D. Reyman

Some β-carboline derivatives as fluorescence standards

Abstract We present in this work a study on the standard properties of five β-carboline derivatives: nor-harmane or β-carboline, harmane, harmine, 2-methyl harmine and harmaline in H2SO4 aqueous solutions. The cationic species absorb and emit in these media, therefore, we have measured the fluorescence quantum yield and lifetime values of cationic species. Fluorescence spectra of these five cationic species cover a region between 400–560 nm. These five spectra show a strong overlapping.

ESR-spin trapping study on the sonochemistry of liquids in the presence of oxygen. Evidence for the superoxide radical anion formation

The sonolysis of water and some organic liquids such as ethylene glycol, methanol and chloroform in the presence of oxygen, at 20 and 475 kHz ultrasound frequencies has been investigated by the ESR-spin trapping technique. 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO), 3,3,5,5-tetramethylpyrroline-N-oxide (TMPO) and N-tert-butyl-alpha-phenyl nitrone (PBN) were able to trap superoxide radical anion, generated as the result of the sonication of the organic media. The addition of superoxide dismutase (SOD) resulted in a dramatic decrease of the ESR signal intensity of the superoxide radical adduct. In addition, the thermolysis of the liquids under ultrasound was shown by ESR detection of the spin adducts of the radicals formed by homolytic fragmentation. Occasionally, the nature of the detected spin adduct was dependent on the sonication time or on the frequency of the ultrasonic radiation. Experiments carried out in the presence of 2-methyl-2-nitrosopropane (MNP) resulted in the detection of radicals originating from thermal decomposition of the spin trap, showing its lability under ultrasonic radiation.

Solvent effects on the photophysical properties of N-substituted 1,8-naphthalimide derivatives

Abstract The photophysical properties of two N -substituted 1,8-naphthalimide derivatives were studied in various solvents (dichloromethane, benzene, dimethyl sulphoxide and water). The variations in the fluorescence spectra, absorption spectra, quantum yield and fluorescence lifetime were analysed. The results obtained are slightly different from the conclusions of previous investigations.

Photophysical properties of methyl β-carboline-3-carboxylate mediated by hydrogen-bonded complexes—a comparative study in different solvents

The hydrogen bonding interactions of methyl beta-carboline-3-carboxylate (BCCM) in both ground and first singlet excited electronic states have been studied in solvents with different properties in the presence of acetic acid, a hydrogen-bonding donor/acceptor. The methyl ester substituent reduces the pyridinic nitrogen basicity of this beta-carboline derivative. This fact has let us study the hydrogen bonding interactions in a higher range of acetic acid concentrations than for other beta-carboline derivatives previously studied. Steady and non-steady photophysical studies have been carried out in two non-polar solvents, benzene and p-dioxane; and in two polar solvents, acetonitrile and dichloromethane. Six different fluorescence emissions have been isolated corresponding to the uncomplexed BCCM, the protonated species and four different complexes between BCCM and acetic acid whose structures we have tried to elucidate.

Quantum yield and fluorescence lifetime measurements of neutral and cationic species for six β-carboline derivatives

Abstract A study of some emission parameters (quantum yield and fluorescence lifetime) for six alkaloid derivatives of β-carboline is presented. Depending on pH value, various species can exist in solution. In this work we have studied only two of them: cationic and neutral species (see fig. 1). The variation of quantum yield and fluorescence lifetime has been analysed for different substituted β-carboline rings and for some derivatives which do not present the aromatic character on the pyrimidinic nucleus.

Hydrogen-bonding interactions of norharmane in mixtures of acetic acid with benzene, p-dioxane and acetonitrile

