Dendritic Gelators Research Articles

Peripheral groups of polyhedral oligomeric silsesquioxane (POSS) core-based dendrimers: a crucial factor for higher-level supra-architecture building.

The role of peripheral groups (PGs) on dendrimers in the spontaneous higher-level organization of hierarchically assembled nanofibers was investigated in a series of POSS-based dendritic gelators (POSS-Lys-X, X: -Boc, -Cbz, -Fmoc, etc.). We demonstrate that the PGs not only affect the gelation ability in solutions, but also the construction of orderly entangled fibrous supramolecular networks, e.g., "loofah-like" networks. Attributed to the PGs (especially the -Boc group) causing a lower cooperative assembly, the steady state with the lowest potential energy of gelators can be easily achieved by the higher ordering of nanofiber entanglement into superstructures. The -Boc group-containing dendrimers show low molar enthalpy and molar entropy of gelation, which help the construction of unique three-dimensional (3D) "loofah-like" superstructures. In contrast, the high cooperative assembly of the dendrimer (-Cbz as the PG) promotes the gelator into a higher enthalpy gelation process, with a constructed normal fibrous network. Hence, the PGs of POSS-based dendrimers act as the crucial factor in controlling the hierarchical self-assembly via a thermodynamics approach. This research presents new perspectives to explicate the relationships between PGs of dendrimers, supra-architectures and gel performances, which further guide the design of functional supramolecular materials via controllable self-assembly.

Triarylamine-Cored Dendritic Molecular Gel for Efficient Colorometric, Fluorometric, and Impedometeric Detection of Picric Acid.

Detection of nitroaromatics at ultralow concentration is a major security concern in defense, forensics, and environmental science. To this end, a new triarylamine-cored dendritic gelator (OGR) was synthesized, which produced thermoreversible, thixotropic, and fluorescent gels in n-octanol. On gelation, both π-π* transitions and the emission peak of the gelator show redshifts with a 4.5-fold increase of fluorescence intensity in the gel state indicating J-aggregation. The nitrogen lone-pair electrons of OGR make it a donor, and electron transfer occurs to acceptor nitroaromatics causing fluorescence quenching, which is further promoted due to its acidity. The Stern-Volmer rate constants measured for different nitroaromatics showed that it senses picric acid (PA) best. The contact-mode technique with OGR-treated paper strips can allow naked-eye detection of PA under UV light down to 10-11 m concentration within 30 s. Reusability of the gel is achieved by treating OGR@PAx with NaOH solution. Impedance spectroscopic results indicated a decrease of both charge-transport resistance and Warburg impedance on successive addition of PA. The limits of detection of PA determined from fluorescence and impedance measurements match well. Thus, the OGR gel is a reusable, low-cost, specific sensor for PA by naked-eye colorimetric, fluorescence, and impedance techniques.

Body Temperature Controlled Optical and Thermal Information Storage Light Scattering Display with Fluorescence Effect and High Mechanical Strength.

A kind of body temperature controlled optical and thermal information storage light scattering display based on super strong liquid crystalline physical gel with special "loofah-like gel network" was successfully prepared. Such liquid crystal (LC) gel was obtained by mixing a dendritic gelator (POSS-G1-BOC), an azobenzene compound (2Azo2), and a phosphor tethered liquid crystalline host (5CB), which could show its best contrast ratio at around human body temperature under UV light because of the phosphor's fluorescence effect. The gel also has quite strong mechanical strength, which could be used in wearable device field especially under sunlight, even under the forcing conditions as harsh as being centrifuged for 10 min at the speed of 2000 r/min. The whole production process of such a display is quite simple and could lead to displays at any size through noncontact writing. We believe it will have wide applications in the future.

