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-pr-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 Tm 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 (similar to 10 Angstrom). In-plane rotational reorientation of SR101 at the interface showed two time constants (tau(rot). The fast component tau(1)(rot)) was similar to that in water irrespective of the nature of PE, while the slow one (tau(2)(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.