Organic–Inorganic Hybrid Materials: From “Simple” Coordination Polymers to Organodiamine-Templated Molybdenum Oxides

A blueprint for the design of oxide materials is provided by nature. By borrowing from nature's ability to influence inorganic microstructures in biomineralization processes and in the hydrothermal synthesis of complex minerals, a new class of materials in which organic components exert a role in controlling inorganic microstructure is evolving. By employing members of the ever-expanding class of polymeric coordination complex cations, novel molybdenum oxide substructures, such as the one shown, may be prepared.     

“Organic Metals”—The Intermolecular Migration of Aromaticity

The electrical conductivity of a wide variety of organic materials is reviewed and a comparison made between those materials which transport charge efficiently and those that do not. This comparison reveals that the most efficient conductors contain molecules (a) whose radical ions form a new aromatic sextet upon one-electron oxidation or reduction and (b) whose aromaticity can migrate by mixed-valence interaction. The electrical data are presented in light of the chemistry of these materials and the importance of chemical bonding interactions in the electron transport process. Several new examples are discussed and directions for further research explored.     

Interfaces between Molecular and Polymeric “Metals”: Electrically Conductive,Structure-Enforced Assemblies of Metallomacrocycles

The design, synthesis, characterization, and understanding of new molecular and macro-molecular substances with “metal-like” electrical properties represents an active research area at the interface of chemistry, physics, and materials science. An important, long-range goal in this field of “materials by design” is to construct supermolecular assemblies which exhibit preordained collective phenomena by virtue of “engineered” interactions between molecular building blocks. In this review, such a class of designed materials is discussed which, in addition, bridges the gap between molecular and polymeric conductors: assemblies of electrically conductive metallomacrocycles. It is seen that efforts to rationally construct stacked metal-like molecular arrays lead logically to structure-enforced macromolecular assemblies of covalently linked molecular subunits. Typical building blocks are robust, chemically versatile metallophthalocyanines. The electrical optical, and magnetic properties of these metallomacrocyclic assemblies and the fragments thereof, provide fundamental information on the connections between local atomic-scale architecture, electronic structure, and the macroscopic collective properties of the bulk solid.     

Synthesis,purification, and characterization of “perfect” star polymers via “Click” coupling

The copper (I)‐catalyzed azide‐alkyne cycloaddition “click” reaction was successfully applied to prepare well‐defined 3, 6, and 12‐arms polystyrene and polyethylene glycol stars. This study focused particularly on making “perfect” star polymers with an exact number of arms, as well as developing techniques for their purification. Various methods of characterization confirmed the star polymers high purity, and the structural uniformity of the generated star polymers. In particular, matrix‐assisted laser desorption ionization‐time‐of‐flight mass spectrometry revealed the quantitative transformation of the end groups on the linear polymer precursors and confirmed their quantitative coupling to the dendritic cores to yield star polymers with an exact number of arms. In addition to preparing well‐defined polystyrene and poly(ethylene glycol)homopolymer stars, this technique was also successfully applied to amphiphilic, PCL‐b‐PEG star polymers. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Synthesis of a “Butterfly Cage” Based on a Double‐Decker Silsesquioxane

Novel polyhedral structures were prepared with a butterfly‐shape composed of oligosiloxane wings and a double‐decker silsesquioxane (DDSQ) body. The compounds were synthesized in two steps from commercially available alkoxysilanes, and their structures were confirmed using spectroscopic methods and X‐ray crystallography. Not like other phenyl‐substituted cage silsesquioxanes, these butterfly cages show very good solubility in common organic solvents. The crystal structures clearly showed their unique features: a larger space with longer siloxane chains and a very flexible framework. Moreover, these compounds are thermally stable with a Td₅ (5 % weight loss temperature) over 320 °C.     

