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Examining Supramolecular Exchange

Microtubules are a natural example of a one-dimensional (1D) supramolecular structure. Synthetic examples of 1D fibrils often have monomers linked by weak noncovalent interactions that allow monomers to exchange in and out of the fibrils. Albertazzi et al. (p. 491) used a combination of super-resolution microscopy on individual fibrils and stochastic simulation to study monomer exchange in fibrils formed from stacked 1,3,5-benzenetricarboxamide motifs. Exchange did not require large-scale depolymerization and repolymerization, or reassembly of fragments, but proceeded through individual monomers exchanging homogeneously throughout the fibrils.

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A series of chiral thio- and oxo-substituted polythiophenes and polythienylethynylenes has been synthesised and investigated. The replacement of an oxo-linkage by a thio-linkage distorts and/or inhibits the helical aggregation behaviour. The quenching of fluorescence upon coordination to palladium is also radically different depending on the presence of oxo- or thio-substitution of the thiophene units. The alkylthio-substituted polythiophenes and polythienylethynylenes do act as sensors for palladium by quenching of their fluorescence to a significantly greater extent than the quenching observed for nonthio functionalised polythiophenes and polythienylethynylenes.

Peptides have been modified with quadruple hydrogen bonding ureido-pyrimidinone moieties to be applied in supramolecular architectures for biomedical applications. A convenient solid-phase synthesis method was developed to functionalize peptide sequences with ureido-pyrimidinone units. Two different ureido-pyrimidinone synthons were used: based on a carbonyldiimidazole-activated amine or on an isocyanate functionality. Oligopeptides were functionalized on the solid support using two coupling strategies: on the N-terminus or selectively on the ϵ-position of a C-terminal lysine. Several peptides were modified to show the generality of the approach, varying from cell adhesion sequences, to collagen binding peptides and cysteine derivatives which can be used for native chemical ligation. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

The dynamic kinetic resolution (DKR) of various primary amine substrates was performed using a modified version of the Bäckvall system. A single equivalent of isopropyl 2-methoxyacetate was used as acyl donor in combination with p-MeO Shvo complex as the racemization catalyst and Novozym 435 as the acylation catalyst. A reaction temperature of 100 °C was employed to ensure a high racemization rate. Adding 2,4-dimethyl-3-pentanol (DMP) as hydrogen donor at a concentration of 0.5 M successfully suppressed side product formation. Under these modified DKR conditions, complete conversion was observed for most substrates within 26 h showing both high ee values and good chemoselectivity, whereas the original system required a reaction time of 72 h.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

C₃-symmetrical molecules are described which consists of a 1,3,5-benzenetricarboxamide core extended with dipeptide fragments bearing peripheral mesogenic groups. Small structural modifications in the dipeptide fragment have been performed to demonstrate their influence on the stability of the stacks and on the order within the self-assemblies formed. Seven C₃-symmetrical discs have been investigated, all with different combinations of glycine, L- and/or D-phenylalanine in the dipeptide fragments. Characterization of these discotics in the neat state using differential scanning calorimetry (DSC), X-ray diffraction (XRD) and polarized optical microscopy (POM) and in solution with circular dichroism (CD), UV-visible spectroscopy, low-concentration proton nuclear magnetic resonance and IR spectroscopy reveals that there is a clear trend in the stack stability, going from the glycine–phenylalanine motifs to the phenylalanine–phenylalanine ones. The combination of a larger hydrophobic core, more confinement of space and the possibility of additional π–π interactions leads to more stable stacks. Surprisingly, the weakest stacks consist of discotics of which the center is extended with L-phenylalanyl–glycines and not of discotics of which the center is extended with the glycyl–L-phenylalanine sequences. Furthermore, the XRD investigations show that it is difficult to form well-ordered self-assemblies in the neat state. And, CD measurements point out that some of the discs have a very complex energy landscape in solution. These observations suggest that small differences in the balance between the secondary interactions originating from the benzenetricarboxamide core and the dipeptide fragments, have a strong influence on the order within the stack. From these results it can be concluded that subtle modifications in the peptide fragments of the discs cause significant changes in the stacking properties, stressing the importance of understanding the self-assembly mechanism of each discotic in order to clarify its self-assembly behavior.

A library of C₃-symmetrical molecules consisting of a 1,3,5-benzenetricarboxamide core extended by dipeptide fragments, which bear peripheral mesogenic groups, have been synthesized. The self-assembling properties of the resulting compounds are reported by E. W. Meijer et al. in their Full Paper on page 8111 ff. Subtle modifications in the dipeptide motifs strongly influence the stability and the order within the self-assemblies, which makes it possible to tune the stacking properties.

