Sukhendu Nath

Effect of solvent polarity on the aggregation of C60

C60 forms aggregates in solution when the dielectric constant of the solvents exceeds some critical value viz. ⩾13. Further, for aggregation the C60 concentration has to exceed some critical value. The aggregation process is reversible and the aggregates exist in equilibrium with monomers. Picosecond laser flash photolysis experiments indicate that the excited (S1) aggregates undergo very fast relaxation without contributing towards the triplet formation.

Intermolecular electron transfer between coumarin dyes and aromatic amines in Triton-X-100 micellar solutions: Evidence for Marcus inverted region

Photoinduced electron transfer (ET) between coumarin dyes and aromatic amines has been investigated in Triton-X-100 micellar solutions and the results have been compared with those observed earlier in homogeneous medium. Significant static quenching of the coumarin fluorescence due to the presence of high concentration of amines around the coumarin fluorophore in the micelles has been observed in steady-state fluorescence studies. Time-resolved studies with nanosecond resolutions mostly show the dynamic part of the quenching for the excited coumarin dyes by the amine quenchers. A correlation of the quenching rate constants, estimated from the time-resolved measurements, with the free energy changes (DeltaG0) of the ET reactions shows the typical bell shaped curve as predicted by Marcus outer-sphere ET theory. The inversion in the ET rates for the present systems occurs at an exergonicity (-DeltaG0) of approximately 0.7-0.8 eV, which is unusually low considering the polarity of the Palisade layer of the micelles where the reactants reside. Present results have been rationalized on the basis of the two dimensional ET model assuming that the solvent relaxation in micellar media is much slower than the rate of the ET process. Detailed analysis of the experimental data shows that the diffusional model of the bimolecular quenching kinetics is not applicable for the ET reactions in the micellar solutions. In the present systems, the reactions can be better visualized as equivalent to intramolecular electron transfer processes, with statistical distribution of the donors and acceptors in the micelles. A low electron coupling (Vel) parameter is estimated from the correlation of the experimentally observed and the theoretically calculated ET rates, which indicates that the average donor--acceptor separation in the micellar ET reactions is substantially larger than for the donor--acceptor contact distance. Comparison of the Vel values in the micellar solution and in the donor--acceptor close contact suggests that there is an intervention of a surfactant chain between the interacting donor and acceptor in the micellar ET reaction.

Effect of solvent polarity on the aggregation of fullerenes: a comparison between C60 and C70

Abstract Effect of solvent polarity on the aggregation behaviour of C70 has been investigated in several mixed solvents using optical absorption, fluorescence, dynamic light scattering and scanning electron microscopic measurements and compared with those observed for the other fullerene analogue, C60. It is seen that similar to C60, aggregation of C70 also requires the solvent polarity to exceed some critical value. In terms of solvent dielectric constant the critical solvent polarity, required for C70 aggregation is found to be in the range of ∼27–31, which is much higher than that required for C60 aggregation (∼12–14). The large difference in the critical solvent polarity required for C60 and C70 aggregation has been rationalized on the basis of the molecular shapes and the polarizabilities of two fullerene molecules.

Ultrafast Bond Twisting Dynamics in Amyloid Fibril Sensor

The fundamental process of bond twisting that is responsible for the fluorescence sensing activity of the most extensively used amyloid fibril sensor, Thioflavin T, has been revealed using ultrafast time-resolved fluorescence spectroscopy. From the wavelength-dependent fluorescence decay kinetics and the subsequently constructed time-resolved emission spectra (TRES), the dynamic Stokes shift and the change in the spectral width were observed. These results are rationalized on the basis of the proposition that, following photoexcitation, Thioflavin T undergoes ultrafast bond twisting to form a twisted intramolecular charge-transfer state that is weakly emissive in nature. Formation of the twisted state from the local excited state was found to occur in the subpicosecond time domain (time constant approximately = 570 fs). Quantum chemical calculations support the proposition of the bond twisting process in the photoexcited Thioflavin T and suggest that the twisting around the central C-C single bond, rather than the C-N single bond, of the Thioflavin T molecule is mainly responsible for the observed ultrafast dynamics in the excited state. Detailed time-resolved fluorescence studies of Thioflavin T incorporated in amyloid fibril show substantial retardation in the bond twisting dynamics, suggesting the involvement of this process in the sensor activity of the dye.

