Host-guest complexes - Boosting the performance of photosensitizers

In this review, we will show the diversity of supramolecular host-guest complexes of cyclodextrins, cucurbit[n]urils, calix[n]- and pillar[n]arenes with photosensitizers, like porphyrins and phthalocyanines. Host-guest complexes are one of the main building blocks in supramolecular chemistry. For example, they have been widely used to encapsulate hydrophobic drug molecules to enhance the bioavailability in the human body. In these days of multiresistant bacteria and difficulties in cancer therapy, supramolecular host-guest systems with photosensitizers for the photodynamic therapy(PDT) gain more and more interest. In general, photosensitizers with a (large) conjugated aromatic π-system are used, which tend to π-πstacking in aqueous media suppressing the cell toxicity by singletoxygen production quenching. This can be overcome by the formation of host-guest complexes. Besides that, encapsulation of the photosensitizers in host molecules can enhance the solubility, increase cellular uptake, lead to hydrogels, rotaxanes, and switchable systems.