The well-known dynamic kinetic resolution of secondary alcohols and esters was extended to secondary diols and diesters to afford chiral polyesters. This process is an example of iterative tandem catalysis (ITC), a polymerization method where the concurrent action of two fundamentally different catalysts is required to achieve chain growth. In order to procure chiral polyesters of high enantiomeric excess value (ee) and good molecular weight, the catalysts employed need to be complementary and compatible during the polymerization reaction. We here show that Shvo's catalyst and Novozym 435 fulfil these requirements. The optimal polymerization conditions of 1,1′-(1,3-phenylene) diethanol (1,3-diol) and diisopropyl adipate required 2 mol % Shvo's catalyst and 12 mg Novozym 435 per mmol alcohol group in the presence of 0.5 M 2,4-dimethyl-3-pentanol as the hydrogen donor. With these conditions, chiral polyesters were obtained with peak molecular weights up to 15 kDa, an ee value up to 99 % and with 1–3 % ketone end groups. Also with the structural isomer, 1,4-diol, a chiral polyester was obtained, albeit with lower molecular weight (8.3 kDa) and slightly lower ee (94 %). Aliphatic secondary diols also resulted in enantio-enriched polymers but at most an ee of 46 % was obtained with molecular weights in the range of 3.3–3.7 kDa. This low ee originates from the intrinsic low enantioselectivity of Novozym 435 for this type of secondary aliphatic diols. The results presented here show that ITC can be applied to procure chiral polyesters with good molecular weight and high ee from optically inactive AA–BB type monomers.

The multiple monovalent binding of adamantyl–urea poly(propyleneimine) dendrimers with carboxylic acid–urea guests was investigated using molecular dynamics simulations and X-ray crystallography to better understand the structure and behavior of the dynamic multivalent complex in solution. The results from the two methods are consistent and suggest a preferred molecular picture of this complicated aggregate of multiple components. The guest molecules can bind to the dendrimer in a variety of ways although most involve hydrogen-bonding interactions between urea groups of the dendrimer with urea and/or carboxylic acid groups of the guest. In addition, acid–base interactions between the carboxylic acid of the guest and the tertiary amine in the interior of the dendritic host are present. Our proposed structure gives important information about the predominant dynamic interactions between the host and guest and illustrates how they fit together and interact with each other.

Two novel monomers, ambrettolide epoxide and isopropyl aleuriteate, encompassing functional groups, were obtained in a single step from commercially available materials. Novozym 435 catalyzed ring opening polymerization of ambrettolide epoxide furnished a polymer of M_n = 9.7 kg/mol and PDI = 1.9 while the epoxide groups remained unaffected during the polymerization. Selective polymerization of the primary hydroxyl groups of isopropyl aleuriteate using Novozym 435 was feasible and a polymer with moderate molecular weight (M_n = 5.6 kg/mol, PDI = 3.2) was isolated in moderate yield (43%). Subsequently, copolymerization of isopropyl aleuriteate with ε-CL in different ratios was performed, resulting in soluble, hydroxy functional polymers with good molecular weights (M_n = 10.4–27.2 kg/mol) in good yield (71–78%). The secondary hydroxy groups in the polymer reacted easily with hexyl isocyanate, showing the potential of isopropyl aleuriteate as a comonomer for the synthesis of functional polyesters. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5968–5978, 2007

Microfluidic biosensor chips that are functionalized with cysteine derivatives are readily modified with peptides and proteins by pulsed native chemical ligation. The chemoselectivity ensures a homogeneous presentation of the biomolecule at the surface, and the ligand density can be tuned in a highly programmable way. The modified surfaces are selectively responsive towards complementary analytes, thus allowing accurate kinetic and thermodynamic information to be obtained.

The quest to understand the origin of chirality in biological systems has evoked an intense search for nonlinear effects in catalysis and pathways to amplify slight enantiomeric excesses in racemates to give optically pure molecules. The amplification of chirality in polymeric systems as a result of cooperative processes has been intensely investigated. Ten years ago, this effect was shown for the first time in noncovalent dynamic supramolecular systems. Since then, it has become clear that a subtle interplay of noncovalent interactions such as hydrogen-bonding, π–π stacking, and hydrophobic interactions is also sufficient to observe amplification of chirality in small molecules. Here we summarize the results obtained over the past decade and the general guidelines we can deduce from them. Predicting amplification of chirality is still impossible, but it appears to be a balance between different types of interactions, the formation of an intrinsically chiral object, and cooperative aggregation processes.

