Total internal reflection fluorescence dynamic anisotropy of sulforhodamine 101 at a liquid/liquid interface: Rotational reorientation times and interfacial structures

The dynamic anisotropy of Sulforhodamine 101 (SR101) at water/phthalate ester (PE, bis(2-ethylhexyl) phthalate, di-n-heptyl phthalate, di-n-butyl phthalate, or di-n-ethyl phthalate) interfaces was studied by using time- resolved total internal reflection (TIR) fluorometry. A magic-angle dependence of the TIR fluorescence dynamics revealed that rotational reorientation of SR101 at the water/PE interface was restricted in the X-Y plane (in-plane) of the interface. The results indicated that the interface was sharp with respect to the molecular size of SR101 (~10 Å). In-plane rotational reorientation of SR101 at the interface showed two time constants (τ(rot)). The fast component (τ₁(rot)) was similar to that in water irrespective of the nature of PE, while the slow one (τ₂(rot)) was affected by the viscosity of PE but not directly by the macroscopic viscosity. The two rotational reorientation times of SR101 characteristic to the water/PE interface were explained in terms of different adsorption modes of the dye on the interface and the chemical structure of PE itself: mobility of the alkyl chains in PE. Fluorescence dynamic anisotropy under the TIR conditions was shown to be a potential means to study molecular motion of a probe molecule at the water/PE interface as well as chemical/physical characteristics of the interface at a molecular level.