FLUORESCENCE QUENCHING OF INDOLES BY N,N-DIMETHYLFORMAMIDE

Abstract. The quenching of the excited state of indoles by small polar molecules is believed to occur via the formation of excited state complexes. The rate of excited state complex formation of indoles with dimethylformamide is shown to be a function of the group appended to the indole ring at the 3- or 5-position. Substituents which can strengthen the charge transfer character of the exciplex are observed to enhance the rate of fluorescence quenching. In contrast to other amides, dimethylformamide appears to be a moderately good quencher of indole fluorescence, and may be a useful probe for proteins.     

THE PHOTOPHYSICS OF 5-METHOXYINDOLE. A NON-EXCIPLEX FORMING INDOLE: ADDITIONAL EVIDENCE FOR TWO CLASSES OF EXCIPLEXES*

Abstract— 5-Methoxyindole is a non-exciplex forming indole, and its excited state behavior is qualitatively different from that of indole and its methyl substituted derivatives. This fact supports the idea that there are two limiting classes of exciplexes, charge-transfer and dipole-dipole stabilized. The fluorescence quantum yield in water is 0.29 with a lifetime of 4.0 ns at 25d?C. The activation energy for fluorescence quenching in water is 15.9 pM 0.5 kJ/mol, which is smaller than for indole and the methyl substituted indoles which have been measured. In cyclohexane at 25d?C, the fluorescence quantum yield is 0.63 with a lifetime of 6.7 ns. The fluorescence is efficiently quenched by electron scavengers, as is the case for other indoles. Some electron ejection to solvent probably occurs in both solvents.     

Proton-transfer mediated quenching of pyrene/indole charge-transfer states in isooctane solutions

The fluorescence quenching of pyrene (Py) by a series of N-methyl and N-H substituted indoles was studied in isooctane at 298 K. The fluorescence quenching rate constants were evaluated by mean of steady-state and time-resolved measurements. In all cases, the quenching process involves a charge-transfer (CT) mechanism. The I(o)/I and tau(o)/tau Stern-Volmer plots obtained for the N-H indoles show a very unusual upward deviation with increasing concentration of the quenchers. This behavior is attributed to the self-quenching of the CT intermediates by the free indoles in solution. The efficiency of quenching of the polyaromatic by the N-H indoles increases abruptly in the presence of small amount of added pyridine (or propanol). A detailed analysis of the experimental data obtained in the presence of pyridine provides unambiguous evidence that the self-quenching process involves proton transfer from the CT states to indoles.     

Electronic Properties of Q-CdS Clusters Stabilized by Adenine

Adenine-capped Q-CdS has been synthesized in an aqueous medium. IR spectroscopy indicates an interaction between Q-CdS and adenine through Cd²⁺. The amount of adenine controls the size of the clusters. A typical 5×10⁻³ mol dm⁻³ of adenine produces nanoclusters having the onset of absorption and an emission band at 2.8 and 2.35 eV, respectively. Adenine binds to the shallower traps and enhances the emission intensity of the 530-nm band without causing any shift in emission. Thermolysis of these colloids leads to the production of larger CdS clusters with changed electronic properties. Relaxation kinetics of charge carriers shows their average lifetime to increase with a decrease in particle size. Illumination of these particles does not lead to their photodissolution. This catalyst is, however, photoactive. The addition of indole causes the quenching of its emission. The photocatalytic oxidation of indole produces indigo with a quantum efficiency of 0.03.     

A FLUORESCENCE STUDY OF THE INTERACTION OF INDOLIC COMPOUNDS WITH SYNTHETIC POLYELECTROLYTES

Abstract— –The interaction of indole derivatives with synthetic polyelectrolytes was investigated using UV absorption and fluorescence spectroscopy. The presence of both sodium poly(styrene sulfonate) (PSS) and sodium polyvinyl sulfonate) (PVS) inhibits the fluorescence quenching of 1-pyrene sulfonic acid by tryptamine. The effect is more marked for PSS than for PVS. There was no polyelectrolyte effect on the quenching by tryptophan. It was also found that aromatic polyelectrolytes strongly quench the fluorescence of indole derivatives of opposite charge by a static mechanism. This is accompanied by a new absorption in the red extreme of the UV spectrum of the mixtures. The systems investigated were tryptamine-PSS and polyvinyl benzyl trimethylammonium chloride) with the anions of indole-3-alkanoic acids. Equilibrium constants for the binding of the indole derivatives to the polyelectrolytes were determined. The fluorescence of zwitterionic tryptophan is not affected by the presence of the polyelectrolytes.     