Two-component dendritic gel: Influence of the aromatic chain length on the organogel stability and supramolecular architecture

Abstract we report the influence of the aromatic chain length on the organogel stability and supramolecular structure. First, we designed and readily prepared a two-component dendritic gelator based on a combination of a third-generation poly(urethane amide) dendron and a rigid aromatic spacer chain ( e.g. , 1,4-diazine, 4,4′-dipyridy, 1,4-bis(pyrid-4-yl)benzene, 4,4-di(4-pyridyl)biphenyl) in 2:1 molar ratio. All two-component gelators were easy to gelate aromatic solvents, especially toluene and p -xylene. When the length of aromatic spacer chain was increased, the gel-to-solution transition temperature first increased, and then decreased. The supramolecular structures and morphologies in xerogel samples of gelators have been carefully studied using the transmission electron microscopy and the atomic force microscopy characterizations. We demonstrated that the stability and supramolecular structures of organogels could be well controlled and regulated by simple length regulation of the aromatic spacer chain in two-component gelator, and thus we believe that applying this kind of method will provide significant new insights for the fabrication of soft materials for various applications.

ChemInform Abstract: Supramolecular Organogels Based on Dendrons and Dendrimers

AbstractReview: [93 refs.

Synthesis of Two-Component Dendritic Gelators and their Self-Assembly Behavior in MMA

Three kinds of amide dendritic branches, Branch1(COOH), Branch2(COOH) and Branch3(COOH), are successfully synthesized. After mixing with diaminoctane, their self-assembly behavior in methyl methacrylate (MMA) is investigated. The structures of the amide dendritic branches are confirmed by1H-NMR and Mass spectra (MS). The gelation ability of the amide dendritic gelators is researched through tube inversion experiment, the results of which show that different structures led to quite different gelation ability, including gel-sol temperature and critical concentration to form a stable gel. SEM experiments show that three kinds of gelators formed different gel morphologies in MMA.

L-Lysine-Based Dendritic Gelator Modified PMMA Nanocomposite

L-lysine-based dendritic gelator(G3-C12-G3) was successfully designed and synthesized using a divergent approach and it can self-assemble into nanofibers in MMA followed by gelation. After thermal polymerization of MMA gels, the self-assembly nanofibers modified PMMA nanocomposite can be obtained. SEM studies indicate the embedded supra molecular nanofibres joint each other and form a relative intact gel networkskeleton in PMMA matrix. Impact strength and DMTAmeasurementindicatethe existence of gel network skeleton make PMMA nanocomposite had higher impact strength and storage modulus than pure PMMA. UV-vis spectrophotometer studies indicate PMMA nano-composite retains high light transmittance.

The key effect of the self-assembly mechanism of dendritic gelators: solubility parameters, generations and terminal effects

A comprehensive investigation of the key effect of the self-assembly mechanism of dendritic gelators.

Supramolecular Organogels Based on Dendrons and Dendrimers

Dendrons and dendrimers have well-defined, discrete structures that can be precisely controlled at the molecular lever. Owing to their unique architectures and multiple functionalities, dendritic molecules have shown intensive self-assembly behavior and functional performance. In particular, they have been shown to be promising candidates for applications in the assembly of gel-phase materials. Furthermore, the introduction of suitable functional moieties into the core, the branches, and/or the periphery of the dendritic gelators enables the construction of smart and functional supramolecular gel materials. Over the past decade, a number of dendritic organogelators that are based on poly(amino acid), poly(amide), and poly(aryl ether) dendrons, or together with multiple alkyl chains on the periphery, have been reported. This review describes the important developments in dendritic organogelators, with an emphasis on new strategies for the molecular design of dendritic gelators, understanding of driving forces for gel formation, and their evolution for potential applications in smart soft materials.

High Toughness and Light Transmittance of PMMA Composite Prepared via In-Situ Polymerization with Incorporating Self-Assembled Dendritic Gel Networks

High toughness and light transmittance PMMA composite was prepared via in-situ thermal polymerization with gel networks self-assembling from dendritic gelators. Significant improvements of PMMA composite's toughness could be achieved by adding a small amount of dendrimers, without losing its light transmittance. Phase morphology, mechanical properties and optical performance of the composite were investigated with SEM, DMTA, impact strength test and UV–Vis spectrophotometer, respectively. The results showed that gel networks constructed from the dendrimers’ self-assembling kept intact during in-situ polymerization, while the PMMA composite containing very small amount(≤1.0 wt%) of G2-C12-G2orgelator-1 exhibits higher storage modulus, toughness and glass transition temperature than those of PMMA homopolymer.

