Fluorescence depolarization to define wobbling motion of oriented molecules and Langmuir-Blodgett films

Abstract

We have formulated a new theory for fluorescence depolarization of rod-like fluorophores embedded in flatly ordered molecular films assuming that the fluorophores wobble in an inclined cone. This theory gives values of the wobbling angle of the fluorophores and the orientation angle, defined as the angle between the averaged molecular axis and the normal to the substrate plane. The data required are the intensity ratios of parallel and perpendicular components of fluorescent light at t = 0 and t → ∞ under vertically polarized excitation light. We have measured the decay of fluorescence emitted from DPH molecules embedded in the stearic acid LB films and have analyzed the results. The DPH molecules are inclined at an average angle of 7–8° to the direction normal to the substrate surface and wobble within 13°, reflecting the states of neighboring stearic acid molecules. The wobbling angle is independent of temperature in the range 5–35°C.