Special Seminar: Tuning the excitonic properties of chromophore aggregates across the visible and shortwave infrared

In extended chromophore self-assemblies, excitations on each molecule Coulombically interact over long distances, forming collective excited states called delocalized Frenkel excitons. The resulting excitonic properties are drastically different from the monomeric chromophores, and depend on the geometric packing arrangements, topology as well as (structural and energetic) disorder within the extended aggregates.  For linear aggregates, co-facial or head-to-tail packing of chromophores leads to blue or red shifts in the optical spectra, known as H- and J- aggregates, respectively. In this talk, I will discuss 2-dimensional sheet-like and tubular aggregates obtained from solution state self-assembly of cyanine dyes, introducing the concept of I-aggregates that show intermediate properties of H- and J-aggregates. I will show how different chromophore packings can be afforded using supramolecular handles and how these packings affect the excitonic band structures, availing strong absorption and emission across a broad spectral region from visible through shortwave infrared (SWIR, 1000-2000 nm). Finally, I will discuss the canonical example of a double-walled nanotubular system that shows micron-scale exciton delocalization and high super radiance. We obtain a high-resolution native-state structure of the nanotubes, directly observing the chromophore packings that control its excitonic properties for the first time.