Layered Nanocomposites of Aggregated Dyes and Inorganic Scaffolding

Abstract

Layer-by-layer thin films were prepared by the alternate deposition of clay particles and ionic organic compounds from solution onto a rigid support. The organic component comprised a monomeric cyanine dye or poly-l-lysine with the same cyanine chromophore appended to it, a dye polymer. Physical measurements indicated that layers of the organic and inorganic materials could be deposited alternately in a repeated fashion with each layer only one to two nanometers thick. Within the film structure the cyanine chromophore exhibits J-aggregate spectroscopic properties, either as the result of adsorption to the clay in the case of the monomeric dye or because it preexisted in this form in the case of the dye polymer. This composite assembly can potentially serve as a model for a light-harvesting photoantenna system. We attempted to demonstrate that by capping the multilayer film with another layer of a different cyanine dye, whose J aggregate absorption band overlapped the fluorescence wavelength of the dye incorporated into the film, acting as an excitation energy donor. Efficient energy transfer was demonstrated for films containing no more than four dye layers for the monomeric donor dye or six layers for the polymeric donor dye. However, close examination of the absorption spectra of the donor−acceptor systems reveals that an interaction appears to occur in the assembly that results in excess acceptor dye being incorporated into the film. This raises the possibility that the acceptor dye penetrates into the film and mixes into the donor dye layers, calling into question whether long-range energy transfer occurs due to an antenna effect.