Ranjit S. Sarpal

Solubilization of two cyano 3H-indole molecular probes in SDS and CTAB micelles: spectral and photophysical characterization

An absorption and fluorescence spectral study on the solubilization behaviour of 2-[(p-amino)phenyl]-3,3-dimethyl-5-cyano-3H-indole (1) and 2-[(p-dimethylamino)phenyl]-3,3-dimethyl-5-cyano-3H-indole (2) in sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) aqueous micelles has been carried out. Using spectral correlations with polarity parameters, bandwidths and photophysical parameters, the nature of the binding site of the molecules in micelles has been discussed. Both molecules occupy sites in SDS and CTAB that are less polar than water but accessible to it. The dimethylamino substituent helps in pushing the molecule toward the micellar core in both micelles. pH effects on the ring-nitrogen protonation (neutral–monocation equilibrium) and terminal nitrogen deprotonation (neutral–monoanion equilibrium) in SDS and CTAB, respectively, have also been discussed and found to be consistent with both the pseudophase ion-exchange (PIE) and electrostatic models.

Effects of solvent and acid concentration on the absorption and fluorescence spectra of α,α-diaminonaphthalenes

Absorption and fluorescence spectra of 1,4-diaminonaphthalene (1,4-DAN), 1,5-diaminonaphthalene (1,5-DAN) and 1,8-diaminonapthalene (1,8-DAN) have been recorded in 11 different solvents and at various acid concentrations (H0/pH/H–, –10 to 16). These studies have indicated that the two amino groups are coplanar to the napthalene ring in 1,5-DAN, are twisted with respect to each other but coplanar to the ring in 1,8-DAN and are non-planar with respect to the naphthalene ring in 1,4-DAN. Based on molar absorption coefficient data the long wavelength band is found to be a mixture of the 1Ag→1La and 1Ag→1Lb transitions in non-polar solvents, whereas in aqueous solvents it is due to the 1Ag→1Lb transition. The change in dipole moment on excitation is zero for 1,5-DAN and greatest for 1,4-DAN. Prototropic equilibrium is not established between mono-cation and neutral species in the S1 state. This is because the radiative decay rate is greater than the protonation/deprotonation rates. Proton-induced fluorescence quenching is not observed for neutral species, whereas it is observed for the mono-cations before these are protonated to form di-cations. k′q for 1,5-DAN and 1,8-DAN are 2.6 × 108 and 1.0 × 109 dm3 mol–1 s–1, respectively. Lifetime data have shown that the proton-induced fluorescence quenching is dynamic in nature. —NH2 and —NH+3 are stronger acids in the S1 state than in S0 state.

Prototropism in aminophenols and anisidines: A reinvestigation

Abstract The absorption and fluorescence spectra obtained in various solvents indicate that aminophenols and anisidines act as proton donors in ether and acetonitrile but as proton acceptors in methanol and water in the S 0 state and as proton donors to all the solvents in the S 1 state. Stretched sigmoid curves are observed for the equilibrium between the monocation and the neutral species for all the compounds, except p -aminophenol (pAMP) and p -anisidine, giving both ground state and excited state p K a values. For pAMP and p -anisidine, proton-induced fluorescence quenching of the neutral species is observed at moderate hydrogen ion concentrations. The values of the quenching constant are 3.3×10 9 dm 3 mol −1 s −1 and 2.0×10 9 dm 3 mol −1 s −1 for pAMP and p -anisidine respectively. The proton transfer reaction studied in methanol-water systems has indicated that, in low dielectric constant solvents, the excited state equilibrium is not established and thus the ground state p K a value is observed from fluorimetric titration curves.

Absorption and fluorescence spectral characteristics of aromatic amines in cetyltrimethylammonium bromide

The spectral characteristics of eight aromatic amines of different hydrophobicity have been studied in CTAB. A. red shift (8–15 nm) in the absorption spectra and a blue shift (ca. 10 nm) in the fluorescence spectra of these amines in CTAB relative to their aqueous solutions indicate that the polarity at the binding site of micelles corresponds to a relative permittivity of 24 and that water molecules are present in small amounts inside the micelles. Fluorescence lifetime data support this conclusion. Fluorescence quantum yields (of) of all the amines (except 2-aminonaphthalene) increase in CTAB, whereas in CTAC, of increases for all the amines relative to their aqueous solutions. The binding constant increases with increasing hydrophobicity of the amine. All the monoanions, with the exception of aniline and 9-aminophenanthrene are fluorescent, indicating that fluorescence quenching of these anions by water molecules in aqueous solution is faster than radiative decay. The pKa values for the deprotonation reactions are lower in 0.02 mol dm–3 CTAB compared to those in aqueous solutions. These results can be explained adequately by the pseudophase ion-exchange model.

Prototropic equilibria of some aromatic amines in miceliar solutions of sodium dodecylsulphate

Abstract The monocation—neutral equilibria of eight aromatic amines were studied in sodium dodecylsulphate in the S 0 and S 1 state. Apparent p K a values for these equilibria increase, reach a maximum value and then decrease with an increase in detergent concentration. Changes observed in the apparent p K a values can be explained by the pseudophase ion exchange model. Aromatic ammonium ions are stronger acids in the S 1 state, even in micelles, but not to the same extent as observed in aqueous media. Proton-induced fluorescence quenching of the neutral aromatic amines is observed in micellar solutions and fluorometric titrations cannot be used to determine the excited state p K a values.

The association parameters of bromide and iodide ions with cationic micelles using steady state fluorescence quenching measurements

Abstract The fluorescence quenching of 2-aminonaphthalene (2-AN) and 2-aminochrysene (2-AC) by Br− and I− ions was studied in different concentrations of cetyltrimethylammonium bromide (CTAB) and cetyltrimethylammonium chloride (CTAC) detergents. The results show that both inorganic ions are partitioned and bound to both micelles, with the exception of 2-AC where I− ions distribute between the aqueous and micellar phases. The partition coefficients and binding constants of I− ions in these micelles are much larger than those of Br− ions. The decrease in the quenching rate constant (kq) in the micelles is larger for 2-AC than for 2-AN, indicating that 2-AC is present in the micelles at a site at which the viscosity is high.

Effect of Cationic Micelles on the Deprotonation Equilibria of Benzimidazoles in the Ground and First Excited Singlet States

Spectral characteristics of six methyrand phenyl substituted benzimidazoles (BIs) have been studied in cetyltrimethyl ammonium bromide (CTAB) at different pH. Absorption and fluorescence band maxima of the BIs are hardly affected in the micelles, but the fluorescence quantum yield of each BI decreases with increase of [CTAB], and this decrease depends on the position of the substituent rather than on their number. On the other hand, the fluorescence quantum yields of monoanions increase by a factor of 10 to 15, and the band maxima are blue shifted in comparison to an aqueous medium. The apparent pKa values for the deprotonation reaction decrease in CTAB, and this effect of detergent concentration can be explained with the help of the pseudophase ion exchange model (PIE).