Molecular Packing Arrangements Research Articles

Exciton delocalization drives rapid singlet fission in nanoparticles of acene derivatives.

We compare the singlet fission dynamics of five pentacene derivatives precipitated to form nanoparticles. Two nanoparticle types were distinguished by differences in their solid-state order and kinetics of triplet formation. Nanoparticles that comprise primarily weakly coupled chromophores lack the bulk structural order of the single crystal and exhibit nonexponential triplet formation kinetics (Type I), while nanoparticles that comprise primarily more strongly coupled chromophores exhibit order resembling that of the bulk crystal and triplet formation kinetics associated with the intrinsic singlet fission rates (Type II). In the highly ordered nanoparticles, singlet fission occurs most rapidly. We relate the molecular packing arrangement derived from the crystal structure of the pentacene derivatives to their singlet fission dynamics and find that slip stacking leads to rapid, subpicosecond singlet fission. We present evidence that exciton delocalization, coincident with an increased relative admixture of charge-transfer configurations in the description of the exciton wave function, facilitates rapid triplet pair formation in the case of single-step singlet fission. We extend the study to include two hexacene derivatives and find that these conclusions are generally applicable. This work highlights acene derivatives as versatile singlet fission chromophores and shows how chemical functionalization affects both solid-state order and exciton interactions and how these attributes in turn affect the rate of singlet fission.

Reversible near-infrared/blue mechanofluorochromism of aminobenzopyranoxanthene.

Mechanochromic organic molecules (MOMs) that exhibit a large difference of fluorescence wavelength between two states have important potential applications, but few such compounds are known. Here, we report a new MOM, cis-ABPX01(0), which shows switchable near-IR and blue fluorescence responses. Detailed spectrophotometric and single-crystal X-ray analyses revealed that the near-IR fluorescence is attributable to fluorescence from slip-stacked dimeric structures in crystals, while the blue fluorescence is attributable to fluorescence from the monomer. Switching between the two is achieved by dynamic structural interconversion between the two molecular packing arrangements in response to mechanical grinding and solvent vapor-fuming.

Porphyrin Derivatives in Transistors and Nano/Micro Sized Coordination Polymers

Recently, the charge-transport phenomena of organic conjugated materials have been intensively investigated because of the potential applications in electronics and optoelectronics. Among them, organic field-effect transistors (OFETs) fabricated from either thin films or well-defined single crystals (SCs) as charge-transporting layers are one of the most promising electronic devices. In particular, the unique anisotropic arrangement of organic semiconducting molecules owing to their strong intermolecular interactions is expected to have a significant influence on the OFET performance, because the larger overlap of pi-orbitals between neighboring molecules may increase the bandwidth and facilitate charge-transport. In this regards, porphyrin is one of the most important p-conjugated planar molecules and they have often been employed in OFETs, organic phototransistors (OPTs), and organic photovoltaics (OPVs). A deeper understanding of such systems has hardly been achieved because of the lack of information on the molecular packing and intermolecular arrangement (which are closely related to the OFET performance), since most porphyrin-based OFET devices are based on thin films or polycrystalline objects prepared by spin-coating or vacuum-deposition processes and showing relatively poor device performances because of the limitation in use of pi-conjugation. For a high degree of crystallinity with an excellent determinacy as well as a high corresponding device performance, the extension of pi-orbitals and the location of conjugative substituents on the porphyrin core may play major roles. Therefore, I like to present a series of new pi-extended porphyrin derivatives and their OFET devices performances along with films and single-crystals. In addition, I like to introduce the rational synthesis of porphyrin based unique structures with a variety of morphologies obtained from the manipulation of secondary substituents (i.e., substituents that are not directly bonded to the porphyrin ring) and the introduction of various metals to engender van der Waals interactions beyond the immediate porphyrin ring and metal meditated coordination bondings. So, I like to introduce porous coordination polymer particles assembled by a simple synthetic manipulation and their applications in heterogeneous catalysis.

