Debnarayan Nath

Effect of environment on the magnetic field modulation of exciplex luminescence

Abstract The effect of a magnetic field on the pyrene-N,N-dimethylaniline exciplex luminescence in methanol has been studied using an improved phase-sensitive detection technique. The wavelength dependence of B 1/2 , the effect of isotopic substitution of the solvent, and the consequences of using mixed donors have been investigated. The results throw light on the environmental perturbation of the intersystem crossing process in solvent-separated radical pairs.

Effect of variation of dielectric constant on the magnetic field modulation of exciplex luminescence

The effect of variation of dielectric constant on the relative magnetic field effect in singlet luminescence has been studied using a typical exciplex system at a saturating field. The study indicates strong specificity in the perturbation of the magnetic field effect by alcoholic solvents. In contrast to alcohols where relative singlet magnetic field effect is of the order of 2% only, the magnetic field effect in non-alcoholic medium reaches as high as 9%. Moreover, dielectric constant variation in alcohols yields curves which are distinctly different from those in non-alcoholic media. It turns out that this dependence of magnetic field effect on dielectric constant is similar in all non-alcoholic solvent mixtures. An analytical study based on Hong and Noolandi’s solution of Smoluchowski equation has been made. Derived expressions can interpret experimental curves reasonably well.

Excited-state proton transfer kinetics of carbazole

Abstract Rate constants for the excited-state proton transfer reaction of carbazole in aqueous alkaline solution have been determined using picosecond single photon counting. Fluorescence decay measurements show that the back reaction is slow compared to the fluorescence decay time and therefore equilibrium is not attained in the excited state. The validity of a pK value for the lowest excited state determined from steady-state fluorescence measurements assuming equilibrium is discussed. It is concluded that the thermodynamic pK* value for carbazole is 10.98.

Picosecond Time Resolved Electron Injection from Excited Cresyl Violet Monomers and Cd3P2 Quantum Dots into TiO2

Abstract Picosecond time resolved luminescence was employed for monitoring the decay of electronic excited reactants on the surface of oxide semiconductors. Decay of cresyl violet monomers was determined on TiO2 and on α-Al2O3 by measuring decay curves over a wide range of very low coverages. The decay curves showed a characteristic opposite dependence on coverage on the oxide semiconductor with a low conduction band edge, i.e. TiO2, compared to the oxide semiconductor with a high conduction band edge, i.e. α-Al2O3. The dominating time constant for cresyl violet monomer decay was in the range of 280 ps on TiO2 and 560 ps on α-Al2O3. In both cases quenching was attributed to electron transfer. At room temperature the decay of excited Cd3P2 quantum dot luminescence was faster than 1 ps when the dots were attached to TiO2 colloidal particles in solution. The ultrafast decay was ascribed to electron injection.

Electrical conduction in nanocomposites of copper in silicate glasses

Nanocomposites of metallic copper in a silica-based glass have been prepared by the hot compaction of ion-exchanged and reduced glass powders. The diameter of copper particles ranges from 5.7 to 9.2 nm. The specimens exhibit a semiconducting behaviour in the temperature range 80-333 K with an unusually low activation energy of the order of 3.0 meV. This result appears to be consistent with a quantum size effect.

Time-resolved studies of the effect of a magnetic field on exciplex luminescence

Time-resolved studies of the magnetic field effect on pyrene-dimethylaniline exciplex luminescence in non-alcoholic solvent mixtures (dielectric constant of 16) have been performed with the help of a time-correlated single-photon counting technique. The shape of the time-variation curve is interpreted in terms of a simple analytical model.

Salt effect on the hydrophobic binding of p-toluidino naphthalene sulphonate with cyclodextrins

Lithium chloride, lithium perchlorate, cesium bromide, tetra-n-butyl ammonium bromide (TBBr) and guanidine hydrochloride (GnCl) affect the fluorescence properties of 2, 6-p-toluidino naphthalene sulphonate (TNS) bound to α- and β-cyclodextrins (CD) in strikingly different ways. This indicates that the salts have considerable influence on the hydrophobic binding of TNS molecules with CDs. The salting-in agents (LiClO4, CsBr and GnCl) markedly reduce the fluorescence quantum yield (Φf) and lifetime (τf) of CD-bound TNS anion. It is suggested that they directly interact with the bound TNS ions. The salting-in agent TBBr increases Φf and τf. This fluorescence enhancement indicates that the local polarity is reduced by wrapping the four n-butyl groups around the CD-bound TNS ions. The salting-out agent LiCl causes a marginal increase (< 15%) and that at a high concentration (7 M). In the absence of CD in aqueous solution on addition of 0.1 M TBBr; the fluorescence yield of TNS increases sixfold. This is attributed to clustering of TB cations around the TNS anion.

Magnetic field effect on exciplex luminescence in liquids

Abstract The effect of a magnetic field on the luminescence of unlinked as well as linked exciplex systems has been discussed. The magnetic field modulated luminescence. (Δφ/φ) is not only dictated by the hyperfine interaction in the radical ion pair, but also by the environment, such as viscosity and dielectric constant (ϵ) of the medium and presence of other molecules in the neighbourhood of the exciplex. A complex interplay between spin evolution, radical pair recombination and diffusion determine the magnitude and nature of the magnetic field effect (MFE). The dependence of (Δφ/φ) on the ϵ could be explained on the basis of simple theoretical models. The concept of Heisenberg spin exchange has been invoked to rationalise the quenching of MFE by lanthanide ions. Time-resolved studies provide useful information regarding the dynamics of the spin-evolution of the system. In viscous medium the rotational diffusion slows down and the MFE becomes dependent on the direction of the field with respect to the exciplex.

Identification of ΔJ= 0, ±1 transitions in Ln3+ through circular dichroism: Er3+ -Diglycollate and Er3+-pyrogermanate

Abstract The 2 K 15/2 and 2 K 13/2 states of Er 3+ have been assigned on the basis of the circular-dichroism/optical-density (CD/OD) ratio in two optically active crystals, Er—pyrogermanate and Er—diglycollate.

Effect of variation of dielectric constant on the magnetic field modulation of exciplex luminescence: Nontypical behavior of alcohols

The effect of variation of dielectric constant (e) on the relative magnetic field effect on singlet luminescence (Δφ/φ) has been studied using a typical exciplex system at a saturating field. The study indicates strong specificity in the perturbation of the magnetic field effect by alcoholic solvents. In constrast to alcohols where relative singlet magnetic field effect is of the order of 1% only, the magnetic field effect in non‐alcoholic medium reaches as high as 4.5%. Moreover, e variation in alcohols yields curves which are distinctly different from those in nonalcoholic media.