Invited for the cover of this issue are Wen-Rong Wu, Guang-Jie Xia, and Hak-Fun Chow of The Chinese University of Hong Kong, Xiao-Ping Cao of Lanzhou University and Dietmar Kuck of Bielefeld University. The image depicts how different routes can lead to the same goal. Read the full text of the article at 10.1002/chem.201501556.

A pair of enantiomerically pure metallosquares based on linear platinum-diacetylene edges and tribenzotriquinacene corner units was synthesized. Their structures were characterized by ¹H-, ¹³C- and ³¹P NMR spectroscopy as well as MALDI-TOF mass spectrometry and circular dichroism. Based on DFT calculation, the optimized geometry possesses a distorted square conformation in which the four edges are not sitting on the same plane. The molecular square further self-assembled in the solid state to afford microspheres with diameter of approximately 300 nm, as determined by scanning electron microscopy.

A homologous series of oligo(amide–triazole)s (OAT) [OAT-CO₂H-2 n and OAT-COPrg-(2 n+1)] with an increasing number of primary amide (CONH) and triazole hydrogen-bonding functionalities was prepared by an iterative synthetic procedure. It was found that their self-assembly and thermoreversible gelation strength had a strong correlation to the number of hydrogen-bonding moieties in the oligomers. There also existed a threshold value of the number of CONH units, above which all the oligomers became organogelators. Hence, oligomers with ≤4 CONH units are devoid of intermolecular hydrogen bonding and also non-organogelating, whereas those that contain >4 CONH units show intermolecular association and organogelating properties. For the organogelators, the T_gel value increases monotonically with increasing number of CONH units. On the basis of FTIR measurements, both the CONH and triazole CH groups were involved in the hydrogen-bonding process. A mixed xerogel that consisted of a 1:1 weight ratio of two oligomers of different lengths (OAT-CO₂H-6 and OAT-CO₂H-12) was found to show microphase segregation according to differential scanning calorimetry, thus indicating that oligomers that bear a different number of hydrogen-bonding units exhibited self-sorting to maximize the extent of intermolecular hydrogen bonding in the xerogel state.

A series of main-chain poly(amide-triazole)s were prepared by copper(I)-catalyzed alkyne–azide AABB-type copolymerizatons between five structurally similar diacetylenes 1–5 with the same diazide 6. The acetylene units in monomers 1–5 possessed different degrees of conformational flexibility due to the different number of intramolecular hydrogen bonds built inside the monomer architecture. Our study showed that the conformational freedom of the monomer had a profound effect on the polymerization efficiency and the thermoreversible gelation properties of the resulting copolymers. Among all five diacetylene monomers, only the one, that is, 1-Py(NH)₂ which possesses the pyridine-2,6-dicarboxamide unit with two built-in intramolecular H bonds could produce the corresponding poly(amide-triazole) Poly-(PyNH)₂ with a significantly higher degree of polymerization (DP) than other monomers with a lesser number of intramolecular H bonds. In addition, it was found that only this polymer exhibited excellent thermoreversible gelation ability in aromatic solvents. A self-assembling model of the organogelating polymer Poly-(PyNH)₂ was proposed based on FTIR spectroscopy, XRD, and SEM analyses, in which H bonding, π–π aromatic stacking, hydrophobic interactions, and the structural rigidity of the polymer backbone were identified as the main driving forces for the polymer self-assembly process.

