Pyrene fluorescence quenching by aromatic azides

Pyrene fluorescence quenching by phenylazide derivatives with donor and acceptor substituents has been studied by fluorescence spectroscopy and flash photolysis. The rate constants of quenching (k _q) in acetonitrile ((0.2–1.2) × 10¹⁰ l mol^?1 s^?1) are found to be close to a diffusion limit; the rate constants were somewhat higher for perfluoro-substituted arylazides. It is found that k _q does not depend on solvent polarity; the formation of the pyrene cation in the course of pyrene fluorescence quenching by tolylazide was not detected. Pyrene fluorescence quenching occurred by an energy-transfer mechanism; this is supported by the coincidence of the quantum yields of the direct and sensitized photodecomposition of tolylazide. As estimated, energy transfer in rigid media occurs at characteristic distances of about 10 Å.     

Subanosecond solvent relaxation studies by oxygen quenching of fluorescence

Quenching by oxygen under a pressure of 100 atmospheres reduces the fluorescence lifetime of ethanolic solutions of 1-anilinonaphthalene-8-sulphonate and indole to 54 and 36 psec from the unquenched values of 13.4 and 5.0 nsec, respectively, while the corresponding fluorescence emissions are blue-shifted by 5 and 8 nm. The latter shift, due to incomplete solvent relaxationduring the shortened fluorescence lifetime, is too fast for detection by the complementary technique of nanosecond time-resolved spectroscopy, but is revealed by the quenching method, which provides for a hundred-fold improvement in time-resolution and is capable of wider application.     

Study of quenching of anthracene fluorescence by [60]fullerene

[60]Fullerene has been shown to have a very high quenching effect on the fluorescence of anthracene at room temperature in n-hexane, n-heptane and carbontetrachloride medium. The possibility that the quenching is due to ground state electron donor-acceptor (EDA) complex formation between [60]fullerene and anthracene has been shown to be untanable in the concentration range used ( approximately 10(-5)moldm(-3) in both anthracene and C(60)). No exciplex formation under the present experimental conditions has been observed. In the non-quenching solvents n-hexane and n-heptane the Stern-Volmer constant follows the right trend with respect to change in solvent viscosity but in case of the quenching solvent CCl(4), the trend is opposite.     

Enantiotopic interactions in the fluorescence quenching of camphor by chiral amines

A study of the quenching of fluorescence of (+)- and (?) camphor in hexane by a series of chiral primary, secondary, and tertiary amines shows only very small differences in quenching rate constants for the two enantiomers. The collision complex which precedes quenching (probably charge transfer) is thus insufficiently tight for enantiomeric recognition.     

Lumry's law and the quenching of anthracene fluorescence by bromide in aqueoust-butyl alcohol

The Stern-Volmer quenching constant for the quenching of anthracene fluorescence by bromide at 298.15 K shows a complex dependence on alcohol mole fraction in water plust-butyl alcohol mixtures. A similar pattern is observed at other temperatures except near 285 K, the Lumry temperature for aqueous systems.     

Wavelength-dependent stereodifferentiation in the fluorescence quenching of asymmetric naphthalene-based dyads by amines

In the present contribution, wavelength has been used as a tunable parameter to achieve selective control of the photophysics of two novel asymmetric bichromophoric dyads composed of naphthalene units, i.e., 6-methoxynaphthalene (NPX) and 1-methylnaphthalene (NAP) derivatives, with different electronic properties, connected by an amide spacer [(S,S) and (S,R)-NPX-NAP]. As model systems, relevant monochromophoric compounds (NPX-M and NAP-M) have also been investigated. While upon excitation at 325 nm the light energy remained in the NPX moiety, at 290 nm an efficient singlet-singlet energy transfer (phi(SSET) of about 97%) from the NAP unit to the NPX chromophore dominated. A remarkable stereodifferentiation was observed in the excited-state quenching by triethylamine via exciplex formation. The results demonstrate that it is possible to control configuration-dependent interactions in the excited state by wavelength tuning. This can be rationalized through intramolecular interactions of pi systems leading to modulation of the redox properties.     

On the mechanism of proton-induced fluorescence quenching of aromatic compounds

The quenching mechanism in polar media has been studied by means of nanosecond time-resolved spectroscopy, fluorimelry, and measurements of isotope exchange. The fluorescence of aromatic compounds having an intramolecular charge-transfer structure is quenched effectively by protons and the proton-induced quenching is caused by electrophilic protonation at one of the carbon atoms of the aromatic ring.     

