Targeting the preparation of 0D cages and 1D polymers based on co-facial porphyrins a variety of tetra- and octa- topic porphyrins P and di- topic linkers L decorated with carboxylic acid, aldehydes, amines, and amidines were synthetized.
Unfortunately, all the attempts to produce 1D polymers resulted in insoluble materials making it impossible to continue the project in this sense. Therefore, this fact produced a deviation from the original objectives, and all the efforts were then focused on 0D porphyrin cages.
Regarding the type of interaction that drives the self-assembly process the cages prepared can be classified as:
i) Amidinium-carboxylated 0D cages, assembled by H-bond interactions between complementary tetra-carboxylic acid P and bis-amidine L, that were soluble in both organic and aqueous media by varying the solubilizing groups. These cages have an accessible active site able to encapsulate guest molecules in very different solvent environments.
ii) NC imine 0D cages formed by imine condensation reaction, that is a dynamic covalent reaction, between tetra-amine Ps and bis-aldehyde Ls, and iii) CN imine 0D cages prepared by imine condensation reaction between tetra-aldehyde Ps and bis-amine Ls. The imine-based molecular cages ii) and iii) have been widely studied and manifold modifications have been introduced in their structure to achieve different objectives.
The characterization of the “original” cages revealed that from the two possible conformations, which depend on the alternance of the imine bonds, only the extended one was observed in solution experiments as well as in the solid state (single X-ray diffraction). To explore the ability of these cages to encapsulate molecules into its inner cavity, host-guest studies were performed for both NC and CN cages, concluding that due to the fixed porphyrin-porphyrin distance, the cage shown selectivity for the encapsulation of guests in the function of their size. In addition, some guest molecules were able to trigger a conformational change from extended to compact conformation.
To control the conformational equilibria slight modifications either in the structure of the porphyrin or the linker were introduced. For instance, the molecular cage CN connected by pyridine linkers allowed to switch from the extended to the compact conformation by the coordination of metal cations to the pyridine N and both imine N. Interestingly, the encapsulation of guest molecules experimented with an allosteric effect where the metal cation acts as an effector molecule.
Furthermore, by introducing hydroxyl groups at the porphyrin for the cage CN, and at the linker for the cage NC led to synthetic conformational control achieving exclusively the compact conformation.
Additionally, chiral induction was achieved by adding chiral host molecules to the “original cages” or introducing chiral moieties in the porphyrin or the linker. Finally, the imine cages are being tested as catalysts for epoxide ring-opening reactions catalyzed by metallic Lewis-acid centers.
These results have been disseminated in National and International conferences by the researchers involved, and the main results are being collected in 5 manuscripts aiming to be published in high-impact factor journals.
