Xue-Hui Dong

Giant surfactants provide a versatile platform for sub-10-nm nanostructure engineering

The engineering of structures across different length scales is central to the design of novel materials with controlled macroscopic properties. Herein, we introduce a unique class of self-assembling materials, which are built upon shape- and volume-persistent molecular nanoparticles and other structural motifs, such as polymers, and can be viewed as a size-amplified version of the corresponding small-molecule counterparts. Among them, “giant surfactants” with precise molecular structures have been synthesized by “clicking” compact and polar molecular nanoparticles to flexible polymer tails of various composition and architecture at specific sites. Capturing the structural features of small-molecule surfactants but possessing much larger sizes, giant surfactants bridge the gap between small-molecule surfactants and block copolymers and demonstrate a duality of both materials in terms of their self-assembly behaviors. The controlled structural variations of these giant surfactants through precision synthesis further reveal that their self-assemblies are remarkably sensitive to primary chemical structures, leading to highly diverse, thermodynamically stable nanostructures with feature sizes around 10 nm or smaller in the bulk, thin-film, and solution states, as dictated by the collective physical interactions and geometric constraints. The results suggest that this class of materials provides a versatile platform for engineering nanostructures with sub-10-nm feature sizes. These findings are not only scientifically intriguing in understanding the chemical and physical principles of the self-assembly, but also technologically relevant, such as in nanopatterning technology and microelectronics.

Selective assemblies of giant tetrahedra via precisely controlled positional interactions

Creating unusual nanostructures Self-assembly often occurs when dissimilar molecular fragments are forced together by covalent bonding. Surfactants or block copolymers are two common examples. Huang et al. grafted four different nanoparticles, based on polyhedral oligomeric silsesquioxanes with slightly different compositions, onto a single tetrahedal core (see the Perspective by Yang). Depending on the type of nanoparticle, they assembled into a range of defined, ordered supramolecular lattices similar to a range of metal alloys. These include phases that have higher coordination numbers than usually found in the packing of spherical objects. Science, this issue p. 424; see also p. 396 Tetrahedrally connected nanoparticles self-assemble into complex ordered phases. [Also see Perspective by Yang] Self-assembly of rigid building blocks with explicit shape and symmetry is substantially influenced by the geometric factors and remains largely unexplored. We report the selective assembly behaviors of a class of precisely defined, nanosized giant tetrahedra constructed by placing different polyhedral oligomeric silsesquioxane (POSS) molecular nanoparticles at the vertices of a rigid tetrahedral framework. Designed symmetry breaking of these giant tetrahedra introduces precise positional interactions and results in diverse selectively assembled, highly ordered supramolecular lattices including a Frank-Kasper A15 phase, which resembles the essential structural features of certain metal alloys but at a larger length scale. These results demonstrate the power of persistent molecular geometry with balanced enthalpy and entropy in creating thermodynamically stable supramolecular lattices with properties distinct from those of other self-assembling soft materials.

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.

Construction of a highly symmetric nanosphere via a one-pot reaction of a tristerpyridine ligand with Ru(II).

A three-dimensional, highly symmetric, terpyridine-based, spherical complex was synthesized via the coordination of four novel, trisdentate ligands and six Ru(2+) ions, and it exhibits excellent stability over a wide range of pH values (1-14). Structural confirmation was obtained by NMR and ESI-TWIM-MS.

Synthesis of fullerene-containing poly(ethylene oxide)-block-polystyrene as model shape amphiphiles with variable composition, diverse architecture, and high fullerene functionality

A series of [60]fullerene (C60)-containing poly(ethylene oxide)-block-polystyrene (PEO-b-PS) with various numbers and different locations of C60 along the polymer chains were designed and synthesized via a combination of “click” chemistry and living/controlled polymerization techniques such as anionic polymerization, atom transfer radical polymerization, and reversible addition–fragmentation chain transfer polymerization. One C60 was tethered either to the end of a PS block (PEO-b-PS-C60) or at the junction point between PS and PEO blocks [PEO-(C60)-PS]; while multiple C60s could be attached randomly along the PS block (PEO-b-PS/C60). The reaction conditions were carefully controlled to ensure a quantitative C60 functionality at precise locations in the case of PEO-b-PS-C60 and PEO-(C60)-PS and to avoid crosslinking in the synthesis of PEO-b-PS/C60. The results have implications in the precision synthesis of fullerene polymers in general. These C60-containing diblock copolymers possess different composition, diverse architecture, and high fullerene functionality. They can serve as model “shape amphiphiles” for the construction of complex hierarchical structures via the interplay between C60–C60 aggregation and block copolymer self-assembly/micro-phase separation.

