Supramolecular chirality in solvent-promoted aggregation of amphiphilic porphyrin derivatives: Kinetic studies and comparison between solution behavior and solid-state morphology by AFM topography

The solvent-promoted aggregation behavior of some amphiphilic porphyrin derivatives bearing chiral functionality in the form of a charged L-proline group has been investigated by UV/Vis, resonance light scattering, fluorescence and circular dichroism spectroscopy. The investigated macro- cycles give rise to aggregates featuring supramolecular chirality with high el- lipticity. Kinetic studies reveal peculiar differences in the fashion of aggregation, depending on the intimate nature of the chiral functionality, namely, cat- ionic (nitrogen-quaternized L-proline, 3H2) or anionic (carboxylate residue, 6H2) group. Formation of anionic 6H2 aggregates shows a diffusion-limited kinetic behavior. AFM topography studies show formation of tighter globular structures. On the other hand, the corresponding 3H2 aggregates are formed by a cooperative, fractal-type decay, and appear as long-fibrous, looser structures. In the templated aggregation of 3H2 over preformed 6H2 aggregates, AFM images show formation of globular structures with reduced sizes, as a likely consequence of shorter in- terchromophore distances, due to favorable Coulombic interactions. The results obtained show an interesting parallelism between the solution behavior and the solid-state aggregate structures, corroborating the sergeant- soldier effect observed in the templat- ed aggregation. The results presented give important insights for understanding the complex mechanisms involved in these issues, which are of key importance for the development of chiral supramolecular materials and stereose- lective sensors and devices. ©2010 Wiley-VCH Verlag GmbH & Co. KGaA.