Determining the orientation and quantum efficiency of dye molecules

Abstract

The use of microcavities to control the emission of light is well understood and is used in such areas as polymer lasers, organic light emitting diodes etc I, and to observe phenomena such as strong coupling. Here we make use of microcavity effects to determine the orientation and quantum efficiency of the emissive dipole moments associated .with dye molecules. Whilst we have used a Rhodamine dye in these experiments the technique is a general one and should be applicable to a wide range of emissive materials. The fluorescence lifetime of a dye molecule is dependent on the optical environment in which it resides. For example, a dye molecule situated in the middle of a wavelength scale microcavity may have its emission enhanced if it is resonant with a microcavity mode (its lifetime will thus be reduced) or inhibited if there is no microcavity to mode to which it may couple at the emission frequency (its lifetime will thus be extended). By placing our molecules at well defined positions within two types of microcavity structure we were able to measure such changes in fluorescence lifetime and use them to determine both the quantum yield and the orientation of the dipole moments. Schematic cross-sections of the two structures are shown in the figures below. The first structure, shown in the inset of figure 1, consists of a silver mirror, thermally deposited on a planar silica substrate, forming one side of the microcavity. We then deposited a staircase structure of inert organic monolayers using the Langmuir-Blodgett (LB) technique. This structure was then coated with a bi-layer of Rhodamine 700 (R700) dye mole,cules and the whole structure finally capped with a further bi-layer of inert molecules to form a half-cavity structure. This structure thus replicates the original work of Drexhage ’. The measured fluorescence lifetime as a function of distance between the silver mirror and the dye molecules is shown in figure I. The lifetime is seen to oscillate with distance from the mirror and the amplitude of the oscillation is largely determined by the quantum efficiency of the molecules. By fitting theory to our data we determined the quantum efficiency of the R700 molecules to be 0.4+0.05. The second structure, shown in the inset of figure 2, is similar, save for the fact that now the silver-R700 distance is kept fixed and the thickness of the capping layer is varied. We have found that the dependence of the fluorescence lifetime on overlayer thickness is very sensitive to dipole orientation in this configuration. By fitting theory to our data we determined that our molecules have emissive dipole moments that lie in the plane of the structure. We will discuss the use of this technique in determining spontaneous emission parameters and look at the impact dipole orientation has on the operation of organic light-emitting diodes.