Ultrafast torsional dynamics of protein binding dye thioflavin-T in nanoconfined water pool.

Photophysical properties and ultrafast excited state torsional dynamics of the protein binding dye, Thioflavin-T, have been investigated in a nanoconfined water pool of reverse micelles using both steady-state fluorescence and the femtosecond fluorescence upconversion technique. It is seen that due to the confined environment in the reverse micelle, the fluorescence yield of Thioflavin-T is dramatically enhanced approximately 250-fold as compared to that in bulk water. The fluorescence yield decreases nonlinearly with the increase in the water pool size of the reverse micelles. Fluorescence lifetime of Thioflavin-T is also seen to decrease sharply with an increase in the water pool size. The results have been rationalized on the basis of the effect of confinement on the ultrafast torsional motion in the Thioflavin-T. The rate constants for the torsional motion in Thioflavin-T molecule in confined water pool have been estimated.

Photoinduced intermolecular electron transfer from aromatic amines to coumarin dyes in sodium dodecyl sulphate micellar solutions

Photoinduced intermolecular electron transfer interaction between coumarin dyes and aromatic amines has been investigated in sodium dodecyl sulphate micellar solutions using steady-state and time-resolved fluorescence quenching measurements. Steady-state fluorescence quenching of the coumarin dyes by the amine quenchers always shows a positive deviation from linear Stern–Volmer relationship, which arises due to the localized high quencher concentrations at the micellar Stern layer. In time-resolved fluorescence measurements, the analysis of the fluorescence decays following a micellar quenching kinetics model assuming a unified quenching constant (kq′) per quencher occupancy does not give satisfactory results, especially for the higher quencher concentrations used. The observed fluorescence decays are, however, seen to fit reasonably well following a bi-exponential analysis for all the quencher concentrations used. The average fluorescence lifetimes of the coumarin dyes in the micellar solution as estimat...

Photophysical Properties of Coumarin-7 Dye: Role of Twisted Intramolecular Charge Transfer State in High Polarity Protic Solvents

Photophysical studies on coumarin‐7 (C7) dye in different protic solvents reveal interesting changes in the properties of the dye on increasing the solvent polarity (Δf; Lippert–Mataga solvent polarity parameter) beyond a critical value. Up to Δf∼0.31, the photophysical properties of the dye follow good linear correlations with Δf. For Δf >∼0.31, however, the photophysical properties, especially the fluorescence quantum yields (Φf), fluorescence lifetimes (τf) and nonradiative rate constants (knr), undergo large deviations from the above linearity, suggesting an unusual enhancement in the nonradiative decay rate for the excited dye in these high polarity protic solvents. The effect of temperature on the τf values of the dye has also been investigated to reveal the mechanistic details of the deexcitation mechanism for the excited dye. Studies have also been carried out in deuterated solvents to understand the role of solute–solvent hydrogen bonding interactions on the photophysical properties of the dye. Observed results suggest that the fluorescence of the dye originates from the planar intramolecular charge transfer (ICT) state in all the solvents studied and the deviations in the properties in high polarity solvents (Δf >∼0.31) arise due to the participation of a new deexcitation channel associated with the formation of a nonfluorescent twisted intramolecular charge transfer (TICT) state of the dye. Comparing present results with those of a homologous dye coumarin 30 (C30; Photochem. Photobiol., 2004, 80, 104), it is indicated that unlike in C30, the TICT state of the C7 dye does not experience any extra stability in protic solvents compared to that in aprotic solvents. This has been attributed to the presence of intramolecular hydrogen bonding between the NH group (in the 3‐benzimidazole substituent) of the C7 dye and its carbonyl group, which renders an extra stability to the planar ICT state, making the TICT state formation relatively difficult. Qualitative potential energy diagrams have been proposed to rationalize the differences observed in the results with C7 and C30 dyes in high polarity protic solvents.

