Solution Self-assembly Behavior Research Articles

Synthesis and Solution Self-Assembly Behavior of Porphyrin Tethered by Fullerene C<sub>60</sub>

This paper focuses on the synthesis, spectroscopic studies and self-assembly behavior of porphyrin phenyl linked fullerene C60 dyad, which included COOH groups in its fullerene unit so as to promote its adsorption onto TiO2. UV-Vis spectrum and steady-state fluorescence spectrum measurement, showed that the self-assembly of this compound in different solvents with different polarities. The results show that the soret absorption peak slightly blue shifted by 5 nm in acetonitrile and 2 nm in tetrahydrofuran, while the soret peaks in toluene solution was the same as in chloroform. Increasing concentration from 1x10-6 mol/L to 2x10-5 mol/L in chloroform resulted into aggregation, also the slight red shifting on cooling from 900C to 100C was observed from a temperature dependant UV-Vis absorption spectra, thus indicates aggregation. All these features may indicate presence of J-type aggregation happening in 4-methylphenyl-H2porhyrin-fullerene C60-COOH. In addition, the fluorescence of 4-methylphenyl-H2Porhyrin-fullerene C60-COOH has been quenched compared to that of pure 4-methylphenyl-H2porhyrin-OH under the same concentration. Therefore, this novel material design may find good application in photoelectric devices.

Giant gemini surfactants based on polystyrene–hydrophilic polyhedral oligomeric silsesquioxane shape amphiphiles: sequential “click” chemistry and solution self-assembly

This paper reports our recent investigations in the synthesis, characterization, and solution self-assembly of giant gemini surfactants consisting of two hydrophilic carboxylic acid-functionalized polyhedral oligomeric silsesquioxane (APOSS) heads and two hydrophobic polystyrene (PS) tails covalently linked via a rigid spacer (p-phenylene or biphenylene) (PS–(APOSS)2–PS). The sequential “click” approach was employed in the synthesis, which involved thiol–ene mono-functionalization of vinyl-functionalized POSS, Cu(I)-catalyzed Huisgen [3 + 2] azide–alkyne cycloadditions for “grafting” polymer tails onto the POSS cages, and subsequent thiol–ene “click” surface functionalization. The study of their self-assembly in solution revealed a morphological transition from vesicles to wormlike cylinders and further to spheres as the degree of ionization of the carboxylic acid groups on POSS heads increases. It was found that the PS tails are generally less stretched in the micellar cores of these giant gemini surfactants than those of the corresponding single-tailed (APOSS–PS) giant surfactant. It was further observed that the PS tail conformations in the micelles were also affected by the length of the rigid spacers where the one with longer spacer exhibits even more stretched PS tail conformation. Both findings could be explained by the topological constraint imposed by the short rigid spacer in PS–(APOSS)2–PS gemini surfactants. This constraint effectively increases the local charge density and leads to an anisotropic head shape that requires a proper re-distribution of the APOSS heads on the micellar surface to minimize the total electrostatic repulsive free energy. The study expands the scope of giant molecular shape amphiphiles and has general implications in the basic physical principles underlying their solution self-assembly behaviors.

Fast bulk click polymerization approach to linear and hyperbranched alternating multiblock copolymers

A novel bulk click polymerization (BCP) approach was determined to prepare alternative multiblock copolymers (AMCs). Poly(e-caprolactone) (PCL) and polyethylene glycol (PEG) were first modified into azide and alkyne terminated macromonomers by esterification, respectively. Subsequent azide–alkyne click polymerization of the two macromonomers in bulk without any solvents using CuBr/N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA) as a catalyst system afforded AMCs with higher degrees of polymerization (DPs) than those obtained via solution polymerization (SP) in the control experiments. Through this approach, within 1 h (normally 5 min), AMCs were obtained with high DPs of up to 16, as revealed by gel permeation chromatography (GPC). Studies on the thermal properties of the AMCs showed that the alternating structure confined the crystallizations of the PCL and PEG blocks. The solution self-assembly behavior of the amphiphilic AMCs was investigated for the first time, and interesting structures such as globules, fibers, and worms were observed by means of atomic force microscopy (AFM). Additionally, hyperbranched AMCs were also prepared via an A2 + B3 strategy.

