Effect of the Bulkiness of the End Functional Amide Groups on the Optical,Gelation, and Morphological Properties of Oligo(p‐phenylenevinylene) π‐Gelators

Herein, we describe the role of end functional groups in the self‐assembly of amide‐functionalized oligo(p‐phenylenevinylene) (OPV) gelators with different end‐groups. The interplay between hydrogen‐bonding and π‐stacking interactions was controlled by the bulkiness of the end functional groups, thereby resulting in aggregates of different types, which led to the gelation of a wide range of solvents. The variable‐temperature UV/Vis absorption and fluorescence spectroscopic features of gelators with small end‐groups revealed the formation of 1D H‐type aggregates in CHCl₃. However, under fast cooling in toluene, 1D H‐type aggregates were formed, whereas slow cooling resulted in 2D H‐type aggregates. OPV amide with bulky dendritic end‐group formed hydrogen‐bonded random aggregates in toluene and a morphology transition from vesicles into fibrous aggregates was observed in THF. Interestingly, the presence of bulky end‐group enhanced fluorescence in the xerogel state and aggregation in polar solvents. The difference between the aggregation properties of OPV amides with small and bulky end‐groups allowed the preparation of self‐assembled structures with distinct morphological and optical features.     

Self-assembled nanotapes of oligo(p-phenylene vinylene)s: sol-gel-controlled optical properties in fluorescent pi-electronic gels

A rational approach to the design of supramolecular organogels of all-trans oligo(p-phenylene vinylene) (OPV) derivatives, a class of well-known organic semiconductor precursors, is reported. Self-assembly of these molecules induced gelation of hydrocarbon solvents at low concentrations (<1 mM), resulting in high aspect ratio nanostructures. Electron microscopy and atomic force microscopy (AFM) studies revealed twisted and entangled supramolecular tapes of an average of 50-200 nm in width, 12-20 nm in thickness, and several micrometers in length. The hierarchical growth of the entangled tapes and the consequent gelation is attributed to the lamellar-type packing of the molecules, facilitated by cooperative hydrogen bonding, pi stacking, and van der Waals interactions between the OPV units. Gelation of OPVs induced remarkable changes in the absorption and emission properties, which indicated strong electronic interaction in the aggregated chromophores. Comparison of the absorption and emission spectra in the gel form and in the solid film indicated a similar chromophore organization in both phases. The presence of self-assembled aggregates of OPVs was confirmed by solvent- and temperature-dependent changes in the absorption and emission properties, and by selective excitation experiments. This is the first detailed report of the gelation-induced formation of OPV nanotapes, assisted by weak, nondirectional hydrogen-bonding motifs and pi-pi stacking. These findings may provide opportunities for the design of a new class of functional soft materials and nanoarchitectures, based on pi-conjugated organic semiconductor-type molecules, thereby enabling the manipulation of their optical properties.     

Novel self-assembled molecular aggregates formed by fluoroalkyl end-capped oligomers and their application

A variety of fluoroalkyl end-capped oligomers were prepared under mild conditions by the use of fluoroalkanoyl peroxide as a key intermediate. These oligomers can form the self-assembled molecular aggregates with the aggregations of end-capped fluoroalkyl groups in aqueous and organic media. Fluorinated self-assembled molecular aggregates containing carboxyl and sulfo groups were suggested to interact with positively charged HIV-1 to exhibit a potent anti-HIV-1 activity in vitro. In contrast, fluoroalkyl end-capped oligomers containing cationic segments exhibited not only the unique surface active properties imparted by fluorine as well as the usual low-molecular fluorinated surfactants, but also high surface antibacterial activity. Fluoroalkyl end-capped oligomers containing betaine-type segments were found to cause a gelation where the strong aggregation of the end-capped fluoroalkyl groups is involved in establishing the physical gel network in water and polar organic solvents under non-crosslinked conditions. Similarly, fluoroalkyl end-capped oligomers containing hydroxyl groups could cause a gelation, where the aggregation of fluoroalkyl groups and hydrogen-bonding interaction is involved in establishing a physical gel network in water and polar organic solvents under non-crosslinked conditions. Fluoroalkanoyl peroxide is also a convenient tool for the preparation of new fluoroalkyl end-capped oligomers containing recognition moieties such as diacetone segments. These fluorinated oligomers containing recognition moieties could form the self-assembled molecular aggregates to recognize selectively the hydrophilic amino and N,N-dimethylamino compounds as guest molecules.     