Spectroscopic studies on the hydrogen-bonding interactions of norharmane with acetic acid (AcH), a proton acceptor/donor molecule, have been carried out in two non-polar solvents (benzene and p-dioxane) and in a polar solvent (acetonitrile). The steady and non-steady photophysical results show three different kinds of norharmane/AcH hydrogen-bonded complexes in the first excited state depending on the norharmane nitrogen implied in the complex formation: 1 ∶ 1 pyridinic nitrogen complex, 1 ∶ 1 pyrrolic nitrogen complex and 1 ∶ 2 pyridinic–pyrrolic nitrogen complex. In the three solvents and from the dependence of the reciprocal of norharmane lifetime on the acetic acid concentration, the quenching deactivation constants have been obtained. These constants are much higher than the diffusion constant indicating that the interaction between norharmane and AcH exists in the ground state. The 1 ∶ 1 stoichiometry of norharmane–AcH complexes in the ground state has been confirmed by the absorbance change at 354 nm. Benzene is the solvent in which complex formation is more favoured. Single, double and tri-exponential fluorescence decays were recorded, depending on the AcH concentration and the emission wavelength. Decay times of the neutral form and the 1 ∶ 1 complexes are quite close, ca. about 1 ns. The cation form has a lifetime of about 17 ns and the complex with two acetic molecules has a decay time of about 3 ns. With the formation of these complexes, it is possible to explain the fact that the emission around 500 nm disappears for high acetic acid concentrations.

Double exponential fluorescence decay in the protonation equilibrium of 4-methoxy-N-[2-(1-pyrrolidin)ethyl]-1,8-naphthalimide]

Abstract Fluorescence decay of 4-methoxy-N-[2-(1-pyrrolidin)ethyl]-1,8-naphthalimide] in water solution is analyzed. At pH values around 10, a double exponential decay function is observed. When pH > 10 or pH

The Impact of Dihydrogen Phosphate Anions on the Excited-State Proton Transfer of Harmane. Effect of β-Cyclodextrin on These Photoreactions

Photoinduced proton transfer reactions of harmane (1-methyl-9H-pyrido[3,4-b]indole) (HAR) in the presence of a proton donor/acceptor such as dihydrogen phosphate anions in aqueous solution have been studied by stationary and time-resolved fluorescence spectroscopy. The presence of high amounts of dihydrogen phosphate ions modifies the acid/base properties of this alkaloid. Thus, by keeping the pH constant at pH 8.8 and by increasing the amount of NaH(2)PO(4) in the solution, it is possible to reproduce the same spectral profiles as those obtained in high alkaline solutions (pH >12) in the absence of NaH(2)PO(4). Under these conditions, a new fluorescence profile appears at around 520 nm. This result could be related to the results of a recent investigation which suggests that a high intake of phosphates may promote skin tumorigenesis. The presence of β-cyclodextrin (β-CD) avoids the proton transfer reactions in this alkaloid by means the formation of an inclusion complex between β-CD and HAR. The formation of this complex originates a remarkable enhancement of the emission intensity from the neutral form in contrast to the cationic and zwitterionic forms. A new lifetime was obtained at 360 nm (2.5 ns), which was associated with the emission of this inclusion complex. At this wavelength, the fluorescence intensity decay of HAR can be described by a linear combination of two exponentials. From the ratio between the pre-exponential factors, we have obtained a value of K = 501 M for the equilibrium of formation of this complex.

Steady-state and time-resolved study of the proton-transfer fluorescence of harmine and 2-methyl-harmine in organic solvents

Excited-state reactions involved in the fluorescence of 2-methyl-harmine have been investigated in different binary mixtures of acetic acid and dichloromethane. The fluorescence decay of these compounds are performed as a function of acetic acid concentration and emission and excitation wavelengths. A two-stage model of reaction is proposed in order to explain a triple-exponential decay for the cation fluorescence. It is suggested that polar solvents block the π indole ring inhibiting the zwitterion formation.

Photochemical and sonochemical reactions of norharmane

In this work, a study on the photochemical and sonochemical behaviour of norharmane in halomethane media (dichloromethane, chloroform, carbon tetrachloride and dibromomethane) is presented. When solutions of norharmane in these media are irradiated with UV light or sonicated with ultrasound at 20 or 50 kHz, the cationic species of norharmane appears in solution. In order to explain the formation of this species by a UV radiation effect, a exciplex (charge transfer complex) between norharmane and halomethane has been proposed. The formation rate constants of the exciplex were calculated using fluorescence quantum yields and lifetime values. The sonochemical formation of cationic species does not occur by formation of a charge transfer complex between norharmane and halomethane. In this paper, we report that an initially neutral medium can quickly develop acidity in a few minutes of exposure in a conventional bath to ultrasound.