Regulating the twist: Tunable supramolecular chirality is induced in an achiral helical self-assembly (P, M; see picture) by hydrogen-bonded chiral guest molecules (yellow and purple rods). Chiroptical probing of the π-conjugated chromophore reveals the mechanistic pathways of chiral induction. “Majority rules” and “sergeant and soldiers” experiments give insight into the chiral amplification in the stacks.

All lined up: Supramolecular assemblies are aligned by a convective flow in clear, nonviscous dilute solutions (see picture). The linear dichroism created by the alignment results in artifacts in circular dichroism measurements, the origins of which are explained.

Die Frage nach dem Ursprung der Chiralität in biologischen Systemen löste die Suche nach nichtlinearen Effekten in der Katalyse aus, und man suchte nach Wegen, um ausgehend von minimalen Enantiomerenüberschüssen optisch reine Verbindungen zu erhalten. Intensiv erforscht wurde bereits die Chiralitätsverstärkung in Polymeren als Folge kooperativer Prozesse. Vor zehn Jahren wurde dieser Effekt erstmals auch für nichtkovalente, dynamische supramolekulare Systeme nachgewiesen. Seitdem hat sich gezeigt, dass auch für niedermolekulare Verbindungen eine Chiralitätsverstärkung möglich ist, wenn nichtkovalente Wechselwirkungen wie Wasserstoffbrücken, hydrophobe und π-π-Stapelwechselwirkungen aufeinander abgestimmt werden. Wir fassen die Resultate der letzten zehn Jahre zusammen und leiten daraus einige allgemeine Regeln ab. Eine Chiralitätsverstärkung kann zwar immer noch nicht sicher vorhergesagt werden, essenziell sind aber wohl ein ausgewogenes Verhältnis der beteiligten Wechselwirkungen und oftmals die Bildung eines intrinsisch chiralen Objekts sowie kooperative Aggregationsprozesse.

In Reih' und Glied: Durch Konvektion werden in klaren, nichtviskosen verdünnten Lösungen supramolekulare Aggregate gebildet (siehe Bild). Die Ausrichtung erzeugt einen Lineardichroismus, der zu Artefakten in Circulardichroismusmessungen führt.

Richtungsweisende Zusätze: Über Wasserstoffbrücken gebundene chirale Gastmoleküle (gelbe und violette Stäbchen) können Chiralität in ein achirales helikales Assoziat (P, M; siehe Bild) einführen. Chiroptische Untersuchungen der π-konjugierten Chromophore zeigen den Mechanismus der Chiralitätsinduktion. Experimente zum Mehrheits- („majority rules“) und Befehlsprinzip („sergeant and soldiers“) beleuchten den Modus der Chiralitätsverstärkung innerhalb der Stapel.

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Hierarchical self-assembly offers a powerful strategy for producing molecular nanostructures. Although widely used, the mechanistic details of self-assembly processes are poorly understood. We spectroscopically monitored a nucleation process in the self-assembly of p-conjugated molecules into helical supramolecular fibrillar structures. The data support a nucleation-growth pathway that gives rise to a remarkably high degree of cooperativity. Furthermore, we characterize a helical transition in the nucleating species before growth. The self-assembly process depends strongly on solvent structure, suggesting that an organized shell of solvent molecules plays an explicit role in rigidifying the aggregates and guiding them toward further assembly into bundles and/or gels.

Poly(ureidophthalimide)s decorated with hydrophilic side chains, that ensure solubility in aqueous media, have been synthesized and characterized by UV/Vis and circular dichroism (CD) spectroscopy. Temperature and concentration dependent CD measurements in water have revealed an almost temperature and concentration independent Cotton effect, indicative for a strong intramolecular organization. Similar studies in THF demonstrate the dynamic nature of the secondary architecture, a characteristic of foldamers. In addition, the bisignated Cotton effect in water is opposite in sign to that in THF, suggestive for a solvent-dependent preference for one helical handedness. Mixing experiments prove the dominance of water in determining the handedness of the helical architecture. The solvent allows for control over the helical architecture and thus governs the supramolecular synthesis.

