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Controlled Hierarchical Self-Assembly and Deposition of Nanoscale Photonic Materials We gratefully acknowledge support from the National Science Foundation (CHE0135509, CHE0095649, and IGERT DGE-9972892), the Israel-US Binational Science Foundation (1999082), and PSC-CUNY. H. C. Chemistry department infrastructure is partially supported by NIH RCMI program GM3037. Dr. Andy Round is thanked for technical assistance
Abstract
Gestapelte Moleküle: Porphyrin-Nonamer-Anordnungen stapeln sich durch Selbstorganisation zu dreidimensionalen Aggregaten (siehe Schema: „L“=L-förmiges Porphyrin (Eckstück), „T“=T-förmiges Porphyrin (Seitenstück), „+“=kreuzförmiges Porphyrin (Zentrum)). Die Größe der Aggregate auf Oberflächen kann von einem einzelnen Nonamer bis zu 50 nm hohen Stapeln reichen; dies hängt zum einen von der Oberfläche ab und lässt sich zum anderen noch chemisch kontrollieren.
Eine begrenzte Zahl Polyethylenoxid-substituierter Perylenbisimide, die wahlweise mit Terpyridin-Liganden für eine Metallkoordination funktionalisiert sind (siehe Beispielstruktur), kombiniert verschiedene nichtkovalente Wechselwirkungen und kann so in eine Vielzahl von optoelektronisch aktiven, organischen Materialien mit unterschiedlichen Überstrukturen verarbeitet werden.
Molecular assemblies (MAs) of oligofurans and oligothiophenes were formed from solutions on various substrates. These films were obtained by alternating deposition of organic chromophores (oligofurans or oligothiophenes) and a palladium salt. These coordination-based MAs were characterized by UV/Vis spectroscopy, spectroscopic ellipsometry, atomic force microscopy (AFM), X-ray reflectivity (XRR), X-ray photoelectron spectroscopy (XPS), and electrochemistry. The MAs exhibit similar electrochemical behavior and their growth and structure are apparently not affected when different organic template layers are used. The density of the MAs is a function of the structure of the molecular component. The oligothiophene density is approximately 50 % higher than that observed for the oligofuran-based assemblies. The optical and electrochemical properties of the MAs scale linearly with their thickness. The UV/Vis data indicate that upon increasing the film thickness, there is no significant conjugation between the metal-separated organic chromophores. DFT calculations confirmed that the HOMO-LUMO gap of the surface-bound oligofuran and oligothiophene metal oligomers do not change significantly upon increasing their chain length. However, electrochemical measurements indicate that the susceptibility of the MAs towards oxidation is dependent on the number of chromophore units.
Vermutlich werden synthetische 2D-Polymere auf vielen Gebieten der Naturwissenschaft und Technik Bedeutung erlangen, und zwar grundlegende ebenso wie praktische. Daher überrascht es, dass auf diesem Gebiet bisher so wenige Fortschritte zu verzeichnen sind. In den letzten Jahrzehnten sind zwar viele Ansätze zur Synthese solcher Polymere beschrieben worden, die tatsächliche Synthese eines kovalent verknüpften, periodischen, molekularen Films mit der Dicke einer Monomereinheit ist bisher jedoch nicht gelungen. Unser Aufsatz liefert einen Überblick über bisherige Strategien sowie eine Analyse, wie die Synthese eines 2D-Polymers wohl gelingen könnte. Bei unserer Analyse wird die Polymerisation in einer (anfänglich) homogenen Phase mit der Polymerisation an Grenzflächen verglichen, und es werden strukturelle Aspekte von Monomeren sowie möglicherweise bevorzugte Verknüpfungsmodi berücksichtigt. Es werden auch Aspekte wie Schrumpfung sowie Domänen- und Rissbildung behandelt, und es wird kurz darauf eingegangen, wie die Chancen für eine erfolgreiche Strukturanalyse des Endprodukts verbessert werden könnten.