PHOTOPHYSICAL STUDIES OF INDOLE ALKANOIC ACIDS and TRYPTAMINE IN REVERSE MICELLES OF SODIUM DIOCTYL SULFOSUCCINATE

Abstract— The fluorescence spectra and emission lifetimes of several 3-alkanoic indoles of different chain length and tryptamine (TA) were studied in sodium dioctyl sulfosuccinate (AOT)/heptane reverse micelles over a wide range of water/AOT ratio (R - 5 to 44). Fluorescence quenching experiments were done using carbon tetrachloride and acrylamide as quenchers. Experiments with TA were carried out using water at pH 3 in order to assure its protonation. Under these conditions, the results indicate that the indole moiety of TA remains at the micellar interface over all the range considered. Furthermore, the results can be interpreted assuming for the TA population a single microenvironment whose properties remain almost invariant when R increases from 11 to 44. The studies employing the 3-alkanoic indoles were carried out at pH 10. Under these conditions, the anions are progressively displaced to the water pool when the R value increases. This displacement is determined by the length of the side alkyl chain of the 3-indole derivatives. For these compounds, the quenching experiments indicate that, even at low R values, the excited indole moieties are distributed among different microenvironments.     

Exciplex-type behavior and partition of 3-substituted indole derivatives in reverse micelles made with benzylhexadecyldimethylammonium chloride, water and benzene

The fluorescence properties of 3-methylindole (MI), 3-indoleacetic acid (IAA), 3-indoleethyltrimethylammonium bromide (IETA), L-tryptophan (Trp) and tryptamine hydrochloride (TA) were studied in reverse micelles solutions made with the cationic surfactant benzylhexadecyldimethylammonium chloride (BHDC) in benzene as a function of the molar ratio water/surfactant R (= [H2O]/[BHDC]). The fluorescence quenching of the model compound MI by benzene in cyclohexane solutions and by BHDC in benzene solutions were also studied in detail. The fluorescence of MI in benzene is characteristic of a charge-transfer exciplex. The exciplex is quenched by the presence of BHDC, due to the interactions of the surfactant ion pairs with the polar exciplex. In reverse micelle solutions at low R values, all the indoles show exciplex-type fluorescence. As R increases, the fluorescence behavior strongly depends on the nature of the indole derivative. The anionic IAA remains anchored to the cationic interface and its fluorescence is quenched upon water addition due to the increases of interface's micropolarity. For IETA, TA and Trp an initial fluorescence quenching is observed at increasing R, but a fluorescence recovery is observed at R > 5, indicating a probe partition between the micellar interface and the water pool. For the neutral MI, the fluorescence changes with R indicate the partition of the probe between the micellar interface and the bulk benzene pseudophase. A simple two-site model is proposed for the calculation of the partition constants K as a function of R. In all cases, the calculation showed that even at the highest R value, about 90% of the indole molecules remain associated at the micellar interface.     

Intra- and intermolecular fluorescence quenching in 7-(pyridyl)indoles

Three isomeric 7-(pyridyl)indoles reveal very different, solvent-dependent photophysical properties. Due to rapid excited state depopulation involving intramolecular hydrogen bonding, 7-(2′-pyridyl)indole is practically nonfluorescent at room temperature. In nonpolar and polar aprotic solvents, 7-(3′-pyridyl)indole and 7-(4′-pyridyl)indole fluorescence strongly, but the emission is quenched in alcohols. Syn and anti rotameric forms of 7-(3′-pyridyl)indole are detected, each quenched to a different degree. This differential quenching is interpreted as evidence of enhanced S₁ → S₀ internal conversion being more efficient in cyclic solvates, with alcohol molecules forming a bridge between the proton donor and acceptor groups of an excited chromophore.     

