Picosecond ground-state rotational diffusion of merocyanine 540 in polar solvents

The ground-state rotational diffusion of merocyanine 540 in polar solvents was studied by using picosecond polarized transient bleaching. The rotational diffusion time τ_rot was obtained from fits of a single exponential to the absorption anisotropy. The value of τ_rot was 78 ± 5 ps in acetone, 130 ± 24 ps in methanol, 190 ± 19 ps in dimethylformamide, 270 ± 30 ps in ethanol, 490 ± 50 ps in 1-propanol, 530 ± 13 ps in dimethyl sulfoxide, and 700 ± 130 ps in 1-butanol. The dependence of the observed rotational diffusion time on the solvent's shear viscosity was the same in protic and aprotic solvents. The reorientational dynamics of this dye in these solvents can be explained by hydrodynamics for a prolate ellipsoid using stick boundary conditions. The absence of effects due to hydrogen bonding and dielectric friction in alcohol solvents is discussed.