Popping open one by one into polymers

We rarely board airplanes by joining the back of a single well-ordered line. More often, we jostle around in one of several bulging crowds that merge haphazardly near the gate. Roughly speaking, these processes are analogous to the chain growth and step growth mechanisms of polymer assembly at the molecular level. Kang et al. present a strategy to link molecular building blocks through hydrogen bonding in accord with the well-controlled chain growth model. The molecules start out curled inward, as they engage in internal hydrogen bonding, until an initiator pulls one open; that molecule is then in the right conformation to pull a partner into the growing chain, poising it to pull in yet another, and so forth down the line.

Science, this issue p. 646

Ions' response to hydrophobic surfaces

The strength of interactions between ions depends on their solvation environment. Schellman postulated in the 1950s that when aqueous solvated ions approached a hydrophobic surface (such as a parts of a protein surface), interactions between ions would be enhanced. Chen et al. experimentally tested this theory by studying the dissociation of organic ions bonded by salt bridges after the addition of acid. The ion pairs were held by tethers at different distances from hydrophobic surfaces. The salt bridge was stronger when it was closer to the hydrophobic surface.

Science, this issue p. 555

Nanotube Engineering

In general, the reversible self-assembly of nanotubes through noncovalent bonding either gives fully assembled or fully disassembled products. Fukino et al. (p. 499, published online 10 April; see the Perspective by Hudson and Manners) developed a system with an intermediate possibility. Hollow nanotube structures were assembled from ferrocene-based tetratopic pyridyl ligands mixed with AgBF₄. Through oxidation of the ferrocene groups, the tubes could be cut into stable, large rings and then reversibly reassembled into nanotubes by reduction of the ferrocene groups.

Finessing Ferroelectric Liquid Crystals

For a material to show a ferroelectric response, it needs to have segments that can be polarized, with a net polarization that remains when the applied field is removed. However, the fluidity that allows liquid crystal molecules to easily move under an applied force also makes it hard to create a ferroelectric response. Miyajima et al. (p. 209) show that a set of columnar liquid crystal molecules, with polar cyano groups tethered to amide-capped nonpolar chains, can assemble into an umbrella-shaped core–shell architecture, in which hydrogen bonding among the amides keeps the cyano groups confined. With only subtle variations in the tether chemistry, the assemblies can be tuned from having a para-electric to a ferroelectric response, which requires only a small coercive field.

Abstract

A supramolecular self-assembly approach is used to make a nanotubular heterojunction.

Iron(II) triazolate coordination polymers with lipophilic sulfonate counterions with alkyl chains of different lengths have been synthesized. In hydrocarbon solvents, these polymers formed a physical gel and showed a thermoreversible spin transition upon the sol–gel phase transition. The formation of a hydrogen-bonding network between the triazolate moieties and sulfonate ions, bridged by water molecules, was found to play an important role in the spin-crossover event. The spin-transition temperature was tuned over a wide range by adding a small amount of 1-octanol, a scavenger for hydrogen-bonding interactions. This additive was essential for the iron(II) species to adopt a low-spin state. Compared with nongelling references in aromatic solvents, the spin-crossover physical gels are characterized by their quick thermal response, which is due to a rapid restoration of the hydrogen-bonding network, possibly because of a dynamic structural ordering through an enhanced lipophilic interaction of the self-assembling components in hydrocarbon solvents.

A mesostructured silica/organic composite 1-MS, constructed from a rodlike micelle of amino acid amphiphile 1 that has a condensable head group and that can be used as a template, was found to be able to catalyze the acetalization of cyclohexanone, in ethanol at 25 °C (50 % in 12 h), whereas no reaction took place with unfunctionalized mesoporous silica. In sharp contrast, hydrolytic removal of the C₁₆ alkyl tail of immobilized 1 resulted in the complete disappearance of the catalytic activity, which suggests the importance of a hydrophobic inner domain for the admission of cyclohexanone. Unsupported peptide amphiphile 2, under identical conditions to those above, was inefficient for acetalization regardless of the absence (2 % in 24 h) or presence of mesoporous silica (7 % in 24 h). Reference composite 2-MS, which is a noncovalently immobilized peptidic micelle, was virtually inactive (1 % in 24 h). These observations indicate the importance of covalent immobilization of the peptidic rod micelle for catalysis. Mesostructured silicate 3-MS hybridized with a nonpeptidic, ammonium ion amphiphile (3) showed a certain catalytic activity, but the yield (12 % in 24 h) of the acetal was much lower than that achieved by using 1-MS as the catalyst. Amorphous silica with immobilized 1 on its surface was much less active than 1-MS for acetalization (5 % in 24 h).

