Host-Guest Systems Based on Silver(I)-Complexed Bis-porphyrin Cages

The design and study of molecular cages and capsules represents an attractive field of research with important applications in catalysis and biomedicine.1a These structures are able to stabilize guest molecules according to their size, shape, and affinity for the host. The encapsulation process is at the basis of the functionality of the molecular container, that can work as a nanoreceptor or nanoreactor for sensing or drug delivery appilcations. Moreover, the capsule can respond to external stimuli such as chemicals or light. Porphyrins are fascinating components of photo-active molecular cages, thanks to their rich photophysics and their mimicry of natural pigments.1b Host-guest systems based on porphyrin containers have shown great potential for driving photo-induced processes such as collection and migration of electronic energy and charge separation.2 Modulation of the receptor activity through controlled conformational changes remains a significant challenge for the development of switchable receptors. In our recent work we studied bis-porphyrin cages with flexible linkers, showing that the distance between the porphyrins can be increased by silver(I) coordination to the peripheral triazoles.3a Silver(I) ions can thus act as chemical triggers, upon binding to the cages, for the encapsulation of guest molecules. Here we report on the encapsulation of a flat aromatic guest (N,N'-dibutyl-1,4,5,8-naphthalene diimide, NDI) into the cavity of two bis-Zn(II) porphyrins cages, differing in the length of the linkers, but fixed in a face-to-face disposition through the coordination of four silver(I) cations. The study has been performed by means of absorption and emission spectroscopy.3b The photoinduced processes occurring in the assembled systems were identified by means of detailed photophysical investigations supported by MD-PMM calculations.