Functional organogel based on a hydroxyl naphthanilide derivative and aggregation induced enhanced fluorescence emission

A new class of low molecular weight organogelator (LMOG) of hydroxyl naphthanilide moiety was suitably designed and synthesized and it forms gels through noncovalent interactions in hydrocarbon solvents. Self-assembly structure, hydrogen bonding interaction, and photophysical properties of organogelator 3-hydroxy-naphthalene-2-carboxylic acid (2-heptylcarbamoyl-phenyl)-amide (2) have been investigated by field emission scanning electron microscope (FE-SEM), FT-IR, UV–vis absorption and photoluminescence combined with theoretical studies by hybrid density-functional theory (DFT) B3LYP and semi-empirical calculations AM1 with CI methods. It was found that gelation is completely thermoreversible, and it occurs due to the aggregation of the organogelator resulting in the formation of a fibrous network due to the π–π stacking interaction complemented by the presence of both inter- and intra-molecular hydrogen bonding. The self-assembled fibrillar networks in the gels were distinctly evidenced by SEM observations. FT-IR studies confirm that the common driving force for aggregation in the organogels and microsegregation in the mesophase is the occurrence of a tight intermolecular hydrogen bonded network that does not persist in diluted solution. Gelator 2 is very weakly fluorescent in solution, but its intensity is increased by almost 30–32 times in their respective gelled state depending on the nature of the gelling solvents. The aggregation induced emission enhancement is ascribed to the formation of J-aggregation and inhibition of intramolecular rotation in the gel state.     

Evolution from lyotropic liquid crystal to helical fibrous organogel of an achiral fluorescent twin-tapered bi-1,3,4-oxadiazole derivative

We report an unprecedented hierarchical self‐assembly of an achiral twin‐tapered bi‐1,3,4‐oxadiazole derivative (2,2‐bis(3,4,5‐trioctanoxyphenyl)‐bi‐1,3,4‐oxadiazole, BOXD‐T8). This molecule can form a layer‐structured lyotropic liquid crystal and further forms a helical fibrous organogel in DMF at concentrations above 0.6 wt %. The self‐assembly process of BOXD‐T8 in DMF is accompanied by a change in its fluorescence. The pitches of the helical fibers are non‐uniform, and both left‐ and right‐handed helical fibers are observed in equal quantities. Intermolecular π–π interactions between aromatic segments have been demonstrated to be the driving force for aggregate formation. This helical structure of BOXD‐T8 is dependent on the solvent, concentration, and the layer‐structured intermediate liquid‐crystalline state.     

Redox‐Controlled Helical Self‐Assembly of a Polyoxometalate Complex

Polyoxometalate (POM) complex (DODA)₂[Mo₆O₁₉] with a symmetrical linear structure was prepared conveniently by replacing the tetrabutylammonium (TBA) counterions of Lindquist‐type cluster (TBA)₂[Mo₆O₁₉] with cationic surfactant dioctadecyldimethylammonium (DODA). A helical self‐assembled structure of the complex was formed in dichloromethane/propanol. The dynamically reversible transformation between helical and spherical assemblies on alternate UV irradiation and H₂O₂ oxidation was characterized by SEM, TEM, and UV/Vis studies. The redox‐controlled morphology change is modulated by variation of the electrostatic interactions between the inorganic polyanion and the organic cation DODA through controlling the redox properties of the POM component, as shown by the XRD, X‐ray photoelectron spectroscopic, and ¹H NMR measurements. The strategy applied herein is a unique example of targeted smart and helical assembly of POM complexes.     

Thermoswitchable fluorescence organogels based on hydrogen bond‐assisted chiral gelators

To clarify the individual effect of secondary forces on the self‐assembly of molecules, a chiral cholesteryl N‐(2‐anthryl) carbamate (CAC) consisting of anthryl, carbamate, and cholesteryl groups was synthesized. From the results of the temperature‐dependent ¹H NMR, the hydrogen bond‐assisted π–π interaction was found to maintain the growth of the axis of the self‐assembled structure, and the three‐dimensional effect from the cholesteryl group induces the rotational structure. Fluorescence behavior of the CAC molecules with and without assistance of secondary forces was investigated. Thermoswitchable fluorescence of gelators was observed. Supramolecular organogels reveal significant enhanced fluorescence strength due to the aggregation‐induced enhanced emission of the CAC molecules. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Helical stacking tuned by alkoxy side chains in pi-conjugated triphenylbenzene discotic derivatives