We describe the synthesis, supramolecular ordering on surfaces and in solution, and photophysical characterization of OPV4UT-PERY, an oligo(p-phenylenevinylene) (OPV) with a covalently attached perylene bisimide moiety. In chloroform, the molecule forms dimers through quadruple hydrogen bonding of the ureido-s-triazine array. This is supported by scanning tunneling microscopy (STM) studies, which reveal dimer formation at the liquid (1,2,4-trichlorobenzene)/solid (graphite) interface. Moreover, contrast reversal in bias-dependent STM imaging provides information on the ordering and different electronic properties of the oligo(p-phenylenevinylene) and perylene bisimide moieties. In dodecane, the molecule self-assembles into H-type aggregates that are still soluble as a result of the hydrophobic shell formed by the dodecyloxy wedges. The donor–acceptor molecule is characterized by efficient energy transfer from the photoexcited OPV to the perylene bisimide. Mixed assemblies with analogous OPVs lacking the perylene bisimide unit have been prepared in dodecane solution and energy transfer to the incorporated perylene bisimides has been studied by fluorescence spectroscopy.

Aus zwei wird eins: Ein AB-Monomer mit jeweils vier H-Brücken bildenden 2-Ureido-4[1H]-pyrimidinon- und 2,7-Diamido-1,8-naphthyridin-Einheiten entsteht durch selektive Olefin-Kreuzmetathese. Mit ihm lässt sich das stöchiometrische Ungleichgewicht bei der Selbstorganisation von supramolekularen AA+BB-Polymeren umgehen. Da der Linker zwischen den beiden Motiven flexibel ist, liegt das Monomer bei niedriger Konzentration als Cyclus vor.

Einblick in supramolekulare Objekte: Mit optischen Methoden und konfokaler Rastermikroskopie wurde der intermolekulare Energietransfer (ENT) in gemischten Vesikeln aus Donor/Acceptor-Oligo(p-phenylenvinylenen) (OPVs) in Wasser verfolgt (siehe Bild). Diese Beobachtung des Austauschprozesses zwischen getrennten Donor- und Acceptorvesikeln belegte, dass sich die Eigenschaften einzelner Vesikel von denen der Lösung unterscheiden.

A chiral supramolecular “sergeant” as the structure-directing agent affects the backbone stereochemistry during polymerization of the achiral “soldier” monomers. The polymer obtained shows preferred helical folding despite its mixed microstructure.

The preorganization of bifunctional 2-ureido-4-pyrimidinones mediated by either 1,3-substituted adamantane or meta-substituted phenylene ring linkers leads to the preferred formation of stable pentameric (1)₅ and hexameric (2)₆ assemblies, respectively. Despite the high binding constant of the 2-ureido-4-pyrimidinone dimers and the highly preorganized structure of the monomer, the predominant formation of cycles (1)₅ and (2)₆ in solution occurs only within a specific concentration range.

Ein chiraler supramolekularer „Aufpasser“ als strukturdirigierendes Agens beeinflusst die Rückgratkonfiguration während der Polymerisation der achiralen monomeren „Untergebenen“ (siehe Bild). Das erhaltene Polymer ist trotz seiner gemischten Mikrostruktur bevorzugt helical gefaltet.

The supramolecular structure of two types of oligo(p-phenylene vinylene) (OPV) building blocks in dodecane solution is studied. Monofunctional chromophores (MOPV) form well-defined helical assemblies, whereas bifunctional molecules (BOPV) aggregate into so-called frustrated stacks, lacking any higher helical order. This difference in organization has a major influence on the transfer of excitation energy through the assemblies. Energy transfer to supramolecularly incorporated guests (MOPV with lower bandgap) is used to probe the intrinsic differences in exciton mobility in these two types of mixed aggregates. From the observed donor fluorescence quenching, it can be concluded that the helically ordered nature of the MOPV stacks facilitates the transfer of excitation energy, yielding evidence for higher exciton mobility in the well-ordered assemblies than in the frustrated stacks. Finally, the concept of energy transfer in supramolecular assemblies is extended to the solid state by the successful implementation in a light-emitting diode (LED).