Scanning tunneling microscopy has been used to study the adsorbed phases of functionalized terpyridines at the solid–liquid interface of highly ordered pyrolitic graphite (HOPG). Terpyridines are well-known for their complexing behavior to transition metal ions, making them widely used ligands in organometallic and supramolecular chemistry. We found that solutions of 2,2′:6′,2″-terpyridine-4′-oxydodecane (tpy-O-C12) and 2,2′:6′,2″-4′-oxyoctadecane (tpy-O-C18) form highly ordered two- dimensional (2D) arrays on HOPG in phenyloctane. For both compounds, large, well-defined lamellar domains have been observed with domain sizes larger than 500 nm. Sequential scans of an area with two grain boundaries indicate that desorption–resorption is taking place along the domain edges. High-resolution images of the lamella have been obtained, and the 2D packing within the lamella was determined in detail. The terpyridines align in long uniform double rows with their alkyl chains packing in an alternating, zipper-like fashion. The terpyridines pack with the polar head-groups head to head, and the observed size and shape of the molecules fit exactly to their modeled geometries.
The molecular interaction of dihydroxo[5,10,15,20-tetrakis(4-tert-butyl-phenyl)porphyrinato]-tin(IV) (SnTTBPP(OH)(2)), the structural order and growth of ultrathin films on Ag(100) have been studied by means of low-energy electron diffraction (LEED) and synchrotron based photoelectron spectroscopy, i.e., X-ray photoemission (XPS) and near-edge X-ray absorption fine structure (NEXAFS/XANES) spectroscopy. For the first time, monolayer adsorption of a metalloporphyrin with octahedral coordination of the metal center by two additional axial hydroxo ligands is investigated in a multi-technique study. The delicate balance of molecule-substrate interactions and intermolecular interactions leads to the formation of a densely-packed organic monolayer which is commensurate with the Ag(100) substrate. From NEXAFS linear dichroism an almost coplanar orientation of the porphyrin system is derived. XPS and NEXAFS clearly indicate that the axial hydroxo ligands are cleaved in monolayer films, i.e., upon adsorption to the Ag substrate. With increasing film thickness orientational order gets lost and leads to polycrystalline growth for thicker films as confirmed by scanning X-ray transmission microscopy (STXM).
A limited number of poly(ethylene oxide)-substituted perylene bisimides, some of which are equipped with terpyridine ligands for transition-metal coordination (see structure), combine different types of noncovalent interactions to yield optoelectronically active organic materials with different types of supramolecular morphologies.
Rings around a ring : The synthesis, characterization, material properties and biological studies of novel hexakis porphyrinato cyclotriphosphazene systems (see scheme) and their metal derivatives are described. magnified image
A simple method has been employed to synthesize a cyclotriphosphazene appended with six porphyrins. The reaction of one equivalent of hexachlorocyclotriphosphazene with six equivalents of 5‐(4‐hydroxyphenyl)‐10,15,20‐tri( p ‐tolyl)porphyrin or ‐21‐thiaporphyrin in tetrahydrofuran in the presence of cesium carbonate, followed by simple column‐chromatography purification, afforded the hexaporphyrinato cyclotriphosphazene systems as the single products in 80–90 % yields. The metal(II) derivatives (Cu II , Zn II ) of the hexaporphyrinato cyclotriphosphazene systems were prepared by treating the hexaporphyrin assemblies with the appropriate metal salts under standard metallation conditions. The compounds are freely soluble in common organic solvents and were characterized by mass spectrometry, NMR, absorption, and fluorescence spectroscopy, and electrochemical techniques. Material studies carried out by using fluorescence microscopy, SEM, and AFM techniques showed that the hexaporphyrinato cyclotriphosphazene systems form ringlike architectures. Preliminary biological studies indicate that the hexacopper(II) porphyrin assembly can be used as an artificial nuclease.
In light of the considerable impact synthetic 2D polymers are expected to have on many fundamental and applied aspects of the natural and engineering sciences, it is surprising that little research has been carried out on these intriguing macromolecules. Although numerous approaches have been reported over the last several decades, the synthesis of a one monomer unit thick, covalently bonded molecular sheet with a long-range ordered (periodic) internal structure has yet to be achieved. This Review provides an overview of these approaches and an analysis of how to synthesize 2D polymers. This analysis compares polymerizations in (initially) a homogeneous phase with those at interfaces and considers structural aspects of monomers as well as possibly preferred connection modes. It also addresses issues such as shrinkage as well as domain and crack formation, and briefly touches upon how the chances for a successful structural analysis of the final product can possibly be increased.
