Nilmoni Sarkar

Solvation dynamics of Coumarin 153 in aqueous and non-aqueous reverse micelles

The solvation dynamics of water, methanol and acetonitrile in n-heptane/AOT/water, n-heptane/AOT/methanol and n-heptane/AOT/acetonitrile reverse micelles have been investigated by picosecond time-resolved emission spectroscopy using rigid and neutral probe, Coumarin 153 (C-153). We have observed a substantially slow dynamics (order of few nanosecond) in all the three reverse micelles compared to pure solvents. The w dependency of solvation time is observed in case of water and methanol reverse micelles. However, a very little w dependency is observed in case of acetonitrile reverse micelles. Moreover, fluorescence anisotropy measurements are used to characterize rotational motion of C-153 in all the three reverse micelles.

Excited-state intramolecular proton transfer and rotamerism of 2-(2'-hydroxyphenyl) benzimidazole

Abstract Excited-state intramolecular proton transfer (ESIPT) and rotamerism of 2-(2′-hydroxyphenyl benzimidazole (HPBI) have been studied using steady-state and time-resolved emission spectroscopy and by semi-empirical quantum-chemical methods. Two ground-state rotamers ( I and II ) with distinct excitation and emission spectra and lifetimes have been identified. Excitation of one of them ( I ) produces the normal emission while the other after excitation undergoes ultrafast ESIPT to form the tautomer ( III ) with Stokes-shifted emission. At 77 K the tautomer emission is markedly supressed as the rotamer, II , responsible for it, is less stable than I . CNDO/S-CI calculations were performed optimising the ground-state geometry by the AM1 method. These calculations give good estimates of the ground- and excited-state energies of the rotamers I and II and the tautomer, III .

Probing the Interaction of 1-Ethyl-3-methylimidazolium Ethyl Sulfate ([Emim][EtSO4]) with Alcohols and Water by Solvent and Rotational Relaxation

The effect of the addition of cosolvents in the room temperature ionic liquid (RTIL) 1-ethyl-3-methylimidazolium ethyl sulfate ([Emim][EtSO(4)]) was probed by the solvent and rotational relaxation studies of coumarin 153 in neat ionic liquid [Emim][EtSO(4)] and [Emim][EtSO(4)]-cosolvent mixtures by using steady-state and time-resolved fluorescence spectroscopy. With gradual addition of cosolvents in the RTIL, both the average solvation time and rotational relaxation times gradually decrease. Addition of cosolvents in the IL decreases the viscosity of the medium. We have optimized the geometry of [Emim][EtSO(4)] and [Emim][EtSO(4)]-cosolvent mixtures by using quantum chemical calculations using density functional theory methods, which show the formation of hydrogen bond between cosolvents with [Emim][EtSO(4)]. With addition of the same amount of alcohols in neat [Emim][EtSO(4)], the rotational relaxation time decreases more compared to the addition of the same amount of water.

Spontaneous Transition of Micelle–Vesicle–Micelle in a Mixture of Cationic Surfactant and Anionic Surfactant-like Ionic Liquid: A Pure Nonlipid Small Unilamellar Vesicular Template Used for Solvent and Rotational Relaxation Study

The micelle-vesicle-micelle transition in aqueous mixtures of the cationic surfactant cetyl trimethyl ammonium bromide (CTAB) and the anionic surfactant-like ionic liquid 1-butyl-3-methylimidazolium octyl sulfate, [C4mim][C8SO4] has been investigated by using dynamic light scattering (DLS), transmission electron microscopy (TEM), surface tension, conductivity, and fluorescence anisotropy at different volume fractions of surfactant. The surface tension value decreases sharply with increasing CTAB concentration up to ∼0.38 volume fraction and again increases up to ∼0.75 volume fraction of CTAB. Depending upon their relative amount, these surfactants either mixed together to form vesicles and/or micelles, or both of these structures were in equilibrium. Fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH), incorporated in this system at different composition of surfactant indicates the formation of micelle and vesicle structures. The apparent hydrodynamic diameter of these large multilamellar vesicles is about ∼200 nm-300 nm obtained by DLS measurement and finally confirmed by TEM micrographs. The large multilamellar vesicles are transformed into small unilamellar ones by sonication using a Lab-line instruments probe sonicator with a diameter of ∼90-125 nm. To investigate the heterogeneity, solvent, and rotational relaxation of coumarin-153 (C-153) have been investigated in these unilamellar vesicles by using picosecond time-resolved fluorescence spectroscopic technique. The solvation dynamics of C-153 in these vesicles is found to be biexponential with average time constant ∼580 ps. This indicates the slow relaxation of water molecules in the surfactant bilayer. In accordance with solvation dynamics, fluorescence anisotropy analysis of C-153 in unilamellar vesicles also indicates hindered rotation compared to bulk water.

