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 excitatio n energy transfer from SR101 to Acid Blue 1 (AB1) at water/carbon tetrachlo ride (CCL4) and water/1,2-dichloroethane (DCE) interfaces were studied by u sing time-resolved total-internal-reflection (TIR) fluorometry, A magic-ang le dependence of the TIR fluorescence dynamic anisotropy revealed that rota tional reorientation of SR101 at the water/CCl4 interface was restricted in the two-dimensional plane of the interface, while that at the water/DCE in terface took place rather freely, similar to reorientation in an isotropic medium. Furthermore, the structural dimension derived fi om the excitation energy transfer dynamics measurements at the water/CCl4 or water/DCE interf ace 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 (simil ar to 10 Angstrom), while the water/DCE interface was relatively rough comp ared to the water/CCl4 interface. Structural dimension analysis of excitati on energy transfer dynamics between dye molecules adsorbed on the water/oil interface measured under the TIR conditions was shown to be a potential me ans to estimate roughness/ thickness of the interface at a molecular level. The present results were also compared with those predicted from molecular dynamics simulations.