Host–guest interactions between the periphery of adamantylurea-functionalized dendrimers (host) and ureido acetic acid derivatives (guest) were shown to be specific, strong and spatially well-defined. The binding becomes stronger when using phosphonic or sulfonic acid derivatives. In the present work we have quantified the binding constants for the host–guest interactions between two different host motifs and six different guest molecules. The host molecules, which resemble the periphery of a poly(propylene imine) dendrimer, have been fitted with an anthracene-based fluorescent probe. The two host motifs differ in terms of the length of the spacer between a tertiary amine and two ureido functionalities. The guest molecules all contain an acidic moiety (either a carboxylic acid, a phosphonic acid, or a sulfonic acid) and three of them also contain an ureido moiety capable of forming multiple hydrogen bonds to the hosts. The binding constants for all 12 host–guest complexes have been determined by using fluorescence titrations by monitoring the increase in fluorescence of the host upon protonation by the addition of the guest. The binding constants could be tuned by changing the design of the acidic part of the guest. The formation of hydrogen bonds gives, in all cases, higher association constants, demonstrating that the host is more than a proton sensor. The host with the longer spacer (propyl) shows higher association constants than the host with the shorter spacer (ethyl). The gain in association constants are higher when the urea function is added to the guests for the host with the longer spacer, indicating a better fit. Collision-induced dissociation mass spectrometry (CID-MS) is used to study the stability of the six motifs using the corresponding third generation dendrimer. A similar trend is found when the six different guests are compared.

Eine Vielzahl gut definierter multivalenter Peptide und Proteine ist erhältlich, indem synthetische Peptide und rekombinant exprimierte Proteine mithilfe der nativen chemischen Ligation an Cystein-funktionalisierte Dendrimere konjugiert werden (siehe Bild). Auf diesem Weg sind Dendrimere zugänglich, die für das Verständnis zentraler Aspekte der Multivalenz in biologischen Wechselwirkungen wie auch für biomedizinische Anwendungen interessant sind.

A wide variety of well-defined multivalent peptides and proteins can be made by conjugating synthetic peptides and recombinantly expressed proteins to cysteine-functionalized dendrimers using native chemical ligation (see picture). This modular approach provides access to dendrimers that are attractive both for understanding fundamental issues of multivalency in biological interactions as well as for biomedical applications.

Blocked isocytosine isocyanates are conveniently obtained by the reaction of 1,1′-carbonyldiimidazole (CDI) with isocytosines. The resulting precursors for quadruple hydrogen-bonded structures can be isolated and stored for further use. Reaction with either aliphatic or aromatic amines gives the corresponding mono-, bi-, or trifunctional ureidopyrimidinone derivatives in good to excellent isolated yields. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Hierarchische Selbstorganisation: Oligo(p-phenylenvinylene) mit Diaminotriazin-Endgruppen bilden durch Selbstorganisation wasserstoffverbrückte hexamere Rosetten (linke Bildhälfte), die sich wiederum zu langen supramolekularen Röhrchen mit idealer Raumausnutzung anordnen (rechts).

A new type of guest has been designed and synthesized for the exo-type supramolecular functionalization of adamantyl-urea-terminated poly(propylene imine) dendrimers. This new type of guest motif features a uriedo methane sulfonic acid moiety that binds very selectively to the surfaces of dendrimers via a combination of noncovalent interactions forming well-defined complexes. The guest–host properties have been examined for a fifth-generation adamantyl-urea-functionalized poly(propylene imine) dendrimer capable of binding 32 guest molecules and for a model host molecule that can bind only one guest molecule. The guest–host chemistry has been studied with ¹H NMR spectroscopy, nuclear Overhauser enhancement spectroscopy NMR spectroscopy, T₁-relaxation NMR experiments, and IR spectroscopy. The 1:32 ratio with the dendrimer has been confirmed unambiguously from ¹H NMR spectra of the complex after size exclusion chromatography. Competition experiments with guests bearing a carboxylic acid instead of a sulfonic acid in the binding motif have demonstrated that the sulfonic acid has superior binding strength. Also, the importance of a combination of noncovalent interactions has been shown via competition experiments with a guest lacking the uriedo moiety. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3792–3799, 2004

Selektive Gastfreundschaft: An der Peripherie eines Dendrimers der dritten Generation können bis zu acht Gastmoleküle gebunden werden. Massenspektrometrische Untersuchungen mit stoßinduzierter Fragmentierung (collision-induced dissociation, CID) haben gezeigt, dass gezielt ein Gastmolekül nach dem anderen entfernt wird (siehe Spektrum).

Selective guest ejection: Up to eight guest molecules are assembled at the periphery of a third-generation dendrimer. Collision-induced dissociation mass spectrometry has demonstrated that the guests are removed very selectively in a one-by-one manner (see spectrum).

Hierarchical self-assembly: Oligo(p-phenylenevinylene)s with pendant diamino triazine moieties can self-assemble through hydrogen-bonding interactions to give hexameric rosettes (see picture, left). These rosettes further organize into large supramolecular tubules (right) with perfect space filling.

