Sonia G. Bertolotti

The Excited-State Interaction of Resazurin and Resorufin with Aminesin Aqueous Solutions. Photophysics and Photochemical Reaction¶

Abstract The photophysics and photochemical behavior of the phenoxazin-3-one dyes, resazurin and resorufin, have been studied in aqueous solutions. The irradiation of resazurin in the presence of amines leads to deoxygenation of the N-oxide group, giving resorufin. This photoreaction is highly dependent on the amine structure and is efficient only in the presence of tertiary aliphatic amines. The absorption and fluorescence properties of these dyes are dependent on pH. At pH above 7.5 both dyes are in their anionic form. For resorufin this form is highly fluorescent (ΦF = 0.75). At lower pH the fluorescence is strongly reduced. The N-oxide dye presents a very weak fluorescence quantum yield (0.11), which also is reduced at low pH. Flash photolysis experiments allowed characterization of the triplet state and the transients formed after irradiation of these dyes in the absence and presence of amines. The triplet quantum yields are 0.08 for resazurin and 0.04 for resorufin. The photodeoxygenation of N-oxide in the presence of amines occurs from the triplet state.

Electron transfer quenching of singlet and triplet excited states of flavins and lumichrome by aromatic and aliphatic electron donors

The quenching of excited singlet and triplet states of riboflavin, lumiflavin and lumichrome was investigated in methanol. The quenchers were aromatic electron donors and aliphatic amines. Bimolecular quenching rate constants were determined from static and dynamic fluorescence measurements. Triplet quenching was studied by laser flash photolysis. Transient absorption spectra showed the presence of semireduced flavins and lumichrome, and the radical cation of quenchers. The results confirm that the quenching rate constants for aliphatic donors are lower than those of aromatic donors of similar oxidation potential. Plots of the quenching rate constants vs. the free energy for the electron transfer reaction, ΔG°, were fitted by the Rehm–Weller model of electron transfer quenching. The aliphatic quenchers needed a higher intrinsic barrier for the fitting that was ascribed to the internal reorganisation of the amines. For the aromatic donors the singlet quenching rate constants reach the diffusional limit at highly negative ΔG°. However, for the triplet quenching the limiting value of the rate constants is lower than the plateau of the singlet quenching. This is explained in terms of the non-adiabaticity of the triplet quenching process.

FLUORESCENCE QUENCHING OF INDOLIC COMPOUNDS IN REVERSE MICELLES OF AOT

Abstract— The fluorescence properties of indole, 1,2‐dimethylindole and tryptophan have been determined in sodium bis(2‐ethylhexyl)sulfosuccinate inverse micelles in heptane. Fluorescence quenching studies were done using CCl4 and acrylamide as quenchers localized in the organic and micellar pseudophase respectively. The Stern‐Volmer plots show departure from linearity and are dependent on the emission wavelength. The results were interpreted using a three pseudophases model. 1,2‐Dimethylindole is predominantly solubilized in the n‐heptane; however a small interaction with the micelles is apparent at high detergent concentration. Indole is distributed between the organic and the interfacial microphase with a partition constant of 34 ± 3, independent of the water content. Tryptophan is entirely localized at the interface. However the quenching experiments show that tryptophan molecules are not uniformly distributed in the interfacial pseudophase.

THE INTERACTION OF THE EXCITED STATES OF SAFRANINE T WITH ALIPHATIC AMINES IN ORGANIC SOLVENTS

Abstract—

The excited states quenching of safranine T by p-benzoquinones in polar solvents

Abstract Safranine is a well known photochemical oxidant. The photoreduction reaction of this dye by various types of electron donors has been studied quite often, and in many cases mechanism accounting for the observed results were proposed and explained in terms of electron transfer theories. Only a few studies seem to exist on reactions of this dye with electrons acceptor. In this work we present results of fluorescence quenching and laser flash photolysis which demonstrate that safranine T may function also as a photochemical reductant with p -benzoquinones as electron acceptors in methanol and acetonitrile. Singlet quenching rate constants are nearly diffusion controlled in both solvents. Triplet quenching rates constants were obtained from the decay of the triplet at 830 nm as a function of quinone concentration. They follow a Rehm and Weller type correlation for electron transfer in acetonitrile, while in methanol the rate constants for triplet quenching are substantially larger.

Photoreduction of resazurin in the presence of aliphatic amines

Abstract Photophysical and photochemical properties of the dye resazurin were measured. The quantum yields of fluorescence and triplet formation were determined to be 0.10±0.01 and 0.2, respectively. The lifetimes of the singlet and triplet states are 1.2 ns and 30 μs. In the presence of aliphatic amines, resazurin is readily photoreduced to resorufin. This reaction proceeds through the triplet state and is inhibited by the presence of oxygen.

Excimer emission of pyrene derivatives induced by ionic detergents below the critical micelle concentration

Abstract The fluorescence of three ionic pyrene derivatives in the presence of CTAC and SDS was investigated at detergent concentrations below the critical micelle concentration (CMC). The characteristic excimer emission was observed for surfactant concentrations more than one order of magnitude lower than the CMC. Lifetime measurements show that excimer formation must be via a static mechanism. This is explained by the formation of mixed premicellar aggregates between the surfactants and the probes. The quenching of the excimer emission by ionic (TI + , S 2 O 2- 3 ) and neutral (N,N-dimethylaniline, fumaronitrile) species was studied. The quenching is only effective when the ions are of opposite charge to that of the detergent. The quenching by neutral species can be explained by a static mechanism which involves the association of the quencher with the aggregates.

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 interfaces 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.

Hydrogen bonding and charge transfer interactions in exciplexes formed by excited indole and monosubstituted benzenes in cyclohexane

Abstract The interaction between the excited singlet state of indole derivatives with simple monosubstituted benzenes has been investigated in cyclohexane by steady state and time resolved fluorescence spectroscopy. The fluorescence quenching efficiency of indole increases in the order benzene, toluene, chlorobenzene and benzonitrile. For indole derivatives substituted on the N atom of the heterocyclic ring the quenching efficiency is much lower. At high concentrations of the benzenes derivatives the emission spectrum is broader and red shifted and the decay of the emission becomes biexponential. The results are analysed in terms of an exciplex forming mechanism. The binding forces in the exciplex are predominantly of hydrogen bonding for the case of benzene and toluene, while a charge transfer interaction is more likely in the case of chlorobenzene and benzonitrile.

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.