Amide-Assisted Polymerization of 1,3-Butadiyne Containing Thiolate Ligands on Small Gold Nanoparticles.

Incorporation of directing amide groups has been shown to facilitate the topochemical polymerization of 1,3-butadiyne (diacetylene) groups in noncrystalline phases such as gels, amorphous solids, and liquid crystals. It remains challenging to polymerize 1,3-butadiyne-containing alkylthiolate ligands within their self-assembled monolayers on gold nanoparticles (AuNPs), which enhances their stability and adds new optical and electronic properties. Especially smaller AuNPs of sizes below 5 nm in diameter have been reported to display sluggish photopolymerization and are susceptible to photodegradation under UV irradiation. To probe the effectiveness of the amide-directed photopolymerization of 1,3-butadiyne ligands, small AuNPs in the 2-4 nm range were synthesized that contain alkylthiolate ligands with and without amide and 1,3-butadiyne groups. Their photopolymerization and photostability were studied by transmission electron microscopy (TEM), UV-vis spectroscopy, and Raman spectroscopy. AuNP with amide-free 1,3-butadiyne ligands templated the polymerization of the 1,3-butadiyne ligands but fused to large and insoluble particles during the polymerization process. AuNPs with ligands containing both 1,3-butadiyne and amide groups polymerized significantly faster, which slowed down photodegradation. A UV irradiation (254 nm and 176 W/m2) for 5-10 min was found to be optimal for the AuNPs with directing amide groups studied here, although their average core sizes grew from 3.8 to 4.0 nm in diameter and about 20% of the attached 1,3-butadiyne ligands remained unreacted after 10 minutes of irradiation. About 75% of the attached 1,3-butadiyne ligands were already polymerized during the first 5 min of UV irradiation. This decrease in reactivity is reasoned with a fast polymerization of ligands attached to facet sites and slower polymerization rates for ligands attached to edge and corner sites. Unexpectedly, photopolymerization occurred only in the presence of solvent, whereas no polydiacetylene was generated when dry powders of any of the diacetylene-containing gold nanoparticles were irradiated.

Columnar Mesomorphism of Board-Shaped Perylene, Diketopyrrolopyrrole, Isoindigo, Indigo, and Quinoxalino-Phenanthrophenazine Dyes.

The properties of organic dyes depend as much on their intermolecular interactions as on their molecular structure. While it is generally predictable what supramolecular structure would be ideal for a specific application, the generation of specific supramolecular structures by molecular design and suitable processing methods remains to be a challenge. A versatile approach to different supramolecular structures has been the application of mesomorphism in conjunction with alignment techniques and self-assembly at interfaces. Reviewed here is the columnar mesomorphism of board-shaped dyes perylene, indigo, isoindigo, diketopyrrolopyrrole, and quinoxalinophenanthrophenazine. They generate a larger number of different supramolecular structures than conventional disc-shaped (discotic) mesogens because of their non-circular shape and directional intermolecular interactions. The mesomorphism of all but the perylene derivatives is systematically and comprehensively covered for the first time.

Stabilization of Columnar Liquid Crystal Phases via Arene-Perfluoroarene Interactions.

We report the synthesis of novel polycatenar dibenzopentacenequinones 1 and 2 that are designed to form columnar liquid crystalline phases. The mesomorphic properties of these compounds were investigated by polarized optical microscopy, differential scanning calorimetry, and powder X-ray diffraction. While compound 1 exhibits two distinct columnar mesophases between 148 and 177 °C, fluorinated 2 exhibits a columnar mesophase from 121 to 336 °C. This dramatic stabilization of the columnar mesophase of 2 can be attributed to improved π-stacking as a result of arene-perfluoroarene interactions.

Topochemical Polymerization of a Nematic Tetraazaporphyrin Derivative To Generate Soluble Polydiacetylene Nanowires.

