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Santiniketan, India

Ghosh M.,Integrated Science Education and Research Center | Mora A.K.,Bhabha Atomic Research Center | Nath S.,Bhabha Atomic Research Center | Chandra A.K.,North - Eastern Hill University | And 2 more authors.
Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy

(Graph Presented) Photophysical properties of free base tetraphenylporphyrin and its zinc analog are investigated in detail in solvents of varying polarity by using steady state and time-resolved techniques. Both the porphyrins are excited at the Soret band to have better signal-to-noise ratio. Also, the fluorescence emission measurements are carried out by using dilute solutions (∼10-7 mol/L) of the fluorophores in order to minimize the self-quenching effect. It is observed that the steady state absorption and emission characteristics of the porphyrin molecules are mainly affected by polarizability (via refractive index) rather than polarity (via dielectric constant) of the moderate to highly polar solvents. As the molecules are highly symmetric in the ground state, the associated dipole moments are found to be very low from quantum chemical calculations performed by density functional theory method by using Gaussian 03 package. The dipole moments associated with the first excited singlet state of the porphyrins are computed by applying solvatochromic Stokes' shift method. To the best of our knowledge, this is the first attempt to calculate the excited state dipole moments of the porphyrins used in the present investigations. Also, fluorescence quantum yield, fluorescence lifetime of the first excited singlet state, radiative and non-radiative rate constants of the porphyrins are reported in solvents of varying polarity. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. Source

Sinha S.S.,Indian Association for The Cultivation of Science | Mondal D.,Indian Association for The Cultivation of Science | Bag B.C.,Siksha Bhavana | Ray D.S.,Indian Association for The Cultivation of Science
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics

We propose a scheme for quantum Brownian motion of a particle in a fermionic bath. Based on the spin coherent-state representation of the noise operators and a canonical thermal distribution of the associated c numbers, we derive a quantum analog of generalized Langevin equation for quantum-mechanical mean position of the particle subjected to an external force field. The approach allows us to map the quantum problem on a classical setting. The quantum dispersion around the mean can be estimated order by order by a set of quantum correction equations up to a desired degree of accuracy for a given nonlinear potential. We derive a quantum diffusion equation for free particle and show that quantization, in general, enhances the mean-square displacement. Increase in temperature leads to suppression of mean-square displacement. The method is based on canonical quantization procedure and may be used for understanding diffusive transport and thermally activated processes in a fermionic bath. © 2010 The American Physical Society. Source

Ghosh M.,Integrated Science Education and Research Center | Nath S.,Bhabha Atomic Research Center | Hajra A.,Siksha Bhavana | Sinha S.,Integrated Science Education and Research Center
Journal of Luminescence

Self-quenching of the fluorescence emission of tetraphenylporphyrin at high concentrations in toluene at the ambient temperature (300 K) is discussed in detail based on steady state and time-resolved fluorescence measurements. The fluorescence self-quenching is mainly attributed to re-absorption effect and the Förster type resonance energy transfer process (homotransfer). The re-absorption effect is found to deform the fluorescence emission spectra significantly in energy positions as well as relative intensities of different peaks at high concentrations. Nearly ideal fluorescence emission spectra are observed at a concentration ∼10-7 mol/L. Moreover, there is an apparent enhancement of the fluorescence lifetime value of tetraphenylporphyrin in toluene at high concentrations, especially on the blue side of the fluorescence emission spectra. To the best knowledge of the authors, this is the first detail report on the fluorescence self-quenching of porphyrins in liquid medium. This finding carries great importance in view of the widespread research on porphyrins in the fields of solar light harvesting, artificial photosynthesis, photodynamic therapy, etc. © 2013 Elsevier B.V. All rights reserved. Source

Ghosh M.,Integrated Science Education and Research Center | Mora A.K.,Bhabha Atomic Research Center | Nath S.,Bhabha Atomic Research Center | Hajra A.,Siksha Bhavana | Sinha S.,Integrated Science Education and Research Center
Journal of Photochemistry and Photobiology A: Chemistry

The possible role of photoinduced electron transfer is investigated in the fluorescence quenching of the donors metallo-octaethylporphyrins (zinc octaethylporphyrin and magnesium octaethylporphyrin) in presence of the acceptor 2-nitrofluorene in polar acetonitrile liquid medium by using steady state and time-resolved techniques at the ambient temperature (300 K). The donors are selectively excited at the Q band (S1 state), where 2-nitrofluorene has negligible absorbance. The observed bimolecular fluorescence quenching rate constants for the present donor-acceptor systems from steady state as well as time-resolved fluorescence quenching data are of the order of 1010 L/(mol s) (less than the diffusion-controlled limit of 1.9 × 1010 L/(mol s)). The moderately negative values of Gibbs free energy change (ΔG°) indicate that photoinduced electron transfer from the metallo-porphyrins to 2-nitrofluorene in acetonitrile solvent is possible upon photoexcitation of the former molecules. Moreover, the fluorescence quenching rates for both the donor-acceptor systems obtained from Marcus theory agree quite well with the observed values from fluorescence quenching measurements (especially time-resolved measurements). © 2014 Published by Elsevier B.V. All rights reserved. Source

Nandi M.,Siksha Bhavana | Uyama H.,Osaka University
Chemical Record

In this article we discuss those materials that have recorded the highest adsorption capacities for the greenhouse gas CO2 under ambient conditions as well as at different temperatures and pressures. For convenience, the materials have been categorized under four categories, viz., porous carbon, metal-organic, zeolite and mesoporous silica, and porous organic frameworks. It has been found that the gas adsorption property significantly relies on several factors such as high surface area and pore volume and the presence of N-, O- and S-containing moieties. The presence of a microporous structure and strong interaction between the CO2 molecules with the framework through H-bonding or dipole-quadrupole interactions facilitates adsorption of the gas. © 2014 The Chemical Society of Japan and Wiley-VCH, Weinheim. Source

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