Tapas Misra

Effect of adsorption of vapours on the electrical conductivity of some polyene semiconductors: adsorption and desorption kinetics

The change in semiconductive properties of β-apo-8′-carotenal, astacene and methyl bixin on adsorption of various vapours on the crystallite surfaces has been studied at a constant sample temperature. The adsorption of vapours enhances the semiconductivity of the polyenes appreciably. This enhancement depends on the chemical nature and also on the pressure of the adsorbed vapour. The adsorption and desorption kinetics follow the modified Roginsky-Zeldovich relation. A two stage desorption process, the first stage of which gives a Lennard-Jones potential energy curve and is followed by a rate-determining transition over a potential energy barrier to the second stage of adsorption forming weakly bound complexes between the vapour molecules and the polyene crystallites, can explain satisfactorily the experimentally observed kinetic data.

Non-radiative depletion of the excited electronic states of 9-cyanoanthracene in presence of tetrahydronaphthols.

Both steady state and time resolved spectroscopic measurements reveal that the prime process involved in quenching mechanism of the lowest excited singlet (S1) and triplet (T1) states of the well known electron acceptor 9-Cyanoanthracene (9CNA) in presence of 5,6,7,8-tetrahydro-1-naphthol (TH1N) or 5,6,7,8-tetrahydro-2-naphthol (TH2N) is H-bonding interaction. It has been confirmed that the fluorescence of 9CNA is not at all affected in presence of 5,6,7,8-tetrahydro-2-methoxy naphthalene (TH2MN) both in non-polar n-heptane (NH) and highly polar acetonitrile (ACN) media. This indicates that the H-bonding interaction is crucial for the occurrence of the quenching phenomenon observed in the present investigations with TH1N (or TH2N) donors and 9CNA acceptor. In ACN solvent both contact ion-pair (CIP) and solvent-separated (or dissociated) ions are formed due to intermolecular H-bonding interactions in the excited electronic states (both singlet and triplet). In NH environment due to stronger H-bonding interactions, the large proton shift within excited charge transfer (CT) or ion-pair complex, 1 or 3(D+-H...A-), causes the formation of the neutral radical, 3(D+H-A)*, due to the complete detachment of the H-atom. It is hinted that both TH1N and TH2N due to their excellent H-bonding ability could be used as antioxidants.

The non-coincidence effect in polar liquids

Abstract The experimental non-coincidence splitting of the CO stretching Raman band of acetone in dichloromethane, chloroform and carbontetrachloride solvents has been compared with the theoretical values predicted by dipolar resonant transfer model of Mirone and coworkers and that by mean spherical approximation model of Logan. It is observed that Logans theory better reflects the origin of the non-coincidence effect in these binary mixtures.

Photophysical properties of 5-hydroxyindole (5HI): Laser flash photolysis study

Steady state fluorescence emission and transient absorption spectra of 9-fluorenone (9FL) were measured in the presence of 5-hydroxyindole (5HI) in highly polar acetonitrile (ACN) environment at ambient temperature. Cyclic voltammetry measurements demonstrate that ground state 5HI as a donor could take part in highly exothermic electron transfer (ET) reactions with excited 9FL, which should serve as electron acceptor. From the transient absorption measurements it is inferred that in geminate ion-pair (GIP) (or contact ion pair), formed initially due to photoinduced ET, the decay of this contact ion-pair occurs not only through ion recombination (back electron transfer to ground state of reactants), but through the other processes also such as proton-transfer (hydrogen abstraction) from radical cation to anion and separation of ion-pair producing the free ions. From the computed reorganisation energy parameter (λ) and experimentally observed -‡ET0 values it is hinted that there is a possibility that highly exothermic forward electron transfer reactions in the singlet stateS1 occur, within present reacting systems, in Marcus inverted region. Back transfer seems to follow the same path. Investigations with similar other reacting systems are underway.

Distance-dependent energy transfer between indole and anthracene moieties in Langmuir–Blodgett films

1,2-Diphenyl indole (DPI) and 9,10-diphenyl anthracene (DPA) are non-amphiphilic molecules but form excellent LB films when mixed with stearic acid (SA). Spectroscopic investigations of these films indicate formation of aggregates of DPI and DPA in the mixed LB films. DPA has been used as the quencher of the fluorescence of the DPI donor. Distance-dependent energy transfer between donor and acceptor monolayers in the LB film, where they can be precisely separated by inert spacers of stearic acid layers of varied thickness, is shown to satisfy Khuns quadratic equation. This suggests that the donor excitations are delocalized. The large critical transfer distance estimated from the experimental results has been attributed to the formation of aggregates of the molecules in a LB monolayer.

Two-stage adsorption and desorption processes in some nitroaromatic semiconductor-vapour systems : implications in compensation effect in dark conduction process

Adsorption and desorption ofn-hexane, cyclohexane, CCl4, C6H6 ethylacetate, methanol and ethanol vapours on the crystallite surfaces of 2-nitrofluorene and three isomeric nitrobenzoic acid semiconductors have been investigated using electrical conductivity as a probe. Kinetic analysis data show that in all these vapour-semiconductor systems, the adsorption is a two-stage process. Existence of any relationship between the adsorption process and the compensation effect in dark conduction process has been examined. The compensation effect is generally observed in cases where the change in semiconduction activation energy is caused by a two-stage adsorption process.

Semiconductive properties of biologically important compounds: Gas adsorption effect on vitamin A (alcohol and acetate)

The change in semiconductive properties of vitamin A (alcohol and acetate) after adsorption of various vapours on its crystallite surface has been studied at a constant sample temperature. A rapid enhancement in the semi-conductivity has been observed. The rise in conductivity has been found to be exponential with increasing vapour pressure of the adsorbed gas. It has been suggested that charge transfer (CT) interaction may be responsible for such conductivity change. The adsorption process being efficiently reversible this CT complex is weakly bound.The biological implication of these observations is discussed.