Structural Characterization of Vapor-Deposited Glasses of an Organic Hole Transport Material with X-ray Scattering

Vapor-deposited organic glasses can be produced with enhanced thermal stability and tunable molecular orientation by controlling the substrate temperature during deposition. Recent work has also shown improved charge carrier mobility associated with anisotropic molecular packing in organic electronics. Here grazing-incidence wide-angle X-ray scattering (GIWAXS) is used to characterize the structural anisotropy in glasses of a hole transport material, N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine, commonly referred to as TPD. The TPD glasses were prepared by physical vapor deposition at substrate temperatures between 0.79 Tg and 0.98 Tg, where Tg is the glass transition temperature. A GIWAXS-derived orientation order parameter is used to quantify the anisotropy observed in the scattering patterns of the glasses. The GIWAXS-order parameter exhibits both positive and negative values as a function of substrate temperature, indicating either face-on or edge-on packing, and correlates well with a spectroscopic ellipsometry-derived order parameter that is sensitive to molecular orientation. We propose molecular packing arrangements based on the combination of the two order parameters and explore the relationship between kinetic stability and glass structure.

Unidirectional coating technology for organic field-effect transistors: materials and methods

Solution-processed organic field-effect transistors (OFETs) are essential for developing organic electronics. The encouraging development in solution-processed OFETs has attracted research interest because of their potential in low-cost devices with performance comparable to polycrystalline-silicon-based transistors. In recent years, unidirectional coating technology, featuring thin-film coating along only one direction and involving specific materials as well as solution-assisted fabrication methods, has attracted intensive interest. Transistors with organic semiconductor layers, which are deposited via unidirectional coating methods, have achieved high performance. In particular, carrier mobility has been greatly enhanced to values much higher than 10 cm2 V−1 s−1. Such significant improvement is mainly attributed to better control in morphology and molecular packing arrangement of organic thin film. In this review, typical materials that are being used in OFETs are discussed, and demonstrations of unidirectional coating methods are surveyed.

Study of liquid-phase molecular packing interactions and morphology of fatty acid methyl esters (biodiesel).

Background1H low field nuclear magnetic resonance (LF-NMR) relaxometry has been suggested as a tool to distinguish between different molecular ensembles in complex systems with differential segmental or whole molecular motion and/or different morphologies. In biodiesel applications the molecular structure versus liquid-phase packing morphologies of fatty acid methyl esters (FAMEs) influences physico-chemical characteristics of the fuel, including flow properties, operability during cold weather, blending, and more. Still, their liquid morphological structures have scarcely been studied. It was therefore the objective of this work to explore the potential of this technology for characterizing the molecular organization of FAMEs in the liquid phase. This was accomplished by using a combination of supporting advanced technologies.ResultsWe show that pure oleic acid (OA) and methyl oleate (MO) standards exhibited both similarities and differences in the 1H LF-NMR relaxation times (T2s) and peak areas, for a range of temperatures. Based on X-ray measurements, both molecules were found to possess a liquid crystal-like order, although a larger fluidity was found for MO, because as the temperature is increased, MO molecules separate both longitudinally and transversely from one another. In addition, both molecules exhibited a preferred direction of diffusion based on the apparent hydrodynamic radius. The close molecular packing arrangement and interactions were found to affect the translational and segmental motions of the molecules, as a result of dimerization of the head group in OA as opposed to weaker polar interactions in MO.ConclusionsA comprehensive model for the liquid crystal-like arrangement of FAMEs in the liquid phase is suggested. The differences in translational and segmental motions of the molecules were rationalized by the differences in the 1H LF-NMR T2 distributions of OA and MO, which was further supported by 13C high field (HF)-NMR spectra and 1H HF-NMR relaxation. The proposed assignment allows for material characterization based on parameters that contribute to properties in applications such as biodiesel fuels.