What a PET! Two well‐defined nanoarchitectures with 2D netlike and 1D linear topological structures are constructed by bis( p ‐sulfonatocalix[5]arenes) and cationic porphyrins, respectively, in which an unambiguous PET process is observed. As a result, the supramolecular aggregates possess, in principle, benign photoelectric properties with the transport of an electron between the building blocks in the nanoscale region. magnified image
Possessing 2D netlike and 1D linear morphologies, two nano‐supramolecular architectures A1 and A2 are constructed by tetracationic porphyrin ( G1 ) and dicationic porphyrin ( G2 ), respectively, upon complexation with the novel water‐soluble bis( p ‐sulfonatocalix[5]arenes) bridged at the lower rim ( H2 ). Corresponding to the molecular design, the aggregation morphologies are well manipulated by the inherent binding sites of the building blocks through host–guest interactions as well as charge interactions. In comparison to the simple p ‐sulfonatocalix[5]arene H1 which can only form particle‐type complexes C1 and C2 with porphyrin guests, H2 provides excellent pre‐organized structure to construct highly complex nano‐supramolecular assemblies. The exhibited electron‐transfer process of the supramolecular systems is further investigated by steady‐state and time‐resolved fluorescence spectroscopy, electrochemical measurements, and transient absorption spectroscopy. The results obtained show that calixarenes are also effective electron donors in PET besides acting as significant building blocks, which gives them many advantages in constructing well‐ordered nanomaterials with the capability of electron and energy transport.
We report the synthesis of molecular sheets based on the photochemically initiated dimerization of monomers with lateral anthracene units. The film thickness and composition were investigated by ellipsometry and X-ray photoelectron spectroscopy (XPS). The mechanical stability of the film was sufficient to span it over 45x45 microm-sized holes. Several model reactions were performed to illustrate the underlying chemistry and to assist in analysis. The reported experiments are considered first steps towards the ultimate goal of the rational synthesis of laterally "infinite", one-monomer-unit-thick molecular sheets with a long-range positional order and a periodic covalent-bonding pattern. Such sheets are referred to as 2D polymers and are considered a prime goal of chemical synthesis with intriguing applications.
Mo takes the tube: A saddle-distorted Mov-porphyrin complex forms a tubular assembly through intermolecular π-π interactions, which has an internal diameter of 1 nm (see picture). Within such tubes, tetranuclear Mo VI-oxo clusters could be size-selectively included through intermolecular hydrogen-bonding interactions.
The synthesis and self-assembly behaviour of porphyrin dodecamers 1H(2) and Zn-1, which consist of twelve porphyrins that are covalently attached to a central aromatic core, is described. According to STM, 1D and 2D NMR studies, and molecular modelling calculations, the porphyrin dodecamers have a yo-yo-shaped structure. Their large pi surface, in combination with their disk-like shape, allows them to form self-assembled structures, which in the case of Zn-1 can be tuned by adding bidentate ligands. The self-assembly of the molecules at the liquid-solid interface of 1-phenyloctane with highly oriented pyrolytic graphite or Au(111) was imaged by using STM. The porphyrin disks in the self-assembled arrays have an edge-on orientation on the surface. The addition of bidentate axial ligands to the Zn-1 molecules in the arrays allows their intermolecular distance to be precisely controlled.
