Guest-Shape-Directed Structural Switching between Two Isomers of a Pd₆ Host and Its Structural Adaptability for...

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 cis-Pd(II) acceptor (A) was designed, which upon self-assembly with the tri-imidazole ligand (L), generated an unusual A₆L₄ octahedral cage (M1O) instead of the expected isomeric double-square architecture that was obtained from the tetramethyl-substituted ethylene diamine blocked cis-Pd(II) acceptor in water. Interestingly, in the presence of planar guests (Gn), M1O showed a transformation to a transient double-square architecture (M1DS), forming host-guest complexes with two such guests, (Gn)₂@M1DS. The transient double-square cage (M1DS) readily converts back to the parent octahedral structure, M1O upon the removal of the guests. On the contrary, tetrahedral guests (G5/G6) stabilized the octahedral isomer of the host (M1O) by acting as suitable templates. Additionally, tetrahedral guests could induce the reverse transformation of M1DS to M1O by driving out planar guests from metastable M1DS. The specific antiparallel orientation and proximity of two anthracene derivatives within M1DS enabled them to be selectively transformed to the trans isomers of their respective dimers under photoirradiation. Upon dimer formation, the nonplanar product was expelled readily from the cavity of M1DS, and the host switched back to its original octahedral form (M1O), 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.