Majority rules: “Chiral amplification” in asymmetric transformations can be realized if a helical supramolecular polymer, derived from the substrate (S), obeys the “majority rule” in determining its handedness. This concept works properly for the asymmetric transformation of a chirality-memorizing D₂-symmetric saddle-shaped porphyrin.

Which way around? A J-aggregated zinc porphyrin dendrimer can be used to detect the macroscopic chirality of a vortex. The sign of the circular dichroism response changes quickly upon switching from clockwise (CW) to counterclockwise (CCW) stirring (see picture). The observed chiroptical activity most likely arises from a macroscopic helical alignment of nanofibers formed from the polymeric J-aggregate.

Die Mehrheit bestimmt: „Chirale Verstärkung“ gelingt in asymmetrischen Umwandlungen, wenn ein helicales supramolekulares Polymer, das sich vom Substrat (S) ableitet, dem „Mehrheitsprinzip“ bei der Festlegung seiner Händigkeit gehorcht. Dieses Konzept funktioniert einwandfrei bei der asymmetrischen Umsetzung eines D₂-symmetrischen sattelförmigen Porphyrins mit Chiralitätsgedächtnis.

„Chiralitätsverstärkung“ durch Umwandlung eines Substrats in eine helicale polymere Vorstufe, in der die Händigkeit durch das “Mehrheitsprinzip” bestimmt wird, schildern T. Aida et al. in der Zuschrift auf S. 6596 ff. Die grundlegende Strategie, die durch die asymmetrische Umwandlung eines D₂-symmetrischen sattelförmigen Porphyrins mit Chiralitätsgedächtnis demonstriert wird, könnte ein neues Grundprinzip für die allgemeine asymmetrische Synthese bilden.

Wie herum? Ein J-aggregiertes Zinkporphyrindendrimer kann zum Nachweis der makroskopischen Chiralität eines Wirbels dienen. Das Vorzeichen des Circulardichroismus ändert sich schnell, wenn von Rühren im Uhrzeigersinn (CW) auf Rühren im Gegenuhrzeigersinn (CCW) gewechselt wird (siehe Bild). Die chiroptische Aktivität resultiert höchstwahrscheinlich aus der makroskopischen helikalen Ausrichtung von Nanofasern, die sich aus dem polymeren J-Aggregat bilden.

Die Form folgt der Konfiguration: Das Zinkporphyrindimer 1 mit Pyridylgruppen kann einen asymmetrischen Kohlenwasserstoff wie Limonen chiroptisch nachweisen, indem es das homochirale schachtelförmige tetramere Assoziat BOX_⊥ bildet. Da das gebildete BOX_⊥ enantiomerenangereichert ist, können die optische Reinheit wie auch die absolute Konfiguration des Solvens Limonen bestimmt werden.

The surrounding shapes the box: The zinc porphyrin dimer 1 bearing pyridyl groups can chiroptically sense an asymmetric hydrocarbon, such as limonene, by forming the homochiral box-shaped tetrameric assembly BOX_⊥. As the BOX_⊥ formed is enantiomerically enriched, the optical purity as well as the absolute configuration of the solvent limonene can be determined.

“Chiral amplification” by temporal conversion of a substrate into a helical polymeric precursor, in which the handedness is determined by the ‘majority rule’, is described by T. Aida and co-workers in their Communication on page 6476 ff. The basic strategy, which is demonstrated by the asymmetric transformation of a chirality-memorizing D₂-symmetric saddle-shaped porphyrin, may give a new rationale for general asymmetric synthesis.