A number of new porphyrins equipped with complementary triple hydrogen-bonding groups were synthesized in good yields. Self-assembly was investigated by NMR spectroscopy, dynamic light scattering (DLS), and atomic force microscopy (AFM). These artificial antenna systems were further characterized by stationary and time-resolved fluorescence techniques to investigate several yet unsolved questions on the mechanism of excitation energy transfer (EET) in supramolecular systems. For example, the photophysics of a simple D--U[triple chemical bond]P--A dyad was studied, in which donor D and acceptor A are ZnII- metalated and free-base porphyrins, respectively, and U (uracyl) and P (2,6-diacetamidopyridyl) are complementary hydrogen-bonding groups linked by flexible spacers. In this dyad, the EET occurs with about 20 % efficiency with a lifetime of 14 ps. Reversal of the nonsymmetric triple hydrogen-bonding groups to give a A--U[triple chemical bond]P--D construct results in an EET efficiency of about 25 % and a lifetime of 19 ps. Thus, there is a slight directionality of EET mediated by these asymmetric triple hydrogen-bonding units tethered to flexible spacers. In polymeric systems of the type P-D-P[triple chemical bond]U-A-U[triple chemical bond]P-D-P, or U-D-U[triple chemical bond]P-A-P[triple chemical bond]U-D-U, the EET efficiency doubles as each donor is flanked by two acceptors. Because doubling the probability of photon capture doubles the EET efficiency, there is no energy amplification, which is consistent with the "antenna effect". For these polymeric systems, AFM images and DLS data indicate large rodlike assemblies of a few hundred nanometers, whereas the components form much smaller aggregates under the same conditions. To understand the importance of the flexible hydrogen-bonding zipper, three different covalently bridged D-B-A molecules were synthesized in which the bridge B is a rigid steroidal system and the same ester chemistry was used to link the porphyrins to each end of the steroid. The geometry inferred from molecular modeling of D-B-A indicates geometric similarities between B and some conformations of the --P[triple chemical bond]U-- supramolecular bridge. Although the EET efficiency is a factor of two greater for the steroidal systems relative to the supramolecular dyads, the rate is 50-80 times slower, but still slightly faster than that predicted by Förster-type mechanisms. Circular dichrosim (CD) spectra provide a conformational sampling of the porphyrin groups appended on the steroidal skeleton, thus allowing an estimation of the orientation factor kappa for the transition dipole moments, which significantly affects the EET rate. We conclude that the flexible hydrogen-bonded linked systems are adaptive and have variable geometries with foldamers in which the D and A groups can approach well under 1 nm. In these folded conformations, a rapid EET process occurs, probably also involving a Dexter-type exchange mechanism, thus explaining the fast EET relative to the rigid steroidal compounds. This study predicts that it is indeed possible to build large supramolecular antennas and the component design and supramolecular dynamics are essential features that dictate EET rates and efficiencies.
Photoformation of metalloporphyrin cations in a lipid bilayer increases the ionic currents of negative and decreases those of positive hydrophobic ions. At low concentrations of the mobile hydrophobic ion, a 30% change in conductivity is observed that decreases with increasing concentration of positive tetraphenylphosphonium ion and increases drastically with increasing concentration of negative tetraphenylboride ion. In the region of saturated conductance of boride ion, the increase in conductivity is 3.6-fold. A 15-fold increase is observed with the protonophore carbonyl cyanide 3-chlorophenylhydrazone. In this case the net charge gated is 300 times greater than the photogenerated charge in the bilayer membrane. Thus there is a net gain in this organic field effect phototransistor. The gating can also be accomplished by continuous light or chemical oxidants. Photogating is explained as space charge effects inside the bilayer.
Crystals are generally considered to grow by attachment of ions to inorganic surfaces or organic templates. High-resolution
transmission electron microscopy of biomineralization products of iron-oxidizing bacteria revealed an alternative coarsening
mechanism in which adjacent 2- to 3-nanometer particles aggregate and rotate so their structures adopt parallel orientations
in three dimensions. Crystal growth is accomplished by eliminating water molecules at interfaces and forming iron-oxygen bonds.
Self-assembly occurs at multiple sites, leading to a coarser, polycrystalline material. Point defects (from surface-adsorbed
impurities), dislocations, and slabs of structurally distinct material are created as a consequence of this growth mechanism
and can dramatically impact subsequent reactivity.
A method of constructing <30-nanometer structures in close proximity with precise spacings is presented that uses the step-by-step
application of organic molecules and metal ions as size-controlled resists on predetermined patterns, such as those formed
by electron-beam lithography. The organic molecules serve as a ruler for scaling down a larger “parent” structure. After metal
deposition and lift-off of the organic multilayer resist, an isolated smaller structure remains on the surface. This approach
is used to form thin parallel wires (15 to 70 nanometers in width and 1 micrometer long) of controlled thickness and spacing.
The structures obtained were imaged with field emission scanning electron microscopy. A variety of nanostructures could be
scaled down, including structures with hollow patterns.
The following article is an edited transcript of the Symposium X: Frontiers of Materials Research addresses by Mark A. Reed, the Harold Hodgkinson Professor of Engineering and Applied Science and Chair of Electrical Engineering at Yale University, presented at the MRS Fall Meeting on December 1, 1999.
