T. N. Misra

A pH dependent surface-enhanced Raman scattering study of hypoxanthine

Surface-enhanced Raman scattering (SERS) of hypoxanthine (HX) molecule on silver colloid confirms that the molecule in the surface adsorbed state exists as [keto–N9 H9] form at acidic and normal pH but as [enol–O6 H1, N9 H9] form at alkaline pH. It is substantiated by the appearance of an intense carbonyl stretching mode at 1690 cm−1, a medium-intense in-plane CO bending mode at 655 cm−1 and disappearance of the C6–O6 H1 stretching mode in SER spectra in acidic and normal pH. Strongly intense Raman band at 1166 cm−1 assigned as C6–O6 H1 stretching mode confirms [enol–O6 H1, N9 H9] form in alkaline pH. The presence of Ag–N and Ag–O stretching modes in SER spectra further infer two modes of orientation of the HX molecule on the silver surface. At acidic pH (pH 2) the HX molecule is adsorbed on the metal surface through the N3 atom with the molecular plane lying almost perpendicular on the silver substrate, while at alkaline pH the molecule is oriented nearly parallel to the silver surface via nonbonding electrons of N1, N3, N7 and O6. Copyright

Energy transfer in triton-X 100 micelles: a fluorescence study.

The study of fluorescence energy transfer from the phenyl groups of the micellar triton X-100 (TX-100) to solubilised 1-pyrene butyric acid (PBA) has been carried out. Through the analysis of the donor fluorescence quenching energy transfer efficiency has been determined. The observed donor-acceptor separation suggests that pyrene molecules are distributed uniformly in the micellar core.

Classical and chemical effects of SERS from 2,2′:5,2″ terthiophene adsorbed on Ag-sols

Abstract Surface-enhanced Raman scattering (SERS) has been observed from 2,2′:5,2″ terthiophene (3T) adsorbed on Ag-sols. Experimental results suggest that the absorbed 3T molecules are anti but twisted and that the twist angle is smaller in the surface-adsorbed state than in solution. The first-layer effect, i.e., maximum enhancement when a monomolecular layer of 3T is adsorbed on silver particles, has been observed for the first time in such SERS active surfaces. A charge-transfer model has been shown to contribute to this effect. On the other hand, the Raman excitation frequency dependence of the enhancement suggests a significant classical electromagnetic contribution to SERS in sol.

A comparative study of SERS from α-sexithiophene and other thiophene oligomers

Abstract Surface enhanced Raman scattering (SERS) is observed from α-sexithiophene (6T) adsorbed on Ag sols and compared with other oligothiophenes. Concentration dependence and Raman excitation frequency dependence of the enhancement factor have been studied and explained in terms of “classical” and “chemical” contributions. Experimental results suggest that smaller oligomers form a monomolecular layer on the silver surface at a lower concentration than the longer oligomers. Also, the decrease in twist angle between the thiophene rings in the surface adsorbed state is larger for the smaller oligomers. Possible orientations of different oligothiophenes, on the silver surface, have been discussed.

Concentration-dependent surface-enhanced resonance Raman scattering of a porphyrin derivative adsorbed on colloidal silver particles

Surface-enhanced Raman spectra (SERS) of 5,10,15,20-tetrakis(1-decylpyridium-4-yl)-21H,23H-porphintetrabromide or Por 10 (H(2)Tdpyp) adsorbed on silver hydrosols are compared with the FTIR and resonance Raman spectrum (RRS) in the bulk and in solution. Comparative analysis of the RR and the FTIR spectra indicate that the molecule, in its free state, has D(2h) symmetry rather than C(2v). The SERS spectra, obtained on adsorption of this molecule on borohydride-reduced silver sol, indicate the formation of silver porphyrin. With the change in the adsorbate concentration, the SERS shows that the molecule changes its orientation on the colloidal silver surface. The appearance of longer wavelength band in the electronic absorption spectra of the sol has been attributed to the coagulation of colloidal silver particles in the sol. The long wavelength band is found to be red-shifted with the decrease in adsorbate concentration. The excitation profile study indicates that the resonance of the Raman excitation radiation with the original sol band is more important than that with the new aggregation band for the SERS activity. This indicates a large contribution of electromagnetic effect to surface enhancement.