EXCITED STATES OF FLAVINES – V. THE QUENCHING OF FLAVINE LUMINESCENCE BY INDOLES

Abstract— The quenching of the fluorescence of flavines by indoles in aqueous solution has been studied as a function of concentration, viscosity and temperature. The quenching shows a negative temperature dependence, and is in part diffusion controlled. Static and dynamic processes both contribute to quenching.
In the presence of alcohol or denaturants static quenching is inhibited and dynamic quenching supervenes. The effect of indoles on the flavine fluorescence lifetimes and phosphorescence characteristics is consistent with the quenching mechanism proposed.     

Fluorescence quenching of carbazole and indole by ethylenetrithiocarbonate

The fluorescence of quenching of carbazoles and indoles by ethylenetrithiocarbonate has been studied. Both steady-state and transient fluorescence experiments were carried out in order to determine rate constants in the case of carbazoles. The results obtained support the charge-transfer mechanism. Ground-state complex formation is assumed to explain the higher rate constants for indole and its derivatives, which appear as positive curvatures in Stem-Volmer plots.     

Rotaxanated conjugated sensory polymers

Two highly emissive conjugated polymers with tethered rotaxane repeat units are reported. Hydrogen bonding between acidic alcohols and the N-heteroaromatic groups in the rotaxanes attenuates polymer fluorescence. In addition, the rotaxane groups create precise three-dimensional pockets for metal binding, which results in fluorescence quenching. Exposing thin films of Zn-doped polymers to alcohol vapors reverses the quenching by up to 25%.     

Photophysics and Photocatalytic Properties of Cd(OH)(2)-Coated Q-CdS Clusters in the Presence of Guanine and Related Compounds

The photophysics of Cd(OH)(2)-coated Q-CdS in the presence of guanine and related compounds has been examined. Guanine and adenine quench the bandgap emission and reduce the emission lifetime of these particles. Approximately 50% of the bandgap fluorescence is quenched by a low [guanine] (2x10(-5) mol dm(-3)). Quenching takes place with a bimolecular rate constant of 2x10(12) dm(3) mol(-1) s(-1). The presence of these additives did not affect the red emission appreciably. The nature of the interaction between Cd(OH)(2) layer of Q-CdS and the additive has been analyzed by fluorescence and absorption spectroscopy. Interception of the hole by guanine and adenine occurs through their adsorption and hydrogen-bonding interaction between the -OH of Cd(OH)(2) and certain functional groups of the additive. Cd(OH)(2)-coated Q-CdS sensitizes the photodecomposition of guanine efficiently in the presence of oxygen under visible light irradiation. Shallowly trapped and direct holes are suggested to participate in the oxidation. The reactivity of the hole is governed by the redox potential of the solute. Copyright 2001 Academic Press.     

Nucleophilic addition of grignard reagents to 3-acylindoles: stereoselective synthesis of highly substituted indoline scaffolds

3-Acylindoles undergo nucleophilic-type reactions with Grignard reagents to efficiently afford either cis- or trans-substituted indolines, depending on the different quenching procedures. The enolate intermediate could be trapped by aryl acyl chlorides to provide indolines bearing a quaternary carbon center with high stereoselectivity. In contrast, the use of benzyl bromide as an electrophile results in the fragmentation of the indole ring. The indoline products could be easily transformed into indoles through oxidation with DDQ in a one-pot manner.     

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.     

Photophysics of anthracene-indole systems in unilamellar vesicles of DMPC and POPC: Exciplex formation and temperature effects

The quenching of anthracene fluorescence by indole (IN), 1,2-dimethylindole (DMI), tryptophan (Trp) and indole 3-acetic acid (IAA) in dimiristoylphophatidylcholine (DMPC) and palmitoyloleoylphosphatidylcholine (POPC) lipid bilayers was investigated. The studies were carried out at 25 degrees C in POPC vesicles and below (15 degrees C) and above (35 degrees C) the phase transition temperature (24 degrees C) of DMPC. A very efficient quenching of the anthracene fluorescence by IN and DMI in the lipid membrane is observed in all cases. It is less efficient in the case of Trp and IAA. 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. Partition constants of the quenchers were obtained from the changes in the fluorescence emission of the indole moiety caused by the presence of the phospholipid. Using the partition constants bimolecular quenching rate constants were determined in terms of the local concentration of quencher in the lipid bilayer. These corrected rate constants are lower than those in homogeneous solvents. In the case of DMPC values the gel phase are higher than in the liquid-crystalline phase. In the quenching by IN and DMI a new, red shifted, emission band appears which could be assigned to an exciplex emission. The exciplex band is absent in the quenching by IAA and Trp.     