Excitation energy migration in oligo(p-phenylenevinylene) based organogels: structure-property relationship and FRET efficiency

Excitation energy migration (EM) and assisted energy transfer (ET) properties of a few oligo(p-phenylenevinylene) (OPV) based organogelators with different end functional groups have been studied using picosecond time-resolved emission spectroscopy (TRES). EM was found to be more efficient in OPV gelators with small end functional groups (OPV3-4) when compared to that of the gelators with bulky end groups (OPV1-2) in the gel state. TRES studies at elevated temperature and in chloroform solution highlight the role of the self-assembled scaffolds in assisting the EM and ET processes. Increase in temperature and solvent polarity leads to the aggregate breaking and hence adversely affects the EM and ET efficiencies. The effect of EM efficiency on the fluorescence resonance energy transfer (FRET) properties of the OPV gels was studied by using OPV1 and OPV3 as the donors and OPV5 as the acceptor. Better transfer of excitation energy was observed in the donor system (OPV3) having higher EM efficiency even at very low concentration (3.1 mol%) of the acceptor molecules, whereas ET efficiency was lower in the donor system (OPV1) with low EM efficiency.     

Star-shaped oligo(p-phenylenevinylene) substituted hexaarylbenzene: purity, stability, and chiral self-assembly

An oligo(p-phenylenevinylene) (OPV)-substituted hexaarylbenzene has been synthesized and fully characterized. Recycling gel permeation chromatography appeared to be a powerful technique to obtain the OPV molecules in a very pure form. X-ray analysis and polarization optical microscopy revealed that the OPV molecule is plastic crystalline at room temperature with an ordered columnar superstructure. In apolar solvents, the molecules self-assemble via a highly cooperative fashion into right-handed chiral superstructures, which are stable even at high temperatures and low concentration. Atomic force microscopy revealed right-handed fibers with a diameter of 6 nm, indicating pi-stacked aggregates; on a silicon oxide substrate, supercoiled chiral structures were observed. STM studies on a liquid-solid interface showed that the star-shaped OPV molecule forms an organized monolayer having a chiral hexagonal lattice.     

Synthesis and Gelation Capability of Fmoc and Boc Mono-substituted Cyclo(L-Lys-L-Lys)s

Fmoc or Boc mono-substituted cyclo(L-Lys-L-Lys)s were synthesized via the reaction of lysine cyclic dipeptide with Fmoc N-hydroxysuccinimide este(Fmoc-OSu) and di-tert-butyl dicarbonate[(Boc)₂O], respectively. The resulted mono-substituted cyclo(L-Lys-L-Lys)s(2-4) by means of test tube inversion method served as organogelators enabled to form stable thermo-reversible organogels in alcoholic, substituted benzene and chlorinated solvents, with the minimum gelation concentration(MGC) in a range of 1%-4%(mass fraction). The transmission electron microscopy(TEM) and scanning electron microscopy(SEM) observations reveal that these gelators self-assembled into 3D nanofiber, nanoribbon or nanotube network structures. The rheological measurement exhibited that the storage modulus of gels is higher than the loss one, and the complex viscosity is reduced linearly with the increasing of scanning frequency. The fluorescence spectrum of compound 2 in 1,2-dichloroethane and benzene demonstrates that the emission peak of Fmoc at 320 nm has red-shifted and the intensity decreases gradually, while the intensity of the emission peak at 460 nm substantially enhances as a function of concentration, indicating the existence of π-π sta- cking interactions and the formation of J-type aggregates. Meanwhile, compound 4 self-assembled into nanotubes via the stacking of multiple bilayer membranes. Fmoc and Boc disubstituted cyclo(L-Lys-L-Lys)(3) holds the relatively lower MGC values, showing the stronger gelation ability in most selected organic solvents due to the presence of both Fmoc and Boc groups.     