Generalized stacking fault energies in the basal plane of triclinic molecular crystal 1,3,5-triamino-2,4,6-trinitrobenzene (TATB)

Molecular dynamics and molecular statics simulations were used to investigate dislocation glide in the basal plane (i.e. the – plane, where , and define the edge vectors of the unit cell) of -triamino--trinitrobenzene (TATB) using generalized stacking fault energy surfaces (-surfaces). Triclinic symmetry and molecular packing arrangement result in two glide plane types for the same glide plane normal. The unstable stacking fault energies () are less than at K and atm, indicating easy dislocation glide. Glide in the and – directions is preferred compared to the direction at atm. A stable stacking fault is predicted in the – direction at atm and 5 GPa. A compound twin with energy is predicted to be stable in the basal plane. Unequal values of about the stable stacking fault in the and – traces indicate an asymmetric barrier to dislocation glide. High pressure ( GPa) results in an increase in the values. The extremely small barriers to twinning and dislocation glide might be sources for observed second-harmonic generation in the nominally centrosymmetric crystal.

Nitrile-Halogen Interactions in Some Bridge-Flipped Isomeric Benzylideneanilines

We designate as "bridge-flipped isomers" those pairs of molecules that differ only in the orientation of a bridge of atoms linking two major molecular fragments: in benzylideneanilines, Ar-CH=N-Ar' vs. Ar-N=CH-Ar'; in phenylhydrazones, Ar-NH-N=CH-Ar' vs. Ar-CH=N-NH-Ar' (Ar = aryl). We use them as a context in which to evaluate the roles of molecular conformation, hydrogen bonding, space-filling requirements, and supramolecular synthons in establishing crystalline isomorphism or non-isomorphism. To examine nitrile-halogen and halogen-halogen interactions in solid isomeric benzylideneanilines, we have determined the structures of 2-cyanobenzylidene-2'-iodoaniline (I), 2-iodobenzylidene-2'-cyanoaniline (II), 2-cyanobenzylidene-2'-bromoaniline (III), 2-cyanobenzylidene-2'-chloroaniline (IV), and 2-chlorobenzylidene-2'-cyanoaniline (V) by single-crystal X-ray diffraction. I/II and IV/V are bridge-flipped isomeric pairs. I, III, and IV are isomorphous; the I/II and IV/V pairs are not. In I, III, and IV, translationally related molecules are linked into chains by C≡N···X contacts; no close X···X contacts occur. Although C≡N···X contacts between translationally related molecules define chains in II similar to those in I, III, and IV, and although II likewise lacks close X···X contacts, the molecular packing arrangements differ (monoclinic for I, III, and IV vs. triclinic for II). V in contrast assumes an orthorhombic structure from which the C≡N···X interaction is absent and which is isomorphous with 2-chlorobenzylidene-2'-chloroaniline (VI), the nitrile group of V exchanged for the aniline-side chlorine of VI, that chlorine atom in VI not involved in a close intermolecular Cl···Cl contact. Although neither C≡N···X nor X···X contacts result in isomorphous bridge-flipped isomers in the cases of I/II and IV/V, the C≡N···X contacts apparently play a major structure-defining role and supersede any potential X···X contacts in I-IV.

Two-Dimensional Brickblock Arrangement in Bis-Fused Tetrathiafulvalene Semiconductors

Molecular packing arrangement is a very important factor in the charge carrier mobility of organic semiconductors, but its rational design has not been established as yet. Two-dimensional (2D) lamellar packing is an advantageous arrangement for high charge mobility, but few examples have been reported thus far. Herein we show crystal structures and the electronic properties of newly designed bis-fused tetrathiafulvalene (TTF) semiconductors with hetero substituent groups with distinct electronic effects. Unprecedented 2D lamellar alignment is achieved in a single crystal, where the bis-fused TTF rings interact three dimensionally with face-to-face and side-by-side intermolecular S···S contacts up to a total of 20 sites per π molecule and form graphitelike “brickblock” structure. The charge mobility of a single crystal is as high as 0.78 cm2 V–1 s–1. Systematic investigations of the semiconductors reveal a key role of intramolecular S···O interaction between a bis-fused TTF ring and a methoxycarbonyl group...

