Guest-Shape-Directed Structural Switching between Two Isomers of a Pd Host and Its Structural Adaptability for Selective Photodimerization.

The structure and functions of metal-organic cages are heavily dependent on the nature of the building blocks. Herein, a dimethyl-substituted propane diamine blocked -Pd(II) acceptor () was designed, which upon self-assembly with the tri-imidazole ligand (), generated an unusual octahedral cage () instead of the expected isomeric double-square architecture that was obtained from the tetramethyl-substituted ethylene diamine blocked -Pd(II) acceptor in water. Interestingly, in the presence of planar guests (), showed a transformation to a transient double-square architecture (), forming host-guest complexes with two such guests, . The transient double-square cage () readily converts back to the parent octahedral structure, upon the removal of the guests. On the contrary, tetrahedral guests (/) stabilized the octahedral isomer of the host () by acting as suitable templates. Additionally, tetrahedral guests could induce the reverse transformation of to by driving out planar guests from metastable . The specific antiparallel orientation and proximity of two anthracene derivatives within enabled them to be selectively transformed to the isomers of their respective dimers under photoirradiation. Upon dimer formation, the nonplanar product was expelled readily from the cavity of , and the host switched back to its original octahedral form (), which functionally and structurally imitates enzymatic activity. Thus, a multifunctional supramolecular host was obtained that showed unique guest-shape-driven reversible structural switching and acted as an adaptive host for selective photodimerization.