Preparation of organic/inorganic hybrid nanomaterials using aggregates of poly(stearyl methacrylate)-b-poly(3-(trimethoxysilyl) propyl methacrylate) as precursor

A novel brush-type amphiphilic copolymer of PSMA-b-PTMSPMA was synthesized via ATRP technique. As-synthesized polymer was characterized by GPC and ¹H NMR. It was of interest that the resultant polymer could self-assemble into micelles with different morphologies and sizes in selective solvents by adjusting the copolymer concentrations. These aggregates could be prepared into novel stable organic/inorganic hybrid nanomaterials by the gelation process. The size and structure of these aggregates and the corresponding hybrid nanomaterials were observed by TEM.     

Hydrogen-bonded oligo(p-phenylenevinylene) functionalized with perylene bisimide: self-assembly and energy transfer

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.     

Chiral bis(amino alcohol)oxalamide gelators-gelation properties and supramolecular organization: racemate versus pure enantiomer gelation

Four new chiral bis(amino alcohol)oxalamides (1-4: amino alcohol=leucinol, valinol, phenylglycinol, and phenylalaninol, respectively) have been prepared as low-molecular-weight organic gelators. Their gelation properties towards various organic solvents and mixtures were determined and these were then compared to related bis(amino acid) oxalamide gelators. Spectroscopic (FTIR, (1)H NMR) and X-ray diffraction studies revealed that the primary organization motif of (S,S)-1 and racemate 1 (rac-1) in lipophilic solvents involved the formation of inverse bilayers. The X-ray crystal structure of (S,S)-1 also shows this type of bilayer organization. The crystal structure of rac-2 reveals meso bilayers of hydrogen-bonded aggregates. Within the bilayers formed, the gelator molecules are connected by cooperative hydrogen bonding between oxalamide units and OH groups, while the interbilayer interactions are realized through lipophilic interactions between the iBu groups of leucinol. Oxalamide meso-1 lacks any gelation ability and crystallizes in monolayers. In dichloromethane rac-1 forms an unstable gel; this is prone to crystallization as a result of the formation of symmetrical meso bilayers. In contrast, in aromatic solvents rac-1 forms stable gels; this indicates that enantiomeric bilayers are formed. Oxalamide rac-1 is capable of gelling a volume of toluene three times larger than (S,S)-1. A tranmission electron microscopy investigation of rac-1 and (S,S)-1 toluene gels reveals the presence of thinner fibers in the former gel, and, hence, a more compact network that is capable of immobilizing a larger volume of the solvent. The self-assembly of these types of gelator molecules into bilayers and subsequent formation of fibrous aggregates can be explained by considering the strength and direction of aggregate forces (supramolecular vectors) in three-dimensional space.     

Gelation characteristics and applications of poly(ethylene glycol) end capped with hydrophobic biodegradable dipeptides

Poly(ethylene glycol) (PEG) end capped with biodegradable hydrophobic dipeptides shows versatile gelation behavior in a wide range of aqueous and organic solvents. This gelation characteristic is attributed to the aggregation of polymer chains induced by dipeptide end groups. Both PEG molecular weight and molecular structure of end groups control this aggregation by striking a balance between two opposing molecular interactions: solubility of the PEG segment which tends to dissolve the polymer while hydrophobic and intermolecular noncovalent interactions between the end groups induce aggregation. Morphologically, this aggregated structure forms interpenetrating nano sheets with characteristic microstructural features. These gels are biodegradable and possess physicomechanical characteristics suitable for biomedical applications. Furthermore, proteins and hydrophobic model drugs can be encapsulated within the gels from aqueous and organic solvents, respectively, and can be released in a controlled fashion which indicates the applicability of the gels as drug delivery vehicles. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1917–1928     

Self‐Assembly of a Chiral Lipid Gelator Controlled by Solvent and Speed of Gelation

