Forster energy transfer studies of polyelectrolyte heterostructures containing conjugated polymers: A means to estimate layer interpenetration

Using a sequential adsorption process, thin film multilayer assemblies of p olymers which photophysically interact via the Forster energy transfer mech anism have been fabricated and characterized in order to determine the leve l of interpenetration between layers. The assemblies consisted of layers of poly(phenylene vinylene) (PPV) which were separated from layers of a sulfo natopropoxy anion derivatized poly(p-phenylene) [(-)PPP] by nonconjugated p olyelectrolyte spacer bilayers. The spacer bilayers were composed of poly(a llylamine hydrochloride) (PAH) with a polyanion of either poly(acrylic acid ) (PAA), poly(methacrylic acid) (PMA), or poly(styrenesulfonate) (PSS). An estimate of the level of interpenetration of the layers was made for each t ype of spacer bilayer by correlating the relative amount of quenching of th e (-)PPP photoluminescence with the measured total thickness of the spacer bilayer(s) utilizing a diffuse layer model which assumed a Gaussian distrib ution of polymer segments. Using this approach, the level of interpenetrati on for the assemblies with the PMA/PAH spacer bilayers was estimated to be between 15 and 53 Angstrom (1-2.5 bilayers). The heterostructure assembly w hich used spacer bilayers of PAA/PAH demonstrated that one sufficiently thi ck bilayer (greater than or equal to 57 Angstrom) could prevent the energy transfer from the(-)PPP to the PPV. The failure of the (-)PPP photoluminesc ence to be fully restored even with eight spacer bilayers (> 53 Angstrom) f or the assemblies containing PSS/PAH spacer bilayers indicated that, for th e processing conditions used, significant layer mixing was obtained. Overal l, this work demonstrated that nonradiative energy transfer offers a valuab le tool for probing the internal structure of sequentially adsorbed polyele ctrolyte films and that the level of interpenetration appears to be depende nt upon the system being examined.