Layered Nanocomposites of Aggregated
Dyes and Inorganic Scaffolding
Thomas Penner*, David Whitten†, and Ileana Place
Center for Photoinduced Charge Transfer, University of Rochester,
Rochester, NY 14627
*Eastman Kodak Company, Rochester, NY 14650
(716) 477-9674
(716) 477-7135 FAX
†Los Alamos National Laboratory
CST-1, MS J565
Los Alamos, NM 87545
(505) 665-6024
(505) 665-4631 FAX
whitten@lanl.gov
The intermolecular self-association of organic dyes under the influence of environmental factors has been recognized and studied for many years. Certain structures can be induced to form highly ordered one and two-dimensional arrays or aggregates in solution under particular conditions of concentration, solvent, ionic strength, and temperature, such that their molecular energy levels are perturbed through dipolar coupling, resulting in drastic changes in photophysical properties such as absorption energy, bandwidth and excited state lifetime. These changes can be understood in terms of molecular excitonic theory, and result in materials that have substantially different properties from the component molecules, in many cases beneficially. In view of the small forces that control the details of the intermolecular packing and the resulting optical properties, it is a desirable goal to be able to influence the course of aggregation and to stabilize it through manipulation of the medium. Adsorption to solid surfaces, particularly of ionic dyes to charged interfaces, is one known technique for achieving this. Both inorganic and organic interfaces have been used for this purpose.
In recent years, the ability to fabricate lamellar structures of organic materials onto a substrate, with layer thicknesses of a few nanometers or less, has extended beyond mechanical techniques such as Langmuir-Blodgett and vacuum deposition to the covalent linking of multifunctional molecules, the layer-by-layer deposition of oppositely charged polyelectrolytes, and the electrostatically-driven buildup of alternate layers of organic polymers and two-dimensional sheets of inorganic minerals such as clays. These techniques allow the fabrication of structures in which two components build a repeating array with nanometer periodicity in a direction normal to the layers, or in which one or both components of the array can be changed in a tailored fashion.
It is our goal to combine these two concepts of spontaneous self-organization of organic dyes into aggregates and the deliberate fabrication of lamellar structures. Choosing the proper inorganic scaffolding material should allow the desired dye aggregate packing to be stabilized on its surface. Then using the build-up approach we will be able to create three-dimensional arrays containing dye aggregates with novel optical behavior which can be changed from layer to layer. This should result in the packaged assembly of multicomposite materials with designed functionality such as photoinduced energy and charge transfer or nonlinear optical properties.
To this end we have been studying the aggregation of organic cyanine dyes onto clay surfaces. We find that with the proper choice of dye structures we can observe a high degree of molecular order of the dye in the presence of the clay manifested by the transformation of its absorption spectrum into a red-shifted narrow intense band exhibiting near-resonant fluorescence ("J" aggregation). We are currently in the process of fabricating multilayer structures of these materials. The particular materials being used in this investigation and the specific structures and properties to be studied should be viewed as demonstrations. But combining the self-organizing order within the organic layer with the stabilizing and scaffolding function of the inorganic layer provides optical features whose properties may be changed by relatively small external stimulus, because of the weak intermolecular forces that cause the aggregate packing and properties.