A serendipitous finding that ionic liquids gel with carbon nanotubes has opened a new possibility of ionic liquids as modifiers for carbon nanotubes. Upon being ground into ionic liquids, carbon nanotube bundles are untangled, and the resultant fine bundles form a network structure. This is due to the possible specific interaction between the imidazolium ion component and the π-electronic nanotube surface. The resultant gelatinous materials, consisting of highly electroconductive nanowires and fluid electrolytes, can be utilized for a wide variety of electrochemical applications, such as sensors, capacitors, and actuators. Ionic liquids allow for noncovalent and covalent modifications of carbon nanotubes and fabrication of polymer composites with enhanced physical properties. The processing of carbon nanotubes with ionic liquids is not accompanied by the disruption of the π-conjugated nanotube structure and does not require solvents; therefore it can readily be scaled up. This article focuses on new aspects of ionic liquids for designer soft materials based on carbon nanotubes.

A crystal structure of the racemic form of chiral molecular scissors 1 with a trans configuration at the azobenzene unit (rac-trans-1), in which the scissors adopt a closed geometry with two blade phenyl groups that overlap each other, was successfully determined. X-ray crystallographic determination of the structure of (1S,1′S)-10, which is a derivative of the key precursor of trans-1, was also successful. On the basis of the crystal structure of (1S,1′S)-10, the absolute configuration of 1 and related molecular machines, such as molecular pedal 2, and self-locking rotor 3, which all contain a chiral tetrasubstituted ferrocene module, were determined. A correlation between the absolute configuration and the circular dichroism properties of these molecular machines and their synthetic precursors was also determined.

Abstract

Controlled self-assembly of a trinitrofluorenone-appended gemini-shaped amphiphilic hexabenzocoronene selectively formed nanotubes or microfibers with different photochemical properties. In these nanotubes, which are 16 nanometers in diameter and several micrometers long, a molecular layer of electron-accepting trinitrofluorenone laminates an electron-donating graphitic layer of π-stacked hexabenzocoronene. The coaxial nanotubular structure allows photochemical generation of spatially separated charge carriers and a quick photoconductive response with a large on/off ratio greater than 10⁴. In sharp contrast, the microfibers consist of a charge-transfer complex between the hexabenzocoronene and trinitrofluorenone parts and exhibit almost no photocurrent generation.

Fishing of graphitic nanotubes with a macroscopic glass hook: A ∼30 mm long fiber (see figure and cover) is readily processed from a suspension of self-assembled nanotubes with one-handed helical chirality, formed from the (R)- or (S)-enantiomer of chiral amphiphile 2. The majority of the nanotubes in the fiber are unidirectionally oriented along the fiber axis. Upon doping with I₂, the fiber displays an anisotropic electrical conduction along the fiber axis more than an order of magnitude greater than that across the fiber axis.

The cover illustrates self-assembly of a hexa-peri-hexabenzocoronene amphiphile to give a graphitic nanotube, formed through helical rolling-up of a bilayer tape consisting of bilaterally coupled columns of π-stacked hexabenzocoronene units. The graphitic layer inside the wall offers a charge-transport pathway. Aida and co-workers report on p. 1297 on the fabrication of a macroscopic fiber by “fishing” the nanotubes with a glass hook. In the fiber, most of the nanotubes are oriented unidirectionally along the fiber axis, thereby allowing an anisotropic electrical conduction upon doping with iodine.

Multiporphyrin dendrimers are among the most promising architectures to mimic the oxygenic light-harvesting complex because of their structural similarities and synthetic convenience. The overall geometries of dendrimers are determined by the core structure, the type of dendron, and the number of generations of interior repeating units. The rigid core and bulky volume of exterior porphyrin units in multiporphyrin dendrimers give rise to well-ordered three-dimensional structures. As the number of generations of interior repeating units increases, however, the overall structures of dendrimers become disordered and randomized due to the flexibility of the repeating units. To reveal the relationship between molecular structure and processes of excitation-energy migration in multiporphyrin dendrimers, we calculated the molecular structure and measured the time-resolved transient absorption and fluorescence anisotropy decays for various hexaarylbenzene-anchored polyester zinc(II) porphyrin dendrimers along with three types of porphyrin dendrons as references. We found that the congested two-branched type dendrimers exhibit more efficient energy migration processes than one- or three-branched type dendrimers because of multiple energy migration pathways, and the three-dimensional packing efficiency of dendrimers strongly depends on the type of dendrons.

