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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
(τ
r
) 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.
Dedicated to Professor M.I. Savadatti on his 77th Birthday. 相似文献
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