysical behavior of a known ESIPT dye of the benzothiazole class, when in solution with uncoated superparamagnetic iron oxide nanoparticles, and when grafted to silica‐coated iron oxide nanoparticles. Uncoated iron oxide nanoparticles promoted the fluorescence quenching of the ESIPT dye, resulting from collisions during the lifetime of the excited state. The assembly of iron oxide nanoparticles with a shell of silica provided recovery of the ESIPT emission, due to the isolation promoted by the silica shell. The silica network gives protection against the fluorescence quenching of the dye, allowing the nanoparticles to act as a bimodal (optical and magnetic) imaging contrast agent with a large Stokes shift.