Effect of temperature and viscosity on rotational diffusion of merocyanine 540 in polar solvents

David R. Bessire, Edward L. Quitevis

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The rotational diffusion time, τrot, of merocyanine 540 (MC540) in the excited state was inferred from steady-state fluorescence anisotropy measurements as a function of temperature T and viscosity η in n-alkyl alcohol and n-alkanenitrile solvents. The rotational diffusion of MC540 is well described by the Debye-Stokes-Einstein (DSE) equation, with τrot varying linearly with η/T. Within experimental error, the slopes of plots of τrot vs η/T for solvents in a homologous series are equal. Taking weighted averages, we obtain a slope of 87 ± 9 ns K/cP for the alcohols and a slope of 117 ± 15 ns K/cP for the nitriles. These values are greater than the predicted value of 73 ns K/cP, based on the assumption that MC540 rotates as a prolate ellipsoid with a volume equal to its van der Waals volume. From the temperature-dependent data, we find that the rotational activation energy is greater than the viscosity activation energy. These results cannot be rationalized by the continuum dielectric friction model. We propose instead a quasi-hydrodynamic model in which the larger slope values are associated either with a local viscosity which is larger than the bulk viscosity or with changing boundary conditions. This quasi-hydrodynamic situation originates from specific solute-solvent interactions with the zwitterionic state of the dye which lead to enhanced solute-solvent coupling in the excited state of the dye. The larger slope in the nitriles is ascribed to greater solvation of MC540 in nitrile solvents than in alcohol solvents, as evidenced by the Stokes shift data.

Original languageEnglish
Pages (from-to)13083-13092
Number of pages10
JournalJournal of physical chemistry
Issue number49
StatePublished - 1994


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