Cucurbit[7]uril Enables Multi-Stimuli-Responsive Release from the Self-Assembled Hydrophobic Phase of a Metal Organic Polyhedron

Mixed self-assembly of ligands 1, 2, 1,6-hexanediamine (HDA), and Pd(NO₃)₂ afforded Fujita-type metal organic polyhedron MOP1 (diameter ≈ 8.2 nm), which is covalently functionalized with an average of 18 cucurbit[7]uril (CB[7]) units, as evidenced by ¹H NMR, diffusion-ordered spectroscopy NMR, and transmission electron microscopy measurements. By virtue of the host-guest properties of CB[7], the inner cavity of MOP can be rendered hydrophobic by using octadecyl HDA (3) as guest during the self-assembly process. The hydrophobic cavity was successfully utilized to trap the hydrophobic dye Nile Red (NR) and the anticancer drug doxorubicin (DOX). The stimuli-responsive release of encapsulated NR or DOX occurs (1) upon addition of a competitive binder (e.g., adamantane ammonium (ADA)) for CB[7], (2) by a dual pH-chemical stimulus involving the protonation state change of adamantane carboxylate at pH 5.8, and (3) by a dual pH-photochemical stimulus involving photoisomerization of trans-6 to cis-6 at pH 5.8. NR is released from NR@MOP2 within HeLa cancer cells. This body of work suggests that the covalent attachment of cucurbit[n]uril to metal organic polyhedra constitutes a promising vehicle for the development of both diagnostic and therapeutic nanoparticles.