Ultraschneller Energietransfer in gemischten supramolekularen Stapeln monofunktionalisierter Oligo(phenylenvinylene) (MOPVs) in Lösung: Columnare MOPV3-MOPV4-Aggregate löschen die MOPV3-Fluoreszenz sehr effizient, was dafür spricht, dass der Energietransfer (ENT) innerhalb der supramolekularen Stapel vom kürzeren zum längeren Oligomer verläuft (siehe Schema).

Ultrafast energy transfer has been observed in mixed supramolecular stacks of mono-functionalized oligo(phenylene vinylene)s (MOPVs) in solution. Extended columnar aggregates of MOPV3 incorporating MOPV4 show a very efficient quenching of the MOPV3 fluorescence, which indicates that energy transfer (ENT) occurs within the supramolecular stacks from the shorter oligomer to the longer one (see schematic representation).

A series of mono- and bifunctional compounds 2–7, based on the ureido pyrimidinone quadruple hydrogen bonding unit, was prepared to study the mode of aggregation of these compounds in the bulk and in solution. Compounds 2–7 exhibit thermotropic liquid crystalline properties, as evidenced by differential scanning calorimetry and optical polarization microscopy. The presence of an ordered hexagonal discotic (D_ho) phase of 2 a was confirmed by X-ray diffraction on an aligned sample. In chloroform, the bifunctional compounds form cyclic dimers at millimolar concentrations, and these dimers exist in equilibrium with linear species above a critical concentration, which may be from 6 mM to greater than 260 mM, depending on the structure of the spacer. Circular dichroism measurements in chloroform did not show a Cotton effect. Dodecane solutions of compounds 3, 4 b, and 7 b display a Cotton effect at the absorption band of the phenyl-pyrimidinone unit. Amplification of chirality was observed in mixtures of 7 a and 7 b, but not in mixtures of 4 a and 4 b, indicating that 7 a and 7 b form mixed polymeric aggregates with a helical architecture in dodecane solution, whereas 4 a and 4 b do not. The Cotton effect is lost upon increasing the temperature. Half of the helicity is lost at 25 °C for 3 and at 60 °C for 4 b, suggesting that 3, bearing the shorter spacer, forms less stable columns than 4 b. Compound 7 b loses half of its helicity at 45 °C. Compounds 2 b, 5, and 6 do not exhibit helical organization, as evidenced by the absence of Cotton effects.

To investigate the potential of di- and tri-azaheterocycles as building blocks for π-conjugated materials with high electron affinity, linear oligomers incorporating pyrazine and a C₃-symmetric discotic molecule based on triazine were synthesized. The tridodecyloxyphenyl end-capped ethynylene pyrazinylene oligomers showed remarkable solvatochroism in absorption and emission in solution. The oligomers containing one and two pyrazine rings displayed liquid crystallinity in the solid state. The largest ethynylene pyrazinylene oligomer containing three pyrazine rings had the lowest first reduction potential at −1.08 V. The triazine-derived discotic molecule exhibited UV/Vis and fluorescence behavior comparable to that of the linear oligomers and featured a first reduction potential at −1.49 V, somewhat lower than expected.

Stereospecific polymerizations of functionalized acetylenes were successfully performed using a Rh complex as the catalyst and triethylamine as a base yielding poly(3,4,5-tridodecyloxyphenylacetylene) (Poly5) and poly[3,4,5-tris((S)-3,7-dimethyloctyloxy)phenylacetylene] (Poly6). Poly5 and Poly6 are liquid crystalline with presumably a bilayer-type architecture in which the mesogens are arranged in an antiparallel overlapping interdigitated manner. Poly6 showed unusual thermal stereomutation in solution in a very narrow temperature range.

Polymerization of a functionalized acetylene was successfully performed using a Rh complex as the catalyst and triethylamine as a base yielding poly{(E,E,E)-4-[4-[4-(3,4,5-tridodecyloxystyryl)-2,5-bis((S)-2-methylbutoxy)styryl]-2,5-bis((S)-2-methylbutoxy)styryl]phenylacetylene} (PAOPV). Films of PAOPV mixed with a fullerene derivative showed electron transfer from the OPV oligomer donor to the fullerene acceptor. The films could be furthermore used in photovoltaic devices.