A simple model of the charge distribution in a π-system is used to explain the strong geometrical requirements for interactions between aromatic molecules. The key feature of the model is that it considers the σ-framework and the π-electrons separately and demonstrates that net favorable π-π interactions are actually the result of π-σ attractions that overcome π-π repulsions. The calculations correlate with observations made on porphyrin π-π interactions both in solution and in the crystalline state. By using an idealized π-atom, some general rules for predicting the geometry of favorable π-π interactions are derived. In particular a favorable offset or slipped geometry is predicted. These rules successfully predict the geometry of intermolecular interactions in the crystal structures of aromatic molecules and rationalize a range of host-guest phenomena. The theory demonstrates that the electron donor-acceptor (EDA) concept can be misleading: it is the properties of the atoms at the points of intermolecular contact rather than the overall molecular properties which are important.
Der erste Schritt der selektiven Bindung eines Substrats an einen Rezeptor unter Bildung eines supramolekularen Verbandes (eines „Übermoleküls” oder „Überkomplexes”) ist die molekulare Erkennung, die durch die in den beteiligten Molekülen gespeicherte Information ermöglicht wird. Die Funktionen von supramolekularen Einheiten umfassen sowohl Erkennung als auch Katalyse und Transport. In Verbindung mit der Organisation von molekularen Einheiten zu geordneten Systemen eröffnen sich so Wege zu molekularen und supramolekularen Funktionseinheiten für die Informationsverarbeitung und Signalerzeugung. Die Entwicklung solcher Funktionseinheiten erfordert die Herstellung von Molekülen, die bestimmte Funktionen (z. B. Photoaktivität, Elektroaktivität, Ionenaktivität, Thermoaktivität oder Chemoaktivität) erfüllen können und die außerdem für den Einbau in geordnete Systeme geeignet sein müssen. Molekulare Funktionseinheiten zur Lichtumwandlung und zur Ladungstrennung konnten mit photoaktiven Cryptaten verwirklicht werden, die aus Rezeptoren mit lichtempfindlichen Gruppen wie Bipyridinen und Porphyrinen bestehen. Zum Transport elektronischer und ionischer Signale sind elektroaktive und ionenaktive Systeme erforderlich. Redoxaktive langkettige Polyene wie die „Caroviologene” können als molekulare Drähte für den Elektronentransport durch Membranen betrachtet werden. Push-pull-Polyene haben ausgeprägte nichtlineare optische Eigenschaften. Geordnete tubuläre Stapel geeigneter Makrocyclen in Mesophasen sowie Makrocyclen mit langkettigen Substituenten und funktionellen Gruppen an deren Enden („Chundles”) sind Prototypen von Ionenkanälen. Mit lipophilen Makrocyelen können Langmuir-Blodgett-Filme hergestellt werden, die eine molekulare Erkennung an der Luft-Wasser-Grenzfläche ermöglichen könnten. Die Supramolekulare Chemie war bisher auf mehr oder weniger vorgeformte („präorganisierte”) Rezeptoren für die molekulare Erkennung, Katalyse und Transportprozesse angewiesen. Einen Schritt weiter geht der Entwurf von Systemen, bei denen auf molekularer Ebene eine Selbstorganisation möglich ist und die spontan eine genau definierte supramolekulare Struktur annehmen. Mehrere selbstorganisierende Systeme wurden beschrieben: 1. Helicale Metallkomplexe/doppelsträngige Helicate: Zwei lineare Polybipyridine koordinieren bestimmte Metall-Ionen unter spontaner Bildung einer Doppelhelix. 2. Mesophasen und flüssigkristalline Polymere: Durch molekulare Erkennung entstehen aus komplementären Komponenten supramolekulare Verbände mit entsprechenden makroskopischen Eigenschaften. 3. Geordnete Festkörperstrukturen: Durch molekulare Erkennung kann der Aufbau geordneter Festkörper gesteuert warden. Baut man in Moleküle mit Photo-, Elektro- oder Ionenaktivität Erkennungselemente ein, so eröffnet sich die Perspektive, „programmierte” molekulare und supramolekulare Systeme herzustellen, die sich selbst — gesteuert durch die molekulare Erkennung — zu geordneten supramolekularen Funktionseinheiten zusammensetzen. Diese Funktionseinheiten könnten dann selektive Erkennung, Reaktion, Übertragung und Strukturerzeugung bei der Signal- und Informationsverarbeitung auf molekularer Ebene übernehmen.