We report on the synthesis and self-assembly of a new series of discotic molecules containing triphenylbenzene as the core and alkoxy side chain with varying length. It was found that compounds 3 a-c, 4 b and 5 b could form stable gels in several apolar solvents. Transmission electron microscopy (TEM) images revealed that their morphologies were very different for the different alkoxy-substituted organogels. In toluene or hexane, 3 b and 3 c resulted in both left- and right-handed helical fibers, whereas 3 a resulted in straight rigid fibers; 4 b and 5 b resulted in most straight fibers with a few twisted fibers. The results from FT-IR and UV/Vis absorption spectroscopy indicated that the hydrogen bonding and pi-pi interactions were the main driving forces for the formation of the self-assembled gels. Further detailed analysis of their aggregation modes were conducted by UV-visible absorption spectra and X-ray diffraction (XRD) measurements. Based on these findings, the influence of these peripheral alkoxy substituents on the gel formation and the aggregation mode were discussed. The special enhanced fluorescent emissions, which resulted from aggregation, were also found in the gel phase.     

Self‐Assembly of a Four‐Helix Bundle on a DNA Quadruplex

Come together : A novel method for assembling monomers and controlling structure of a de novo helix bundle protein is described. A guanine (G)‐rich oligodeoxynucleotide scaffold forms a hydrogen‐bonded DNA quadruplex in the presence of potassium counterions, thereby inducing a helical structure and fourfold stoichiometry in conjugated, amphiphilic peptide sequences. The DNA scaffold shows potential for rapidly assembling designed proteins.

     

Mirror helices and helicity switch at surfaces based on chiral triangular-shape oligo(phenylene ethynylenes)

The self‐assembly of triangular‐shaped oligo(phenylene ethynylenes) (OPEs), peripherally decorated with chiral and linear paraffinic chains, is investigated in bulk, onto surfaces and in solution. Whilst the X‐ray diffraction data for the chiral studied systems display a broad reflection centered at 2θ ～20° (λ=Cu_Kα), the higher crystallinity of OPE 3 , endowed with three linear decyl chains, results in a diffractrogram with a number of well‐resolved reflections that can be accurately indexed as a columnar packing arranged in 2D oblique cells. Compounds (S)‐ 1 a and (R)‐ 1 b —endowed with (S)‐ and (R)‐3,7‐dimethyloctyloxy chains—transfer their chirality to the supramolecular structures formed upon their self‐assembly, and give rise to helical nanostructures of opposite handedness. A helicity switch is noticeable for the case of chiral (S)‐ 2 decorated with (S)‐2‐methylnonyloxy chains which forms right‐handed helices despite it possesses the same stereoconfiguration for their stereogenic carbons as (S)‐ 1 a that self‐assembles into left‐handed helices. The stability and the mechanism of the supramolecular polymerization in solution have been investigated by UV/Vis experiments in methylcyclohexane. These studies demonstrate that the larger the distance between the stereogenic carbon and the aromatic framework is, the more stable the aggregate is. Additionally, the self‐assembly mechanism is conditioned by the peripheral substituents: whereas compounds (S)‐ 1 a and (R)‐ 1 b self‐assemble in a cooperative manner with a low degree of cooperativity, the aggregation of (S)‐ 2 and 3 is well described by an isodesmic model. Therefore, the interaction between the chiral coil chains conditions the handedness of the helical pitch, the stability of the supramolecular structure and the supramolecular polymerization mechanism of the studied OPEs.     

Periodic Mesoporous Organosilicas with Helical and Concentric Circular Pore Architectures

This study systematically investigates periodic mesoporous organosilicas (PMOs) with controlled helical and concentric circular (CC) pore architectures prepared through a basic‐catalyzed sol–gel process by using an achiral cationic surfactant trimethyloctadecylammonium bromide (C₁₈TAB) as a structure‐directing agent, perfluorooctanoic acid (PFOA) as an additive, and 1,2‐bis(triethoxysilyl)ethane (BTEE) as a hybrid silica precursor. By increasing the weight ratio of PFOA/C₁₈TAB, a pore architecture transition of PMO materials from hexagonal‐arrayed, straight longitudinal channels to helical and CC mesostructures is achieved; such a transition has not been observed before in PMO materials. Our discovery is helpful in understanding the supramolecular cooperative assembly of hybrid materials and their structural and morphological evolution, which are important in the future applications of PMO materials.     