Adamantyl urea and adamantyl thiourea modified poly(propylene imine) dendrimers act as hosts for N-terminal tert-butoxycarbonyl (Boc)-protected peptides and form chloroform-soluble complexes. Investigations with NMR spectroscopy show that the peptide is bound to the dendrimer by ionic interactions between the dendrimer outer shell tertiary amines and the C-terminal carboxylic acid of the peptide, and also through host-urea to peptide-amide hydrogen bonding. The hydrogen-bonding nature of the peptide–dendrimer interactions was further confirmed by using Fourier transform IR spectroscopy, for which the NH- and CO-stretch signals of the peptide amide moieties shift towards lower wavenumbers upon complexation with the dendrimer. Spatial analysis of the complexes with NOESY spectroscopy generally shows close proximity of the N-terminal Boc group of the peptide to the peripheral adamantyl groups on the dendrimer host. The influence of side-chain motif on interactions with the host is analyzed by using seven different N-Boc-protected tripeptides as guests for the dendrimer. Downfield shifts of up to 1.3 ppm were observed for the guest amide NH-proton signals. These shifts decreased with increasing 'bulkiness' of the amino acid side chains. Despite this, the dendrimer was capable of making multiple peptide–dendrimer complexes when presented with a library of seven peptides. The different peptides were all present in the host, which did not show specific preferences, and could be released under mild acidic conditions. These results show the general nature of the peptide–dendrimer interactions in the formation of either single- or multiple-peptide–dendrimer complexes.

An oligo(p-phenylene vinylene) that contains terpyridine ligands has been synthesized. Upon addition of metal ions, a π-conjugated metallo polymer is formed in which the well-defined character of oligomers and the material properties of polymers are combined. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4020–4023, 2002

A donor-acceptor-donor triad molecule with a perylene bisimide derivative as electron acceptor, and an oligo(p-phenylene vinylene) (OPV) derivative as electron donor was synthesized (OPV-PERY-OPV). The structure of the triad was characterized by ¹H and ¹³C NMR spectroscopy, size-exclusion chromatography (SEC), and MALDI-TOF spectrometry. Absorbance spectra and CD spectroscopic measurements of the triad molecule indicated the formation of aggregates in solvents such as toluene, chloroform, and tetrachloroethane, whereas it was present in the molecularly dissolved state in THF. The ¹H NMR spectra of the molecule in chloroform had, unexpectedly, four doublet peaks for the perylene protons, instead of the two doublets that is generally seen in N,N′-substituted perylene molecules. To understand the aggregation behavior and the splitting of the signals in the ¹H NMR spectra, a simple model compound was synthesized, in which the OPV units were replaced by phenyl groups (Ph-PERY-Ph). ¹H NMR spectra in CDCl₃ and tetrachloroethane again had four doublet peaks for the perylene protons, whereas in THF the perylene protons gave only a single peak. NOE and COSY spectroscopy were used to assign the peaks to their corresponding perylene protons. UV/Vis and CD spectroscopic measurements indicated that, similar to the OPV-PERY-OPV triad molecule, the model compound Ph-PERY-Ph was also present in the aggregated form in solvents such as toluene, chloroform, and tetrachloroethane, and in the molecularly dissolved state in THF. IR measurements of the model molecule in the first set of solvents indicated carbamate bond (OCONH)-induced intermolecular hydrogen bonding, whereas in THF, the molecule was mostly present in the free form. CPK models with a dimeric structure, in which two perylene molecules are held together by intermolecular hydrogen bonding with the perylene core shifted slightly with respect to one another, could account for the optical properties and the observation of the four different peaks in the ¹H NMR spectra in polar solvent. Temperature-dependent ¹H NMR spectroscopic, UV/Vis, and CD measurements indicated that the transition from the aggregated to the molecularly dissolved state took place at higher temperatures. The electrochemical studies indicated that OPV-PERY-OPV was both p- and n-dopable, whereas Ph-PERY-Ph was only n-dopable. Cyclic voltammetry measurements of Ph-PERY-Ph in THF had two reduction peaks corresponding to the reduction of the perylene core to the monoanion and dianion, respectively. In dichloromethane, however, an additional reduction peak at lower potential was observed. This new reduction peak might arise from the hydrogen-bonded species.

Vom Tetraeder bis zur Thermodynamik reichten die Konzepte des niederländischen Physikochemikers Jacobus Henricus van 't Hoff, der einen einmaligen Einfluss auf die Chemie ausübte und vor nunmehr hundert Jahren der erste Träger des Chemie-Nobelpreises wurde. In seinen jungen Jahren gab er der Chemie als Wissenschaftsdisziplin ein ganz neues Gesicht. Van 't Hoff inspirierte über die Jahre hinweg viele niederländische Naturwissenschaftler zur Beschäftigung mit neuartigen stereochemischen Konzepten. Das Bild zeigt zeitgenössische stereochemische Modelle, die Van 't Hoff selbst hergestellt hat.

