Excitation energy transfer from sulforhodamine 101 to Acid Blue 1 at a liquid/liquid interface: Experimental approach to estimate interfacial roughness

Dynamic fluorescence anisotropy of Sulforhodamine 101 (SR101) and excitation energy transfer from SR101 to Acid Blue 1 (AB1) at water/carbon tetrachloride (CCl₄) and water/1,2-dichloroethane (DCE) interfaces were studied by using time-resolved total-internal-reflection (TIR) fluorometry. A magic-angle dependence of the TIR fluorescence dynamic anisotropy revealed that rotational reorientation of SR101 at the water/CCl₄ interface was restricted in the two-dimensional plane of the interface, while that at the water/DCE interface took place rather freely, similar to reorientation in an isotropic medium. Furthermore, the structural dimension derived from the excitation energy transfer dynamics measurements at the water/CCl₄ or water/DCE interface was 1.93 or 2.48, respectively. These results indicated that the water/CCl₄ interface was sharp with respect to the molecular size of SR101 (~10 Å), while the water/DCE interface was relatively rough compared to the water/CCl₄ interface. Structural dimension analysis of excitation energy transfer dynamics between dye molecules adsorbed on the water/oil interface measured under the TIR conditions was shown to be a potential means to estimate roughness/thickness of the interface at a molecular level. The present results were also compared with those predicted from molecular dynamics simulations.