THE PHOTOREDUCTION OF METHYLENE BLUE BY ARYLAMINOMETHANE-SULFONATES

Abstract— The photoreduction of methylene blue in the presence of arylaminomethanesulfonates (RAMS = RC₆H₄NHCH₂SO₃Na) was studied by laser and conventional flash photolysis. These compounds quenched the methylene blue triplet deviating from a normal Stern-Volmer behaviour. For low quencher concentrations, a Rehm-Weller relationship was found between the k _q's and the DL G 's obtained for the electron transfer reactions. The lack of further quenching at higher [RAMS] is ascribed to the formation of a ground state ion pair between the dye and the anionic quencher which, on excitation, forms a triplet state unable to under go electron transfer for steric reasons. A second order decay rate constant was found for the semireduced species (MB') ( ca. 5 × 10⁹ M _-1 s_-1, independent of the RAMS used) and is attributed to a proton transfer from the radical zwitterion (RC₆H₄NH CH₂SO₃⁻) to MB. The overall dependence on the substituent of the bleaching observed by continuous irradiation follows the triplet behaviour.     

Influences of Acid on Molecular Forms of Fluorescein and Photoinduced Electron Transfer in Fluorescein‐Dispersing Sol–Gel Titania Films

Fluorescein‐dispersing titania gel films were prepared by the acid‐catalyzed sol–gel reaction using a titanium alkoxide solution containing fluorescein. The molecular forms of fluorescein in the films, depending on its acid–base equilibria, and the complex formation and photoinduced electron transfer process between the dye and titania surface were investigated by fluorescence and photoelectric measurements. The titanium species were coordinated to the carboxylate and phenolate‐like groups of the fluorescein species. The quantum efficiencies of the fluorescence quenching and photoelectric conversion were higher upon excitation of the dianion species interacting with the titania, i.e. the dye–titania complex. This result indicated that the dianion form was the most favorable for formation of the dye–titania complex exhibiting the highest electron transfer efficiency. Using nitric acid as the catalyst, the titania surface bonded to the fluorescein instead of the adsorbed nitrate ion during the steam treatment. The dye–titania complex formation played an important role in the electron injection from the dye to the titania conduction band.     

The effect of oxygen on the three‐component radical photoinitiator system: Methylene blue,N‐methyldiethanolamine,and diphenyliodonium chloride

In this article, we extend our mechanistic study of the three‐component radical photoinitiator system, consisting of methylene blue (MB), N‐methyldiethanolamine, and diphenyliodonium chloride, by investigating the influence of oxygen on the rate of the consumption of MB dye. The mechanism involves electron transfer/proton transfer from the amine to the dye as the primary photochemical reaction. Oxygen quenches the triplet state of the dye, leading to retardation of the reaction. We used time‐resolved steady‐state fluorescence monitoring to observe the MB concentration in situ in both a constant oxygen environment and a sealed reactor as the dye is consumed via photoreaction. In the sealed reactor, we observed a retardation period (attributed to the presence of oxygen) followed by rapid exponential decay of the MB fluorescence after the oxygen was depleted. On the basis of the impact of the amine and iodonium concentrations on the fluorescence intensity and the duration of the retardation period, our proposed mechanism includes an oxygen‐scavenging pathway, in which the tertiary amine radicals formed in the primary photochemical process consume the oxygen via a cyclic reaction mechanism. The iodonium salt is an electron acceptor, acting to reoxidize the neutral dye radical back to its original state and allowing it to reenter the primary photochemical process. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3336–3346, 2000     

Nucleophilic substitution reaction of benzyl benzenesulfonates with anilines in MeOH-MeCN mixtures-I : Effects of substituent and solvent on the transition-state structure

Kinetic studies on nucleophilic substitution reaction of benzyl tosylates with anilines are reported. The reaction was found to proceed via a dissociative S_N2 mechanism with less than 50 % bond formation and extensive bond breaking at the transition state. It was found that positive charge development at the benzylic carbon is substantial and para-substituent effect on the substrate is predominantly of resonance type. Bond formation is shown to be favored by a better nucleophile, by an electron withdrawing group on the substrate and by the more polar(higher MeCN content) solvent. The substrate, nucleophile and solvent were found to follow the RSP.     

Stereospecific [2 + 2]-cross-photocycloaddition in a supramolecular donor–acceptor complex

A bis(propylammonium) derivative of (E)-4-(4-mercaptostyryl)pyridine, which was synthesized for the first time, forms a highly stable bimolecular complex with a bis(18-crown-6 ether) derivative of (E)-stilbene in solution owing to ditopic coordination via hydrogen bonds. The complex formation results in much faster deactivation of the excited states of both compounds, which is explained by photoinduced electron transfer from the stilbene derivative to the styrylpyridinium dye. Despite this, the complexed olefins undergo [2?+?2]-cross-photocycloaddition upon selective excitation of the dye to afford solely the syn-cycloadduct. The retro-photocycloaddition occurs readily upon UV irradiation of the cycloadduct and leads to the initial bimolecular complex.     