Exploring shape amphiphiles beyond giant surfactants: molecular design and click synthesis

This paper reports the molecular design and click syntheses of novel shape amphiphiles with molecular architectures beyond conventional giant surfactants. They include (1) the giant bolaform surfactant which consists of a polystyrene (PS) chain tethered with one hydrophilic POSS cage at each end of the chain (DPOSS–PS–DPOSS); (2) the giant gemini surfactant which contains two hydrophilic POSS cages and two PS tails tethered at one junction point (2DPOSS–2PS); and (3) the multi-headed giant surfactant which is composed of three hydrophilic POSS cages tethered at one end of a PS chain (3DPOSS–PS). The syntheses were achieved in a modular and efficient fashion following the sequential click approach in good yields, providing easy access to a family of shape amphiphiles with precise chemical structures and fine-tuned interactions for a systematic study of structure–property relationships.

Toward Controlled Hierarchical Heterogeneities in Giant Molecules with Precisely Arranged Nano Building Blocks

Herein we introduce a unique synthetic methodology to prepare a library of giant molecules with multiple, precisely arranged nano building blocks, and illustrate the influence of minute structural differences on their self-assembly behaviors. The T8 polyhedral oligomeric silsesquioxane (POSS) nanoparticles are orthogonally functionalized and sequentially attached onto the end of a hydrophobic polymer chain in either linear or branched configuration. The heterogeneity of primary chemical structure in terms of composition, surface functionality, sequence, and topology can be precisely controlled and is reflected in the self-assembled supramolecular structures of these giant molecules in the condensed state. This strategy offers promising opportunities to manipulate the hierarchical heterogeneities of giant molecules via precise and modular assemblies of various nano building blocks.

Pathway toward Large Two-Dimensional Hexagonally Patterned Colloidal Nanosheets in Solution

We report the solution self-assembly of an ABC block terpolymer consisting of a polystyrene-block-poly(ethylene oxide) (PS-b-PEO) diblock copolymer tail tethered to a fluorinated polyhedral oligomeric silsesquioxane (FPOSS) cage in 1,4-dioxane/water. With increasing water content, abundant unconventional morphologies, including circular cylinders, two-dimensional hexagonally patterned colloidal nanosheets, and laterally patterned vesicles, are sequentially observed. The formation of toroids is dominated by two competing free energies: the end-cap energy of cylinders and the bending energy to form the circular structures. Incorporating the superhydrophobic FPOSS cages enhances the end-cap energy and promotes toroid formation. Lateral aggregation and fusion of the cylinders results in primitive nanosheets that are stabilized by the thicker rims to partially release the rim-cap energy. Rearrangement of the parallel-aligned FPOSS cylindrical cores generates hexagonally patterned nanosheets. Further increasing the water content induces the formation of vesicles with nanopatterned walls.

Self-Assembly of Fullerene-Based Janus Particles in Solution: Effects of Molecular Architecture and Solvent

Two molecular Janus particles based on amphiphilic [60]fullerene (C60 ) derivatives were designed and synthesized by using the regioselective Bingel-Hirsh reaction and the click reaction. These particles contain carboxylic acid functional groups, a hydrophilic fullerene (AC60 ), and a hydrophobic C60 in different ratios and have distinct molecular architectures: 1:1 (AC60 -C60 ) and 1:2 (AC60 -2C60 ). These molecular Janus particles can self-assemble in solution to form aggregates with various types of micellar morphology. Whereas vesicular morphology was observed for both AC60 -C60 and AC60 -2C60 in tetrahydrofuran, in a mixture of N,N-dimethylformamide (DMF)/water, spherical micelles and cylindrical micelles were observed for AC60 -C60 and AC60 -2C60 , respectively. A mechanism of formation was tentatively proposed based on the effects of molecular architecture and solvent polarity on self-assembly.

Tunable Affinity and Molecular Architecture Lead to Diverse Self-Assembled Supramolecular Structures in Thin Films

The self-assembly behavior of specifically designed giant surfactants is systematically studied in thin films using grazing incidence X-ray scattering and transmission electron microscopy, focusing on the effects of molecular nanoparticle (MNP) functionalities and molecular architectures on nanostructure formation. Two MNPs with different surface functionalities, i.e., hydrophilic carboxylic acid functionalized [60]fullerene (AC60) and omniphobic fluorinated polyhedral oligomeric silsesquioxane (FPOSS), are utilized as the head portions of the giant surfactants. By covalently tethering these functional MNPs onto the end point or junction point of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) diblock copolymer, linear and star-like giant surfactants with different molecular architectures are constructed. With fixed length of the PEO block, changing the molecular weight of the PS block leads to the formation of various ordered phases and phase transitions. Due to the distinct affinity, the AC60-based and FPOSS-based giant surfactants form two- or three-component morphologies, respectively. A stretching parameter for the PS block is introduced to characterize the PS chain conformation in the different morphologies. The highly diverse self-assembled nanostructures with high etch resistance between components in small dimensions obtained from the giant surfactant thin films suggest that these macromolecules could provide a promising and robust platform for nanolithography applications.