Polydiacetylenes are well-established one-dimensional organic semiconductors that have been generated by photochemical and thermal polymerizations of diacetylenes in single crystals, gel phases, thin films, and membranes. Their formation in mesophases, such as liquid crystals, has been surprisingly little studied although higher-ordered mesophases should support the topochemical polymerization of diacetylenes (1,3-butadiyne groups) and may give access to large domains of uniformly aligned materials. The polymerization of diacetylenes in a mesophase may also increase the stability of the self-assembled supramolecular structure. Here, the dye and discotic mesogen tetraazaporphyrin was decorated with eight diacetylene-containing alkyl chains to probe its mesomorphism and conversion into multifunctional polydiacetylene materials. While the incorporation of diacetylene groups supports columnar mesomorphism, successful photopolymerization required the presence of directing amide groups that suppressed columnar in favor of nematic mesomorphism. Still, the polymerization of the nematic mesophase generated a soluble nematic polydiacetylene of significantly higher molecular weight (Mn = 77 kDa or 25 monomer units by gel permeation chromatography) than what has been obtained in gel phases of related compounds. The formation of polydiacetylene was confirmed by Raman spectroscopy, and its nematic structure was verified by UV-vis spectroscopy, polarized optical microscopy, and X-ray diffraction. Both its nematic structure and the incorporation of eight side chains per discotic unit provide the polydiacetylene with sufficient solubility for casting thin films on substrates. Atomic force microscopy studies of films on silicon wafers revealed a grid-like structure of connected nanofibers. This study demonstrates the requirements for the formation of multifunctional mesomorphic polydiacetylene materials from mesomorphic precursors and their advantages. Optimization of the presented molecular design should give access to other mesophases and, consequently, functional polydiacetylene materials with tunable structures and optoelectronic properties.

Synthesis and Self-Assembly of Liquid Crystalline Triphenylenedicarboxythioimides.

We report the synthesis and properties of a series of novel triphenylenedicarboxyimides and thioimides (4-6) to probe the effect of thionation on the formation of columnar mesophases. These materials display broad columnar mesophases and high clearing points and self-associate in solution to form dimers. Overall, thionation improved the self-assembly in solution and led to a stabilization of the columnar mesophase. Furthermore, increasing the thionation of these materials led to a lowering of the lowest unoccupied molecular orbital (LUMO) energy level and a narrowing of the highest occupied molecular orbital-LUMO gap.

Self-Assembly of Board-Shaped Diketopyrrolopyrrole and Isoindigo Mesogens into Columnar π-π Stacks.

Diketopyrrolopyrrole and isoindigo are commercially important dyes that have recently found broad application as electron acceptor and light-absorbing groups in organic semiconductors. Their self-assembly into specific supramolecular structures to control optoelectronic properties has been hampered by limited options for substitution and their high propensity for crystallization. Reported here is a molecular design that directs self-assembly into previously elusive columnar mesophases of π-π stacking cores. Although attachment of bis(trisoctyloxyphenyl)-1,3,5-triazine groups to both ends of diketopyrrolopyrrole-thiophene and isoindigo cores generated mesomorphic dyes of similar overall shapes and dimensions, distinct differences in their mesomorphism and optoelectronic properties were observed.

Synthesis and Characterization of Liquid-Crystalline Tetraoxapentacene Derivatives Exhibiting Aggregation-Induced Emission.

A series of new tetrakis(dialkoxyphenyl) dicyanotetraoxapentacene derivatives (1 a-c) were prepared by reaction of the appropriate terphenyl diols with tetrafluoroterephthalonitrile in good yields. Compounds 1 b and 1 c, which bear hexyloxy and decyloxy side chains, exhibited columnar hexagonal mesophases, as shown by polarized optical microscopy, variable-temperature powder X-ray diffraction, and differential scanning calorimetry. Single-crystal X-ray diffraction of methoxy-substituted 1 a revealed that the dicyanotetraoxapentacene core is highly planar, consistent with the notion that these molecules are able to stack in columnar mesophases. A detailed photophysical characterization showed that these compounds exhibit aggregation-induced emission in solution, emission in nonpolar solvents, weak emission in polar solvents, and strong emission in the solid state both as powder and in thin films. These observations are consistent with a weakly emissive charge-transfer state in polar solvents and a more highly emissive locally excited state in nonpolar solvents.

Influence of Chromophore Length and Acceptor Groups on the Optical Properties of Rigidified Merocyanine Dyes.

Invited for this month's cover are the three collaborating groups from the University of Stuttgart and the Fraunhofer Institute for Applied Polymer Research in Potsdam, both from Germany as well as the University of Windsor, Canada. The cover picture shows the shift of frontier orbital energies and symmetries upon excitation with light for three different chromophore lengths. Read the full text of the article at 10.1002/cplu.201300308.

Influence of Chromophore Length and Acceptor Groups on the Optical Properties of Rigidified Merocyanine Dyes.

