Subhasis Chakrabarti

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.

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.

Surface enhanced Raman spectroscopic study of α-bithiophene and α-quaterthiophene molecules adsorbed on silver sols

Abstract Surface enhanced Raman scattering (SERS) has been observed from α-bithiophene (2T) and α-quaterthiophene (4T) adsorbed on Ag sols. Experimental results suggest that the adsorbed oligothiophene molecules are anti but twisted and the twist angle is smaller in the surface adsorbed state than in solution. It has been observed that the enhancement factor decreases with the increase of the chain length and maximum enhancement is obtained at a higher concentration for the longer oligomer. The Raman excitation frequency dependence of the enhancement suggests a significant classical electromagnetic contribution to the SERS of oligothiophenes in Ag sols

Lattice phonon mediation of solid state photoreaction in 7-methoxycoumarin crystal: laser Raman spectroscopic study

Abstract The photodimerization reaction in 7-methoxycoumarin (7MC) in the crystalline state is shown to be mediated by a lattice phonon. The phonon participation is through a mode softening in the excited state of the crystal on photoirradiation and not via strong exciton—phonon coupling as in generally observed. IR and Raman spectroscopy were used to characterize the reactant and product, and it is confirmed that the reaction occurs by cyclobutane ring formation through the bond of the pyrone ring. Electronic spectroscopy suggests that the exciton—phonon coupling is weak in the monomer crystal.

Topochemically controlled solid state photoreaction in 7-acetoxy coumarin : A Raman phonon spectroscopic study

The solid state photodimerization reaction mechanism in 7-acetoxy coumarin (7AC) crystal has been investigated. Raman phonon spectroscopic studies reveal that the reaction mechanism is homogeneous and the lattice becomes disordered with the increase of product concentration. No mode softening is observed. Electronic excitation and emission spectra show that exciton-phonon coupling is weak in the monomer crystal. Thus the reaction seems to be topochemically controlled and lattice phonons do not take part in the reaction. Formation of cyclobutane rings through the > CC < of the pyrone moiety is established by infrared and Raman spectroscopy.

Phonon participation in solid state photoreaction in 6-bromocoumarin : Laser Raman spectroscopic study

Abstract The photodimerization reaction in 6-bromocoumarin in the crystalline state is shown to be mediated by lattice phonon. Significant overlap between the electronic absorption and emission spectra suggests that the exciton-phonon coupling is not very strong in the monomer crystal. Raman phonon spectra show softening of a specific phonon mode with photoreaction progress. This suggests that phonon participation in the reaction is through a mode softening in the excited state of the crystal on photoirradiation, not via strong exciton-phonon coupling. Raman phonon spectroscopic study also reveals that the reaction mechanism is homogeneous in the initial stage of reaction when the reactant and the product form solid solution and is heterogeneous in the later stage when the product segregates out. IR and Raman spectroscopy are used to characterize the reactant and the product, and the results confirm that reaction occurs by cyclobutane ring formation through the > C  C

Spectroscopic study of photoreaction dynamics inp-bromo cinnamic acid

The photodimerization reaction inp-bromocinnamic acid (p-BCA) in the crystalline state has been probed by Raman phonon spectroscopy. Monotonic redshift of the phonon bands and nonappearance of any new phonon bands on reaction progress suggest that the reaction is homogeneous and the reactant and the product form a solid solution. It is further observed that a low frequency phonon at 18·6 cm−1 softens on photoexcitation and progress of reaction. The photoreaction inp-BCA crystal therefore seems to be mediated by the lattice phonon. In the IR spectrum disappearance of the aliphatic > C=C < stretching frequency and appearance of cyclobutane ring deformation and ring stretching modes on reaction confirm photodimerization by cyclobutane ring formation. Considerable overlap between the absorption and emission bands shows that the exciton-phonon coupling is not very strong in the monomer crystal.

Laser Raman spectroscopic study of photoreaction dynamics inp-chloro cinnamic acid crystal

Raman phonon spectroscopy has been used to study photodimerization reaction inp-chloro cinnamic acid (pCCA) crystal. The β-form of the crystal yields the 4,4′-dichloro-β-truxinic acid dimer. Six distinct low frequency phonon bands are observed in thepCCA monomer crystal. On reaction progress, these bands show a monotonic shift to lower frequencies and broaden out. Finally, in the dimer crystal the phonon spectrum shows two weak broad bands. These results suggest that the reaction is homogeneous in the initial stages and, as the product concentration increases, the lattice becomes highly disordered. The reactant and the product were characterised by infrared and Raman spectroscopy. The disappearance of aliphatic C=C bond stretching vibration and appearance of cyclobutane ring deformation and cyclobutane ring-breathing vibrations on reaction confirm photodimerization by cyclobutane ring formation. The large Stokes shift between the absorption and emission band suggest strong exciton-phonon coupling in the monomer lattice. This reaction seems to be phonon-mediated.