Amphiphilic mesogen‐jacketed liquid crystalline polymers: Design, synthesis, and self‐assembly behaviors

AbstractWe synthesized a series of amphiphilic mesogen‐jacketed liquid crystalline (LC) polymers with a biphenyl side‐chain mesogen containing a carboxylic acid group on one side and an octyloxy group on the other, and the number of methylene units between the biphenyl core and the exterior carboxylic acid group was varied to adjust the mesophases and the amphiphilic nature. The polymers were obtained through conventional radical polymerizations and characterized by a combination of different techniques such as thermogravimetric analysis, differential scanning calorimetry, polarized light microscopy, and X‐ray scattering. The results revealed that the polymer without any methylene spacer, POBP‐0C, did not exhibit LC properties while POBP‐1C (n = 1) and POBP‐7C (n = 7) formed double layer smectic A (SA) phases. The hydrogen bonding among the carboxylic acid groups and the segregation between the carboxylic acid groups and the alky chains played important roles in forming the mesophases. In addition, the solution self‐assembly behaviors were also preliminarily investigated through the fluorescent probe technique and transmission electron microscopy, and vesicles with uniform sizes were observed. The weak hydrophilicity and large degree of freedom of the carboxylic acid group and the relative rigidity of the polymer chain due to the “jacketing” effect were responsible for the formation of the structures. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Controlled Self-Assembly of C3-Symmetric Hexa-peri-hexabenzocoronenes with Alternating Hydrophilic and Hydrophobic Substituents in Solution, in the Bulk, and on a Surface

In this work, we introduce a class of C(3)-symmetric hexa-peri-hexabenzocoronenes (HBCs) 1 with alternating hydrophilic and hydrophobic substituents to achieve control over the self-assembly of discotic nanographene molecules. Our studies show that the following structural parameters and experimental conditions are essential for tailoring the formation of the liquid-crystalline phase in the bulk as well as the self-assembly in solution and on surfaces: (1) steric demand of alkyl and alkylphenyl substituents; (2) noncovalent hydrophilic-hydrophobic interactions of the substituents; and (3) processing conditions, such as the type and mixture of solvents of different polarities along with the nature of the surface. The substitution of HBC with linear alkyl side chains possessing less steric demand (1b) leads to high crystallinity in the bulk solid state and at the liquid-solid interface, and the additional feature of alternating hydrophilic and hydrophobic substituents promotes a high aggregation tendency in polar/apolar solvent mixtures. In contrast, bulky branched alkyl chains (1a) and alkylphenyl substituents (1c) induce liquid crystallinity over the whole temperature range measured. While 1a does not show pronounced self-assembly in solution, compound 1c displays, even at high temperatures, aggregation in polar/apolar solution due to the intermolecular "locking" of peripheral phenyl groups. After solution deposition on a surface, distinct fiber formation is observed for the HBC derivatives, which is related to the solution self-assembly behavior. The present work provides further insight into the molecular design and self-assembly of discotic nanographene materials.

Molecular Self-Assembly into One-Dimensional Nanostructures

Self-assembly of small molecules into one-dimensional nanostructures offers many potential applications in electronically and biologically active materials. The recent advances discussed in this Account demonstrate how researchers can use the fundamental principles of supramolecular chemistry to craft the size, shape, and internal structure of nanoscale objects. In each system described here, we used atomic force microscopy (AFM) and transmission electron microscopy (TEM) to study the assembly morphology. Circular dichroism, nuclear magnetic resonance, infrared, and optical spectroscopy provided additional information about the self-assembly behavior in solution at the molecular level. Dendron rod-coil molecules self-assemble into flat or helical ribbons. They can incorporate electronically conductive groups and can be mineralized with inorganic semiconductors. To understand the relative importance of each segment in forming the supramolecular structure, we synthetically modified the dendron, rod, and coil portions. The self-assembly depended on the generation number of the dendron, the number of hydrogen-bonding functions, and the length of the rod and coil segments. We formed chiral helices using a dendron-rod-coil molecule prepared from an enantiomerically enriched coil. Because helical nanostructures are important targets for use in biomaterials, nonlinear optics, and stereoselective catalysis, researchers would like to precisely control their shape and size. Tripeptide-containing peptide lipid molecules assemble into straight or twisted nanofibers in organic solvents. As seen by AFM, the sterics of bulky end groups can tune the helical pitch of these peptide lipid nanofibers in organic solvents. Furthermore, we demonstrated the potential for pitch control using trans-to-cis photoisomerization of a terminal azobenzene group. Other molecules called peptide amphiphiles (PAs) are known to assemble in water into cylindrical nanostructures that appear as nanofiber bundles. Surprisingly, TEM of a PA substituted by a nitrobenzyl group revealed assembly into quadruple helical fibers with a braided morphology. Upon photocleavage of this the nitrobenzyl group, the helices transform into single cylindrical nanofibers. Finally, inspired by the tobacco mosaic virus, we used a dumbbell-shaped, oligo(phenylene ethynylene) template to control the length of a PA nanofiber self-assembly (<10 nm). AFM showed complete disappearance of long nanofibers in the presence of this rigid-rod template. Results from quick-freeze/deep-etch TEM and dynamic light scattering demonstrated the templating behavior in aqueous solution. This strategy could provide a general method to control size the length of nonspherical supramolecular nanostructures.