Raman Spectral Study of Vibrational Relaxation of the CN Stretching Band of Acetonitrile and Benzonitrile

Raman spectroscopic studies on acetonitrile and benzonitrile as the liquids and in dilute solutions were used to investigate the vibrational relaxation processes which lead to broadening of the C≡N stretching vibrational band. Line widths of isotropic Raman spectra in a number of solvents revealed the quantitative dependence of vibrational relaxation rate on the hydrodynamic and dispersion forces.

CRYSTALLINE STATE PHOTOREACTION IN 4-METHYLCINNAMIC ACID : A RAMAN PHONON SPECTROSCOPIC STUDY

The solid-state photodimerization reaction mechanism in 4-methylcinnamic acid crystal was investigated. Raman and infrared spectroscopy were used to study the intramolecular vibrations of the reactant and the product in order to characterize them. The absence of any large Stokes shift between the excitation and emission bands of the monomer crystal shows that the exciton–phonon coupling is weak. Raman phonon spectroscopic studies reveal that the reaction proceeds by a homogeneous mechanism in the initial stage and subsequently by a heterogeneous mechanism. The appearance of new sharp phonon bands suggests that the product lattice is ordered. No mode softening is observed. Hence the reaction seems to be topochemically controlled and lattice phonons do not take part in the reaction.

Photophysical characteristics of 9-cyanoanthracene molecules organised in Langmuir[ndash ]Blodgett films

Multilayers of 9-cyanoanthracene (CNA) mixed with stearic acid (SA) have been incorporated in Langmuir–Blodgett (LB) films. The surface pressure–area isotherms in the mixed systems in different mole fractions, form stable floating compressible layers on the water subphase. The low area per molecule at high concentration of CNA in the mixed systems indicates multilayer or aggregate formation of CNA molecules in the film within the SA matrix. Steady-state and time-resolved spectroscopic studies reveal that CNA forms aggregates in LB films at room temperature. Presence of nascent trans- and cis- excimeric sites in these aggregates has been confirmed by the two excimeric emissions observed in the LB film at 77 K. Scanning electron microscopy confirms the presence of such aggregates in the LB films.

Photophysical properties of lycopene organized in Langmuir-Blodgett films: formation of aggregates

Abstract Mixed films of lycopene and stearic acid (SA) form highly stable floating monolayers at the air-water interface which may be easily transferred to glass or quartz substrates with a high transfer ratio. The surface pressure vs. area per molecule isotherms indicate that the lycopene molecules have a preferred orientation at the air-water interface. Polarized absorption studies support such an orientation of lycopene molecules in Langmuir-Blodgett (LB) films. Repulsive-type interactions between lycopene and SA have been observed by Gibbs free energy measurements. Spectroscopic studies of lycopene in solution, binary solvents and LB films suggest the formation of both J- and H-type aggregates.

Electrical properties of organised assemblies of pyrrole-N-methylpyrrole copolymer in Langmuir-Blodgett films

Abstract An organised assembly of two dimensional conducting pyrrole-N-methylpyrrole copolymer network interposed by insulating fatty acid layers has been fabricated by Langmuir-Blodgett technique. The electrical properties of such assembly has been investigated. Measured in-plane conductivity show a variation between 0.68-0.01 S cm−1 as the percentage of N-methylpyrrole in the copolymer is varied from 0– 100%. In-plane and transverse conductivity show a large anisotropy of 1010. Temperature dependence of conductivity study show operational definition of semiconduction to hold, the conductivity increases with decrease of activation energy. The experimental results, however, can not identify the conduction mechanism with certainty.