Bottom-Up Construction and Reversible Structural Transformation of Supramolecular Isomers based on Large Truncated Tetrahedra.

A rational synthetic strategy to construct two supramolecular isomers based on polyoxovanadate organic polyhedra with tetrahedral symmetries is presented. VMOP-α, a low-temperature product, has an extremely large cell volume (470 842 Å3 ), which is one of the top three for well-defined MOPs. The corner-to-corner packing of tetrahedra leads to a quite low density of 0.174 g cm-3 with 1D channels (ca. 5.4 nm). The effective pore volume is up to 93.6 % of cell volume, nearly the largest found in MOPs. For the high-temperature outcome, VMOP-ß, the cell volume is only 15 513 Å3 . The packing mode of tetrahedra is corner-to-face, giving rise to a high-density architecture (1.324 g cm-3 ; channel 0.8 nm). Supramolecular structural transformation between VMOP-α and VMOP-ß can be reversibly achieved by temperature-induced solvent-mediated transformation. These findings give a good opportunity for understanding 3D supramolecular aggregation and crystal growth based on large molecular tectonics.

Supramolecular Isomers of Coordination-Directed Side-Chain Polypseudorotaxanes Based on Trimeric Uranyl Oxalate Nodes.

Although the prosperity of rotaxane coordination polymers with rotaxane molecules serving as main-chain linkers is known, side-chain metal-organic polypseudorotaxanes incorporating macrocyclic host molecules have not been reported to date. Herein a new type of coordination-driven cucurbit[6]uril-bearing side-chain polypseudorotaxane, with two-dimensional trimeric uranyl-oxalate as main chains, has been synthesized. This was carried out through hydrothermal reactions of uranyl components with an in situ-formed carboxylated pseudorotaxane ligand in the presence of oxalate co-ligands. Varying the substitution site of coordination groups led to two different supramolecular isomers. Further mechanistic analysis indicated that condition-dependent hydrolysis of the cyano groups of the pseudorotaxane ligand, as well as the participation of oxalate groups into the coordination sphere of uranyl moieties, contributes to the formation of this new type of side-chain polypseudorotaxane.

Disappeared supramolecular isomer reappears with perylene guest.

Among different types of polymorphism, disappearing polymorphism deals with the metastable kinetic form which can not be reproduced after its first isolation. In the world of coordination polymers (CPs) and metal-organic frameworks (MOFs), despite the fact that many types of supramolecular isomerism exist, we are unaware of disappearing supramolecular isomerism akin to disappearing polymorphism. This work reports a MOF with dia topology that could not be reproduced, but subsequent synthesis yielded another supramolecular isomer, a double-pillared-layer MOF. When perylene was added in the same reaction, the disappeared dia MOF reappeared with perylene as a guest in the channels. Interestingly, the photoluminescence of the dia MOF with a perylene guest is dominated by the emission of the guest molecule. The influence of guest molecules on the stabilization of the supramolecular isomers of a MOF opens up a strategy to access MOFs with different structures.

PEG-Induced Synthesis of Coordination-Polymer Isomers with Tunable Architectures and Iodine Capture.

Controllable synthesis of coordination polymer (CP) isomers and revealing their structure-property relationships remain enormous challenges. Three new supramolecular isomers have been synthesized by tuning the poly(ethylene glycol) (PEG) content in the feed. These supramolecular isomers have the same framework formula of [Cu2 I2 (tppe)] and different architectures from the classical 2D stacking framework to a 3D entangled system with the coexistence of interpenetration and polycatenation, and a 3D topological framework. Interestingly, these CPs could be utilized for capturing iodine molecules. According to multiple complementary experiments and crystallographic analyses, iodine capture is mainly based on halogen-bond interactions in the inorganic {Cu2 I2 } building blocks of the framework. The present study describes a structure-property relationship in supramolecular isomerism with distinct topological structures.

Photochromic and Nonphotochromic Luminescent Supramolecular Isomers Based on Carboxylate-Functionalized Bipyridinium Ligand: (4,4)-Net versus Interpenetrated (6,3)-Net.

Photochromic and nonphotochromic luminescent bipyridinium-based supramolecular isomers [Cd(CPBPY)(BDC)(H2O)]n (1) and {[Cd(CPBPY)(BDC)]·H2O}n (2) (CPBPY = N-(3-carboxyphenyl)-4,4'-bipyridinium, BDC= terephthalate) have been successfully obtained by solvothermal reactions at 100 °C via tuning stoichiometric ratios of starting reagents. Isomer 1 features (4,4)-topological layer constructed by edge-shared Cd2 SBUs and BDC linkers attached by N-pendent CPBPY groups. Isomer 2 has (6,3)-topological layers with Cd atoms as nodes and BDC and double CPBPY as linkers, which are 4-fold interpenetrated into 3D network. Although both 1 and 2 contain bipyridinium ligands, only isomer 1 possesses reversible photochromic behavior with quick-switchable luminescence in the solid state. Compound 2 does not show photochromic behavior even after exposure to UV light for more than 2 h. Photochromism process of 1 originates from photostimulated reduction of CPBPY ligands to generate CPBPY(•-) radicals after irradiation, confirmed by EPR spectra. Careful check on structure reveals that the offset π-π stacking interaction between the pyridine ring of CPBPY and benzene ring of BDC with inter-ring shortest C···C distance of 3.214 Å in 1 is responsible for electron transfer to form the CPBPY(•-) radicals. The speculation is further supported by DFT calculation of frontier orbital matching of electron donor and acceptor. HOMO and HOMO-2 orbitals of BDC involve the carbon atoms of benzene ring while LUMO and LUMO+1 orbitals of CPBPY involve the carbon atoms of pyridinium ring. Importantly, the photoinduced formed CPBPY(•-) radicals in 1 have a long-lived lifetime (at least six months in air and room temperature condition), which is mainly attributed to the close packing mode.