Exploring molecular recognition pathways in one- and two-component gels formed by dendritic lysine-based gelators

This paper provides an integrated overview of the factors which control gelation in a family of dendritic gelators based on lysine building blocks. In particular, we establish that higher generation systems are more effective gelators, amide linkages in the dendron are better than carbamates, and long alkyl chain surface groups and a carboxylic acid at the focal point enhance gelation. The gels are best formed in relatively low polarity solvents with no hydrogen bond donor ability and limited hydrogen bond acceptor capacity. The dendrons with acid groups at the focal point can form two component gels with diaminododecane, and in this case, it is the lower generation dendrons which can avoid steric hindrance and form more effective gels. The stereochemistry of lysine is crucial in self-assembly, with opposite enantiomers disrupting each other's molecular recognition pathways. For the two-component system, stoichiometry is key, if too much diamine is present, dendron-stabilised microcrystals of the diamine begin to form. Interestingly, gelation still occurs in this case, and the systems with amides/alkyl chains are more effective gels, as a consequence of enhanced dendron–dendron intermolecular interactions allowing the microcrystals to form an interconnected network.

The Synthesis of Amide Dendritic Gelators and its Self-assembly Behavior in MMA

Three kinds of amide dendritic gelators, G1-C12-G1, G2-C12-G2 and gelator-1, were synthesized, and their self-assemble behavior in methyl methacrylate (MMA) was firstly investigated. The structures of the amide dendritic gelators were confirmed by 1H-NMR and Mass spectra (MS). The gelation ability of the amide dendritic gelators was researched through tube inversion experiment, the results of which showed that different structures led to quite different gelation ability, including gel-sol temperature and critical concentration to form a stable gel. SEM experiments showed that three kinds of gelator formed different gel morphologies in MMA, all of which were nanoscale gel. All the three amide dendritic gelators could not only form stable gel network at certain temperature with different concentrations in MMA, but also in each case, an optically transparent gel was formed, which indicated dendrimers in the MMA had a good compatibility.

Correlation between gel-forming ability, supramolecular aggregates and main-chain conformation of dendronized polymer gelators

Dendronized polymer (DP) gelators were prepared via complexation of a dendritic gelator of the poly(urethane amide) dendron, which contains an ionizing carboxyl group on the apexes, with three positively charged polyelectrolytes. All these complexes could form organogels in toluene, and their gel properties were studied through determination of their minimum gelation concentration and characterization of the networked supramolecular structures formed in their gel-phase using TEM, AFM and SAXS. Our findings showed great differences in the gel-forming ability and supramolecular structure among these gelators. A simple simulation on the conformations of the three polyelectrolytes illustrated that the differences came mainly from the polyelectrolyte conformations which produced different distribution and orientation of the dendritic gelators along the backbones. When the backbone could induce the dendritic gelators to form a pre-ordered structure before gelation, the DP gelator would form nice supramolecular aggregates showing an enhanced gel-forming ability. Without this, the gel-forming ability would not be enhanced.

New dendritic gelator bearing carbazole in each branching unit: selected response to fluoride ion in gel phase

A new dendritic gelator with carbazole as the building block (HBCD) was synthesized. It was found that H-bonding between the amide groups and π-π interaction between the aromatic rings played predominant roles in the gel formation. Meanwhile, significant aggregation-induced emission enhancement was observed in the gel state due to the formation of J-aggregates and the restricted molecular motion. Notably, the gel state of HBCD can be destroyed upon addition of F(-), accompanied by fluorescence enhancement on account of the formation of N-HF(-), which could further lead to the increased coplanarity of HBCD. The sensory capability of HBCD exhibited a high selectivity towards F(-) instead of the Cl(-), Br(-), I(-) and AcO(-) anions, which could be explained by the fact that the steric hindrance of the dendrimer would go against the interactions between the larger anions and HBCD.