Nine dendronized poly(amide–triazole)s 2-Gm Gn (m=1–3, n=1–3), were prepared by the 1:1 copolymerization between AA-type dendritic diazides 4-Gm (m=1–3) and BB-type dendritic diacetylenes 5-Gn (n=1–3) under the copper(I)-mediated click coupling conditions. The degree of polymerization value of the polymers was found to range from 15–50, and decreased with increasing size of the dendron, suggesting steric hindrance had a retardation role on the copolymerization efficiency. Based on FT-IR and ¹H NMR studies, it was found that significantly strong, interchain hydrogen bonding between the amide units was present in the solution state after copolymerization, whereas the monomers 4-Gm and 5-Gn were devoid of any intermolecular hydrogen-bonding interaction. Hence a positive allosteric hydrogen-bonding effect was observed after polymerization, and could be rationalized by the zip effect. The strength of the interchain association in polymers 2-Gm Gn was found to decrease with increasing size of the dendron (i.e., 2-G1 G1>2-G1 G2>2-G2 G1≈2-G2 G2>2-G1 G3≈2-G3 G1>2-G2 G3≈2-G3 G2>2-G3 G3). Among the nine polymers, only 2-G1 G2 and 2-G2 G1 were good organogelators for aromatic solvents, while the 2-G2 G2 polymer, bearing the closest structural resemblance to the previously reported organogelator 1-G2 prepared from the polymerization of AB-type monomers, was devoid of gelating power. Careful analysis of structures of the present polymer series 2-Gm Gn and the previously reported series 1-Gn suggested that the polymer backbone symmetry played a subtle role in controlling their self-assembling and gelating properties.

Two series of aliphatic hydrocarbon-based G1–G3 dendritic 2-ureido-4-pyrimidinones (UPy) (S-Gn)₂ and (L-Gn)₂, differing from one another by the distance between the branching juncture to the urea end, were prepared and characterized. These hydrocarbon dendrons were also appended to a p-aminonitrobenzene solvatochromic chromophore in order to probe their microenvironment polarity. While positive solvatochromism was observed which indicated the chromophore was solvent accessible, there was no significant difference between the microenvironment polarities on going from the G1 to the G3 dendrons. The self-assembling behavior and tautomeric preference of the dendritic UPy derviatives were examined by ¹H NMR spectroscopy. The dimerization constants (K_dim*) of the DDAA tautomers were unchanged at 10⁷ M⁻¹ in CDCl₃ at both 25 and 50 °C, which were comparable to those of UPy compounds bearing other nonpolar substitutents. Furthermore, the lower limits on the K′_dim* of the DADA tautomeric forms of the (S-Gn)₂ and (L-Gn)₂ series were determined to be 10⁶ and 10⁵ M⁻¹ in CDCl₃, respectively. It was found that a closer proximity of the dendron branching juncture to the UPy unit could lead to a destabilization effect on the dimeric states. Hence, the (L-Gn)₂ dimers are more stable than those of (S-Gn)₂ in the DDAA form, but the latter are more stable than the former in the tautomeric DADA state. This study showed that both the highly nonpolar microenvironment and the proximity of the dendritic branching juncture to the UPy motif could alter the strength and profile of the hydrogen bond-mediated self-assembling process.

The copolymerizations of two series of surface functionalized bis(acetylene) G1–G3 dendrimers, one (S-Gn) having a structural rigid skeleton and the other (L-Gn) a relatively more flexible architecture, with two platinum linkers, cis-[(Et₂PCH₂CH₂PEt₂)PtCl₂] (2) and [Cl(Et₃P)₂Pt-CC-p-C₆H₄-]₂ (3) were investigated. For both series of dendrimers, only linear and/or cyclic oligomers were formed when the cis-platinum linker 2 was used. However, high molecular weight (100–200 kD) organoplatinum poly(dendrimer)s were obtained from both series when the elongated linear rod-liked platinum linker 3 was employed and the formation of cyclic oligomers was greatly suppressed for both the structural rigid S-Gn and the structural flexible L-Gn series. These results are in sharp contrast to our earlier findings (S.-Y. Cheung, H.-F. Chow, T. Ngai, X. Wei, Chem. Eur. J.2009, 15, 2278–2288) obtained by using a shorter linear platinum linker trans-[Pt(PEt₃)₂Cl₂] (1), where a larger amount of cyclic oligomers was formed from the structural flexible L-Gn dendrimers. A model was proposed to rationalize how the geometry and size of the platinum linker could control the copolymerization behaviours of these dendritic macromonomers.