Subit K. Saha

Fluorescent probe studies of micropolarity, premicellar and micellar aggregation of non-ionic Brij surfactants.

The dipolar nature of trans-2-[4-(dimethylamino)styryl]benzothiazole (DMASBT) in its twisted intramolecular charge transfer (TICT) excited state makes it useful as a surface probe for phenomena such as premicellar and micellar aggregation of non-ionic Brij surfactants. The process of micellization of Brij 35, Brij 58, Brij 78, and Brij 98 through the formation of smaller premicellar aggregates results in a progressive change in the nature of the DMASBT molecule and its location in the aggregates, reflecting the changes in its photophysical properties, which have been studied using steady-state fluorescence, fluorescence anisotropy, and time-correlated single-photon counting measurements. The microenvironment polarity around the DMASBT probe in the micellar phase is greater than that in corresponding premicellar phases. The orders of premicellar as well as micellar concentrations are Brij 35 > Brij 58 > Brij 98 > Brij 78. The lower fluorescence anisotropies observed in the case of Brij 78 aggregates compared to those in other Brijs studied could be due to the accessibility of a nonrigid environment as a result of a folded conformation of a part of the nonpolar long chain of surfactant molecules near the core of aggregates. Three different locations of DMASBT were noted for Brijs 35, Brij 78, and Brij 98, whereas for Brij 58 only two locations are observed. The micropolarity of the environment around DMASBT in aggregation states has been determined.

Solvatochromic effects in the absorption and fluorescence spectra of indazole and its amino derivatives

Abstract Absorption and fluorescence spectra of Indazole (In) in five solvents and those of 5-amino, 6-amino, and 7-aminoindazoles (5-AI 6-AI, 7-Al) in thirteen solvents have been studied. Using the multiparametric approach of Taft et al., spectral characteristics of these molecules have been analysed on the basis of electrostatic effects, hydrogen bond donor ability and hydrogen bond accepting ability of the solvents. In the excited singlet state, all three effects are nearly equally prominent, whereas in the ground state 5-AI acts as a better hydrogen bond acceptor than the other amines. Stokes shifts and the difference between the radiative decay constants determined experimentally and using Strickler and Bergs equation indicate different geometries of the amines in the S 0 and S 1 states. A decrease in the non-radiative decay constants of 5-AI and 6-AI with an increase in the polarity of the solvents predict better planar geometry in the S 1 state in comparison to that in the S 0 state. Semi-empirical quantum mechanical calculations have been used to find the nature of transitions, total atomic charges at the basic centres and dipole moments of all the aromatic amines in the S 0 and S 1 states. Results so obtained are compared and discussed.

Fluorescence quenching of aromatic amines by chloromethanes

The fluorescence quenching of indazole, 5-amino and 6-aminoindazoles and 2-aminofluorene by dichloro-(CH2Cl2), trichloro-(CHCl3) and tetrachloromethane (CC14) have been studied in cyclohexane, acetonitrile and methanol as solvents. The quenching rate constants for this process have also been obtained by measuring the lifetimes of the fluorophores as a function of quencher concentration. For CCl4, the quenching rate constants depend upon the viscosity, whereas in case of CH2Cl2 and CHCl3, quenching rate constants are neither the function of viscosity nor solvent polarity. Linear plot is observed between log kq and free-energy change in the quenching mechanism for kq ⩽ 1010 M−1 s−1. This suggests the formation of complex involving a charge transfer interaction between the electronically excited fluorophores and the ground-state chloromethanes.

Fragmentation of molecule-induced γ-cyclodextrin nanotubular suprastructures due to drug dosage

The guest-host concentration has already been proved to be a very important factor in the drug delivery process. In the present work we demonstrate the formation of compound induced gamma-cyclodextrin nanotubular suprastructure. The nanotubes formed are found to be highly sensitive to the concentration of the guest molecule. The increasing concentration of the compound in solution initiates a competition toward their existence inside the core of the nanotubes affecting the extent of nanotubular cluster formation. The hydrogen bonding responsible for the building of the cyclodextrin nanotubes is found to be partially disrupted because of this increasing competition. The continuous replacement of the guest molecules inside the nanochannels is supposed to be responsible for the instability in some of the hydrogen bondings that develop during the primary and the secondary interactions between the formed nanotubes resulting into fragmentation of the suprastructures. The steady state and time-resolved fluorescence experiments coupled with fluorescence anisotropy and atomic force microscopy illustrate the guest concentration dependence of the formation of the gamma-cyclodextrin nanotubes.

Defining the different phases of premicellar aggregation using the photophysical changes of a surface-probing compound.

Photophysical changes of a cylindrical compound undergoing twisted intramolecular charge transfer may be used as a surface probe to study the different phases of premicellar aggregate formation. The probe molecule, trans-2-[4-(dimethylamino)styryl] benzothiazole (DMASBT), attaches itself to the premicellar and the micellar aggregates of cationic, anionic, and neutral surfactants in different orientations because of its dipolar nature in the excited state. The micelle formation is preceded by a few typical rearrangements of the surfactant molecules. These events need proper inspection that can only be done by compounds that sense environmental changes by residing in the vicinity of the surface of those aggregates. Steady-state and time-resolved fluorescence spectroscopy coupled with steady-state fluorescence anisotropy measurements serve as a very useful tool to monitor premicellar aggregate formation. The dipolar interaction of DMASBT with the surface of the aggregate and its extraordinary capability to sense the polarity of the environment make it a very efficient molecule to use for the purpose.