Glutamine derivative 1 with two‐photon absorbing units has been synthesized and was found to show gelation ability in some solvents. Its self‐assembly in the gel phase could be controlled by the solvent and speed of gelation. For example, in DMSO the organogelator self‐assembled into H‐aggregates with weak exciton coupling between the aromatic moieties. On the other hand, in DMSO/diphenyl ether (1:9, v/v) the molecules formed 1D aggregates, but with strong exciton coupling due to the small distance between the chromophores. Moreover, the formation of these two kinds of aggregates could be adjusted by the ratio of DMSO to diphenyl ether. In DMSO/toluene, DMSO/butanol, DMSO/butyl acetate, and DMSO/acetic acid systems similar results were observed. Therefore, conversion of the packing model occurs irrespective of the nature of the solvent. Notably, a unique sign inversion in the CD spectra could be realized by controlling the speed of gelation in the DMSO/diphenyl ether (1:9, v/v) system. It was found that a low speed of gelation induces the gelator to adopt a packing model with strong π–π interactions between the aromatic units. Moreover, the gels, when excited at 800 nm, emit strong green fluorescence and the quantum chemical calculations suggest that intramolecular charge transfer leads to two‐photon absorption of the gelator molecule.     

Supramolecular polymerization and polymorphs of oligo(p-phenylene vinylene)-functionalized bis- and monoureas

Bis- and monoureas hybridized with the oligo(p-phenylene vinylene) (OPV) pi-electronic segment and 3,4,5-tridodecyloxyphenyl wedge were synthesized and their supramolecular polymerization in diluted solution, gel formation in concentrated solution, and liquid crystallinity in bulk state were investigated. Bisurea 1a featuring a hexamethylene linker showed the highest supramolecular polymerization ability and formed tapelike nanofibers that can gelate various organic solvents. On the other hand, bisurea 1b featuring a dodecamethylene linker and monourea 2 showed a lower degree of supramolecular polymerization, resulting in gel formations in a smaller variety of solvents. These results clearly reflect a high level of cooperativity between the two urea sites and the two OPV segments of 1a upon hydrogen-bonding and pi-pi stacking interactions, respectively. When the gels of 1a, 1b, and 2 were dried, all the compounds self-organized into multilamellar superstructures. Thermal treatment of these lamellae at high temperatures induces columnar liquid-crystalline mesophases as a result of microsegregation between the rigid OPV parts and the molten aliphatic wedges. These results demonstrate that the present molecular constituent is very useful for fabricating dye-based functional assemblies providing nanoscale pi-electronic fibers, and solvent-incorporated and bulk soft materials.     

Synthesis and photophysical properties of self-assembled metallogels of platinum(II) acetylide complexes with elaborate long-chain pyridine-2,6-dicarboxamides

A series of platinum(II) acetylide complexes with elaborate long-chain pyridine-2,6-dicarboxamides was synthesized. These metal complexes are capable of immobilizing organic solvents to form luminescent metallogels through a combination of intermolecular hydrogen bonding, aromatic π-π, and van der Waals interactions. Fibrillar morphologies were identified by TEM for these metallogels. Unique photophysical properties associated with the sol-to-gel transition have been disclosed with luminescence enhancement at elevated temperatures, which is in sharp contrast to typical thermotropic organogels or metallogels reported in the literature. Such unusual luminescence enhancement is attributed to the increased degree of freedom at higher temperatures that results in the formation of favorable molecular aggregates in the excited state through enhanced aromatic π-π and metallophilic Pt(II)···Pt(II) interactions. Structurally similar Pt-bp3 is not able to gel any common organic solvents. The inability of Pt-bp3 to form gels illustrates the importance of gelation to the macroscopic photophysical properties; Pt-bp3 does not show emission enhancement at elevated temperatures due to its low tendency to form strong aggregates in the ground state.     

