DIPOLE ORIENTATION DISTRIBUTIONS IN LANGMUIR-BLODGETT-FILMS BY PLANARWAVE-GUIDE LINEAR DICHROISM AND FLUORESCENCE ANISOTROPY

A method to determine the orientation distribution of fluorescent mole cules in a thin, substrate-supported film is described. Attenuated tot al reflection spectrometry on the surface of a planar waveguide is use d to measure absorption LD, from which the mean dipole tilt angle in t he film is obtained. Steady-state fluorescence anisotropy is measured in a total internal reflection geometry on a film supported on fused s ilica but prepared under otherwise identical conditions. The angular d istribution about the mean can be recovered from the anisotropy measur ement by modeling the distribution as a probability density that is sp ecified by two adjustable parameters. The method was tested on Langmui r-Blodgett (LB) films of arachidic acid doped with the fluorescent amp hiphiles DiI and BODIPY. In the DiI-doped films, the mean tilt angle w as 75 degrees from the surface normal. Assuming a Gaussian distributio n, the standard deviation was 12 degrees, indicating a high degree of macroscopic order. In the BODIPY-doped films the distribution was 59 /- 17 degrees, which indicates a less ordered assembly. Larger angular distributions were calculated using a step function model. The result s show that dipoles in the headgroup region of arachidic acid LB films are more ordered than dipoles in the alkyl chain region. The method s hould prove useful in studying relationships between assembly techniqu e, structure, and function in two-dimensional molecular arrays.