Photophysical Characterization of Porphyrinic Covalent Cages and Their Ag(I) Complexes

Functional systems based on molecular capsules have shown a great potential as nanoreactors, molecular recognitions systems or drug carriers since they provide a confined environment that enhances molecular reactivity and catalysis. The components that define these functional systems are not only structural, constituting the three-dimensional hollow space, but active species can be selected to shape the activity within the cavity.1a One manner of controlling the binding and delivery of a guest molecule is to incorporate constituents of the molecular capsule that can respond to an external stimulus by provoking a conformational change that modifies the host encapsulation properties. Among the possibilities of designing molecular cages, metalated or free-base porphyrins are attractive due to their chemical stability provided by covalent structures and the expected ?-? interactions or coordination bonds with guest molecules.1b Our previous studies on flexible bis-porphyrin covalent cages incorporating either Zn metalated or free-base porphyrins, linked by 1,2,3-triazolyl ligands, showed that the addition of silver(I) ions, which bind to the peripheral ligands, opens the flattened structures in solution and locks the two porphyrins in a face-to-face disposition.2 Here we report on the photophysical characterization of covalent porphyrinic cages formed by either two free-base porphyrins or by one free-base and one Zn(II) porphyrin (Figure 1). The effect of coordination of silver(I) ions to the peripheral triazole ligands and the spectrophotometric and spectrofluorimetric analysis of the binding processes is discussed.