A hexabenzocoronene derivative consisting of lipophilic dodecyl side chains and thioacetyl-appended hydrophilic oxyethylene chains was newly synthesized. The self-assembly of this hexabenzocoronene amphiphile in tetrahydrofuran gave a nanotubular object, for which the thioacetyl functionalities were located on the inner and outer surfaces. The removal of the acetyl group under alkaline conditions, followed by autoxidation, allowed the conversion of the noncovalent architecture into a covalently stabilized form via disulfide linkages. The resulting surface-polymerized nanotubes were completely insoluble in common organic solvents. Scanning and transmission electron microscopy showed that the tubular morphology mostly remained after the oxidative polymerization. Disulfide oligomers involved in the polymerized nanotubes were transformed into a thiol monomer upon reduction with dithiothreitol. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5120-5127, 2006

The dynamic dual chirality of D₂-symmetric saddle-shaped porphyrin 1 decreases the stereochemical complexity of cocrystallization with racemic mandelic acid derivatives, such as BrMA, and allows spontaneous optical resolution (see picture). Each resulting cocrystal is porous and consists of one-handed double-helical porphyrin arrays in which chiral guests are enantioselectively included.

Ganz schön mobil: Als Gäste in einem engen cyclischen Dimer eines Organorhodiumporphyrins 1 beginnen Fullerene wie C₆₀ und C₇₀ zu oszillieren, indem sie durch den Wirthohlraum gleiten. Ein weiteres Molekül 1 behindert diese Bewegung erheblich, da es einen kapselartigen Einschlusskomplex bildet. Dagegen können größere Fullerene wie C₇₆ kaum durch den Wirthohlraum gleiten, sondern werden einfach von zwei Molekülen 1 eingeschlossen.

Die dynamische duale Chiralität des D₂-symmetrischen sattelförmigen Porphyrins 1 verringert die stereochemische Komplexität bei der Cokristallisation mit racemischen Mandelsäurederivaten wie BrMA und ermöglicht eine spontane Enantiomerentrennung (siehe Bild). Jeder Cokristall ist porös und besteht aus einhändig-doppelhelicalen Porphyrinanordnungen, in die chirale Gäste enantioselektiv eingeschlossen werden.

Frage der Orientierung: Bei der Selbstorganisation eines frei rotierenden Dialkinyl-verbrückten Zink(pyridylporphyrin)-Dimers entsteht bevorzugt das cyclische Tetramer, in dem das orthogonale Konformer (⊥) vorliegt, und nicht das üblichere planare Konformer ( // , siehe Bild). Der Grund für dieses Verhalten ist wahrscheinlich ein effizienter Ausgleich der Dipolmomente in der selbstorganisierten Struktur.

Flüssige Filme: Ionische Flüssigkeiten, in denen einwandige Kohlenstoffnanoröhren dispergiert sind (Bucky-Gele), ermöglichten den ersten schichtweisen Aufbau eines vollständig aus Kunststoff bestehenden Aktuators. Dieser Aktuator hat eine einfache Dreischicht-Konfiguration aus weichen Elektroden und Elektrolytschichten (siehe Bild) und kann an Luft bei niedrigen Spannungen betrieben werden.

A question of orientation: The self-assembly of a dialkynylene-bridged zinc pyridylporphyrin dimer with rotational freedom preferentially leads to a cyclic tetramer consisting of the perpendicular conformer (⊥), rather than the more usual planar conformer ( // , see picture). This preference most likely arises from an efficient cancellation of dipole moments in the self-assembled structure.

Liquid films: Ionic liquids containing dispersed single-walled carbon nanotubes (bucky gels), allow the first layer-by-layer casting fabrication of a fully plastic actuator. This actuator adopts a simple three-layered configuration of soft electrodes and electrolyte layers (see picture) and can operate in air at low voltages.

Abstract

An amphiphilic hexa-peri-hexabenzocoronene self-assembles to form a π-electronic, discrete nanotubular object. The object is characterized by an aspect ratio greater than 1000 and has a uniform, 14-nanometer-wide, open-ended hollow space, which is an order of magnitude larger than those of carbon nanotubes. The wall is 3 nanometers thick and consists of helical arrays of the π-stacked graphene molecule, whose exterior and interior surfaces are covered by hydrophilic triethylene glycol chains. The graphitic nanotube is redox active, and a single piece of the nanotube across 180-nanometer-gap electrodes shows, upon oxidation, an electrical conductivity of 2.5 megohms at 285 kelvin. This family of molecularly engineered graphite with a one-dimensional tubular shape and a chemically accessible surface constitutes an important step toward molecular electronics.