Stereochemistry of Reaction Paths as Determined from Crystal Structure Data—A Relationship between Structure and Energy

The three-dimensional structure of frequently occurring molecular fragments has been studied systematically. Molecular subunits which were examined include hydrogen bridges (O—H?O), triiodide anions (I^?₃), other linear triatomic fragments (Cl—Sb?Cl, S—S?S, Mo—O?Mo), tetrahedral ions (SO^2?₄, PO^3?₄, AlCl^?₄) and related species (MSO₃, etc.), molecules containing both keto- and amino-groups (mostly alkaloids), substituted [10]annulenes and cycloheptatrienes, organic five-membered rings, and five-coordinated metal and nonmetal atoms. The bond distances and angles describing the structure of a giving fragment cover a range that is many times larger than the range of the experimental standard deviations. The changes of the various structural parameters of a fragment are correlated with each other. The observed mutual dependence (structural correlation) may be described by means of Pauling's equation relating bond length r to bond number n:r = r₀ - clogn. The bond numbers n are expressed in terms of bond angles. The sum of bond numbers at a given atom is roughly constant and does not depend on the environment. “Standard bond lengths” of a fragment are determined by a least-squares procedure based on all available data. They are supplemented by curves that describe the observed distortions. The shape of these correlation curves is reminiscent of the structural changes occurring along the pathways of chemical reactions, e.g. nucleophilic substitution at tetrahedrally coordinated atoms (S_N1 and S_N2), nucleophilic addition to carbonyl groups, electrocyclic ring closure of polyenes, pseudo-rotation of five-membered rings or Berry pseudo-rotation. For many of these reactions approximate energy hypersurfaces have been obtained from quantum mechanical calculations, model force fields and from spectroscopic information (IR, NMR). Comparison between reaction pathways determined from structural correlations with those obtained from models of the energy surfaces shows fair agreement.     

Synthesis of arborescent polystyrene by “click” grafting

A method was developed for the synthesis of arborescent polystyrene by “click” coupling. Acetylene functionalities were introduced on linear polystyrene (M_n = 5300 g/mol, M_w/M_n = 1.05) by acetylation and reaction with potassium hydroxide, 18‐crown‐6 and propargyl bromide in toluene. Polymerization of styrene with 6‐tert‐butyldimethylsiloxyhexyllithium yielded polystyrene (M_n = 5200 g/mol, M_w/M_n = 1.09) with a protected hydroxyl chain end. Deprotection, followed by conversions to tosyl and azide functionalities, provided the side chain material. Coupling with CuBr and N,N,N′,N″,N″‐pentamethyldiethylenetriamine proceeded in up to 94% yield. Repetition of the grafting cycles led to well‐defined (M_w/M_n ≤ 1.1) polymers of generations G1 and G2 in 84% and 60% yield, respectively, with M_n and branching functionalities reaching 2.8 × 10⁶ g/mol and 460, respectively, for the G2 polymer. Coupling longer (M_n = 45,000 g/mol) side chains with acetylene‐functionalized substrates was also examined. For a linear substrate, a G0 polymer with M_n = 4.6 × 10⁵ g/mol and M_w/M_n = 1.10 was obtained in 87% yield; coupling with the G0 (M_n = 52,000 g/mol) substrate produced a G1 polymer (M_n = 1.4×10⁶ g/mol, M_w/M_n = 1.38) in 28% yield. The complementary approach using azide‐functionalized substrates and acetylene‐terminated side chains was also investigated, but proceeded in lower yield. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1730–1740     