Pyridiniumchlorochromate oxidations: Studies on the kinetics and substituent effect in the oxidation of aniline in nonaqueous systems

Pyridiniumchlorochromate (PCC) oxidizes aniline and substituted anilines except nitro anilines smoothly in chlorobenzene–nitrobenzene mixtures in the presence of dichloroacetic acid. The reaction has unit dependence on each of the aniline, PCC, and dichloroacetic acid concentrations. Electron-releasing substituents accelerate the reaction, whereas electronwithdrawing groups retard the reaction, and the rate data obey Hammett's relationship. The reaction constant ρ is -3.75. Azobenzene and p-benzoquinone have been obtained as products. The observed experimental data have been rationalized in terms of the formation of an intermediate complex involving PCC–amine undergoing a rapid decomposition to products.     

Electron transfer reaction of 4,4′-bipyridine with triethylamine in acetonitrile: effect of water addition on the reaction dynamics

The photo-induced electron-transfer reaction of 4,4-bipyridine (BPY) with triethylamine (TEA) in acetonitrile is studied by laser flash photolysis. The reaction mechanism and kinetics are found very sensitive to the presence of a small amount of water. At low water concentrations (i.e. <0.003 M), an extremely fast-rising metastable product is detected for the first time. A triplet charge transfer complex (³ECT) is found to be the primary intermediate preceding the electron transfer process. Up to about 0.1 M, water facilitates the electron transfer rate, whereas higher water concentrations retard the rate of electron transfer. The Stern-Volmer plot of the triplet decay rate versus the TEA concentration is consistent with the presence of ³ECT in equilibrium with the free excited triplet state of BPY.     

Iron(III)–salen–H<Subscript>2</Subscript>O<Subscript>2</Subscript> as a peroxidase model: electron transfer reactions with anilines

Abstract  

Iron(III)–salen complexes catalyze the H₂O₂ oxidation of various ring-substituted anilines in MeCN have been studied, and [O=Fe^IV(salen)]^+· is proposed as the active species. Study of the kinetics of the reaction by spectrophotometry shows the emergence of a new peak at 445 nm in the spectrum which corresponds to azobenzene. Further oxidation of azobenzene by H₂O₂ leads to the formation of azoxybenzene. ESI–MS studies also support the formation of these products. The rate constants for the oxidation of meta- and para-substituted anilines were determined from the rate of decay of oxidant as well as the rate of formation of azobenzene, and the reaction follows Michaelis–Menten kinetics. The rate data show a linear relationship with the Hammett σ constants and yield a ρ value of −1.1 to −2.4 for substituent variation in the anilines. A reaction mechanism involving electron transfer from aniline to [O=Fe(salen)]^+· is proposed. The presence of axial ligands modulates the activity of the complex.     

Factors affecting the TMPTA radical polymerization photoinitiated by phenyltrialkylborates paired with tri-cationic hemicyanine dye. Kinetic studies

This article introduces tri-cationic hemicyanine dye employed as a visible-light photoinitiator of acrylic monomers polymerization. This dye, in combination with borate anions, was found to be a very effective photoinitiating system. The kinetics of trimethylolpropane triacrylate polymerization was studied by a microcalorimetric method. The photoredox pair concentration, the co-initiator structure as well as the light intensity strongly affected the progress of the polymerization, leading, for example, to an increase in the polymerization rate and quantum yield of the process. The efficiency of these photoinitiators was discussed on the basis of the free energy change for electron transfer from a borate anion to an excited hemicyanine dye cation. The ?G _el values were estimated for photoredox pairs containing a series of phenyltrialkylborate anions and one styrylpyridinium dye cation. The relationship between the rate of polymerization and the free energy of activation for electron transfer reaction gives the dependence predicted by the classical theory of electron transfer. The photoinitiating abilities of the selected novel photoredox pairs (BPB61, BPB7, BPB8, and BPB9) are comparable with the photoinitiating efficiency of commercially available photoinitiators.     

