Measurement of the rotational relaxation of octadecylrhodamine B adsorbed at a liquid-liquid interface by time-resolved fluorescence anisotropy underthe total internal-reflection condition

We made an apparatus for measuring the time-resolved fluorescence anisotrop y of a fluorophore at a liquid-liquid interface with polarized incident lig ht under the total internal-reflection condition, s-Polarized and pulsed la ser light (wavelength, 536 nm; pulse width, 1 ns) was irradiated from the t oluene phase to the interface at an angle of incidence of 70 degrees, which is larger than the critical angle (63 degrees) for the interface. The deca ys of s- and p-polarized fluorescences emitted from protonated octadecylrho damine B (C18RB) adsorbed at the toluene/water (0.1 M H2SO4) interface were measured directly from the bottom of a drum-shaped sample cell. From the d ifference between the polarized fluorescence decays, the in-plane rotationa l relaxation of C18RB at the interface could be evaluated. Although C18RB m olecules rotated freely at the interface, the rotating region of a C18RB mo lecule was restricted by the adsorption of the nonionic surfactant (Triton X-100),which was added (2.5 x 10(-8) M) to the aqueous phase. C18RB molecul es rotated at the interface with a time constant of less than 1 ns regardle ss of the co-adsorption of Triton X-100.