S. Doraiswamy

Diffusion controlled reactions: Fluorescence quenching of cationic dyes by charged quenchers

Rate coefficients of diffusion‐limited fast reactions of charged reactants are predicted to be time dependent or time independent in different theoretical approaches to solve the Debye–Smoluchowski equation. Different rate coefficient expressions are also predicted in different time domains. Fluorescence quenching experiments using two cationic dyes (rhodamine B and cresyl violet) and four efficient ionic quenchers (iron complexes) in aqueous solutions were carried out to verify the theoretical prediction. A detailed data analysis of the quenched fluorescence decay over a range of concentration of the quenchers supports the prediction that the ‘‘long’’ time rate coefficient is k(t)∼a+bt−1/2. Exact agreement with the theoretical predictions is not obtained in any of the fluorophore–quencher systems described in this work. Cresyl violet–ferrocyanide and cresyl violet–ferricyanide are two systems for which the experimental values of diffusion coefficient D and distance parameter RHN are justifiably close to ...

Rotational reorientation dynamics of polar dye molecular probes by picosecond laser spectroscopic technique

Fluorescence lifetimes and rotational reorientation times for four structurally similar dye molecules—three monocations: cresyl violet, nile blue, and oxazine 720 and one neutral but polar: nile red—have been measured by picosecond time‐resolved fluorescence depolarization spectroscopy using the single‐photon counting technique, in a number of solvents, which included a wide range of alcohols, other hydrogen‐bonding liquids, and a few aprotic liquids. The rotational reorientation of the dye probes (assumed to be oblate ellipsoids) are sought to be explained in terms of the Stokes–Einstein–Debye theory and dielectric friction. The individual contributions to the rotational friction due to the above two factors were calculated using reasonable values for the molecular volume and dipole moment of the solute. The rotational behavior of all the four dyes in amides and aprotic solvents is reasonably well explained in terms of the simple stick hydrodynamic model with the ‘‘molecular volume’’ obtained by using th...

Molecular reorientation dynamics of polar dye probes in tertiary‐butyl alcohol–water mixtures

The rotational motion of the four dye probe molecules—nile blue and thionine (monocations), resorufin (monoanion), and nile red (polar but neutral) have been investigated in aqueous mixtures of tertiary‐butyl alcohol using picosecond fluorescence depolarization spectroscopy. The simple hydrodynamic theory due to Stokes–Einstein–Debye is unable to describe the entire profile of the plot of rotational reorientation time vs viscosity, but is adequate in the water rich region (low viscosity). If the dielectric friction theory proposed for pure solvents is considered, then it is possible to mimic the bivalued profile seen in the alcohol rich region (high viscosity). The inability to produce a satisfactory fit over the entire composition range possibly demands a suitable dielectric friction theory for binary mixtures.

Dielectric relaxation study of hexamethylphosphoramide-water mixtures using time domain reflectometry

The dielectric relaxation studies of hexamethylphosphoramide‐water mixtures have been carried out at different temperatures using time domain reflectometry technique in the frequency range of 10 MHz to 10 GHz. A calibration method based on the least squares fit has been suggested to determine the complex permittivity at the high frequency region for the aqueous solutions. The excess dielectric, Kirkwood correlation factor and thermodynamic properties in the mixture have been determined. These properties suggest a Cage‐type structure of hexamethylphosphoramide molecules around water molecules. The structure has a tendency to break down in the presence of water.

Rotational dynamics of nonpolar solutes in different solvents: Comparative evaluation of the hydrodynamic and quasihydrodynamic models

The rotational reorientation times (τr) of substituted quaterphenyl and quinquephenyl molecules (neutral nonpolar solutes) of 4–5 A radius have been measured in a series of n alkanes, n nitriles, and n alcohols, by picosecond fluorescence depolarization technique. The experimental data were compared with hydrodynamic and quasihydrodynamic models of Gierer Wirtz (GW) and Dote Kivelson Schwartz (DKS). Our studies reveal that these molecules rotated faster in alcohols compared to alkanes and nitriles. A nonlinear variation of τr vs viscosity (η) in alcohols was also observed, being more prominent for the smallest solute. The transition towards the stick hydrodynamic limit with increase in solute size was evident in the chosen set of solute probes. Both GW and DKS theories reproduce the nonlinear variation of τr vs η; however, DKS theory alone correctly predicted the faster rotation in alcohols. A test for quantitative prediction of τr of nonpolar solutes in alkanes and alcohols, using both GW and DKS models ...