Three series of rigidified tri-, penta- and heptamethine merocyanine dyes were synthesised. A piperidyl moiety was chosen as the electron-donating substituent while the electron-accepting group was varied from ketones to malononitriles and cyanoacetates. The structures of the compounds in the solid state and in solution were elucidated by X-ray diffraction and NMR spectroscopy, respectively, while optical properties were investigated by absorption and emission spectroscopy. As a general trend, the acceptor properties decrease in the series malononitrile>cyanoacetate>ketone based on the analysis of their solvatochromic behaviour. The experimental results were further supported by calculations at the B3LYP 6-311+G(d) level of theory.

New dihexadecyldithiophosphate SAMs on gold provide insight into the unusual dependence of adsorbate chelation on substrate morphology in SAMs of dialkyldithiophosphinic acids.

We report the formation and characterization of new self-assembled monolayers (SAMs) formed from dihexadecyldithiophosphate (C16)2DDP and compare their properties with those of SAMs formed from the structurally similar adsorbate dihexadecyldithiophosphinic acid (C16)2DTPA. The new (C16)2DDP SAMs were characterized using X-ray photoelectron spectroscopy, reflection-absorption infrared spectroscopy, contact angle measurements, and electrochemical impedance spectroscopy. The data indicate that (C16)2DDP forms SAMs on gold films formed by e-beam evaporation in which all adsorbates chelate to gold, in contrast to (C16)2DTPA SAMs, in which 40% of the adsorbates are monodentate. The alkyl chains of the (C16)2DDP SAM are also less densely packed and ordered than those of the (C16)2DTPA SAM. To understand these differences, we present density functional theory calculations that show that there are only minimal differences between the geometric and electronic structures of the two adsorbates and that the energetic difference between monodentate and bidentate binding of a gold(I) ion are surprisingly small for both adsorbates. This study leads to the conclusion that differences in intermolecular interactions within the SAM are the driving force for the difference in chelation between the two adsorbates.

Columnar mesomorphism of fluorescent board-shaped quinoxalinophenanthrophenazine derivatives with donor-acceptor structure.

Quinoxalino[2',3':9,10]phenanthro[4,5-abc]phenazine (QPP) dyes have been studied as electron acceptor materials, fluorophores, and building blocks for self-organizing organic semiconductors. Condensation of tetraketopyrene with electron-rich diamino-terphenylene and -triphenylene derivatives generates new donor-acceptor QPP derivatives that display columnar mesomorphism over wide ranges of temperature; are fluorescent in solution, liquid crystal, and solid phases; and have electron acceptor properties. Also reported are the synthesis and properties of the first diamino-(tetraalkoxy)triphenylene as a valuable new synthon.

Mesomorphic [2]rotaxanes: sheltering ionic cores with interlocking components.

Two types of liquid crystalline [2]rotaxanes based on a conventional tetracatenar motif (a rod-shaped molecule with two side chains at each end) have been prepared. Dicationic compounds with ester stoppers and tetracationic materials with pyridinium stoppers are compared to each other and their dumbbell shaped analogs. Since the ionic core contributes about 70% to the overall length and molecular weight of the molecules, sheltering the ionic cores with an interlocked neutral macrocycle has considerable effect on the mesomorphism and thermal stability of the materials. The influence of the sheltering macrocycle, the numbers of charges on the core and the size and nature of the side chains (aliphatic vs siloxane) were probed. [2]Rotaxanes with linear side chains and minimum ratios of chain-to-core volumes of about 0.35 and 0.30 for tetra- and dicationic compounds, respectively, display smectic liquid crystal phases. Larger ratios increase the temperature range of the smectic A phases beyond the decomposition temperatures; a disadvantage for processing because no stable isotropic liquid phase is available. The change from tetra- to dicationic [2]rotaxanes increased not only the fluidity of their smectic A phases but also their thermal and chemical stability. Branched side chains (2-hexyldecyl) disfavor the formation of lamellar mesophases and, instead, induce higher ordered soft crystal phases. No liquid crystal phases but soft crystal phases are observed for the analogous di- and tetracationic compounds without an ion sheltering interlocked macrocycle (dumbbells).

Synthesis of 2,6-disubstituted tetrahydroazulene derivatives.

Synthesis of hydroazulene derivatives has been carried out through a ring-enlargement route by using carbene adduct intermediates. The protocol can be applied for the construction of functionalized hydroazulene skeletons as key components of many natural products as well as the core system of novel liquid-crystalline materials.

Perhydroazulene-based liquid-crystalline materials with smectic phases.