Variation in the Helical Structure of Native Collagen

The structure of collagen has been a matter of curiosity, investigation, and debate for the better part of a century. There has been a particularly productive period recently, during which much progress has been made in better describing all aspects of collagen structure. However, there remain some questions regarding its helical symmetry and its persistence within the triple-helix. Previous considerations of this symmetry have sometimes confused the picture by not fully recognizing that collagen structure is a highly complex and large hierarchical entity, and this affects and is effected by the super-coiled molecules that make it. Nevertheless, the symmetry question is not trite, but of some significance as it relates to extracellular matrix organization and cellular integration. The correlation between helical structure in the context of the molecular packing arrangement determines which parts of the amino acid sequence of the collagen fibril are buried or accessible to the extracellular matrix or the cell. In this study, we concentrate primarily on the triple-helical structure of fibrillar collagens I and II, the two most predominant types. By comparing X-ray diffraction data collected from type I and type II containing tissues, we point to evidence for a range of triple-helical symmetries being extant in the molecules native environment. The possible significance of helical instability, local helix dissociation and molecular packing of the triple-helices is discussed in the context of collagen's supramolecular organization, all of which must affect the symmetry of the collagen triple-helix.

Temperature-Pressure Phase Behavior of Triglycerides Revealed by Synchrotron X-Ray Scattering Studies

Triglycerides, such as cocoa butter (also called theobroma oil), are widely used in food industry as they are a main part of foodstuff such as chocolate. This ingredient affects important properties like gloss, texture and mouth feeling. Cocoa butter consists mainly of three triacylglycerols (TAGs): 1,3-dipalmitoyl-2-oleoylglycerol (POP), 1-palmitoyl-2-oleoyl-3-stearoylglycerol (POS) and 1,3-di-stearoyl-2-oleoylglycerol (SOS). Besides, smaller amounts of mono-, di-glycerides and free fatty acids are present.The TAG composition of cocoa butter or a fat in general is one of the most important parameters since it governs the physical properties and determines its polymorphic form. Polymorphism is defined here as the ability of the TAG molecules to crystallize in different molecular packing arrangements corresponding to different unit cell structures. Fat polymorphs are typically classified by three main forms (α, β', β and variations within these types). The different polymorphs determine the physical-chemical properties and hence the texture, long-term stability and consequently shelf-life of the products used in technological applications. In a combined synchrotron small- and wide-angle x-ray scattering (SAXS/WAXS) study, we have explored the use of the thermodynamic parameter pressure in modulating and controlling these properties, and to determine the temperature-pressure phase diagram of a triglyceride system.

Similarities and differences in the crystal packing of methoxybenzyl and methoxyphenylethyl-1,3,4-oxadiazole-2(3H)-thiones

This work reports the crystal and molecular structures of five 1,3,4-oxadiazol-2(3H)-thiones derived from 2, 3 and 4-methoxy substituted 2-phenylacetic acid and 2- and 4-methoxy substituted 3-phenylpropanoic acid. The methoxybenzyl-1,3,4-oxadiazol-2(3H)-thiones are V-shaped while the corresponding methoxyphenylethyl-systems are close to planar, which impacts the solid state molecular packing arrangement. For molecules 1–4, inversion related N–H⋯S hydrogen bonds generating R22(8) rings dominate the packing, supported by an eclectic mix of C–H⋯O, C–H⋯S, C–H⋯π and π⋯π contacts that stack the molecules into interconnected columns. In contrast, each of the two unique molecules in the asymmetric unit of 5 forms N–H⋯O contacts with like molecules augmented by C–H⋯S contacts for one molecule and C–H⋯O contacts for the other to generate planar layers that are interconnected though a series of π⋯π stacking interactions.

Ordered Structures from Crystalline Carbon Disulfide Solvates of the Nano-Tubular Fullerenes D5h(1)-C90 and D5h-C70