Study on metal nanoparticles synthesis and orientation of gemini surfactant molecules used as stabilizer

In the present study, we report the synthesis of gold (Au), silver (Ag), and gold-silver alloy (Au-Ag) nanoparticles (NPs) by seed-mediated method using gemini surfactant, containing diethyl ether spacer group as a stabilizer. As-synthesized NPs are found very much stable and have been characterized using UV-vis spectroscopy, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and zeta potential techniques. The orientation of gemini surfactant molecules surrounding the metal NPs has been investigated exploiting twisted intramolecular charge transfer (TICT) fluorescence properties of a probe 4-(N,N-dimethylamino) cinnamaldehyde (DMACA). The quenching efficiencies of different NPs have been performed in the fluorescence of DMACA and are found to be different. This effect can be related to the location of DMACA as well as the electro-negativity of the metals as the extent of orientation of the surfactant molecules around NPs controls the location of DMACA in a bilayer. To support the location of DMACA, fluorescence quenching studies with cetylpyridinium chloride (CPC) as an external quencher have also been carried out.

Photoluminescence of 2-aminofluorene: a relook

Abstract The absorption and fluorescence spectra of 2-aminofluorene (2-AF) have been studied as a function of solvent polarity and acid concentration. Using the multiparametric approach of Taft et al., it is clear that 2-AF is a better proton acceptor in the S0 state and a proton donor in the S1 state. Excitation of 2-AF to three electronically excited states has shown that fluorescence is always observed from the lowest excited singlet state, but fluorescence quantum yield increases with the increase of λexc. The decrease in fluorescence quantum yield with increase in solvent polarity and hydrogen bonding is due to solvent-induced quenching. A correspondence is observed between the decrease and increase in fluorescence intensities of the neutral and monocation, respectively, in the pH range from 6 to 3. Proton-induced fluorescence quenching of neutral 2-AF is noticed in the pH range 3 to 1. pKa and p K ∗ a values were determined for different prototropic equilibria and are discussed.

Fluorescence quenching of 2-aminofluorene by cetylpyridinium chloride, iodide ion and acrylamide in non-ionic micelles: Tweens

Abstract The fluorescence quenching of 2-aminofluorene by cetylpyridinium chloride (CPy), iodide ion (I − ) and acrylamide (AC) has been studied in different concentrations of Tween-20, Tween-40, Tween-60 and Tween-80 surfactants. The results show that CPy and I − ion bind as well as get partitioned, whereas AC only gets partitioned. The high values of binding constants for CPy in comparison to I − ion are due to the long hydrocarbon chain length. The binding and partitioning coefficients of CPy increase, whereas those of I − ion decrease with the increase in the Tween number. The values of partitioning coefficient of AC also decrease with increase in the Tween number. On the other hand, quenching rate constants for each quencher in each Tween decrease in going from Tween-20 to Tween-80.

Prototropic equilibrium of 2-aminofluorene in non-ionic micelles: Tweens

Abstract Spectral characteristics of 2-aminofluorene (2-AF) have been studied as a function of the concentration of four Tweens at a given pH, and as a function of pH at a given concentration of the Tweens. The effective dielectric constant (e eff ) at a site of neutral 2-AF and the site where a prototropic reaction is occurring has been evaluated using the Stokes shifts of neutral 2-AF and the p K a values of the monocation–neutral equilibrium, respectively. These results, along with the single exponential decay of the fluorescence intensity of 2-AF, have clearly established that 2-AF is present at the interface of the micelles and water, and is bound to the oxygen atom of the polyoxy ethylene group through hydrogen bonding. The amino group of 2-AF is oriented towards the polar region and the fluorene ring towards the non-polar region of the micelles. The slight disagreement between the values of e eff determined by both the methods is due to a small percentage (5%) of the prototropic reaction occurring in the aqueous phase. In both cases, the solvent systems used have been obtained by employing different compositions of dioxane–water mixtures. The values of the binding constants, non-radiative decay constants and e eff determined by the above methods confirm that the hydrophobic character of the Tweens increases with increase of Tween number.

Solvation dynamics and rotational relaxation of coumarin 153 in mixed micelles of Triton X-100 and cationic gemini surfactants: effect of composition and spacer chain length of gemini surfactants

Solvation dynamics and rotational relaxation of coumarin 153 (C-153) in mixed micelles of non-ionic surfactant, Triton X-100 and a series of cationic gemini surfactants, 12-s-12, 2Br with varying polymethylene spacer chain length (s = 3, 6, 8, 12) at different bulk mole fractions of a surfactant were studied. Studies were carried out by means of UV-Vis absorption, steady-state fluorescence and fluorescence anisotropy, time-resolved fluorescence and fluorescence anisotropy, and dynamic light scattering measurements. While micropolarity of the environment around C-153 in mixed micelles increased, the microviscosity decreased with increasing amount of a gemini surfactant. This is because the thickness of the Stern layer of micelles increases as a result of greater extent of penetration of water molecules. Solvation dynamics and rotational relaxation of C-153 become faster with increasing mole fraction of a gemini surfactant in the mixed micelles. Increasing the thickness of the Stern layer leads to an increase in the number of water molecules hydrogen bonded among themselves, resulting in an increase in polarity and microfluidity of the environment. At a given bulk mole fraction of a surfactant, the microviscosity of micelles decreases with increasing the spacer chain length of the gemini surfactant resulting in an increase in the rate of the rotational relaxation process. However, at a given bulk mole fraction of a surfactant, solvation dynamics becomes slower with increasing spacer chain length from s = 3 to 8 because of the increasing degree of counter ion dissociation. The slow rotational relaxation process is mainly due to the lateral diffusion of C-153 along the surface of the micelles. Rotationalmotion of the micelle as a whole is much slower than the lateral diffusion of C-153.