Inter- and Intralayer Energy Transfer in Zirconium Phosphate−Poly(allylamine hydrochloride) Multilayers: An Efficient Photon Antenna and a Spectroscopic Ruler for Self-Assembled Thin Films

Molecular self-assembly provides a useful synthetic route to many interesting supramolecular structures and devices.1 Covalent2 and ionic3 self-assembly methods have been used to make multilayer thin films with cooperative electronic and optical properties, including electroluminescence,4 secondharmonic generation,5 and photoinduced electron transfer.6 Because these effects depend critically on supramolecular structure, an important goal is to develop simpler and more reliable techniques for juxtaposing the active componentssphotoactive and electroactive moleculessin welldefined structural arrangements. Another ongoing challenge is the development of better tools for characterization of these assemblies on the nanometer length scale. Probes such as ellipsometry, X-ray diffraction, and UV-vis spectroscopy provide information that is averaged over macroscopic dimensions and are often unable to differentiate between possible structures. Ionic multilayer films made from organic polyelectrolytes contain oppositely charged polymer strands that intertwine, and there are no sharp boundaries between sequentially grown layers.7 Related thin films made from two-dimensional inorganic polyanions such as clays8 and R-zirconium phosphate (RZrP)9 are thought to be structurally analogous to ionic intercalation compounds, in which the stratification of inorganic and organic components is well defined. It is not known, however, whether the contents of these layers are mixed in the adsorption process. Each adsorption step appears to produce a single monolayer in some cases9 and multiple layers in others.8 In this paper, we report a study of R-ZrP/poly(allylamine hydrochloride) (PAH) thin films, in which energy transfer between fluorescent probes serves as a “spectroscopic ruler”. A comparison of theoretical and experimental energy transfer efficiencies for many different layer sequences shows that there is little or no bilayer formation or interlayer mixing during the adsorption process. These results underscore the usefulness of this surface assembly technique for device applications. Specifically, this study shows that it is possible to construct solid state photon antenna composites with energy transfer efficiencies approaching unity. The polycation PAH was tagged with fluorescein (Fl) and rhodamine B (RhB) using modified Schotten-Bauman conditions.10 UV-vis absorption spectra gave chromophore loadings of ca. 1:20 and 1:50 for the Fl-PAH and RhB-PAH polymers, respectively. Sequential adsorption of these polymers and colloidal R-ZrP onto glass slides, using conditions similar to those described previously for R-ZrP/PAH films,9,11 resulted in polymer coverages of (3-4) × 10-10 equiv/cm2 on each side of the glass and ellipsometric film thicknesses (measured on silicon substrates) of 22 A per anion-cation layer pair. The increase in thickness of these bilayers relative to R-ZrP/PAH (14-16 A per layer pair) reflects the size of the probe molecules and possibly their effect on the coiling of the polymers at low ionic strength. There was no observable birefringence in the absorption spectra of these films, suggesting random chromophore orientation within the layers.