Filled doughnuts: Molybdenum crown cluster MC accommodates up to three molecules of aminophenyl-substituted metalloporphyrins through hydrogen-bonding interactions, to form discrete inorganic–organic nanocomposite materials (see picture; blue polyhedra=MC, space-filling model=metalloporphyrin). Ultrahigh-vacuum scanning tunneling microscopy, in conjunction with scanning tunneling spectroscopy, at 80 K confirms the formation of the inclusion complex.

Supramolecular polymerization of dendritic zinc porphyrins with two carboxylic acid functionalities leads to the formation of J-aggregates through multipoint π-stacking interactions. Spin-coating of the J-aggregate solutions gives optically active films, whose sense of chirality can be selected by the spinning direction (see picture). The chiroptical memory is thermally stable up to the melting temperatures of the spin-coated films.

The helical sense of the peptidic parts of a cyclodimeric zinc porphyrin host with helical oligopeptide units (1) determines whether a right- or left-handed oligopeptide guest (2) is selectively bound (see picture). In contrast, enantiomers of nonhelical peptidic guests containing a point chirality are hardly discriminated.

Third-generation dendrimers have been used to enable the synthesis and isolation of a series of spatially isolated, discrete conjugated wires G₃-n (where n is the number of repeating monomer units, see schematic representation), including 147-nm long G₃-64, which contains 192 aromatic rings and 256 triple bonds in the backbone. Photochemical studies show there is enhanced π-electronic conjugation in these systems.

Recent progress in fundamental studies on multiporphyrin arrays has provided structural parameters for the molecular design of artificial light-harvesting antennae which mimic the wheel-like antenna complexes of photosynthetic purple bacteria. Covalent and noncovalent approaches have been employed for the construction of artificial light-harvesting multiporphyrin arrays. Such arrays are categorized into ring-shaped, windmill-shaped, star-shaped, and dendritic architectures. In particular, dendritic multiporphyrin arrays have been proven to be promising candidates for both providing a large absorption cross-section and enabling the vectorial transfer of energy over a long distance to a designated point. Such molecular and supramolecular systems are also expected to be potent components for molecular electronics and photonic devices.

Gefüllte Donuts: Ein Molybdän-Kronencluster MC bindet bis zu drei Moleküle Aminophenyl-substituierter Metalloporphyrine über Wasserstoffbrücken, wobei diskrete anorganisch-organische Nanokomposite entstehen (siehe Bild; blaue Polyeder=MC, Kalottenmodell=Metalloporphyrin). Ultrahochvakuum-Rastertunnelmikroskopie bei 80 K bestätigt in Kombination mit der Rastertunnelspektroskopie die Bildung des Einschlusskomplexes.

Supramolekulare Polymerisation dendritischer Zinkporphyrine mit zwei Carbonsäurefunktionalitäten (links) führt durch Multipunkt-π-Stapelwechselwirkungen zur Bildung von J-Aggregaten. Rotationsbeschichtung mit deren Lösungen liefert optisch aktive Filme, deren Chiralitätssinn über die Rotationsrichtung vorgegeben werden kann (rechts). Das chiroptische Gedächtnis ist bis zur Schmelztemperatur der Filme thermisch stabil.

Der Helixsinn der peptidischen Komponenten eines cyclodimeren Zinkporphyrin-Wirts mit helicalen Oligopeptid-Einheiten (1) legt fest, ob ein rechts- oder ein linksgängiger Oligopeptid-Gast (2) selektiv gebunden wird (siehe Bild). Dagegen werden die Enantiomere nichthelicaler peptidischer Gäste mit Inversionszentrum kaum unterschieden.

Dendrimere der dritten Generation wurden zur Synthese und Isolierung einer Serie von räumlich getrennten, diskreten molekularen Drähten, G₃-n (siehe Bild, n=Zahl der Monomereinheiten), herangezogen. Der längste Vertreter, das 147 nm lange G₃-64, enthält 192 aromatische Ringe und 256 Dreifachbindungen in der Hauptkette. Photochemische Studien belegen eine verstärkte π-Konjugation.