The structures of three crystalline solvates, D5h(1)-C90·CS2, D5h-C70·3CS2, and 2(D5h-C70)·3CS2, of nanotubular fullerenes have been determined by single crystal X-ray diffraction. Despite the marked tendency for fullerenes to disorder, the carbon cages in all three structures are fully ordered at 100(2) K for D5h(1)-C90·CS2 and 90 K for the other two crystals. Moreover, the carbon disulfide molecules are also ordered, except for the case of D5h(1)-C90·CS2, where there is a minor disorder in the solvate location. The molecular packing in D5h(1)-C90·CS2 reflects the nanotubular nature of the fullerene component with channels of alternating fullerenes and carbon disulfide molecules running along the crystallographic b axis. The molecular packing arrangements for D5h-C70·3CS2 and 2(D5h-C70)·3CS2 do not show such channels. In D5h-C70·3CS2, the carbon disulfide molecules form chains that snake between the fullerenes and along the crystallographic a axis. In 2(D5h-C70)·3CS2, there are two crystallographically distinct fullerene cages, which are segregated into individual layers. Within each layer, the fullerenes show hexagonal close packing and the carbon disulfide molecules form chains that snake between the fullerene layers in a zigzag fashion. The presence of diiodine in solution was essential for the formation of crystals of D5h-C70·3CS2 and 2(D5h-C70)·3CS2 that were suitable for structure determination, although no diiodine was incorporated in these crystals.

Influence of Anions and Alkyl Chain Lengths ofN-Alkyl-n-(R)-12-Hydroxyoctadecyl Ammonium Salts on Their Hydrogels and Organogels

The self-assembly and gelating characteristics of a set of N-alkyl-(R)-12-hydroxyoctadecylammonium salts (n-HOA-X, where n = 0-6, 18 is the length of the alkyl chain on nitrogen, X = Cl, n = 3, and X = Br, NO3, and BF4) are described. Solid-solid phase transitions were observed for powders of n-HOA-Cl, and orthorhombic-type crystal packing arrangements and lattice spacings were calculated from X-ray diffractograms at 22 °C. The diffractogram of 3-HOA-Br indicates the presence of more than one morph at room temperature, and that of 3-HOA-I corresponds to a lamellar packing arrangement. Differences in the molecular packing arrangements of 3-HOA-X are reflected in their gelation abilities. The melting temperatures (T(gel)) of the hydrogels of 3-HOA-Br are higher than those of 3-HOA-Cl at the same concentrations, and 3-HOA-I failed to gelate any of the investigated liquids. 3-HOA-NO3 gelated only water and CCl4 and 3-HOA-BF4 formed only hydrogels. Plots of changes in conductivities of the 3-HOA-X salts (where X = Cl, Br, NO3 and BF4) as a function of temperature were used to calculate the critical aggregation concentrations (CGCs). Because the CGCs from the 'falling drop' method are nearly the same as those from the conductivity measurements, aggregation, nucleation, and gelation must occur within a very narrow 3-HOA-X concentration range. T(gel) values of 2 wt % 3-HOA-Cl hydrogels (prepared by fast cooling of the sol phase) increased upon adding KCl up to 0.1 M. The effects can be attributed principally to the chloride anion rather than its cation partners. The properties of the hydrogels of 3-HOA-X do not follow the Hofmeister ranking rule. The variations in the counterions afford detailed insight into the behavior of 3-HOA-X in their neat solids and assemblies in gels as well as the processes accompanying gel formation in water and organic liquids.

Quaternary Ammonium-Type Gemini Surfactants Synthesized from Oleic Acid: Aqueous Solution Properties and Adsorption Characteristics

Cationic gemini surfactants having a quaternary ammonium headgroup have been synthesized from oleic acid. The hydrocarbon chain is covalently bound to the terminal carbonyl group of oleic acid via an amide bond, while the quaternary ammonium headgroup is introduced onto the cis double bond of oleic acid. The Krafft temperature of these surfactants drops below room temperature (ca. 25°C) when the counterion is exchanged from Br⁻ to Cl⁻. The aqueous solution properties of the Cl series of surfactants have been assessed by means of pyrene fluorescence, dynamic light scattering (DLS), and static surface tension measurements. An increased hydrocarbon chain length results in a lower critical micelle concentration (cmc) and a higher adsorption efficiency at the air/aqueous solution interface. Surface tension measurements suggest the formation of premicelles at concentrations below cmc, whereas, above cmc, DLS indicates the formation of micellar aggregates whose diameter ranges from 5 to 10 nm. We, furthermore, characterized the adsorption of these surfactants to the silica/aqueous solution interface and observed their spontaneous adsorption to the solid surface by electrostatic and intermolecular hydrophobic interactions. The combination of soft-contact imaging atomic force microscopy (AFM) and force-curve data suggests bilayer formation above cmc, which is reflective of the large packing parameter of the gemini surfactants. Interestingly, we found the repulsive interaction observed during compression of the adsorbed layer to be relatively weak, as a result of the low adsorption density and/or the loose molecular packing arrangement, which arises from the asymmetric structure.

