Excitation energy transfer between molecular thin layers of poly(phenylene vinylene) and dye labeled poly(allylamine) in layer-by-layer self-assembled films

Steady state Förster energy transfer is investigated in multilayer structures of self-assembled films prepared via the layer-by-layer deposition of oppositely charged polyelectrolytes. The samples consist of a single layer of the conjugated polymer poly(p- phenylene vinylene) (PPV) as a donor, several layers of poly(styrene sulfonate) (PSS), and poly(allylamine hydrochloride) (PAH) as a transparent spacer, and a single layer of dye labeled PAH as an acceptor. The dyes are rhodamine B and fluorescein, whereas the molar ratio with respect to the PAH monomers is less than 0.2% for both systems. The steady state fluorescence intensity of the PPV is measured for a series of samples with different spacer thicknesses. The spacer thicknesses are evaluated from x-ray reflectivity measurements. At large thickness of the spacer the donor intensity I versus distance d follows the 1/(1+(d0/d)4) law as theoretically predicted for the case of ideal two-dimensional layers. At short distances a different behavior is observed which is explained first, by the low lateral density of the acceptor molecules and second, by a distribution of the dyes normal to the layer plane. Good agreement between the experimental data and computer simulations is obtained under the assumption of a Gaussian distribution of both donor and acceptor molecules, in the direction normal to the film plane with a width of 25–30 Å. However, a mismatch is observed between the Förster radius derived from the spectroscopic properties and that obtained from the layer-to-layer energy transfer. This is discussed under the consideration of a nonperfect layer structure and the photophysics of the PPV.