The interfacial reaction of the terpyridyl-pendant dendrimers (dend-n-tpy; n = 4, 8, 32) and of the bridging ligand 1,4-bis[4,4‘ ‘-bis(1,1-dimethylethyl)-2,2‘:6‘,2‘ ‘terpyridine-4‘-yl]benzene (BBDTB) dissolved in CH2Cl2 with aqueous Fe2+ or Co2+ gives rise to film formation on highly oriented pyrolytic graphite surfaces. Molecularly resolved scanning tunneling microscopy (STM) images reveal that these films form highly ordered 2-D hexagonal arrays which appear to be composed of one-dimensional polymeric strands with a repeat unit of (tpy-dend-tpy-M)x in the case of dendrimers or (tpy-bridge-tpy-M)x in the case of BBDTB (M = Fe2+, Co2+). The extension of the ordered domains appears to be delimited by terrace width. The dimensions obtained from an analysis of STM images is consistent with the size of the dendrimer or the bridging ligand (obtained from molecular modeling) from which the films are derived. In the case of films derived from dend-n-tpy the ordering is dependent on the dendrimer generation. In all cases, the films are electrochemically active and exhibit a reversible wave at a formal potential that corresponds to the respective [M(tpy)2]2+ complex.
A combination of oligomeric porphyrin copolymers and bidentate Lewis bases has been used to construct 2-dimensional multilayer molecular architectures on siloxane-derivatized glass substrates. The key to this supramolecular chemistry is coordinate covalent bonding between zinc-metalated porphyrins and bipyridyl “spacer” ligands, generating layers in an alternating, iterative fashion. UV−vis, XPS, AFM, and X-ray diffraction data support the formation of ordered thin film arrays. The stepwise deposition of each porphyrin layer is tracked by the linear increase in the absorbance of the porphyrin Soret band. XPS results demonstrate that the pyridinium coupling in the first monolayer is susceptible to X-ray decomposition during prolonged exposure. Extremely weak emission in the 600−800 nm region is attributed to the low chromophore concentration in the multilayer films. These results contrast sharply with those of spin-coated polymer thin films and represent an encouraging first step toward the design of new multilayer architectures.
In this paper it is demonstrated that the stabilizing effect of linear alkanes can be utilized to achieve very high stability in the adsorption and assembly of planar organic molecules on inert surfaces under ambient conditions, by direct deposition from solutions. Using peripherally alkylated phthalocyanines and porphyrins as the examples, optimal resolutions can be achieved with complex molecular systems. Submolecular features of the molecular cores and the interdigitated alkyl parts are clearly visible. Distinctly different packing symmetries were also observed and could be attributed to the intermolecular and adsorbate−substrate interactions. Appreciable contrast variations were also recorded with changing bias voltages. This approach could be adapted to the studies of other molecules to observe submolecular features and could be helpful in obtaining two-dimensional assemblies of monodispersed molecules, especially planar molecules.
A simple method to prepare porphyrin “wheels” on a micrometer scale is described. It is possible to arrange several types of porphyrin derivatives, including Protoporphyrin IX and Chlorophyll-a, in the form of rings using this method. A mechanism for the formation of these types of assemblies is proposed that is based on the generation of “2D-gas bubbles” which induces aggregation of the porphyrin molecules.
Well-defined mesoscopic structures are obtained by self-organization of a multifunctional perylene chromophore that is highly photostable, has a high fluorescence quantum yield, and distinct redox activity. Transmission electron microscopy (see Figure) of such a mesoscopic superstructure, along with NMR and optical spectroscopy, reveals a hierarchical process involving various intermolecular interactions.
