Rina Dutta

Effect of cyclodextrine cavity size on twisted intramolecular charge transfer emission: dimethylamino benzonitrile in β -cyclodextrine

Abstract Emission spectra of p -N,N-dimethylamino benzonitrile (DMABN) in aqueous β-cyclodextrine (β-CD) solutions are reported. In β-CD two distinct kinds of emission are observed, one with enhanced nonpolar emission (at 355 nm) and the other exhibiting enhanced TICT emission (at 480 nm). The former is attributed to those DMABN molecules which are totally inside the β-CD cavity while the latter is due to DMABN molecules only partially enclosed in the cavity.

An excited state proton transfer reaction in micellar media

The excited state proton transfer reaction was studied for carbazole (CAZL) in non-ionic (Triton X-100), cationic (cetyltrimethylammonium bromide (CTAB)) and anionic (sodium dodecylsulphate (SDS) micelles. The micelles strongly affect the equilibrium quotient of the process. Taking the excited state proton transfer reaction as a probe, the critical micelle concentrations (CMCs) for the three micellar solutions were estimated. Furthermore, a phase transition was observed at high ionic strengths for Triton X-100.

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.

Magnetic-field effect on the diphenyl hexatriene-dimethyl aniline exciplex system: anisotropic effects in viscous media

Abstract The exciplex formed between diphenyl hexatriene and dimethyl aniline has been found to be magnetic-field sensitive. The effect of the dielectric constant of the medium and of addition of an alcohol on the magnetic-field modulated luminescence of the exciplex has been discussed. Theoretical B 1 2 values have been calculated by the INDO-UHF method. The lack of correspondence between the theoretical and the experimental B 1 2 values has been explained on the basis of electron hopping. In a viscous polymeric medium, anisotropic interactions between the radical partners have been observed. A qualitative explanation of this novel phenomenon, emphasizing the importance of anisotropic dipolar interactions, has been discussed. To the best of our knowledge, this is the first report on magnetic anisotropy of exciplex luminescence.

Study of the magnetic-field-dependent behaviour of radicals generated photochemically from benzil in micellar media

Abstract Excited benzil, encaged in sodium dodecylsulphate (SDS) surfactant in aqueous solution, generates a radical pair (benzil ketyl radical and SDS radical) on hydrogen abstraction from the micelle. The lifetime of this triplet-born radical pair is dependent on the magnetic field, reaching saturation at fields greater than 1 kG. An intermediate radical pair, as demonstrated by the occurrence of a magnetic field effect, is also observed on ππ* excitation of this fairly labile molecule. In addition, Cu2+ ions quench the effect of the magnetic field on the decay of the radical pair. From the temporal response and the magnetic field effect, it is concluded that radicals initially generated by nπ* and ππ* excitation are different.

Solvent dependence of magnetic-field effect in a polymer-linked exciplex system

Abstract The response of pyrene-polystyrene-dimethylaniline (Py-PS-DMA) polymer (Mn = 4770) to an externally applied magnetic field has been examined in different solvents. Although the dielectric-constant (ϵ) effect is similar to that for the unlinked system, a closer look shows an interesting dependence on the nature of the solvent mixture. In pure acetone Δφ/φ is higher than that in a mixed solvent (tetrahydrofuran/dimethylformamide) of the same ϵ. The results have been explained on the basis of the fact that the influences of coil extension on the magnetic-field effect and the dielectric-constant effect compete with each other, resulting in different behaviours depending on the nature of the solvent mixture.

Magnetic field effect on the benzophenone-sodium dodecyl sulphate system: influence of external additives

Abstract Addition of small amphiphiles such as 1,4-dioxane is found to affect radical recombination and escape rates in micelles such as sodium dodecyl sulphate (SDS). This has been demonstrated with the hydrogen-abstracted product of triplet benzophenone as the probe molecule. This result has been interpreted on the basis of the ability of dioxane to form “mixed” micelles, which thereby affect the size, viscosity and reflectivity of the boundary of SDS. Studies in presence of magnetic fields up to 15 kG show that the magnetic field effect remains unaffected by the addition of dioxane. This his been explained as due to the combined effect of size, viscosity and reflectivity of the boundary of the micelle.

Effect of magnetic field on luminescent exterplexes: the diphenylbutadiene-dimethylaniline system

Abstract The excited state complexation reaction between 1,4-diphenyl-1,3-butadiene and N,N-dimethylaniline was studied. In a sufficiently polar medium, 1: 2 complexes (or exterplexes) were formed. This exterplex system is magnetic field (MF) sensitive. The various parameters which characterize the MF effect in radical ion pair systems, namely wavelength dependence of the MF, field dependence, dielectric constant dependence and time dependence, were studied. The anomaly between the theoretically estimated and experimentally determined B 1 2 values was explained on the basis of the hopping mechanism.

Magnetically sensitive non-polar exciplex: the perfluoronaphthalene–anthracene system

Abstract The complexation reaction between anthracene (AN) and perfluoronaphthalene (PFN) has been found to be magnetically sensitive. The various characteristics of magnetic field effect in the radical ion pair system, namely wavelength, field and dielectric constant dependence, were investigated. The mismatch between theoretically estimated and experimentally determined B 1/2 , has been ascribed to the out-of-plane floppy motion of the perfluoranaphthalene radical anion, which bestows a σ character on the latter thereby affecting the hyperfine coupling constant. The low magnetic sensitivity of this exciplex system has been ascribed to a combined effect of static complexation of AN-PFN and an out-of-plane butterfly motion of the PFN radical.

Effect of salt and solvents on the ionic solvation of p-toluidinonaphthalene sulfonate

Fluorescence properties of p-toluidinonaphthalene sulfonate (TNS) are studied in the presence of tetra-n-butylammonium bromide (TBBr) in different solvents. In the presence of TBBr, in solvents of low polarity (tetrahydrofuran, THF), the fluorescence decay of TNS at the blue end (395 nm) is distinctly faster than that at the red end (500 nm). This is attributed to the formation of ionic aggregates involving negatively charged TNS and positively charged tetrabutylammonium ions. From the difference in the lifetimes at the red and blue ends, the ionic relaxation rates are obtained. In solvents of moderately high polarity (dimethylformamide, DMF) no such aggregates are detected and the decay at the red end is very similar to that at the blue end. In highly polar aqueous media, however, micellar aggregates of TBBr are formed, but there is no difference in the decay at the red and blue ends. The micellar aggregates in aqueous media remain unaffected in the presence of 3 mol dm–3 urea.