Department of Physics
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Item Effect of nanosize micelles of ionic and neutral surfactants on the photophysics of protonated 6-methoxyquinoline: Time-resolved fluorescence study(Elsevier, 2015-03) Pant, Debi D.The excited state dynamic studies have been carried out to investigate the effects of micellar surface charge on the photophysics of protonated 6-methoxyquinoline (6MQ+) in anionic, sodium dodecylsulphate (SDS), cationic, cetyltrimethylammonium bromide (CTAB) and neutral, triton X-100 (TX100) surfactant at premicellar, micellar and postmicellar concentrations in aqueous phase at room temperature. At premicellar concentrations of SDS, there is a slight decrease in emission intensity and at micellar and postmicellar concentrations, increase in emission intensity and blue shift of spectrum has been observed. The blue shift in fluorescence spectrum and slight increase in quantum yield are attributed to incorporation of solute molecule to the micelles. Edge excitation red shift (EERS) in fluorescence maximum of 6MQ+ has been observed in all the surfactant solutions studied. The EERS has been ascribed in terms of solvent relaxation process. In SDS surfactant system, due to heterogeneous restricted motion of solvent molecules, the solvent viscosity increases which results in an increase in net magnitude of EERS. The fluorescence decay components of 6MQ+ fit with multi exponential functions in all the micellar systems studied. The location of the probe molecule in micellar systems is justified by a variety of spectral parameters such as refractive index, dielectric constant, ET (30), EERS, average fluorescence decay time, radiative and non radiative rate constants, and rotational relaxation time.Item Interaction of quinine sulfate with anionic micelles of sodium dodecylsulfate: A time-resolved fluorescence spectroscopy at different pH(Elsevier, 2015-09) Pant, Debi D.Photophysical behavior and rotational relaxation dynamics of quinine sulfate (QS) in anionic surfactant, sodium dodecylsulfate (SDS) at different pH have been studied using steady state and time resolved fluorescence spectroscopy. It has been observed that the cationic form of quinine sulfate (at pH 2) forms a fluorescent ion pair complex with the surfactant molecules at lower concentrations of surfactant. However, for higher concentrations of SDS, the probe molecules bind strongly with the micelles and reside at the water–micelle interface. At pH 7, QS is singly protonated in bulk aqueous solution. At lower concentrations of SDS aggregation between probe and surfactant molecules has been observed. However, for higher concentrations of SDS, an additional fluorescence peak corresponding to dicationic form of QS appears and this has been attributed to double protonation of the QS molecule in micellar solution. At pH 7, in the presence of SDS micelles, the photophysical properties of QS showed substantial changes compared to that in the bulk water solution. At pH 12, an increase in fluorescence intensity and lifetime has been observed and this has been attributed to the increase in radiative rate due to the incorporation of QS at the micelle–water interface. The local pH at micellar surface has been found different from the pH of bulk solution.Item Steady state and time-resolved fluorescence spectroscopy of quinine sulfate dication in ionic and neutral micelles: Effect of micellar charge on photophysics(Elsevier, 2013-05) Pant, Debi D.Steady state and time-resolved fluorescence studies have been carried out to investigate the effects of micellar surface charge on the photophysics of a well known fluorescent molecule quinine sulfate dication (QSD) in cationic, cetyltrimethylammonium bromide (CTAB), anionic, sodium dodecylsulphate (SDS) and neutral, triton X-100 (TX100) surfactants at concentrations above the critical micelle concentrations (c.m.c) in aqueous phase. Edge excitation red shift (EERS) in fluorescence maximum of QSD has been observed in all the surfactant solutions studied. The magnitude of observed EERS is less in anionic SDS surfactant solution compared to the EERS in CTAB and TX100 surfactants. The magnitude of EERS in CTAB and TX100 is almost the same as in bulk water solution. The EERS has been ascribed in terms of solvent relaxation process. The observed multi-exponential decay of fluorescence is due to the different locations of QSD in micellar systems. In SDS surfactant system, due to heterogeneous restricted motion of solvent molecules the solvent relaxation rate decreases which results in a decrease in net magnitude of EERS and fluorescence decay components fit in three exponentials. Following the two step and wobbling in a cone model for the analysis of the temporal fluorescence anisotropy decay of QSD in SDS micelles allows determination of restriction on the motion of fluorophore. Further, we have shown that the extraordinary capability to sense the surrounding environment makes QSD molecule very efficient for surface and interface studies and can also be used as a probe to investigate the mobility of solvent molecules around the excited molecules.Item Photophysics of the dications of cinchonine and cinchonidine(Elsevier, 1993-11) Pant, Debi D.Steady state and time-dependent fluorescence studies have been carried out for the dications of cinchonine (C2+) and cinchonidine (Cd2+). A red shift in the emission maximum is observed on excitation at the red edge of the absorption band. The fluorescence decay shows double-exponential behaviour. The shorter-lifetime (τ1) component remains almost constant across the emission band whereas the longer-lifetime (τ2) component increases with increase in emission wavelength. The presence of double-exponential decay in C2+ and Cd2+ has been shown to be due to the presence of two different isomers in the ground state. The photophysical behaviours of C2+ and Cd2+ have also been compared with the dications of other cinchona alkaloids such as quinidine (Qd2+) and quinine sulphate (QS2+).Item Photophysics of protonated 6-methoxyquinoline: steady state and time-dependent fluorescence(Elsevier, 1990-11) Pant, Debi D.Nanosecond time-resolved emission spectroscopy was used to investigate the excited state solute—solvent interaction in 6-methoxyquinoline. A red shift in the emission maximum is observed on excitation at the red edge of the absorpton band which depends