BITS Faculty Publications
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Item Solvation Dynamics in Restricted Environments: Solvent Immobilization in Reverse Micelles(Springer, 1998) Pant, Debi D.Polar solvation dynamics in various reverse micellar environments are investigated. The solvent is immobilized in the smallest micelles and becomes more mobile with increasing water content and micellar size.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 Time resolved fluorescence spectroscopy of quinine sulphate, quinidine and 6-methoxyquinoline: pH dependence(Elsevier, 1992-03) Pant, Debi D.The excited state dynamics of quinine sulphate (QS), quinidine (Qd) and 6-methoxyquinoline (6MQ) has been studied as a function of pH in steady state and nanosecond time resolved fluorescence experiments. The solvent relaxation process is a dominant process for all the molecules studied, irrespective of pH. Moreover, 6MQ undergoes a proton transfer reaction in the excited state at pH 7 whereas QS and Qd do not exhibit excited state protonation.Item The molecular dynamics of binary liquid mixtures of C6F6 and C6H6 were studied by using optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) with 45 fs laser pulses at room temperature and ambient pressure. The data give evidence for the effect of interspecies interactions in the short-time, nondiffusive part of the OHD-RIKES response(Elsevier, 1997-01) Pant, Debi D.The molecular dynamics of binary liquid mixtures of C6F6 and C6H6 were studied by using optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) with 45 fs laser pulses at room temperature and ambient pressure. The data give evidence for the effect of interspecies interactions in the short-time, nondiffusive part of the OHD-RIKES response.Item Reorientational and intermolecular dynamics in binary liquid mixtures of hexafluorobenzene and benzene: femtosecond optical Kerr effect measurements(Elsevier, 1997-01) Pant, Debi D.The molecular dynamics of binary liquid mixtures of C6F6 and C6H6 were studied by using optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) with 45 fs laser pulses at room temperature and ambient pressure. The data give evidence for the effect of interspecies interactions in the short-time, nondiffusive part of the OHD-RIKES response.Item Polar Solvation Dynamics of H2O and D2O at the Surface of Zirconia Nanoparticles(ACS, 1999-08) Pant, Debi D.Time-resolved fluorescence-upconversion spectroscopy has been used to study the polar solvation dynamics of H2O and D2O at the surface of zirconia (ZrO2) nanoparticles. While an isotope effect is observed for the solvation dynamics of bulk D2O, there is no isotope effect on the interfacial solvation dynamics. The interfacial solvation dynamics are the same for H2O and D2O and are faster than the bulk solutions. The bulk isotope effect is due to stronger hydrogen bonding in D2O compared to H2O, slowing the reorientation of the excited-state dipoles in the bulk D2O. The lack of isotope effect for the interfacial dynamics is explained in terms of the solvent interacting with the ZrO2 surface.Item Structure and Intermolecular Dynamics of Liquids: Femtosecond Optical Kerr Effect Measurements in Nonpolar Fluorinated Benzenes(Elsevier, 1998-07) Pant, Debi D.Polar solvation dynamics of a 95:5% (v/v) water/acetone mixture have been measured at the ZrO2 nanoparticle surface by time-resolved fluorescence of a probe molecule adsorbed to the particle surface. The interfacial solvent response displays two sub-picosecond diffusive components with the same time constants as bulk solution. However, the relative amplitudes for the individual relaxation components are significantly different, leading to a faster average solvation response for molecules at the ZrO2 surface. Furthermore, the overall fluorescence Stokes shift is approximately three times smaller for dye molecules adsorbed to the nanoparticle surface. Implications for electron injection into semiconductor nanoparticles are discussed.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.