One-component gels based on peptidic dendrimers: dendritic effects on materials properties

This paper describes the gelation of symmetric dendrimers based on building blocks constructed from L-lysine. These dendrimers form gel-phase materials in nonpolar organic solvents. The thermal properties and concentration dependence of the gelation were investigated, and it was found that there was a clear dendritic effect on the behavior of the soft materials formed, with higher generation dendrimers giving rise to more thermally stable gels. Variable temperature 1H NMR studies indicated that this behavior was probably a consequence of more extensive interdendrimer hydrogen bonding occurring between the peptidic groups in the higher generation dendrimers. The supramolecular aggregates were found to have a fibrillar structure, with the dimensions and alignment of the fibers being dependent on dendritic generation. Circular dichroism measurements confirmed that these fibers possessed chiral organization of the peptidic groups on the supramolecular (nano) scale, assigned as helicity. This paper indicates that dendritic functionalization provides a useful way of tuning gel-phase materials properties, with clear dendritic effects on gel formation being quantified for the first time, hence illustrating the way dendritic functionalization can play a positive role in the formation of highly functional organic materials with desirable properties.     

Cyclobutane-based peptides/terpyridine conjugates: Their use in metal catalysis and as functional organogelators

Two new conjugates, hcptpyDP and hcptpyTP, of a terpyridine derivative incorporating artificial peptide moieties, have been synthesized and their use in the preparation of metal catalysts and organogelators has been investigated. Ru(II) complexes derived from these ligands showed electrochemical behavior and activity as catalysts in the epoxidation of olefins similar to that of Beller's catalyst. As organogelators, these conjugates were able to gelate a variety of solvents, from toluene to methanol, with satisfactory mgc (minimum gelation concentration) values. The presence of 4′-(4-carboxy)phenylterpyridine (hcptpy) moiety allows tuning the gelling properties and also influences the supramolecular self-assembling mode to produce chiral aggregates with respect to parent peptides DP and TP. In the case of the conjugates, π?π interactions provided by the aromatic moieties cooperate with inter-molecular hydrogen bonding between NH and CO in the amide groups. Further properties of peptide/terpyridine conjugates are under investigation in view of future applications.     

Self‐assembly of oligo(p‐phenylenevinylene)‐block‐poly(ethylene oxide) in polar media and solubilization of an oligo(p‐phenylenevinylene) homooligomer inside the assembly

Rod–coil amphiphilic diblock copolymers, consisting of oligo(p‐phenylenevinylene) (OPV) as a rod and hydrophobic block and poly(ethylene oxide) (PEO) as a coil and hydrophilic block, were synthesized by a convergent method. The aggregation behavior of the block copolymers in a selective solvent (tetrahydrofuran/H₂O) was probed with the absorption and emission of the OPV block. With increasing H₂O concentration, the absorption maximum was blueshifted, the emission from the molecularly dissolved OPV decreased, and that from the aggregated OPV increased. This indicated that the OPV blocks formed H‐type aggregates in which the OPV blocks aligned in a parallel orientation with one another. The transmission electron microscopy observation revealed that the block copolymers with PEO weight fractions of 41 and 62 wt % formed cylindrical aggregates with a diameter of 6–8 nm and a length of several hundreds nanometers, whereas the block copolymer with 79 wt % PEO formed distorted spherical aggregates with an average diameter of 13 nm. Furthermore, the solubilization of an OPV homooligomer with the block copolymer was studied. When the total polymer concentration was less than 0.1 wt %, the block copolymer solubilized OPV with a 50 mol % concentration. The structure of the aggregates was a cylinder with a relatively large diameter distribution. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1569–1578, 2005     

Water-induced physical gelation of organic solvents by N-(n-alkylcarbamoyl)-L-alanine amphiphiles

