PubMed Health⌕ Search

Biomedical subjects

Paresh Chandra Ray

Publications and source records attributed to Paresh Chandra Ray.

6 recordsLinked to original sources
PubMed2006

Very large infrared two-photon absorption cross section of asymmetric zinc porphyrin aggregates: Role of intermolecular interaction and donor-acceptor strengths.

Very large two-photon absorption (TPA) cross sections at the infrared region have been revealed for J-aggregates of asymmetric zinc porphyrin using quantum-chemical calculation. The TPA properties are evaluated for monomer and aggregates of a series of push-pull porphyrins, whose syntheses are known in the literature. The two-photon absorption cross section can be greatly enhanced by increasing the strengths of the electron donor/acceptor. We also present a quantum-chemical analysis on porphyrin aggregates to understand the role of intermolecular interactions and the relationship between structural and collective nonlinear optical properties. It has been observed that the TPA properties change tremendously as monomers undergo J-aggregation and the magnitudes of TPA cross sections are highly dependent on the nature of aggregates. The importance of our results with respect to the design of photonic and photodynamic therapy materials has been discussed.

Journal Article↗
PubMed2006

Gold nanoparticle based FRET assay for the detection of DNA cleavage.

We report gold nanoparticle based FRET assay to monitor the cleavage of DNA by nucleases. Fluorescence signal enhancement is observed by a factor of 120 after the cleavage reaction in the presence of S1 nuclease. The mechanism of distant dependent fluorescence quenching has been discussed. Our experimental results on distance dependent fluorescence quenching match quite well with theoretical findings obtained from the fluorescence quenching model by Gersten and Nitzan (Surf. Sci. 1985, 158, 165). Our experimental observation paradigm for the design of optical based molecular ruler strategies at distances more than double the distances achievable using traditional dipole-dipole Columbic energy transfer based methods.

DNA↗
PubMed2005

Nonlinear optical properties of zwitterionic merocyanine aggregates: role of intermolecular interaction and solvent polarity.

We present a time-dependent quantum-chemical analysis on merocyanine aggregates to understand the insight of the intermolecular interactions and to find the relationship between structural and collective nonlinear optical properties. The first hyperpolarizabilities are evaluated for monomer and aggregates of a series of zwitterionic merocyanine dyes, whose synthesis and formation of H and J type aggregates in solvents are reported recently in the literature (J. Am. Chem. Soc. 2002, 124, 9431). The molecular geometries are obtained via B3LYP/6-31G (hybrid density-functional theory) optimization including PCM approach, while the dynamic NLO properties are calculated with the TD-DFT/SOS and ZINDO/CV method including solvent effects. It has been observed that the first hyperpolarizability changes tremendously as monomers undergo aggregation, and the magnitude of first hyperpolarizabilities highly depends on the nature of the aggregates. It is found that solvents play a remarkable role on the structure and first hyperpolarizabilities of merocyanine monomers and aggregates. Changing the solvent from low to high dielectric causes not only an increase in magnitude of beta but also a change in sign, therefore passing through zero at intermediate dielectric. The importance of our results on the design of electrooptic materials have been discussed.

Journal Article↗
PubMed2005

Two-photon absorption and first nonlinear optical properties of ionic octupolar molecules: structure-function relationships and solvent effects.

We present a quantum-chemical analysis of the two-photon absorption properties and first hyperpolarizabilities of a series of ionic octupolar molecules and a comparison of their characteristics with corresponding neutral molecules. The molecular geometries are obtained via BL3YP/6-31G (d,p) level optimization including the SCRF/PCM approach, while the dynamic NLO and two-photon absorption properties are calculated with the ZINDO/CV method including solvent effects. The effects of donor or acceptor substitution and elongation of the conjugation path length are established to demonstrate the engineering guidelines for enhancing two-photon absorption cross section and molecular optical nonlinearities. It is found that the chain length dependence of the two-photon absorption and the first nonlinearity follow the same trend, displaying a saturation limit at n = 5. The solvent induced effect on the two-photon absorption and NLO properties are studied using the ZINDO/CV/SCRF method. It has been observed that two-photon absorption and the first nonlinearity peaks at epsilon approximately = 20 and then decreases slightly, approaching saturation. We also compare our theoretical findings with the experimental results wherever available in the literature.

Journal Article↗
PubMed2004

The 1:1 adduct of hexamethylenetetramine (HMT) with racemic trans-1,2-cyclohexanedicarboxylic acid (CDA).

Hexamethylenetetramine and rac-trans-1,2-cyclohexanedicarboxylic acid crystallize in a 1:1 ratio as a neutral molecular adduct, C6H12N4.C8H12O4. Two dicarboxylic acid molecules and two tetramine molecules form a hydrogen-bonded ring, in the shape of a rhombus, which lies on a crystallographic twofold axis bisecting the two diacid molecules. The O-H...N hydrogen bonds have lengths 2.6808 (19) and 2.6518 (19) A, and, in each ring, both acid molecules have the same handedness.

Journal Article↗