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WM Keck Science

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The disappearance of the exciting-wavelength dependence of the phosphorescence spectra of polar, aromatic chromophores in supercooled glycol–water mixtures is utilized to monitor the kinetics of solvent reorientation. Reorientation times in the nanosecond to second range are obtained for (3:2 v/v) glycerol–water and (1:1 v/v) ethylene glycol–water at 140–240 °K. The results suggest that the process is one involving a cluster of solvent molecules and in which the chromophore plays a relatively passive role. Steady-state data and direct measurements of phosphorescence shifts as a function of time indicate that the solvent reorientation process is nonexponential in nature. The decay function derived from the temperature dependence of the steady state data is consistent with the decays observed directly as a function of time. Interpretation of this nonexponential decay in terms of a summation of rate processes leads to a distribution dominated by two reorientation rate constants. The relative contributions of the slow and faster reorientation rate constants in addition to their activation parameters differ for the glycerol–water and ethylene glycol–water mixtures.


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