A series of amino acid-based gelators N-(n-alkylcarbamoyl)-L-alanine were synthesized, and their gelation abilities in a series of organic solvents were tested. No gelation was observed in pure solvents employed. All the amphiphilic molecules were found to form stable organogels in the solvents in the presence of a small amount of water, methanol, or urea. The volume of solvent gelled by a given amount of the gelator was observed to depend upon the volume of added water. The gelation behavior of the amphiphiles in a given solvent containing a known volume of water was compared. The effects of chirality and substitution on the acid group on the gelation ability were examined. Although the corresponding N-(n-tetradecylcarbamoyl)-DL-alanine was found to form only weak organogel in pure solvents, the achiral amphiphilic compound N-(n-tetradecylcarbamoyl)-β-alanine, however, did not form gel in the absence of water. The methyl ester of N-(n-tetradecylcarbamoyl)-L-alanine was also observed to form gels in the same solvents, but only in the presence of water. The organogels were characterized by several techniques, including (1)H NMR, Fourier transform IR, X-ray diffraction, and field emission scanning electron microscopy. The thermal and rheological properties of the organogels were studied. The mechanical strength of the organogel formed by N-(n-tetradecylcarbamoyl)-DL-alanine was observed to increase upon the addition of water. It was concluded that water-mediated intermolecular hydrogen-bonding interaction between amphiphiles caused formation of supramolecular self-assemblies.     

Self-assembled pi-nanotapes as donor scaffolds for selective and thermally gated fluorescence resonance energy transfer (FRET)

Self-assembled nanotapes of a few tailor-made oligo(p-phenylenevinylene)s (OPVs) have been prepared and used as supramolecular donor scaffold to study the fluorescence resonance energy transfer (FRET) to a suitable acceptor. In nonpolar solvents, FRET occurs with nearly 63-81% efficiency, exclusively from the self-assembled OPVs to entrapped Rhodamine B, resulting in the quenching of the donor emission with concomitant formation of the acceptor emission at 625 nm. The efficiency of FRET is considerably influenced by the ability of the OPVs to form the self-assembled aggregates and hence could be controlled by structural variation of the molecules, and polarity of the solvent. Most importantly, FRET could be controlled by temperature as a result of the thermally reversible self-assembly process. The FRET efficiency was significantly enhanced (ca. 90%) in a xerogel film of the OPV1 which is dispersed with relatively less amount of the acceptor (33 mol %), when compared to that of the aggregates in dodecane gel. FRET is not efficient in polar solvents due to weak self-organization of the chromophores. These results indicate that energy transfer occurs exclusively from the self-assembled donor and not directly from the individual donor molecules. The present study illustrates that the self-assembly of chromophores facilitates temperature and solvent controlled FRET within pi-conjugated nanostructures.     

多重环境刺激响应性聚苄醚型树状分子凝胶制备及性能研究

环境刺激响应性超分子凝胶材料在传感器、光开关、人工触角、药物缓释等领域表现出潜在的应用前景。本文设计合成了一种新型的核心含偶氮苯官能团聚苄醚型树枝状分子凝胶因子CA-G2。成胶性能测试表明,该凝胶因子在23种有机溶剂和混合溶剂中均可以形成稳定的淡黄色凝胶,其中在苯中表现出最优的成凝胶性能,临界成胶浓度(CGC)可达2.0mg/mL(0.23(wt)%),相当于一个树枝状分子可以固定1.5×10⁴个溶剂分子,表明该凝胶因子具有非常优异的成凝胶性能。并且,该类凝胶材料能够同时对热、超声和触变等外界环境刺激产生响应,并伴随着宏观上凝胶-溶胶的相互转变。此外,该类凝胶对罗丹明B染料分子具有优异的吸附性能,吸附效率高达96.7%。     

Low molecular mass organogel from mesomorphic N-(4-hexyloxybenzoyl)-N'-(4'-nitrobenzoyl)hydrazine

The mesomorphic compound N-(4-hexyloxybenzoyl)-N'-(4'-nitrobenzoyl)hydrazine (C6-NO₂), containing a dihydrazide unit in the rigid core, exhibited a highly stable SmA₁ phase (between 172.2 and 259.5°C) and strong gelation ability in chloroform and other non-protonic organic solvents. Both SEM observations and X-ray diffraction data indicated that the molecules self-assembled into fibrous aggregates with a diameter of about 50 nm, and retained a head-to-tail configuration within layers. FTIR and ¹H NMR studies confirmed that intermolecular hydrogen bonding played a key role in the formation of the supra-structures, and this was considered to be the driving force. Additionally, aggregation-induced enhanced emission was observed in the organogels, and this was attributed to aggregation induced planarization and J-aggregate formation.