Photoreduction of Ru(bpy)(3)2+ by amines in aqueous solution. Kinetics characterization of a long-lived nonemitting excited state

The photochemistry of Ru(bpy)(3)+2 in the presence of amines was investigated in water by laser flash photolysis. N,N'-Dimethylaniline and p-phenylenediamine quench the luminescent metal to ligand charge transfer (MLCT) excited state of the complex by an electron transfer reaction that produces the semireduced form Ru(bpy)3+ in relatively high yields. On the other hand, triethylamine (TEA) and aniline do not quench the MLCT. Nevertheless, when laser flash irradiation at 532 nm is carried out in the presence of these amines, the formation of Ru(bpy)3+ is clearly detected by its transient absorption at 510 nm. These results are interpreted by an electron transfer reaction with the participation of a nonemitting excited state of the complex, formed independently of the MLCT from the Franck-Condon or the relaxed singlet excited state. The rate constants for the quenching of this state by TEA and aniline and the quantum yields for Ru(bpy)(3)+ were determined. The new state is formed in a very fast process and has a lifetime of ca 4 micros in water.     

A continuous two-phase reaction system coupled on-line with capillary chromatography for the determination of polar solutes in water

Gas chromatographic procedures are described for the determination of carboxylic acids and chlorinated anilines in water samples. Propionic acid and 2,6-difluorobenzoic acid in aqueous solution have been simultaneously alkylated and extracted by means of a continuous two-phase reaction system, and then quantitated by on-line coupled capillary gas chromatography; tetrahexyl-ammonium hydrogen sulfate was used as phase transfer catalyst and pentafluorobenzyl bromide as reagent. A factorial design approach was used to optimize on-line derivatization of aqueous propionic acid with regard to pH and concentration of phase transfer catalyst. Alkylation and extraction, under optimized conditions, followed by quantitation of the pentafluorobenzyl ester by flame ionization detection furnished a linear calibration for concentrations between 0.1 and 10 μg/ml. The relative standard deviation was 9–15 %. The continuous two-phase reaction system was also used to determine (chlorinated) anilines present in water at concentrations of 0.1–1 μg/ml; pentafluorobenzoyl chloride was used as reagent and analysis was performed by capillary gas chromatography with flame ionization or electron capture detection. The on-line acylation of p-chloroaniline was optimized with regard to pH, reagent concentration, and reaction time. The on-line reaction system worked satisfactorily for both applications, although excess reagent caused some problems with the chromatography.     

Primary photoprocesses of 3,3’-diethylthiacarbocyanine in the presence of cucurbit[7]uril: Laser flash photolysis and quantum chemical calculations

The effect of cucurbit[7]uril on the phototransformation of 3,3′-diethylthiacarbocyanine iodide dye in aqueous solution has been investigated by nanosecond laser flash photolysis. The presence of cucurbit[7]uril results in the formation of its complex with the dye molecule producing a dimer. The dimer formation is evident from the ground and triplet-triplet absorption spectra. The dimers in the triplet state are capable of electron transfer. The structure of the complexes is suggested on the basis of quantum chemical calculations.     

THE ROLE OF GROUND STATE COMPLEXATION IN THE ELECTRON TRANSFER QUENCHING OF METHYLENE BLUE FLUORESCENCE BY PURINE NUCLEOTIDES

The effect of three purine nucleotides on the fluorescence of methylene blue in aqueous buffer has been investigated. Guanosine-5'-monophosphate (GMP) and xanthosine-5'-monophosphate cause fluorescence quenching while adenosine-5'-monophosphate causes a red shift in the fluorescence maximum. All three nucleotides form ground state complexes with the nucleotides as indicated by absorption spectroscopy. The fluorescence changes at nucleotide concentrations less than 30 mM are best described by a static mechanism involving the formation of non-fluorescent binary and ternary complexes in competition with dimerization of the dye. Quenching of the fluorescence decay (tau = 368 ps) at high GMP concentrations (10-100 mM) occurs at the rate of diffusion. The mechanism of fluorescence quenching may involve electron transfer within the singlet excited dye-nucleotide complex although published values of the oxidation potentials of various purine derivatives would suggest that all three nucleotides should cause quenching. Evidence for electron transfer was obtained from flash photolysis experiments in which 100 mM GMP was found to cause the appearance of a long lived transient species absorbing in the region expected for semimethylene blue.     

Analysis of association constant for ground‐state dye‐electron acceptor complex of photoinitiator systems and the association constant effect on the kinetics of visible‐light‐induced polymerizations

We investigated the formation of ground‐state donor/acceptor complexes between xanthene dyes [rose bengal (RB) and fluorescein (FL)] and a diphenyliodonium (DPI) salt, which is dissolved in 2‐hydroxyethyl methacrylate (HEMA) monomer. To characterize the association constant of the complex, we have suggested a new analysis model based upon the Benesi–Hildebrand model. Because the assumption of the original Benesi–Hildebrand model is that the absorption bands are only due to the presence of the complex and that the absorption by the free component is negligible, the model cannot be applied to our systems, which is a dye‐based initiator system. For each dye, the molar absorptivity of the ground‐state complex was evaluated as a function of wavelength, and this analysis confirmed the validity of the modified Benesi–Hildebrand model. In addition, we observed that the RB/DPI photoinitiator system failed to produce a perceptible polymerization rate but the FL/DPI photoinitiator system provided high rates of polymerization. On the basis of the association constant for these complexes, we concluded that the observed kinetic differences arise from the different association constant values of the ground‐state dye‐acceptor complex, resulting in back‐electron transfer reaction. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1429–1439, 2009     