Eine Kombination aus den wohldefinierten Merkmalen π-konjugierter Oligomere und den Materialeigenschaften von Polymeren weisen die supramolekularen Wasserstoffbrücken-gebundenen π-konjugierten Architekturen der Titelverbindung auf (siehe Bild). Diese Verbindungen wurden erstmals erfolgreich in elektronischen Bauelementen eingesetzt.

The first Nobel Prize in Chemistry was awarded to the Dutch physical chemist Jacobus Henricus van 't Hoff in 1901, exactly hundred years ago. His impact on the chemical sciences is unparalleled with concepts ranging from tetrahedra to thermodynamics. He reshaped the discipline of chemistry at a very young age. Through the years, Van 't Hoff has inspired many Dutch scientists to perform research on novel stereochemical concepts. The picture shows stereochemical models, which Van 't Hoff produced himself.

A combination of the well-defined character of π-conjugated oligomers and the material properties of polymers is displayed by the supramolecular hydrogen-bonded π-conjugated architectures of the title compounds (see scheme). These compounds have been successfully used for the first time in electronic devices.

A novel, fast, and simple synthetic procedure for polycarbamate/urea dendrimers, based on an AB–CD₂ coupling strategy, is presented. The reactivity difference of the two isocyanate functionalities of the AB building block allows the construction of these dendrimers without the necessity of activation or deprotection steps. This makes it possible to construct dendrimers within 2–3 days, even the largest dendrimers. The resulting dendrimers could be fully characterized by ¹³C NMR, IR spectroscopy, and mass spectrometry. The synthetic strategy necessitates only techniques such as stirring, heating, and accurate dosing, and there is no workup required for the purification of the compounds. On account of a wide variety of polyols, amines, and aminoalcohols, this new procedure is not limited to the synthetic strategy followed but allows the incorporation of a large variety of functional molecules in the core, in the branching units, or at the end groups. The method is even applicable when organometallic species are incorporated into the dendritic structure, thereby showing its versatility. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3112–3120, 2001

Symmetrical 2,5-bis(2-aminophenyl)pyrazines have been synthesized by application of the Stille coupling strategy. These cotrimers feature three important properties, namely strong intramolecular hydrogen bonding, push-pull character, and high electron affinity. The presence of intramolecular hydrogen bonds has been confirmed by ¹H NMR, IR spectroscopy, and single-crystal X-ray diffraction. The hydrogen bond strength can be increased by substituting the amino groups with stronger electron-withdrawing functionalities. Despite the anticipated enhanced π-conjugation through planarization, a hypsochromic shift was observed in the UV/Vis spectra, explained by a decrease in push–pull character. The electron affinity of the cotrimers was deduced from the first reduction potentials measured by cyclic voltammetry and is related to the electron-withdrawing character of the amino substituents. The results obtained have been compared with those of the corresponding 4-aminophenyl analogues and show that intramolecular hydrogen bonds can be used to design polymers with enhanced π conjugation as well as a high electron affinity.

Morethan50yearsafterOttoBayer'sdetaileddescription of [m,n]-polyurethanes, the first general synthesis of [n]-polyurethanes 1 is described. This series of aliphatic [n]-polyurethanes is synthesized by the in situ polymerization of the corresponding α,ω-isocyanato alcohol monomers, which in turn are made out of linear α,ω-amino alcohols and di-tert-butyltricarbonate. Polymers of high molecular weight possessing a uniform microstructure are obtained, while their melting points show a strong odd–even effect.

Über 50 Jahre nach Otto Bayers detaillierter Beschreibung der [m,n]-Polyurethane wird nun eine allgemeine Synthese der [n]-Polyurethane 1 beschrieben. Diese Reihe aliphatischer [n]-Polyurethane wird durch In-situ-Polymerisation der entsprechenden α,ω-Isocyanatalkoholmonomere erhalten, die ihrerseits aus linearen α,ω-Aminoalkoholen und Di-tert-butyltricarbonat hergestellt werden. Man erhält so Polymere mit hohem Molekulargewicht und einheitlicher Mikrostruktur, deren Schmelzpunkte einen starke Alternanz (Gerade-Ungerade-Effekt) zeigen.