The development, characterization, and exploitation of novel materials based on the assembly of molecular components is an exceptionally active and rapidly expanding field. For this reason, the topic of molecule-based materials (MBMs) was chosen as the subject of a workshop sponsored by the Chemical Sciences Division of the United States Department of Energy. The purpose of the workshop was to review and discuss the diverse research trajectories in the field from a chemical perspective, and to focus on the critical elements that are likely to be essential for rapid progress. The MBMs discussed encompass a diverse set of compositions and structures, including clusters, supramolecular assemblies, and assemblies incorporating biomolecule-based components. A full range of potentially interesting materials properties, including electronic, magnetic, optical, structural, mechanical, and chemical characteristics were considered. Key themes of the workshop included synthesis of novel components, structural control, characterization of structure and properties, and the development of underlying principles and models. MBMs, defined as “useful substances prepared from molecules or molecular ions that maintain aspects of the parent molecular framework” are of special significance because of the capacity for diversity in composition, structure, and properties, both chemical and physical. Key attributes are the ability in MBMs to access the additional dimension of multiple length scales and available structural complexity via organic chemistry synthetic methodologies and the innovative assembly of such diverse components. The interaction among the assembled components can thus lead to unique behavior. A consequence of the complexity is the need for a multiplicity of both existing and new tools for materials synthesis, assembly, characterization, and theoretical analysis. For some technologically useful properties, e.g., ferro- or ferrimagnetism and superconductivity, the property is not a property of a molecule or ion; it is a cooperative solid-state (bulk) property—a property of the entire solid. Hence, the desired properties are a consequence of the interactions between the molecules or ions, and understanding the solid-state structure as well as methods to predict, control, and modulate the structure are essential to understanding and manipulating such behaviors. As challenging as this is, molecules enable a substantially greater ability of control than atoms as building blocks for new materials and thus are well positioned to contribute significantly to new materials. The diversity of components and processes leads to the recognition of the critical role of cross-disciplinary research, including not only that between traditionally different areas within chemistry, but also between chemistry and biochemistry, physics, and a number of engineering disciplines. Enhancing communication and active collaboration between these groups was seen as a critical goal for the research area.
The structure and conformation of a molecule determine its chemical and
physical properties. Molecular conformation at interfaces is of
particular importance in organic thin films1: in organic
optoelectronic devices2,3, for example, charge carrier
injection is influenced by interfacial properties4. Here we
present a real-space conformational analysis of individual porphyrin
molecules using scanning tunnelling microscopy5,6. Porphyrins
have been used as model systems to study charge transfer7 and
in vivo photoactivation of drug precursors8, and have also
been used in organic light-emitting diodes9. We find that
changes in the porphyrins' conformation occur predominantly by rotations
around the bonds to four tertiary butyl appendages, which differ on
different metal substrates. On corrugated gold (110) surfaces, we
identify two different conformations as the precursory (metastable) and
final states of adsorption. This kind of conformational adaptation to a
surface may be general for adsorbed organic molecules, and might have
important consequences for the technological applications of organic
thin films.
The photogating of hydrophobic ion currents across the lipid bilayer membrane allows the direct study of their kinetics by symmetrically forming charge within the membrane and across each interface, rather than across the membrane. We find that the photoinduced conductance continues to increase beyond the region where the tetraphenylboride charge density in the membrane exceeds the estimated porphyrin cation density. This photoconductance is proportional to the tetraphenylboride charge density raised to the second to third power. The risetime of the photogating effect increases with increasing concentration of tetraphenyl boride. The porphyrin cation mobility is increased when the tetraphenylboride anion is present, and low concentrations of tetraphenylphosphonium cation increase the dark conductivity while inhibiting the photoconductivity. The activation energy for both the porphyrin and phosphonium cation induced conductance is more positive than that of the tetraphenylboride conductance. From these results we conclude that in addition to some cancellation of space charge within the membrane, the mechanism of increased conductance involves the transport of these hydrophobic anions via an alternating anion-cation chain, analogous to the Grotthuss mechanism for excess proton conduction in water. This ion chain conductance can be viewed as an evolutionary prototype of an ion channel across the membrane. It also underscores the importance of the counter ion in the transport of large ions such as peptides across the lipid bilayer.
Anisotropic thin film materials of metallosupramolecular polyelectrolyte-amphiphile complexes (denoted PACs) with structures at several length scales were fabricated through a multistep self-assembly process. Metal ion-mediated self-assembly of the ditopic ligand 1,4-bis(2,2':6',2"-terpyridine-4'-yl)benzene and electrostatic binding with the amphiphile dihexadecyl phosphate result in a PAC with tailored surface chemical properties, including solubility and surface activity. The PAC forms a stable monolayer at the air-water interface that is readily transferred and oriented on solid supports with the Langmuir-Blodgett technique. The presented strategy unifies colloid and metallosupramolecular chemistry and opens a versatile route to hierarchical materials with tailored structures and functions.
So long as it avoids a Panglossian view of nature, the science of biomimetics has the potential to enrich many areas of technology. But accurate mimicry will require greater understanding of natural mechanisms at the molecular scale. As this continues to unfold, emulation may increasingly give way to assimilation of biological machinery.