Superquenching of SYBRGreen dye fluorescence in complex with DNA by gold nanoparticles

The influence of gold nanoparticles (diameter of about 2.5 nm) on the complex between the SYBRGreen dye and double-stranded DNA in solutions has been investigated by fluorescence spectroscopy. Strong quenching of dye fluorescence by nanosized gold particles (??superquenching??), characterized by a high Stern-Volmer constant of K _SV ?? 3.3 × 10⁷ L/mol, has been found. The superquenching effect in the test system is explained in terms of contribution of several processes: electron transfer, formation of aggregates of gold nanoparticles involving dye dications, and enhancement of intersystem crossing by a heavy atom (gold atoms of nanoparticles).     

On the regioselectivity of the nucleophilic aromatic photosubstitution. The photoreaction of 4-nitroveratrole with n-hexylamine.

4-Nitroveratrole is photosubstituted with n-hexylamine giving rise to two isomeric anilines, N-hexyl-2-methoxy-5-nitroaniline and N-hexyl-2-methoxy-4-nitroaniline. Mechanistic evidence indicates that the first is produced in an SN₂Ar reaction through singlet and triplet excited states, whereas the second arises from a radical ion pair via electron transfer from the amine to a triplet excited state.     

Vibrational spectroscopic study of the electron transfer for the Cl- + CH3Cl S(N)2 reaction in the gas phase

The vibrational spectra study of electron transfer of Cl- + CH3Cl --> ClCH3 + Cl- reaction has been examined by density function theory (DFT) calculations at 6-311 + +G** level in this paper. This reaction includes old bond rupture, new bond formation and electronic transfer in the intermolecular. The vibrational frequencies and vibrational modes of reactant, precursor complex, transition state, successor complex and product are analyzed. The relationship and the change among them can confirm the rupture of bond, the formation of bond and the process of electron transfer.     

Excited-state intramolecular charge transfer in 9-aminoacridine derivative

A new fluorochromic dye was obtained from the reaction of 9-aminoacridine with ethyl-2-cyano-3-ethoxyacrylate. It displays complex fluorescence that is ascribed to normal emission from the acridine chromophore in addition to excited-state intramolecular charge transfer (ESICT) formed upon light excitation. The analysis of the fluorescence decays in different solvents reveals two short-lived components in the range of 80-450 ps and 0.7-3.2 ns, ascribed to the formation and decay of the intramolecular charge transfer (ICT) state, in addition to a third component of about 9.0 ns, which is related to the normal emission from the acridine singlet excited state, probably in an enol-imine tautomeric form. The ICT emission is readily quenched by water addition to polar solvents, and this effect is ascribed to changes in the keto-amine/enol-imine equilibrium of this fluorochromic dye.     

Acceleration of solid state Diels-Alder reactions by incorporating the reactants into crystalline charge transfer complexes

Diels-Alder reactions in a solid state between anthracene (AN) derivatives and p-benzoquinone (BQ) under mechanical stressing are accelerated by adding a catalytic amount of 2-naphthol (NP) or (rac)-1,1′-bis-2-naphthol (BN). Their catalytic effects are based on the formation of the charge transfer complex with strong hydrogen bonds. BN is capable of incorporating BQ together with its reaction partner, AN derivatives, simultaneously. The resulted molecular complex with BN provides crystallographically ordered homogenic reaction fields, resulting in the higher rates of the present solid state Diels-Alder reaction.     

Ruthenium‐catalyzed formation of quinolines via reductive cyclization of nitroarenes with tris(3‐hydroxypropyl)amine in an aqueous medium

Nitroarenes react with tris(3‐hydroxypropyl)amine in an aqueous medium (dioxane/H₂O) at 180° in the presence of a catalytic amount of a ruthenium catalyst and tin(II) chloride along with isopropanol as hydrogen donor to afford the corresponding quinolines in good yields. The presence of tin(II) chloride is essential for the formation of quinolines. A reaction pathway involving initial reduction of nitroarenes to anilines, propanol group transfer from tris(3‐hydroxypropyl)amine to anilines to form 3‐anilino‐1‐propanols, N‐alkylation of anilines by 3‐anilino‐1‐propanol to form 1,3‐dianilinopropane and intramolecular heteroannulation of 1,3‐dianilinopropane is proposed for this catalytic process.