Self-Assembly of Octanuclear Pt/Pd Coordination Barrels and Uncommon Structural Isomerization of a Photochromic Guest in Molecular Space.

Two tetragonal molecular barrels and were successfully synthesized by coordination-driven self-assembly of a tetrapyridyl donor () of the thiazolo[5,4-]thiazole backbone with -blocked 90° Pd(II) and Pt(II) acceptors, respectively. The single-crystal structure analysis of revealed the formation of a two-face opened tetragonal Pd molecular barrel architecture. In contrast, the isostructural Pt(II) barrel () is water-soluble. The large confined hydrophobic molecular cavity including wide open windows and good water solubility of the barrel made it a potential molecular container for the encapsulation of guests with different sizes and properties. This has been exploited to encapsulate and stabilize the open form of a photochromic molecule () in water, while the same photochromic molecule exists exclusively in a cyclic zwitterionic form in aqueous medium in the absence of the barrel . This cyclic form is very stable in water and does not go back to its parent open form under common external stimuli. Surprisingly, reverse switching of the cyclic form to a colored hydrophobic open form was also possible instantly in water upon addition of the solid barrel into an aqueous solution of . Such a fast reverse isomerization of an irreversible process in aqueous medium by utilizing host-guest interaction of the barrel and the guest is interesting. The barrel was also capable of encapsulating the water-insoluble radical initiator in aqueous medium.