J.R. Mannekutla

Fluorescence Quenching of UVITEX‐OB by Aniline in Alcohols and Alkanes

Abstract Fluorescence quenching of UVITEX‐OB [2,5‐thiophenediylbis(5‐tert‐butyl‐1,3‐benzoxazole)] by aniline in different polar and nonpolar solvents was examined at room temperature by steady‐state fluorescence measurements. Positive deviations from the nonlinear Stern–Volmer plots were observed in most of the solvents indicating the extent of quenching to be large. The quencher concentration dependence data were analyzed using ground‐state complex and sphere of action static quenching models in order to interpret the results. The magnitudes of the quenching rate parameters suggest that a sphere of action static quenching model is expected to describe the data most accurately. Also, the results are suggestive of both static and dynamic quenching processes being responsible for the observed positive deviation in the Stern–Volmer plot. Experimental results are described by an equation derived using the finite sink approximation model, which allows the evaluation of diffusion‐limited interaction and the estimation of encounter distance and mutual diffusion coefficient independently.

Rotational Diffusion of Coumarins in Alcohols: A Dielectric Friction Study

The rotational dynamics of three structurally similar polar molecules viz., coumarin 440, coumarin 151 and coumarin 450 has been studied in alcohols at room temperature using steady-state fluorescence depolarization method and time correlated single photon counting technique. The observed reorientation times of all the three probes are found to be longer than those predicted by Stokes–Einstein–Debye (SED) hydrodynamic theory with stick boundary condition and a deviation towards super-stick behavior is noted. Dielectric friction theories of Nee–Zwanzig and van der Zwan–Hynes, which treat the solute as a point dipole, overestimate the dielectric friction contribution exhibited by all the three coumarins in alcohols. Results are discussed in the light of theoretical models and the possibility of hydrogen bonding between the amino group of the probe molecules and the hydroxyl group of the alcohols.

Rotational Diffusion of Coumarins: A Dielectric Friction Study

The rotational diffusion of three probes: coumarin 522B (C522B), coumarin 307 (C307) and coumarin 138 (C138) with nearly identical size was studied at room temperature employing steady-state and time-resolved fluorescence anisotropy techniques in series of alcohols and alkanes. Experimental observations indicate faster rotation of C138 compared to the other two dyes in alcohols and a faster rotation of C522B than C307 in alkanes. The dielectric friction theories of Nee-Zwanzig (NZ) and van der Zwan-Hynes (ZH) were employed to estimate the friction experienced by the probes in alcohols, in addition to the mechanical friction calculated using Stokes-Einstein-Debye (SED) hydrodynamic with slip boundary condition and Dote-Kivelson-Schwartz (DKS) quasihydrodynamic theories. The observed reorientation times for the three probes do not follow the trend predicted by dielectric friction theories of NZ and ZH. The dipole moments determined from solvatochromic techniques were found to be different for the three probes.

Rotational Diffusion of Coumarins in Aqueous DMSO

The rotational dynamics of four structurally similar polar molecules viz., coumarin 440, coumarin 450, coumarin 466 and coumarin 151 has been studied in binary mixtures comprising of dimethyl sulphoxide and water at room temperature using the steady state fluorescence depolarization method and time correlated single photon counting technique. The binary mixtures are characterized by the fact that at a particular composition the viscosity (η) of the solution reaches a maximum value that is higher than the viscosities of either of the two co-solvents. The dielectric properties of the solution change across the composition range and the qualitative features of the solvent relaxation dynamics in complex systems are known to differ from those in simple solutions. A hook type profile of rotational reorientation time (τr) vs viscosity (η) is obtained for all the solutes in dipolar aprotic mixture of dimethyl sulphoxide-water, with the rotational reorientation times being longer in organic solvent-rich zone, compared to the corresponding isoviscous point in water-rich zone due to strong hydrogen bonding. Fluorescence lifetimes as well as rotational reorientation times are sensitive to the composition of the binary solvent system under study than to the viscosity suggesting the importance of local structure. The results are discussed in the light of hydrodynamic and dielectric friction models.