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 macrocycles give rise to aggregates featuring supramolecular chirality with high ellipticity. Kinetic studies reveal peculiar differences in the fashion of aggregation, depending on the intimate nature of the chiral functionality, namely, cationic (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 interchromophore 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 templated 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 stereoselective sensors and devices.