Influence of the temperature on the fluorescence quenching of pheophytin α by toluquinone

The influence of the temperature on the constant for the fluorescence quenching of pheophytin a by toluquinone in various solvents has been investigated. It has been shown that the quenching occurs in alcohols according to both static and dynamic mechanisms. The static quenching is caused by the formation of complexes of two types: charge-transfer complexes of ordinary structure and more stable complexes, in which the donor and acceptor are bound to one another by the alcohol molecules.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 1, pp. 115–118, January–February, 1985.     

Characterization of polycyclic aromatic hydrocarbons in environmental samples by selective fluorescence quenching

Selective fluorescence quenching is used to profile polycyclic aromatic hydrocarbons (PAHs) in samples of environmental origin. After separation by high-efficiency capillary liquid chromatography, the PAHs are detected by laser-induced fluorescence spectroscopy. Nitromethane is added to selectively quench the fluorescence of alternant PAHs, whereas diisopropylamine is added to quench nonalternant PAHs. The chromatograms in the absence and presence of fluorescence quenching are evaluated by means of the product moment correlation method to quantify the statistical similarities and differences. This method is demonstrated by application to three samples: a standard mixture of 16 priority pollutants, a coal-derived fluid, and a contaminated soil. The correlation coefficients (r) are typically 0.99 or higher for samples that are identical in origin, 0.90-0.50 for closely related samples, and less than 0.50 for samples that are distinctly unrelated. This method can be used to confirm with high statistical confidence the cause or source of an event with environmental impact, such as an oil leak or spill, contamination or waste by-products from petroleum fuel production and processing, etc.     

Orientation effects in fluorescence quenching for adsorbed aromatic hydrocarbons

Fluorescence quenching by a potential electron donor or acceptor has been examined for an aromatic hydrocarbon adsorbed on aerosil, for which unusual temperature effects have been observed, where the process is dependent on the nature of the quencher. These phenomena are examined in terms of reaction stereospecificity. By this is meant the resultant displacement from a series of jumps between localization positions. Surface Chemistry Institute, Ukrainian National Academy of Sciences, Prospekt Nauki 31, 252039 Kiev, Ukraine. Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 33, No. 1, pp. 12–16, January–February, 1997.     

The mechanism of fluorescence quenching of aromatic compounds by acids: ZDO calculations

The fluorescence quenching of cyano, hydroxy, methoxy, and amino derivatives of naphthalene, anthracene and pyrene by acids in polar solvents has been studied by quantum-chemistry methods in semi-empirical approximation. Quenching mechanisms, including protonation of the aromatic nucleus and electron transfer have been considered. It is shown that the mechanism of radiationless deactivation in encounter complexes of reagents consists in the specific interaction of the solvated proton with certain carbon atoms of the aromatic nucleus, not in the electron transfer. It is found that the rate constant of radiationless deactivation correlates with the charge at the corresponding carbon atom of the excited molecule. It is shown that the fluorescence quenching is determined mainly by the nature of the fluorescent state and electron donor-acceptor properties of the aromatic nucleus and the substituent. The present model makes it possible to predict the fluorescent properties of some aromatic compounds in the presence of acids.     

Electron-transfer fluorescence quenching of aromatic hydrocarbons by europium and ytterbium ions in acetonitrile

To make the effects of molecular size on photoinduced electron-transfer (ET) reactions clear, the ET fluorescence quenching of aromatic hydrocarbons by trivalent lanthanide ions M3+ (europium ion Eu3+ and ytterbium ion Yb3+) and the following ET reactions such as the geminate and free radical recombination were studied in acetonitrile. The rate constant k(q) of fluorescence quenching, the yields of free radical (phi(R)) and fluorescer triplet (phi(T)) in fluorescence quenching, and the rate constant k(rec) of free radical recombination were measured. Upon analysis of the free energy dependence of k(q), phi(R), phi(T), and k(rec), it was found that the switchover of the fluorescence quenching mechanism occurs at deltaG(fet) = -1.4 to -1.6 eV: When deltaG(fet) < -1.6 eV, the fluorescence quenching by M3+ is induced by a long-distance ET yielding the geminate radical ion pairs. When deltaG(fet) > -1.4 eV, it is induced by an exciplex formation. The exciplex dissociates rapidly to yield either the fluorescer triplet or the geminate radical ion pairs. The large shift of switchover deltaG(fet) from -0.5 eV for aromatic quenchers to -1.4 to -1.6 eV for lanthanide ions is almost attributed to the difference in the molecular size of the quenchers. Furthermore, it was substantiated that the free energy dependence of ET rates for the geminate and free radical recombination is satisfactorily interpreted within the limits of the Marcus theory.     