Jüngste Fortschritte bei Grundlagenstudien zu Multiporphyrinsystemen geben Aufschluss über die maßgeblichen Strukturparameter für das Molekulardesign von künstlichen Lichtsammelantennen, die die radähnlichen Antennenkomplexe photosynthetischer Purpurbakterien imitieren. “Kovalente” und “nichtkovalente” Ansätze kamen für den Aufbau von künstlichen lichtsammelnden Multiporphyrinsystemen zum Einsatz. Diese lassen sich in die Kategorien ring-, windmühlen-, sternförmig und dendritisch einteilen, wobei insbesondere die dendritischen Multiporphyrinsysteme als vielversprechende Kandidaten gelten. Sie verfügen über einen hohen Absorptionsquerschnitt und ermöglichen einen zielgerichteten vektoriellen Energietransfer über große Entfernungen. Erwartet wird, dass sich solche molekularen und supramolekularen Systeme als leistungsstarke Komponenten für die molekulare Elektronik und in photonischen Bauelementen eignen.

Abstract

When mixed with imidazolium ion-based room-temperature ionic liquid, pristine single-walled carbon nanotubes formed gels after being ground. The heavily entangled nanotube bundles were found to untangle within the gel to form much finer bundles. Phase transition and rheological properties suggest that the gels are formed by physical cross-linking of the nanotube bundles, mediated by local molecular ordering of the ionic liquids rather than by entanglement of the nanotubes. The gels were thermally stable and did not shrivel, even under reduced pressure resulting from the nonvolatility of the ionic liquids, but they would readily undergo a gel-to-solid transition on absorbent materials. The use of a polymerizable ionic liquid as the gelling medium allows for the fabrication of a highly electroconductive polymer/nanotube composite material, which showed a substantial enhancement in dynamic hardness.

Energietrichter: Ein großes dendritisches Zink-Porphyrin-Heptamer (7P_Zn) in einem C₆₀-terminierten Elektronentransfersystem 7P_Zn-C₆₀ (siehe Abbildung) sammelt nicht nur sichtbares Licht für den Elektronentransfer zur Fullerengruppe, sondern verzögert auch den Rücktransport, sodass eine längere Lebensdauer des ladungsgetrennten Zustands (7P-C) resultiert .

Vollkommen unterschiedliche elektronische Eigenschaften weisen die hexagonal und lamellar angeordneten Nanokompositfilme auf (siehe Bild), die durch Polymerisation Pyrrol-funktionalisierter Tenside mit zwei verschiedenen, nicht polymerisierbaren Tensiden als Templaten in einem Sol-Gel-Prozess erhalten wurden. Die hexagonalen Kompositfilme lassen sich als dünne Polypyrrolnanokabel mit einer Silicatummantelung als Isolierung ansehen.

Very different electronic properties are found in the hexagonal and lamellar nanocomposite films (see schematic representations) prepared by dilution of a pyrrole-containing surfactant with nonpolymerizable surfactants in a sol–gel-based template polymerization. The hexagonal composite film comprises a thin polypyrrole nanocable covered with an insulating silicate channel.

Cobalt(II) complexes of poly(aryl ester) dendrimer porphyrins [(m-[Gn]TPP)Co^II] (generation number n=0–4), in the presence of azobisisobutyronitrile (AIBN) at 60 °C, underwent alkenylation with several alkynes at the metal center. A complete inhibition of double-bond migration (secondary transformation) was observed for [(m-[Gn]TPP)Co^II] (n=3 and 4), which gave [(m-[Gn]TPP)Co^IIIC(CH₂)R] (n=3 and 4) exclusively. Overall reaction rates for [(m-[Gn]TPP)Co^II] (n=0–3) were hardly dependent on the size of the dendritic substituents, while a notable retardation was observed for the largest dendrimer, [(m-[G4]TPP)Co^II]. Mechanistic studies on double-bond migration with pure [(m-[Gn]TPP)Co^IIIC(CH₂)Bu] (n=0–4) demonstrated that the secondary transformation involves participation of [(m-[Gn]TPP)Co^IIIH] (n=0–4), derived from [(m-[Gn]TPP)Co^II] and AIBN, rather than [(m-[Gn]TPP)Co^II] alone. Crossover experiments using [(m-[Gn]TPP)Co^IIIC(CH₂)Bu] (n=2–4), in combination with nondendritic [(m-[G0]TPP)Co^II] and AIBN, indicated a high level of steric protection of the active center by a robust [G4]-dendritic cage, as suggested by a ¹H NMR pulse relaxation time profile of m-[G4]TPPH₂.