Laser‐induced fluorescence spectroscopy of jet cooled p‐aminophenol

The laser‐induced fluorescence spectra of p‐aminophenol both in excitation and emission have been studied in a supersonic jet apparatus. The characterization of the observed spectra was done by comparison with other related substituted anilines and the IR data available in the literature. The excitation spectrum resembles that of aniline with optical activity mainly confined to 6a, 1, and the NH2 inversion mode. In addition, the C–X in‐plane bending mode 9b was also found to be optically active. The 6a mode dominates in most of the dispersed fluorescence spectra and shows a strong Franck–Condon activity. Unlike other similar molecules, the Δv=0 transitions were weak in the single vibronic level fluorescence spectra of 6a1 and 11, which has been qualitatively explained in terms of Franck–Condon analysis. The onset of intramolecular vibrational redistribution occurs at 1135 cm−1, which is much higher than many substituted anilines. The van der Waals complexes viz. p‐aminophenol–Ar1 and p‐aminophenol–Ar2 wer...

Picosecond reorientational dynamics of polar dye probes in binary aqueous mixtures

Picosecond time dependent fluorescence method has been used to measure the rotational reorientation times (τr) of three kinds of dye probes—oxazine 720 (a monocation), nile red (neutral but polar), and resorufin (a monoanion)—in a series of binary mixtures of water–amides, water–dipolar aprotics, and water–alcohols at 298 K. Most of the binary mixtures are characterized by the fact that at a particular composition (between 25% to 40% of the organic solvent in water), the viscosity (η) of the solution reaches a maximum value that is higher than the viscosities of the two solvents. As a consequence, the viscosity profile of the solution as a function of the organic solvent exhibits a bivaluedness, the extent of which is more, if the liquids are nearly isoviscous. The dielectric properties of the solution also change across the composition range. All the dyes show a near linear behavior of τr vs η in formamide–water, N‐methylformamide–water, and methanol–water mixtures. A dual‐valued profile for τr vs η is o...

Diffusion controlled reactions: Experimental verification of the time‐dependent rate equation

The transient effect in a diffusion limited reaction is described by a time‐dependent rate coefficient: k(t)=a+b exp(c2t)erfc(ct1/2), where a, b, and c are expressed in terms of the diffusion coefficient (D), the encounter distance (R), and the absolute rate coefficient (ka); and for ionic reactions, the Onsager length (rc). Time resolved fluorescence quenching studies on cresyl violet–potassium iodide system in water confirm the validity of the above equation. The values of D and R obtained from the fluorescence quenching studies are in good agreement with the values inferred from other sources, and the value of ka is reasonable.

Jet spectroscopy of p-methoxyaniline and intramolecular vibrational relaxation dynamics in the p-alkoxyaniline series

The vibronic spectroscopy of p-methoxyaniline (p-anisidine) in the S1 and S0 states has been studied using laser induced fluorescence in a supersonic jet apparatus. The band origin is found at 31 581 cm−1. Vibrational modes 6a, 6b, 1, I, and 10b are found to be active in the excitation and emission spectrum and their frequencies are 395, 565, 821, 668(v=2), and 499(v=2) cm−1 in the excited state and 426, 642, 845, 472(v=2), and 525(v=2) cm−1 in the ground state, respectively. The intramolecular vibrational relaxation (IVR) dynamics discerned from the single vibronic level fluorescence (SVLF) spectra is compared with p-aminophenol and p-ethoxyaniline, the other two members of the p-alkoxyaniline series. It is postulated that the IVR dynamics in this series is dependent on the changes in the electronic structure in the excited state rather than the increase in the density of states alone.

Effect of substitution on the pyridine ring geometry - a microwave spectroscopic study

Abstract Pyridine ring is affected differently by different substituents for the same substituent the ring distortion depends upon the position of the substituent.