Direct synthesis of terminally “clickable” linear and hyperbranched polyesters

1,3‐Dipolar cycloaddition of an organic azide and an acetylenic unit, often referred to as the “click reaction”, has become an important ligation tool both in the context of materials chemistry and biology. Thus, development of simple approaches to directly generate polymers that bear either an azide or an alkyne unit has gained considerable importance. We describe here a straightforward approach to directly prepare linear and hyperbranched polyesters that carry terminal propargyl groups. To achieve the former, we designed an AB‐type monomer that carries a hydroxyl group and a propargyl ester, which upon self‐condensation under standard transesterification conditions yielded a polyester that carries a single propargyl group at one of its chain‐ends. Similarly, an AB₂ type monomer that carries one hydroxyl group and two propargyl ester groups, when polymerized under the same conditions yielded a hyperbranched polymer with numerous “clickable” propargyl groups at its molecular periphery. These propargyl groups can be readily clicked with different organic azides, such as benzyl azide, ω‐azido heptaethyleneglycol monomethylether or 9‐azidomethyl anthracene. When an anthracene chromophore is clicked, the molecular weight of the linear polyester could be readily estimated using both UV–visible and fluorescence spectroscopic measurements. Furthermore, the reactive propargyl end group could also provide an opportunity to prepare block copolymers in the case of linear polyesters and to generate nanodimensional scaffolds to anchor a variety of functional units, in the case of the hyperbranched polymer. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3200–3208, 2010     

“Click polyester”: Synthesis of polyesters containing triazole units in the main chain via safe and rapid “click” chemistry and their properties

Click Cu(I)‐catalyzed polymerizations of diynes that contained ester linkages and diazides were performed to produce polyesters (click polyesters) of large molecular weights [(～1.0–7.0 ) × 10⁴], that contained main‐chain 1,4‐disubstitued triazoles in excellent yields. Incorporation of triazole improved the thermal properties and magnified the even‐odd effect of the methylene chain length. We also found that, by changing the positions of the triazole rings, the thermal properties of the polyesters could be controlled. The use of in situ azidation was a safe reaction, as explosive diazides are not used. In addition, the microwave heating was found to accelerate the polymerization rates. This is the first study that has applied click chemistry for the synthesis of a series of polyesters. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4207–4218, 2010     

A General Method for Growing Two‐Dimensional Crystals of Organic Semiconductors by “Solution Epitaxy”

Two‐dimensional (2D) crystals of organic semiconductors (2DCOS) have attracted attention for large‐area and low‐cost flexible optoelectronics. However, growing large 2DCOS in controllable ways and transferring them onto technologically important substrates, remain key challenges. Herein we report a facile, general, and effective method to grow 2DCOS up to centimeter size which can be transferred to any substrate efficiently. The method named “solution epitaxy” involves two steps. The first is to self‐assemble micrometer‐sized 2DCOS on water surface. The second is epitaxial growth of them into millimeter or centimeter sized 2DCOS with thickness of several molecular layers. The general applicability of this method for the growth of 2DCOS is demonstrated by nine organic semiconductors with different molecular structures. Organic field‐effect transistors (OFETs) based on the 2DCOS demonstrated high performance, confirming the high quality of the 2DCOS.     

Discovery and Syntheses of “Superbug Challengers”—Platensimycin and Platencin

Bacteria have developed resistance to almost all existing antibiotics known today and this has been a major issue over the last few decades. The search for a new class of antibiotics with a new mode of action to fight these multiply‐drug‐resistant strains, or “superbugs”, allowed a team of scientists at Merck to discover two novel antibiotics, platensimycin and platencin using advanced screening strategies, as inhibitors of bacterial fatty acid biosynthesis, which is essential for the survival of bacteria. Though both these antibiotics are structurally related, they work by slightly different mechanisms and target different enzymes conserved in the bacterial fatty acid biosynthesis. This Focus Review summarizes the synthetic and biological aspects of these natural products and their analogues and congeners.     

The “Precursor Approach” to Patterned Fluorescence Images in Polymer Films

In contrast to the conventional two‐step method, which involves the generation of reactive functional groups followed by incubation in a dye solution (a wet developing process), the “precursor approach” enables the rapid and cost‐effective generation of patterned images in one step, without the need for an additional wet process. By using the “precursor approach”, the fluorescence of precursor molecules in polymer films can be effectively manipulated by: (1) photoinduced removal of transient protecting groups; (2) photoinduced protonation or intramolecular proton transfer; (3) photochromism; (4) photoinduced formation of fluorophores; (5) photoinduced oxidative degradation or molecular orientation.