The microscopic viscosity of water-alcohol binary solvents studied by ultrafast spectroscopy utilizing diffusive phenyl ring rotation of malachite green as a probe

The microscopic viscosity of water-alcohol binary solvents was studied by probing the excited state (S1) lifetime of malachite green (MG), the nonradiative decay of which is coupled to the diffusive rotation of the phenyl rings. The lifetime of the S1 state elongated with increasing alcohol mole fraction up to XA∼0.2, while it became insensitive to the mole fraction at higher concentrations. In the range of XA≤0.2, formation of the alcohol clusters caged inside a hydrogen-bond network of water molecules induced an increase in microscopic viscosity with increasing alcohol mole fraction. For XA≥0.3, the MG molecule was almost completely surrounded by the alcohol molecules and the S1 state lifetime was insensitive to the destruction of the solvent structure.