Study of polymorphism in imatinib mesylate: A quantum chemical approach using electronic and vibrational spectra

Imatinib mesylate, 4-(4-methyl-piperazin-1-ylmethyl)-N-u[4-methyl-3-(4-pyridin-3-yl)pyrimidine-2-ylamino)phenyl]benzamide methanesulfonate is a therapeutic drug that is approved for the treatment of chronic myelogeneous leukemia (CML) and gastrointestinal stromal tumors (GIST). It is known that imatinib mesylate exists in two polymorphic forms α and β. However, β-form is more stable than the α-form. In this work, we present a detailed vibrational spectroscopic investigation of β-form by using FT-IR and FT-Raman spectra. These data are supported by quantum mechanical calculations using DFT employing 6-311G(d,p) basis set, which allow us to characterize completely the vibrational spectra of this compound. The FT-IR spectrum of α-form has also been discussed. The importance of hydrogen-bond formation in the molecular packing arrangements of both forms has been examined with the vibrational shifts observed due to polymorphic changes. The red shift of the NH stretching bands in the infrared spectrum from the computed wavenumber indicates the weakening of the NH bond. The UV–vis spectroscopic studies along with the HOMO–LUMO analysis of both polymorphs (α and β) were performed and their chemical activity has been discussed. The TD-DFT method was used to calculate the electronic absorption spectra in the gas phase as well as in the solvent environment using IEF-PCM model and 6-31G basis set. Finally, the results obtained complements to the experimental findings.

Nonisomorphous X-Ray Structures of Tritylnitrile and Tritylisonitrile

The crystal structures of a related series of isosteric compounds, namely tritylnitrile 1, tritylisonitrile 2, and a solid solution of tritylnitrile/tritylisonitrile 3 have been determined by single crystal X-ray diffraction. The bond lengths, angles, and intermolecular interactions do not deviate significantly from previously reported nitriles and isonitriles. These isosteric compounds pack in nonisomorphous but very similar structures. Subtle differences between the packing of 1 and 2 are detailed. The solid solution of tritylnitrile/tritylisonitrile 3 is isomorphous with the structure of tritylnitrile 1. This solid solution possesses substitutional disorder involving the nitrile and isonitrile groups (Z′ = 2, nitrile/isonitrile occupancy ratios 53.7/46.3 and 54.6/45.4). The structure of tritylisonitrile 2 possesses positional disorder over two sites (Z′ = 2, occupancy ratio of 93.1/6.9). A comparison with previously reported nearly isosteric structures reveals that other chemically dissimilar compounds crystallize in each of the two molecular packing arrangements described in this report. Structure 1: monoclinic, P21/c, a = 18.049(11) A, b = 9.583(6) A, c = 17.824(11) A, β = 108.680(10)°, V = 2921(3) A3, Z = 8, R 1 = 0.0467, wR 2 = 0.1491. Structure 2: triclinic, P $$ \overline{1} $$ , a = 9.5877(13) A, b = 10.7479(14) A, c = 14.809(2) A, α = 88.457(2)°, β = 80.303(2)°, γ = 77.128(2)°, V = 1466.4(3) A3, Z = 4, R 1 = 0.0423, wR 2 = 0.1151. Structure 3: monoclinic, P21/c, a = 18.03464(15) A, b = 9.5739(8) A, c = 17.8251(11) A, β = 108.5920(10)°, V = 2917.4(4) A3, Z = 8, R 1 = 0.0587, wR 2 = 0.1734. The crystal structures of the isosteric pair tritylnitrile 1 and tritylisonitrile 2 are nonisomorphous while the structure of a solid solution of tritylnitrile/tritylisonitrile 3 is isomorphous with the structure of tritylnitrile 1.