Cetylpyridinium chloride micelles as a selective fluorescence quenching solvent media for discriminating between alternant versus nonalternant polycyclic aromatic hydrocarbons

Fluorescence behavior of 41 polycyclic aromatic hydrocarbons (PAHs) dissolved in aqueous micellar cetyltrimethylammonium chloride (CTAC) solvent media and in five different cetyltrimethylammonium chloride + cetylpyridinium chloride (CPC) surfactant mixtures is reported. Experimental fluorescence measurements reveal that CPC is a selective fluorescence quenching agent for alternant PAHs. The cetylpyridinium ion effectively quenched emission intensities of the 21 alternant PAHs studied. Emission intensities of nonalternant PAHs, with a few noted exceptions, were unaffected by the presence of CPC in the mixed cationic surfactant micelles.     

Absorption,fluorescence, and photoinitiating properties of the aromatic ethers and aromatic amines complexes of cyclopentadienyliron

The aromatic ethers complexes of cyclopentadienyliron (O-CpFe) and aromatic amines complexes of cyclopentadienyliron (N-CpFe) were prepared by coupling several aniline derivatives and phenol derivatives with (η6-chlorobenzene) (η5-cyclopentadienyl) iron hexafluorophosphate through nucleophilic aromatic substitution reaction. The UV/vis absorption, fluorescence, and excitation spectra of the compounds were measured. The abilities of the photoinitiator in photopolymerizing the diglycidyl ether of the bisphenol-A epoxy oligomer under a halogen lamp were evaluated by near-infrared spectroscopy. The results show that the absorption of N-CpFe is higher than that of O-CpFe in the visible region. However, the photoinitiation ability of N-CpFe is lower than that of O-CpFe.     

Influence of temperature on quenching of fluorescence in compounds of the coumarin series

A study has been made of the influence of temperature on the quantum yield of fluorescence of 3,4-benzocoumarin in ethanol or n-hexane, in the temperature interval 77–300 K. A mechanism of fluorescence quenching is proposed, consisting of thermal activation of intercombination conversion between neighboring energy levels that differ in orbital nature.Poltava Branch, Academy of Sciences of Engineering Cybernetics of the Ukraine, 3 Koval' Street, Poltava 314601, Ukraine. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 32, No. 4, pp. 247–250, July–August, 1996. Original article submitted September 26, 1995.     

Studies on the fluorescence quenching of polysilane copolymers by chlorohydrocarbons

The room-temperature solution fluorescence quenching of polysilane copolymers by chlorohydrocarbons such as CCl₄, CHCl₃, C₂Cl₆, and Cl₂CHCHCl₂ was studied. The existence of dynamic quenching was preliminarily demonstrated by the experiment of fluorescence lifetime quenching. The fluorescence quenching data were in conformity with the equation: F₀/F = (1+K_SV[Q])exp(NV[Q]), where F and F₀ are the fluorescence intensity with and without the addition of quencher, K_SV is the Stern-Volmer constant, [Q] is the quencher concentration, N is the Avogadro constant, and V is the volume of the active sphere. The fluorescence quenching by the first three chlorohydrocarbons was attributed to the contemporaneous effect of dynamic quenching and static quenching. There exists, at least mathematically, a critical quencher concentration [Q]_C. When the quencher concentration [Q] < [Q]_C, the fluorescence quenching is dominated by the dynamic quenching part; when [Q] > [Q]_C, it is dominated by the static quenching part. However, the fluorescence quenching by Cl₂CHCHCl₂ was attributed to only static quenching. Furthermore, it was proposed that the dynamic quenching may be related with the electrical positivity of the central carbon nucleus of the quenching molecules while the static quenching may be caused by the “outside heavy atom effect” of the Cl element. © 1996 John Wiley & Sons, Inc.     

Determination of aromatic amines in water samples by capillary electrophoresis with electrochemical and fluorescence detection

Two capillary electrophoresis methods have been compared for the determination of aniline derivatives in environmental water samples. With the first method the anilines were separated as cations by free zone electrophoresis at low pH, and detected by amperometry. For this, the separation capillary was connected through a palladium field decoupler to an electrochemical detection cell which had been modified to match the volume scale of the separation. Most anilines tested, except chlorinated compounds, could be detected with full sensitivity at a detection potential of +0.7 V. Detection limits with this detection scheme were on a low microg/l level. The alternative method involved the derivatization of the anilines with fluorescamine, the separation of the derivatives formed by micellar electrokinetic chromatography, and fluorescence detection. For detection a lamp-based, fibre optics instrument was used. Detection limits with fluorimetry were comparable with those obtained with amperometric detection (in the order of 1 microg/l). Still, this method was preferred since it gave a higher separation efficiency and shorter analysis times (approximately 4 min). The most important argument, however, was its higher reliability and ease-of-handling. Preliminary experiments with water samples collected in areas where pollution with anilines may be expected showed that the method is highly specific, with few interferences showing up in the electropherograms.