Energy Transfer between Conjugated Polyelectrolytes in Layer-by-Layer Assembled Films

We present a study of Forster resonance energy transfer (FRET) between two emissive conjugated polyelectrolytes (CPEs) in layer-by-layer (LbL) self-assembled films as a means of examining their organization and architecture. The two CPEs are a carboxylic acid functionalized polyfluorene (PFl-CO2) and thienylene linked poly(phenylene ethynylene) (PPE-Th-CO2). The PFl-CO2 presents a maximum emission at 418 nm, while the PPE-Th-CO2 has an absorption lambda(max) centered at 431 nm, in sufficient proximity for effective FRET. Several LbL films have been constructed using varied concentrations of the deposition solutions and identity of the buffer layers separating the two emissive layers, using a system of either weak polyelectrolytes, poly(allylamine hydrochloride) (PAH)/poly(sodium methacrylate) (PMA), or strong polyelectrolytes, poly(diallylammonium chloride) (PDDA)/poly(styrene sulfonate) sodium (PSS). The efficiency of FRET has been monitored using fluorescence spectroscopy. Initially, the fluorescence of the PFl-CO2 (E-g similar to 3.0 eV), which emits at 420 nm, is quenched by the lower band gap PPE-Th-CO2 (E-g similar to 2.5 eV). For films using the PAH/PMA system as buffer bilayers and deposited from 1 mM solutions, the PFl-CO2 fluorescence is progressively recovered as the number of intervening buffer bilayers is increased. Ellipsometry measurements: indicate that energy transfer between the two emissive layers is efficient to a distance of ca. 7 nm.