Modulation of the Photophysical Properties of Curcumin in Nonionic Surfactant (Tween-20) Forming Micelles and Niosomes: A Comparative Study of Different Microenvironments

The modulation of the photophysical properties of curcumin inside two different types of microenvironments provided by nonionic surfactant forming micelles and vesicles (niosomes) has been investigated using steady state and time-resolved fluorescence spectroscopy. The formation of small unilamellar Tween-20/cholesterol niosomes with narrow size distribution has been successfully demonstrated by means of dynamic light scattering (DLS) and transmission electron microscopy (TEM) techniques. Our results indicate that niosomes are a better possible delivery system than the conventional surfactants forming normal micelles to suppress the level of degradation of curcumin. The enhanced fluorescence intensity along with the significant blue-shift in the emission maxima of curcumin upon encapsulation into the hydrophobic microenvironments of micelles and niosomes is a consequence of the reduced interaction of curcumin with the water molecules. We found that the more rigid and confined microenvironment of niosomes enhances the steady state fluorescence intensity along with the fluorescence lifetime of curcumin more than in micelles. The rigidity of the niosome membrane which arises basically due to the presence of cholesterol molecules increases the level of interaction between curcumin and the oxoethylene units of Tween-20 molecules. It is also possible for the hydroxyl groups of the cholesterol moieties to form intermolecular hydrogen bonds with curcumin to perturb nonradiative deactivation mechanism through excited state intramolecular hydrogen atom transfer (ESIHT).

Designing a New Strategy for the Formation of IL-in-Oil Microemulsions

Due to the increasing applicability of ionic liquids (ILs) as different components of microemulsions (as the polar liquid, the oil phase, and the surfactant), it would be advantageous to devise a strategy by which we can formulate a microemulsion of our own interest. In this paper, we have shown how we can replace water from water-in-oil microemulsions by ILs to produce IL-in-oil microemulsions. We have synthesized AOT-derived surface-active ionic liquids (SAILs) which can be used to produce a large number of IL-in-oil microemulsions. In particular, we have characterized the phase diagram of the [C(4)mim][BF(4)]/[C(4)mim][AOT]/benzene ternary system at 298 K. We have shown the formation of IL-in-oil microemulsions using the dynamic light scattering (DLS) technique and using methyl orange (MO), betaine 30, and coumarin-480 (C-480) as probe molecules.

Intramolecular charge transfer processes and solvation dynamics of coumarin 490 in reverse micelles

The relative retardation of intramolecular charge transfer (ICT) rate and solvation dynamics of coumarin 490 (C-490) in the water pool of the Aerosol OT (AOT)-heptane reverse micelle compared to normal water have been investigated using picosecond time resolved emission spectroscopy. We have observed a maximum 3.5 times change in the ICT rate of C-490 in the water pool of the reverse micelle compared to normal water. The retardation of ICT rate is much smaller compared to the several thousands fold decrease in the solvation dynamics in the water pool of the reverse micelle compared to pure water.