Molecular Arrangements in Two-Dimensional J-Aggregate Monolayers of Cyanine Dyes

Atomic force microscopy was used for the study of two-dimensional J-aggregates monolayer structures of two cyanine dyes with a high-resolution. Two coexisting morphologically different types (i.e. the bilayer rectangular stripes and monolayer curve shaped leaves) were observed for 3,3’-di(γ-sulfopropyl)-4,4’,5,5’-dibenzo-9-ethylthiacarbocyanine betaine pyridinium (dye D-1). The morphological diversity, the crystallographic peculiarities and the monolayer height ~ 1.35 nm are selfconsistently explained by the difference in the molecular packing arrangements, i.e. the ladder for stripes and the staircase for leaves. A building unit of both J-aggregate types is a symmetric monolayer with the same height and slip angle ~ 15o, composed of all-trans monomers with anti-parallel up-down orientation. For the two-dimensional sheets of the second cyanine dye (5,5'-dichloro-3,3'-disulfopropyl thiacyanine sodium salt) Dye-2 the fine substructure formed by close packed parallel narrow threads was resolved. The stripes were inclined at the angle ~30o to the J-aggregates long side providing for the first time the direct microscopic support for the probable staircase molecular arrangement. It is speculated that the quazi-one-dimensional staircase-type and ladder-type units are building blocks of the two-dimensional aggregates of various cyanine dyes.

Correlations of Properties and Structures at Different Length Scales of Hydro- and Organo-gels Based onN-Alkyl-(R)-12-Hydroxyoctadecylammonium Chlorides

The self-assembly and gelating ability of a set of N-alkyl-(R)-12-hydroxyoctadecylammonium chlorides (NCl-n, where n = 0-6, 18 is the length of the alkyl chain on nitrogen) are described. Several are found to be ambidextrous (gelating both water and a variety of organic liquids) and very efficient (needing less than ca. 0.5 wt % at room temperature). Structure-property correlations at different distance scales of the NCl-n in their hydro- and organo-gels and neat, solid states have been made using X-ray diffraction, neutron scattering, thermal, optical, cryo-SEM and rheological techniques. The self-assembled fibrillar networks consist of spherulitic objects with fibers whose diameters and degrees of twisting differ in the hydro- and organo-gels. Increasing n (and, thus, the molecular length) increases the width of the fibers in their hydrogels; an irregular, less pronounced trend between n and fiber width is observed in the corresponding toluene gels. Time-dependent, small angle neutron scattering data for the isothermal sol-to-gel transformation of sols of NCl-18/toluene to their gels, treated according to Avrami theory, indicate heterogeneous nucleation involving rodlike growth. Rheological studies of gels of NCl-3 in water and toluene confirm their viscoelastic nature and show that the hydrogel is mechanically stronger than the toluene gel. Models for the different molecular packing arrangements within the fibrillar gel networks of the hydro- and organogels have been inferred from X-ray diffraction. The variations in the fibrillar networks provide a comprehensive picture and detailed insights into why seemingly very similar NCl-n behave very differently during their self-assembly processes in water and organic liquids. It is shown that the NCl-n provide a versatile platform for interrogating fundamental questions regarding the links between molecular structure and one-dimensional self-aggregation, leading to gelation.

CO2 as an Oxygen Source for Polysiloxanes – Preparation, Crystal Structure and Thermal Decomposition of Two Novel Silylcarbamates

Diaminosilanes (CH3)2Si(NHR)2 1a, b are obtained from dichlorodimethylsilane and primary amines (R = n-hexyl, iso-propyl) in good yields. Bis-insertion of CO2 into the Si-N bonds of the aminosilanes quantitatively gives the silylcarbamates (CH3)2Si(OCONHR)2, 2a, b. Oligo- and polydimethylsiloxanes 3a, b as well as N,N`-substituted ureas CO(NHR)2 4a, b are formed upon heating the silylcarbamates 2a, b to ~ 150 ◦C. The results of comprehensive NMR analyses of the aminosilanes, the two novel bis-silylcarbamates, the siloxanes and the ureas, and of single-crystal structure analyses of 2a and 4b are presented. In the crystal the n-hexyl silylcarbamate 2a shows a similar molecular packing arrangement as the iso-propyl urea derivative 4b.