Chemically Induced Breathing of Flexible Porphyrinic Covalent Cages

The synthesis of two flexible bis-porphyrin cages 3 and 4, incorporating respectively Zn(II) porphyrins and free-base porphyrins, is reported. In both cages, the four covalent linkers that bind the two porphyrins are functionalized with two 1,2,3-triazolyl ligands. These cages were characterized by NMR and HRMS, and for cage 3 incorporating 1,4-diazabicyclo[2.2.2]octane (DABCO), an X-ray crystallographic structure was obtained. Chemically induced conformational changes are studied and compared to those of two related cages with longer flexible linkers. Binding of four silver(I) ions to the peripheral ligands opens the flattened structures in solution and locks the two porphyrins in a face-to-face disposition. Addition of an excess of acid fully expands the cages due to electrostatic repulsion between the positively charged sites. These two reversible processes allow for a chemically induced breathing of the flexible structures.