Poly(benzyl ether) dendrimers with o-, m-, and p-isomers of dialkoxybenzene at their focal points [o-, m-, and p-(Gn)₂Ar], having generation numbers (n) of 0–3, were synthesized. ¹H NMR pulse relaxation times (T₁) of the exterior MeO groups of o- and m-(Gn)₂Ar (n = 0–3) all remained in the range of 0.92–1.43 s. In sharp contrast, an exceptionally short T₁ value (0.23 s) was observed for p-(G3)₂Ar. Although their absorption spectral profiles were slightly different from one another, an essential difference was observed for their fluorescence properties. When the generation number was increased, the fluorescence efficiency of o-(Gn)₂Ar increased, but that of p-(Gn)₂Ar decreased, whereas m-(Gn)₂Ar exhibited a relatively small change in the fluorescence efficiency. Fluorescence depolarization studies showed a highly efficient intramolecular energy migration in p-(G3)₂Ar as compared with o-(G3)₂Ar and m-(G3)₂Ar. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3524–3530, 2003

Energy funnels: A large dendritic zinc porphyrin heptamer (7P_Zn) in a C₆₀-terminated electron-transfer relay system 7P_Zn-C₆₀ (see picture) not only harvests visible light for the electron transfer to the C₆₀ terminus, but also retards the back-electron-transfer process, thus resulting in a longer lifetime of the charge-separated state (7P-C).

A series of star- and cone-shaped dendritic multiporphyrin arrays, (n P_Zn)₄P_FB and (n P_Zn)₁P_FB, respectively, that contain energy-donating dendritic zinc porphyrin (P_Zn) wedges of different numbers (n=1, 3, and 7) of the P_Zn units, attached to an energy-accepting free-base porphyrin (P_FB) core, were synthesized by a convergent growth approach. For the cone-shaped series ((n P_Zn)₁P_FB), the efficiency of energy transfer (Φ_ENT) from the photoexcited P_Zn units to the focal P_FB core, as evaluated from the fluorescence lifetimes of the P_Zn units, considerably decreased as the generation number increased: (1P_Zn)₁P_FB (86 %), (3P_Zn)₁P_FB (66 %), and (7P_Zn)₁P_FB (19 %). In sharp contrast, the star-shaped series ((n P_Zn)₄P_FB) all showed high Φ_ENT values: (1P_Zn)₄P_FB (87 %), (3P_Zn)₄P_FB (80 %), and (7P_Zn)₄P_FB (71 %). Energy transfer efficiencies of (3P_Zn)₄-ester-P_FB, (1P_Zn)₄-ester-P_FB, and (3P_Zn)₁-ester-P_FB, whose dendritic P_Zn wedges are connected by an ester linkage to the P_FB core, were almost comparable to those of the corresponding ether-linked versions. Fluorescence depolarization (P) studies showed much lower P values for star-shaped (7P_Zn)₄P_FB and (3P_Zn)₄P_FB than cone-shaped (7P_Zn)₁P_FB and (3P_Zn)₁P_FB, respectively, indicating a highly efficient energy migration among the P_Zn units in the star-shaped series. Such a morphology-assisted photochemical event is probably responsible for the excellent light-harvesting activity of large (7P_Zn)₄P_FB molecules.

Physical stabilization of triphenylene-based charge-transfer complexes by hexagonally aligned silica channels enables the preparation of color-tunable, highly transparent mesoporous silica films (see picture). The donor/acceptor molar ratio was varied over a wide range (1:1 to 9:1) with the included charge-transfer complexes exhibiting unique red-shifted absorption bands.

Farbiges SiO₂: Wenn Charge-Transfer-Komplexe auf Triphenylen-Basis in hexagonal angeordneten SiO₂-Kanälen stabilisiert werden, entstehen durchsichtige, mesoporöse Filme mit einstellbarer Färbung (siehe Bild; CT=Charge Transfer). Bei Variation des Donor-Acceptor-Molverhältnisses von 1:1 bis 9:1 verschieben sich die Absorptionsbanden ins Rote.

