Porphyrinic covalent cages and their Ag(I) complexes as size-tunable molecular containers: a photophysical study

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 units that compose these functional systems are structural, constituting the three-dimensional hollow space, but they can also define the host-guest properties within the cavity.1 One way of controlling the encapsulation and delivery of a guest molecule is to incorporate constituents of the molecular capsules which 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 their expected ?-? interactions or coordination bonds with guest molecules.2 Here we report on the photophysical characterisation of covalent porphyrinic cages formed by either two free-base porphyrins or by one free-base and one Zn(II) porphyrin, presenting different lengths in the lateral linkers (Figure 1). The photophysical properties of the cages are discussed in relation to their different composition and structure. The addition of silver(I) ions, which bind to the peripheral ligands, is expected to open the flattened structures in solution and lock the two porphyrins in a face-to-face disposition.3,4 The spectrophotometric and spectrofluorimetric analysis of the binding processes is here presented and discussed, as well as the photophysical characterisation of the complexed cages.