竹红菌甲素和吲哚类化合物的相互作用

本文利用吸收光谱,荧光光谱研究了竹红菌甲素(以下简称HA)和吲哚类化合物的相互作用。发现HA和吲哚类化合物在一定条件下可形成复合物。这种复合物带有电荷转移性质。吲哚环上带有给电子取代基,位阻小,溶剂的极性大和溶液呈中性有利于该复合物的形成。     

Specific features of luminescence of Q-CdS colloids with different sizes

The specific features of luminescence of colloidal solutions of Q-CdS with particles of different size and the regularities of luminescence quenching by quenchers of various nature were studied. The luminescence spectra of Q-CdS consist of several bands, which are shifted to the long-ware region as the particle size increases. The dependence of the integral quantum yield of luminescence on the particle size has a sharp maximum at a particle diameter of ?23Å. A Stem—Volmer-type equation including the adsorption isotherm of the quencher molecules on the surface of the Q-CdS colloidal particles was used to describe the regularities of luminescence quenching of Q-CdS colloidal solutions. The CdS particle size was found to affect the efficiency of luminescence quenching. The regularities of luminescence quenching depend both on the rate constant of electron transfer to the quencher molecules and on the ability of the quencher molecules to be adsorbed on the surface of the CdS colloidal particle.     

A two step route to indoles from haloarenes--a versatile variation on the Fischer indole synthesis

In a new variation on the Fischer indole synthesis, readily available haloarenes are converted into a wide range of indoles in just two steps by halogen-magnesium exchange, quenching with di-tert-butyl azodicarboxylate, followed by reaction with ketones under acidic conditions.     

Q-CdS/聚合物纳米复合膜的制备与荧光性能

采用配位化学合成原理 ,分离制备出颗粒尺寸小于 10nm的单分散性的Q态CdS(Q CdS)纳米粒子 ,将Q CdS纳米粒子与聚合物复合成膜 ,制备出一系列Q CdS 聚合物纳米复合膜 .用紫外可见吸收光谱与透射电镜研究了纳米复合膜的量子尺寸效应和分散性 .通过荧光光谱探讨了不同聚合物基体材料和不同Q CdS含量的纳米复合膜的荧光发光性能 .结果表明 ,一方面这种以聚合物为基体的纳米复合膜 ,由于聚合物与Q CdS之间的相互作用 ,使纳米复合膜表现出与单一相组分完全不同的特征荧光发射峰 ;另一方面 ,随着纳米复合膜中Q CdS含量的不断增大 ,纳米复合膜的荧光强度不断增强 ,在一定浓度时达到最大值 .     

Indole synthesis by radical cyclization of o-alkenylphenyl isocyanides and its application to the total synthesis of natural products

Development of indole synthesis by tin-mediated radical cyclization of o-alkenylphenyl isocyanide is described. Upon heating o-alkenylphenyl isocyanide in the presence of tri-n-butyltin hydride and AIBN, 2-stannyl-3-substituted indole was formed via 5-exo-trig cyclization of the imidoyl radical intermediate. After acidic workup, 3-substituted indoles were isolated. For substrates bearing simple alkyl groups, a substantial amount of tetrahydroquinoline derivatives were generated through 6-endo-trig cyclization. This undesired cyclization was suppressed by using an excess amount (five equivalents based on o-alkenylphenyl isocyanide) of ethanethiol instead of tri-n-butyltin hydride. The 2-stannylindole intermediates proved to be a suitable substrate for Stille coupling, giving 2,3-disubstituted indoles in a one-pot procedure. In addition, the 2-stannylindole intermediates could be converted to 2-iodoindoles by treatment with iodine or N-iodosuccinimide. The 2-iodoindoles thus obtained served as good substrates for Heck reactions, Stille couplings, Suzuki couplings, and palladium-mediated carbonylations, to afford a variety of 2,3-disubstituted indoles. The utility of this protocol was demonstrated by application to synthetic studies on gelsemine and discorhabdin A, and the total synthesis of an aspidosperma alkaloid, (-)-vindoline.