New liquid-crystalline materials with a perhydroazulene core were synthesized and the stereochemistry of these compounds was investigated. The mesomorphic properties of the new LC compounds were investigated by differential scanning colorimetry, polarizing optical microscopy and X-ray diffraction. We report here on the LC properties of nonchiral materials, which predominantly exhibit smectic phases and display nematic phases only within narrow temperature ranges. The dependence of the mesogenic behavior of the new materials on the stereochemistry of the core system was also investigated. All newly synthesized compounds were fully characterized by the usual spectroscopic and analytical methods.

Experimental and computational insights into the stabilization of low-valent main group elements using crown ethers and related ligands.

A series of tin(II) triflate and chloride salts in which the cations are complexed by either cyclic or acyclic polyether ligands and which have well-characterized single-crystal X-ray structures are investigated using a variety of experimental and computational techniques. Mössbauer spectroscopy illustrates that the triflate salts tend to have valence electrons with higher s-character, and solid-state NMR spectroscopy reveals marked differences between superficially similar triflate and chloride salts. Cyclic voltammetry investigations of the triflate salts corroborate the results of the Mössbauer and NMR spectroscopy and reveal substantial steric and electronic effects for the different polyether ligands. MP2 and DFT calculations provide insight into the effects of ligands and substituents on the stability and reactivity of the low-valent metal atom. Overall, the investigations reveal the existence of more substantial binding between tin and chlorine in comparison to the triflate substituent and provide a rationale for the considerably increased reactivity of the chloride salts.

The effect of incorporating Fréchet dendrons into rotaxanes and molecular shuttles containing the 1,2-bis(pyridinium)ethane⊂[24]crown-8 templating motif.

Fréchet-type dendrons (G0-G3) were added as both axle stoppering units and cyclic wheel appendages in a series of [2]rotaxanes, [3]rotaxanes, and molecular shuttles that employ 1,2-bis(pyridinium)ethane axles and 24-membered crown ethers wheels. The addition of dendrimer wedges as stoppering units dramatically increased the solubility of simple [2]rotaxanes in nonpolar solvents. The X-ray structure of a G1-stoppered [2]rotaxane shows how the dendritic units affect the structure of the interlocked components. Increased solubility allows observation of how the interaction of dendritic units on separate components in interlocked molecules influences switching properties and molecular size. In a series of [2]rotaxane molecular shuttles incorporating two recognition sites, it was demonstrated that an increase in generation on either the stoppering unit or cyclic wheel could influence both the rate of shuttling and the site preference of the wheel on the axle.

Application of solid-state 35Cl NMR to the structural characterization of hydrochloride pharmaceuticals and their polymorphs.

Solid-state (35)Cl NMR (SSNMR) spectroscopy is shown to be a useful probe of structure and polymorphism in HCl pharmaceuticals, which constitute ca. 50% of known pharmaceutical salts. Chlorine NMR spectra, single-crystal and powder X-ray diffraction data, and complementary ab initio calculations are presented for a series of HCl local anesthetic (LA) pharmaceuticals and some of their polymorphs. (35)Cl MAS SSNMR spectra acquired at 21.1 T and spectra of stationary samples at 9.4 and 21.1 T allow for extraction of chlorine electric field gradient (EFG) and chemical shift (CS) parameters. The sensitivity of the (35)Cl EFG and CS tensors to subtle changes in the chlorine environments is reflected in the (35)Cl SSNMR powder patterns. The (35)Cl SSNMR spectra are shown to serve as a rapid fingerprint for identifying and distinguishing polymorphs, as well as a useful tool for structural interpretation. First principles calculations of (35)Cl EFG and CS tensor parameters are in good agreement with the experimental values. The sensitivity of the chlorine NMR interaction tensor parameters to the chlorine chemical environment and the potential for modeling these sites with ab initio calculations hold much promise for application to polymorph screening for a wide variety of HCl pharmaceuticals.

The interplay of bent-shape, lateral dipole and chirality in thiophene based di-, tri-, and tetracatenar liquid crystals.

A range of mesogenic molecules varying in both bend angle and strength of lateral dipole were synthesized, and their phase behavior was characterized by polarizing microscopy, thermal analysis, X-ray diffraction, and electrooptical measurements. We find the general destabilization of the liquid crystallinity caused by strong lateral dipolar groups and the bent molecular shape are off-set in mesomorphic tetracatenars, which display stable nematic, smectic, columnar, and cubic mesophases. The broad mesomorphism of the tetracatenars containing lateral dipoles and their incompatibility with chiral induction are explained by considering that loosely correlated dimers exist within the mesophases. Chiral mesophases of derivatives with strong lateral dipoles were achieved by attaching fewer or different side chains to each end of the mesogen.