Turn-Dependent Folding and Contrasting Guest Encapsulation Behavior of Tris-Porphyrin Oligomers

We report the design and synthesis of two tris-porphyrin oligomers, 1 and 2, 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 1 adopts a folded conformation stabilized by intramolecular hydrogen bonds between the endocyclic nitrogens of the phenanthroline units and the amide-NHs, 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 2 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 1 shows a strong binding toward guests G1-G3, with the first binding event more feasible than the second, while guests G4-G6 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.