EFFECT OF TEMPERATURE AND VISCOSITY ON ROTATIONAL DIFFUSION OF MEROCYANINE-540 IN POLAR-SOLVENTS

The rotational diffusion time, tau(rot), of merocyanine 540 (MC540) in the excited state was inferred from steady-state fluorescence anisotr opy measurements as a function of temperature T and viscosity eta in n -alkyl alcohol and n-alkanenitrile solvents. The rotational diffusion of MC540 is well described by the Debye-Stokes-Einstein (DSE) equation , with tau(rot) varying linearly with eta/T. Within experimental error , the slopes of plots of tau(rot) vs eta/T for solvents in a homologou s 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 t he nitriles. These values are greater than the predicted value of 73 n s K/cP, based on the assumption that MC540 rotates as a prolate ellips oid with a volume equal to its van der Waals volume. From the temperat ure-dependent data we find that the rotational activation energy is gr eater than the viscosity activation energy. These results cannot be ra tionalized by the continuum dielectric friction model. We propose inst ead al quasi-hydrodynamic model in which the larger slope values are a ssociated either with a local viscosity which is larger than the bulk viscosity or with changing boundary conditions. This quasi-hydrodynami c situation originates from specific solute-solvent interactions with the zwitterionic state of the dye which lead to enhanced solute-solven t coupling in the excited state of the dye. The larger slope in the ni triles is ascribed to greater solvation of MC540 in nitrile solvents t han in alcohol solvents, as evidenced by the Stokes shift data.