An Understanding of the Modulation of Photophysical Properties of Curcumin inside a Micelle Formed by an Ionic Liquid: A New Possibility of Tunable Drug Delivery System

The present study reveals the modulation of photophysical properties of curcumin, an important drug for numerous reasons, inside a micellar environment formed by a surfactant-like ionic liquid (IL-micelle) in aqueous solution. Higher stability of the drug inside IL-micelle in the absence and presence of a simple salt (sodium chloride) as well as considerably large partition coefficient (K(p) = 8.59 × 10(3)) to the micellar phase from water make this system a well behaved drug loading vehicle. Remarkable change in fluorescence intensity with a strong blue-shift implies the gradual perturbation of intramolecular hydrogen bond (H-bond) present within the keto-enol group of curcumin along with considerable formation of intermolecular H-bond between curcumin and the headgroup of surfactant-like IL. Very fast nonradiative decay channels in curcumin mainly caused by the excited state intramolecular proton transfer (ESIPT) are thus depleted remarkably in the presence of IL-micelle of reduced polarity and as a result of restricted rotational and vibrational degrees of freedom when bound to the micelle. Moreover, time-resolved results confirm that not only the keto-enol group of curcumin is playing here but also the phenolic hydroxyl groups are also responsible for such modulation in photophysical properties. From a thermodynamic point of view, our system shows good correlation with its stability parameters (higher binding constant with very less hydrolytic degradation rate ~1%) and higher negative value of binding enthalpy of interaction (-ΔH) than total free energy change (-ΔG) implies that the nature of binding interaction is enthalpy driven not entropy alone. Summarizing all the above observations, we have concluded that the modulation of the intramolecular proton transfer is due to the presence of both intermolecular proton transfer as well as strong hydrophobic interaction between curcumin and the IL-micelle.

Ionic Liquid Containing Microemulsions: Probe by Conductance, Dynamic Light Scattering, Diffusion-Ordered Spectroscopy NMR Measurements, and Study of Solvent Relaxation Dynamics

Room-temperature ionic liquid (RTIL), N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([P(13)][Tf(2)N]), was substituted for polar water and formed nonaqueous microemulsions with benzene by the aid of nonionic surfactant TX-100. The phase behavior of the ternary system was investigated, and microregions of [P(13)][Tf(2)N]-in-benzene (IL/O), bicontinuous, and benzene-in-[P(13)][Tf(2)N] (O/IL) were identified by traditional electrical conductivity measurements. Dynamic light scattering (DLS) revealed the formation of these IL microemulsions because with gradual increase of RTIL contents the droplet sizes of the microemulsions are also gradually increasing. Pulsed-field gradient spin-echo NMR have been studied to measure the diffusion coefficients of neat [P(13)][Tf(2)N] and [P(13)][Tf(2)N] in microemulsions which indicate ionic liquid containing microemulsions is formed. Moreover, the dynamics of solvent relaxation have been investigated in [P(13)][Tf(2)N]/TX100/benzene microemulsions using steady-state and time-resolved fluorescence spectroscopy using coumarin 153 (C-153) and coumarin 480 (C-480) fluorescence probe with variation of RTIL contents in microemulsions. For both of the probes with increasing amount of ionic liquids in microemulsions the relative contribution of the fast components increases and the slow components contribution decreases; therefore the average solvation time decreases.

Interaction of ionic liquid with water with variation of water content in 1-butyl-3-methyl -imidazolium hexafluorophosphate ( [bmim ] [PF6 ] )/TX-100/water ternary microemulsions monitored by solvent and rotational relaxation of coumarin 153 and coumarin 490

The interaction of water with room temperature ionic liquid (RTIL) [bmim][PF6] has been studied in [bmim][PF6]/TX-100/water ternary microemulsions by solvent and rotational relaxation of coumarin 153 (C-153) and coumarin 490 (C-490). The rotational relaxation and average solvation time of C-153 and C-490 gradually decrease with increase in water content of the microemulsions. The gradual increase in the size of the microemulsion with increase in w0 (w0=[water]/[surfactant]) is evident from dynamic light scattering measurements. Consequently the mobility of the water molecules also increases. In comparison to pure water the retardation of solvation time in the RTIL containing ternary microemulsions is very less. The authors have also reported the solvation time of C-490 in neat [bmim][PF6]. The solvation time of C-490 in neat [bmim][PF6] is bimodal with time constants of 400 ps and 1.10 ns.