Dipole orientation distributions in Langmuir-Blodgett films by planar waveguide linear dichroism and fluorescence anisotropy

A method to determine the orientation distribution of fluorescent molecules in a thin, substrate-supported film is described. Attenuated total reflection spectrometry on the surface of a planar waveguide is used to measure absorption LD, from which the mean dipole tilt angle in the film is obtained. Steady-state fluorescence anisotropy is measured in a total internal reflection geometry on a film supported on fused silica but prepared under otherwise identical conditions. The angular distribution about the mean can be recovered from the anisotropy measurement by modeling the distribution as a probability density that is specified by two adjustable parameters. The method was tested on Langmuir-Blodgett (LB) films of arachidic acid doped with the fluorescent amphiphiles DiI and BODIPY. In the DiI-doped films, the mean tilt angle was 75 degrees from the surface normal. Assuming a Gaussian distribution, 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 results show that dipoles in the headgroup region of arachidic acid LB films are more ordered than dipoles in the alkyl chain region. The method should prove useful in studying relationships between assembly technique, structure, and function in two-dimensional molecular arrays.