Continuous intra- and intermolecular energy transfer in light-harvesting gels from natural amino acids-based dendrons

The gels and co-gels from glycine ( Gly) and glutamic acid ( Glu)-based dendrons with either tyrosine ( Tyr) or tryptophan ( Trp), two of the luminescent amino acid residues in natural proteins, at the focal point were prepared ( Tyr- AB 4 and Trp- AB 4 ). It was found that such gels, especially the co-gels from Tyr- AB 4 / Trp- AB 4 , showed a high efficient energy transfer (ET) and light-harvesting behaviors. Moreover, luminescent gels with tunable emission ranging from blue to green were also observed owing to the cascade intra- and intermolecular ET from dendritic gelators to the guest molecules ( PDNS) in the host–guest gel sample (co-gel with PDNS as the guest molecule), which mimicked the natural light-harvesting systems.

Rigid dendritic gelators based on oligocarbazoles

The first rigid dendritic gel with strong emission based on oligocarbazole was reported. It is interesting that the second generation of dendron exhibited gelation ability under ultrasound, and two-component gel could be formed from the first generation of dendron assisted by 1,6-diaminohexane.

Enhancing Gelation Ability of a Dendritic Gelator through Complexation with a Polyelectrolyte

A poly(urethane amide) (PUA) dendron with long alkyl chains on its periphery was synthesized and then attached to the backbone of a polyelectrolyte, in which each unit contained a positive charge, by ionizing the carboxyl groups on the apexes of the dendrons to form a dendronized polymer. We found that both the PUA dendron and the dendronized polymer could form organogels in toluene. Interestingly, both the minimum gelation concentration and the gelation time of the dendronized polymer gelator were greatly reduced compared with the dendron alone. Our investigations showed that in the gel phase the intermolecular hydrogen bonding between adjacent dendrons creates similar supramolecular structures in both the dendron and the dendronized polymer gelator, which immobilize solvent molecules by means of interactions between dendrons and solvent molecules. Further studies on the gelation kinetics indicated that the polyelectrolyte backbone plays an important role in prearranging the attached dendritic gelators orderly and quickly into the supramolecular structures through a nucleation-elongation mechanism. Therefore, the gel-forming ability of the dendritic PUA gelator is enhanced by being complexed with the polyelectrolyte. In this work, this positive macromolecular effect is discussed in detail.

Butylamide-terminated poly(amidoamine) dendritic gelators

Butylamide-terminated poly(amidoamine) dendrons with either a Boc group ( C-n ( n = 1, 2, 3)) or a carboxyl group ( E-n ( n = 1, 2)) at the focal point, as a new kind of dendritic gelators, were synthesized and their gelation properties were studied by TEM, WAXD, SAXS, NMR,and FTIR spectroscopy. It was found that the structure of focal groups impacted greatly on their gelation ability and the dendrons with higher generations facilitated the gel phase assembly. Hydrogen-bonding and hydrophobic interactions were proved to be the main driving forces responsible for the fibrous assembly with the diameter in the range of 30–100 nm. The molecular packing pattern of the xerogels of C-2, C-3, E-1, and E-2 all showed a lamellar structure, which was revealed by WAXD and SAXS.

Glycine and l-glutamic acid-based dendritic gelators

Novel dendrons based on glycine and l-glutamic acid from the first generation ( G1) to the third generation ( G3) were synthesized and studied for their gelation properties by using transmission electron microscopy (TEM), atomic force microscopy (AFM), fluorescence, IR, circular dichroism (CD), and 1H NMR spectroscopy. It was found that the gelation capability of these dendrons increased from the first generation ( G1) to the third generation ( G3), and that G3 exhibited the highest efficiency in forming gels. Both the focal and peripheral groups of dendrons had great effects on the formation of organogels. Hydrogen bonding and π–π stacking interactions were proved to be the main driving forces to form the fibrous networks at low concentrations (0.5 wt %). Small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) measurements indicate that the xerogels of the second generation ( G2) from ethyl acetate and ethanol, and G3 xerogel from CH 2Cl 2 all display lamellar structures with the interlamellar spacing of ca. 36.0 Å for G2 and 40.5 Å for G3, respectively.