Scientists in both labs made up green-tinctured solutions of each of the 30 molecules in small test tubes and then poked and prodded the solutions by means of analytical techniques to see whether the pigment had aggregated and, if so, how much had formed the assemblies.
Holten's lab studied their absorption of light and their fluorescence (which indicated the presence of monomers, since assemblies don't normally fluoresce) and Bocian's lab studied their vibrational properties, which are determined by the network of bonds in the molecule or pigment aggregate as a whole.
In one crucial test Joseph Springer, a PhD student in Holten's lab, compared the absorption spectrum of a pigment in a polar solvent that would prevent it from self-assembling to the spectrum of the pigment in a nonpolar solvent that would allow the molecules to interact with one another and form assemblies.
"You can see them aggregate," Springer says. "A pigment that is totally in solution is clear, but colored a brilliant green. When it aggregates, the solution becomes a duller green and you can see tiny flecks in the liquid."
The absorption spectra indicated that some pigments formed extensive assemblies and that the steric and electronic properties of the molecules predicted the degree to which they would assemble.
Although this project focused on self-assembly, the PARC scientists already have taken the next step toward a practical solar device.
"With Pratim Biswas, PhD, the Lucy and Stanley Lopata Professor and chair of the Department of Energy, Environmental & Chemical Engineering, we've since demonstrated that we can get the pigments to self-assemble on surfaces, which is the next step in using them to design solar devices," says Holten.
|Contact: Diana Lutz|
Washington University in St. Louis