on the temperature and viscosity of the medium. The fluorescence lifetime is dependent on the emission and excitation wavelengths. A significant change in the energy of emission is observed on the nanosecond time scale. The room temperature data can be explained using the Bakhshiev formulation of solvent relaxation. However, transient and steady state fluorescence studies from 80 to 290 K reveal that, at 160 K, a rapid relaxation process occurs (not solvent relaxation). The photophysics of 6-methoxyquinoline are similar to those of the quinine dication which exhibits two relaxation processes (from 80 to 290 K) — a charge-transfer process around 160 K and a solvent relaxation process at ambient temperature.Item Photophysics of doubly-charged quinine: Steady state and time-dependent fluorescence(Elsevier, 1990-04) Pant, Debi D.The quinine dication in aqueous solution (1 N H2SO4) gives two fluorescence lifetimes (τ1 = 2.80 ns and τ2 = 19.36 ns) at ambient temperature. τ2 shows a small increase with an increase in acid concentration between 0.1 N and 15 N. Quenching by Cl− shows that τ1 and τ2 are differentially quenched. The Stern—Volmer quenching constant KSV for τ1 is 10 M−1 and for τ2 is 75 M−1. In addition, KSV is dependent on emission wavelength. In acidified solution, τ2 increases with an increase in emission wavelength, whereas τ1 exhibits a behaviour which resembles a two-state mechanism with a negative amplitude in the region of longer emission wavelength. However, the two-state theory does not give an entirely satisfactory mechanism for the time-dependent emission. Time-resolved emission spectroscopy (TRES) shows a spectral relaxation which partially explains the dependence of τ2 on emission wavelength in accordance with Bakhshiev formulation. Transient and steady state fluorescence studies from 80 to 290 K show that at 160 K there is a rapid relaxation process resulting in an increase in τ2 and a sudden spectral shift. We propose that the complex behaviour of quinine decay consists of two major relaxation processes: a charge-transfer process which occurs around 160 K and a solvent reorientation process which occurs in the fluid medium.Item Photophysics of quinidine dication in relation to quinine dication and 6-methoxyquinoline monocation(Elsevier, 1991-10) Pant, Debi D.Nanosecond time resolved emission spectroscopy was used to investigate the excited state solute-solvent interaction in quinidine dication. The emission spectrum is susceptible to the wavelength of excitation and the viscosity of the medium. The fluorescence lifetime is dependent on the emission wavelength. Spectral relaxation is observed on a nanosecond time scale. The room temperature data have been explained using Bakshiev's formulation of solvent relaxation. However, transient and steady state fluorescence studies from 80 to 290 K reveal that at 160 K, a rapid relaxation process other than the solvent relaxation occurs. A comparison of the photophysical data of protonated quinidine, quinine and 6-methoxyquinoline shows close similarities among these three molecules. The major two relaxation processes in these molecules are solvent relaxation and charge transfer.Item Photophysics of doubly-charged quinine: Steady state and time-dependent fluorescence(Elsevier, 1990-04) Pant, Debi D.The quinine dication in aqueous solution (1 N H2SO4) gives two fluorescence lifetimes (τ1 = 2.80 ns and τ2 = 19.36 ns) at ambient temperature. τ2 shows a small increase with an increase in acid concentration between 0.1 N and 15 N. Quenching by Cl− shows that τ1 and τ2 are differentially quenched. The Stern—Volmer quenching constant KSV for τ1 is 10 M−1 and for τ2 is 75 M−1. In addition, KSV is dependent on emission wavelength. In acidified solution, τ2 increases with an increase in emission wavelength, whereas τ1 exhibits a behaviour which resembles a two-state mechanism with a negative amplitude in the region of longer emission wavelength. However, the two-state theory does not give an entirely satisfactory mechanism for the time-dependent emission. Time-resolved emission spectroscopy (TRES) shows a spectral relaxation which partially explains the dependence of τ2 on emission wavelength in accordance with Bakhshiev formulation. Transient and steady state fluorescence studies from 80 to 290 K show that at 160 K there is a rapid relaxation process resulting in an increase in τ2 and a sudden spectral shift. We propose that the complex behaviour of quinine decay consists of two major relaxation processes: a charge-transfer process which occurs around 160 K and a solvent reorientation process which occurs in the fluid medium.Item Interaction of 6-methoxyquinoline with anionic sodium dodecylsulfate micelles: Photophysics and rotational relaxation dynamics at different pH(Elsevier, 2016-04) Pant, Debi D.Interactions of different species of 6-methoxyquinoline (6MQ) with anionic micelles have been studied at different pre-micellar, micellar and post-micellar concentrations using steady state, time resolved fluorescence and fluorescence anisotropy techniques. The sensitivity of fluorescence of 6MQ to change in its local environment was used to probe sodium dodecylsulfate (SDS) micelles. At post-micellar concentrations of SDS, the observed blue shift in the fluorescence spectrum and increase in quantum yield are attributed to the incorporation of solute molecule to micelles. 6MQ has been found to bind to the surface of the anionic micelles instead of penetrating inside the core of micelles. The binding constant (Kb) calculated for 6MQ revealed that the electrostatic forces mediate charged probe–micelle association, whereas, hydrophobic interaction allowed neutral 6MQ to associate with SDS micelles. The charged 6MQ gets inserted deeper into the micelle surface than its neutral form. The fluorescence anisotropy decay of 6MQ in SDS micelles studied at different pH allowed determination of restriction of motion of the fluorophore. The location of the probe molecule in micellar systems is justified by a variety of spectral parameters such as refractive index, dielectric constant, ET(30), average fluorescence decay time, radiative and non-radiative rate constants, and rotational relaxation time. The micro-environment around the fluorophore reveals that the photophysics of 6MQ is very sensitive to the microenvironment of SDS and probe molecules reside at the water–micelle interface.