Die supramolekulare Wechselwirkung von C₆₀und C₇₀ mit cyclischen Metalloporphyrindimeren 1-M hängt sehr vom Metallion M ab, wobei bei 1-RhMe/Fulleren-Systemen besonders hohe Assoziationskonstanten (ca. 10⁸ M⁻¹) und geringe Dissoziationsneigung festgestellt wurden. Sowohl Kristallstrukturanalysen als auch NMR-Untersuchungen sprechen für das gleichzeitige Vorliegen von Charge-Transfer- und van-der-Waals-Wechselwirkungen.

Sticklike silicate materials containing aligned nanodomains of conjugated polymers (see picture) were synthesized by sol–gel-based in situ polymerization of ammonium ion terminated diacetylenic surfactant monomers. Upon photoexcitation at 450 nm, the silicate microsticks emit a yellowish green luminescence centered at 550 nm.

π-Electronic conjugation is not necessary to facilitate long-range energy migration and transfer: A rapid (k_ENT=1.04×10⁹ s⁻¹) and efficient (Φ_EXT=71 %) energy transfer from the 28 photoexcited zinc porphyrin units to the focal free-base porphyrin unit occurs in a large, star-shaped dendritic multiporphyrin array in which the chromophore units are linked through flexible ether groups (see picture).

Auch ohne π-Konjugation gelingen Energiewanderung und -transfer über große Entfernungen: Schnell (k_ENT=1.04×10⁹ s⁻¹) und effizient (Φ_EXT=71 %) ist der Energietransfer von den 28 photoangeregten Zinkporphyrin-Einheiten auf die zentrale freie Porphyrinbase in einer großen, sternenförmigen dendritischen Multiporphyrin-Anordnung, in der die Chromophor-Einheiten durch flexible Ethergruppen verknüpft sind (siehe Bild).

Stabförmige Silicatmaterialien, die ausgerichtete Nanodomänen aus konjugierten Polymeren enthalten (siehe Bild), wurden aus tensidartigen Diacetylen-Monomeren mit terminalen Ammoniumgruppen durch eine In-situ-Polymerisation auf der Grundlage eines Sol-Gel-Prozesses hergestellt. Nach Anregung mit Licht der Wellenlänge 450 nm zeigen die Silicatmikrostäbe gelblich-grüne Lumineszenz bei 550 nm.

A novel molecularly hybridized polyethylene/silica composite thin film was obtained by the gas-phase polymerization of ethylene with a titanocene-mounted mesoporous silica layer on a mica plate with mesoscopic pores arranged on the film surface. However, the use of titanocene-mounted hexagonal domains of mesoporous silica on a glass plate for the gas-phase polymerization of ethylene resulted in the formation of an islanded polyethylene/silica hybridized material. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4821–4825, 2000

A unique helix-bundled architecture with an exceptionally high conformational stability is exhibited by apocytochrome b₅₆₂. Reconstitution of this new type of chiral host molecule with the racemate of a chiral zinc mesoporphyrin II having a 1,4-xylylenediamide strap resulted in perfect enantioselection of (R)-1.

Je 32 Carboxylat- und Ammonium-Funktionalitäten ermöglichen die elektrostatische Wechselwirkung zwischen unterschiedlich geladenen dendritischen Elektrolyten und Porphyrinkernen, die in protischen Medien zu einem supramolekularen, fluoreszenzaktiven Aggregat mit vorausbestimmbarer Anordnung zweier kommunizierender Chromophore führt (siehe rechts, A = Acceptor, D = Donor). Im Unterschied zur Komplexierung linearer Polyelektrolyte besteht der Kontakt zwischen zwei Dendrimermolekülen allerdings nur über einen kleinen Bereich, wie man es bei der Zusammenlagerung kugelförmiger Moleküle erwarten würde.

Carboxylate and ammonium functionalities—32 of each—facilitate electrostatic interaction between oppositely charged dendrimer electrolytes and porphyrin cores, which leads in protic media to a supramolecular, fluorescence-active aggregate of two chromophores that communicate and have a predictable topology (see drawing on the right; A=acceptor, D=donor). In contrast to the complexation of linear polyelectrolytes, the contact area for the two dendrimer molecules is very limited, just as one would expect for an assembly of spherical molecules.