Turn-Dependent Folding and Contrasting Guest Encapsulation Behavior of <i>Tris</i> -Porphyrin Oligomers

We report the design and synthesis of two <i>tris</i>-porphyrin oligomers, <b>1</b> and <b>2</b>, incorporating phenanthroline and pyridine turn units, respectively, demonstrating their critical role in controlling the folding behavior of the oligomers and, consequently, guest binding ability. Oligomer <b>1</b> adopts a folded conformation stabilized by intramolecular hydrogen bonds between the endocyclic nitrogens of the phenanthroline units and the amide-<i>NH</i>s, leading to a cofacial arrangement of the porphyrins. It resulted in the formation of two cleft-like cavities that encapsulate electron-deficient planar guests having binding constants up to 10⁶ M^-1 in a 1:2 stoichiometry. In contrast, oligomer <b>2</b> remains in the open state, despite intramolecular H-bonding, due to steric interactions between adjacent sheets, and exhibits negligible nonspecific interactions with the guests. It is evident from the nuclear magnetic resonance titration that oligomer <b>1</b> shows a strong binding toward guests <b>G1</b>-<b>G3</b>, with the first binding event more feasible than the second, while guests <b>G4</b>-<b>G6</b> show weak interactions. A combination of experimental investigations, density functional theory calculations, and 1 μs all-atom molecular dynamic simulations provides further insights into the structural aspects of the folded architecture and the guest recognition behavior.