An evaluation of the efficacy of aspirin and benzydamine hydrochloride gargle for attenuating postoperative sore throat: a prospective, randomized, single-blind study.

Postoperative sore throat (POST), although a minor complication, remains a source of postoperative morbidity. We compared the efficacy of dispersible aspirin gargle to benzydamine hydrochloride (a topical nonsteroidal anti inflammatory drug) gargles for prevention of POST. We enrolled 60 consecutive female patients, 16–60 yr of age, ASA physical status I or II, undergoing elective modified radical mastectomy under general anesthesia in this prospective, randomized, placebo-controlled, single-blind study. Patients were randomly divided into 3 groups of 20 subjects each: Group 1 (C) mineral water; Group 2 (AS) tab aspirin 350 mg; and Group 3 (BH) 15 mL of benzydamine hydrochloride (0.15%). All the medications were made into 30 mL of solution. Patients were asked to gargle this mixture for 30 s, 5 min before induction of anesthesia. Grading of POST was done at 0, 2, 4, and 24 h postoperatively on a 4-point scale (0–3). Aspirin gargles reduced the incidence of POST for 4 h whereas benzydamine hydrochloride gargles reduced POST for 24 h. POST was more severe in the control group at 0 and 2 h (P < 0.05). Aspirin and benzydamine hydrochloride gargles significantly reduced the incidence and severity of POST (P < 0.05).

Effect of solvent polarity on the photophysical properties of coumarin-1 dye

Effects of solvent polarity on the photophysical properties of coumarin-1 (C1; 7-NEt2-4-CH3-1,2-benzopyrone) dye have been investigated using steady-state and time-resolved fluorescence measurements. In moderate to higher polarity solvents the properties like Stokes’ shifts (Δν), fluorescence quantum yields (Φf), fluorescence lifetimes (τf), radiative rate constants (kf), and nonradiative rate constants (knr) follow more or less linear correlation with the solvent polarity function Δf [={(D−1)/(2D+1)}−{(n2−1)/(2n2+1)}]. In nonpolar solvents, namely, hexane, cyclohexane, methylcyclohexane, 3-methylpentane, and decalin, however, all the above-mentioned properties show unusual deviation in comparison to the trend observed in moderate to higher polarity solvents. Thus, the Δν and knr values are unusually lower and the Φf, τf, and kf values are unusually higher in nonpolar solvents. Comparing the results of C1 with those of its lower analogue, namely, coumarin-120 (C120; 7-NH2-4-CH3-1,2-benzopyrone), it has ...

Solubilization of Parabens in Aqueous Pluronic Solutions: Investigating the Micellar Growth and Interaction as a Function of Paraben Composition

The influence of methyl paraben (MP) and butyl paraben (BP) on the aggregation characteristics of Pluronics in an aqueous medium has been investigated by DLS, SANS, viscometry, and fluorescence measurement techniques. Parabens are extensively used as preservatives in cosmetic, pharmaceutical, and food products. In this paper, we show that their influence on the restructuring and growth of Pluronics micelles vary quite significantly with their aqueous solubility and with the composition of Pluronics. In the case of P105 and P104, MP reduces the sphere-to-rod transition temperature down to room temperature, but BP with significantly less aqueous solubility than MP suppresses such micellar transition and leads to the formation of micellar clusters due to the onset of intermicellar attractive interaction. In the case of more hydrophobic Pluronic P103, on the other hand, both MP and BP are able to induce rapid room temperature sphere-to-rod micellar growth, which is not observed in the presence of water structure making salts like NaCl and Na(3)PO(4). These observations have been attributed to modulation of growth and restructuring processes of the Pluronic micelles arising due to different locations of parabens within the micellar corona as determined by their aqueous solubility and the hydrophobicity of the Pluronics.