Synthesis,self‐assembly,fluorescence, and thermosensitive properties of star‐shaped amphiphilic copolymers with porphyrin core

Star‐shaped amphiphilic poly(ε‐caprolactone)‐block‐poly(oligo(ethylene glycol) methyl ether methacrylate) with porphyrin core (SPPCL‐b‐POEGMA) was synthesized by combination of ring‐opening polymerization (ROP) and atom transfer radical polymerization (ATRP). Star‐shaped PCL with porphyrin core (SPPCL) was prepared by bulk polymerization of ε‐caprolactone (CL) with tetrahydroxyethyl‐terminated porphyrin initiator and tin 2‐ethylexanote (Sn(Oct)₂) catalyst. SPPCL was converted into SPPCLBr macroinitiator with 2‐bromoisobutyryl bromide. Star‐shaped SPPCL‐b‐POEGMA was obtained via ATRP of oligo(ethylene glycol) methyl ether methacrylate (OEGMA). SPPCL‐b‐POEGMA can easily self‐assemble into micelles in aqueous solution via dialysis method. The formation of micellar aggregates were confirmed by critical micelle formation concentration, dynamic light scattering, and transmission electron microscopy. The micelles also exhibit property of temperature‐induced drug release and the lower critical solution temperature (LCST) was 60.6 °C. Furthermore, SPPCL‐b‐POEGMA micelles can reversibly swell and shrink in response to external temperature. In addition, SPPCL‐b‐POEGMA can present obvious fluorescence. Finally, the controlled drug release of copolymer micelles can be achieved by the change of temperatures. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Messages in molecules: ligand/cation coding and self-recognition in a constitutionally dynamic system of heterometallic double helicates

Double helicates are known to exhibit self-recognition characteristics determined by the coordination geometry of the metal involved as well as by the topicity of the ligands. Combining tridentate (terpyridine, T) or bidentate (bipyridine, B) subunits in a tritopic strand affords a set of ligands able to assemble by pairs to form double helicates, homo- or heterostranded, homo- or heterotopic, depending on the coordination properties of the metals involved. The four ligand strands, BBB, TTT, BBT, and TBT form constitutionally dynamic sets of double helicates with the metal ions Cu(I), Cu(II), and Zn(II); these helicates correspond to the correct coding of the BB, BT, and TT pairs for tetra-, penta-, and hexacoordinate Cu(I), Cu(II), and Zn(II) cations, respectively.     

Two‐Component Fibers/Gels and Vesicles Formed from Hetero‐Double‐Helices of Pseudoenantiomeric Ethynylhelicene Oligomers with Branched Side Chains

A methodology for the formation of fibers/gels and vesicles by molecular assembly and for controlling their properties is presented. Two‐component systems of pentamer (P)‐ 5 and tetramer (M)‐ 4 pseudoenantiomeric ethynylhelicenes with decyloxycarbonyl (D) and 4‐methyl‐2‐(2‐methylpropyl)‐1‐pentyloxycarbonyl (bD) side‐chains have been examined. Distinct aggregates were formed by changing the solvent for the three combinations of (P)‐bD‐ 5 /(M)‐bD‐ 4 , (P)‐D‐ 5 /(M)‐bD‐ 4 , and (P)‐D‐ 5 /(M)‐D‐ 4 . In toluene, (P)‐bD‐ 5 /(M)‐bD‐ 4 , (P)‐D‐ 5 /(M)‐bD‐ 4 , and (P)‐D‐ 5 /(M)‐D‐ 4 all formed gels and fibrous assemblies were observed by AFM. The minimum gel‐forming concentration (MGC) decreased in the order (P)‐bD‐ 5 /(M)‐bD‐ 4 >(P)‐D‐ 5 /(M)‐bD‐ 4 >(P)‐D‐ 5 /(M)‐D‐ 4 . In diethyl ether, vesicular formation was observed by dynamic light scattering (DLS), AFM, and TEM, and the size of the vesicles decreased in the order (P)‐bD‐ 5 /(M)‐bD‐ 4 >(P)‐D ‐ 5 /(M)‐bD‐ 4 >(P)‐D ‐ 5 /(M)‐D ‐ 4 . Both fiber/gel and vesicle formation were accompanied by enhanced CDs and redshifted UV/Vis absorption bands with a change in color to deep yellow. These are novel two‐component oligomeric systems that form assemblies of fibers/gels or vesicles depending on the solvent, and the structures and properties of the assemblies can be fine‐tuned by changing the combination of oligomers. In m‐difluorobenzene, a homogeneous solution was obtained with (P)‐D‐ 5 /(M)‐bD‐ 4 , which again exhibits enhanced CDs and redshifted UV/Vis absorptions. Vapor pressure osmometry analysis showed the formation of a bimolecular heteroaggregate. The study has indicated that pseudoenantiomeric oligomers form hetero‐double‐helices that hierarchically assemble to form fibers/gels and vesicles.