Xueyan Feng

Tuning “thiol-ene” reactions toward controlled symmetry breaking in polyhedral oligomeric silsesquioxanes

The convenient synthesis of nano-building blocks with strategically placed functional groups constitutes a fundamental challenge in nano-science. Here, we describe the facile preparation of a library of mono- and di-functional (containing three isomers) polyhedral oligomeric silsesquioxane (POSS) building blocks with different symmetries (C3v, C2v, and D3d) using thiol-ene chemistry. The method is straightforward and general, possessing many advantages including minimum set-up, simple work-up, and a short reaction time (about 0.5 h). It facilitates the precise introduction of a large variety of functional groups to desired sites of the POSS cage. The yields of the monoadducts increase significantly using stoichiometric amounts of bulky ligands. Regio-selective di-functionalization of the POSS cage was also attempted using bulky thiol ligands, such as a thiol-functionalized POSS. Electrospray ionization (ESI) mass spectrometry coupled with travelling wave ion mobility (TWIM) separation revealed that the majority of diadducts are para-compounds (∼59%), although meta-compounds (∼20%) and ortho-compounds (∼21%) are also present. Therefore, the thiol-ene reaction provides a robust approach for the convenient synthesis of mono-functional POSS derivatives and, potentially, of regio-selective multi-functionalized POSS derivatives as versatile nano-building blocks.

Macromolecular structure evolution toward giant molecules of complex structure: tandem synthesis of asymmetric giant gemini surfactants

Precise control of primary chemical structures, especially those of complex structures, is a prerequisite to understand the structure–property relationships of functional macromolecules. In this article, we report the rational design and tandem synthesis of three asymmetric giant gemini surfactants (AGGSs) of complex macromolecular structures based on polyhedral oligomeric silsesquioxane (POSS). In two cascading processes (typically within 5 hours), AGGSs can be synthesized where the length of the two polymer tails and the identity of the two POSS heads can be independently controlled and systematically varied. It represents a convenient, efficient, and modular way to prepare giant molecules with rigorous structural precision in only a few steps. This study expands the scope of synthetically available giant surfactants and facilitates further structural evolution toward even more complex macromolecules.

Thiol-Michael “click” chemistry: another efficient tool for head functionalization of giant surfactants

One of the challenges in the precise synthesis of giant surfactants lies in the homogenous functionalization of a head with bulky ligands. In this article, we report the use of thiol-Michael “click” chemistry as a facile, modular and robust approach to address this issue. A giant surfactant with acryloxyl-functionalized POSS (ACPOSS) head was conveniently constructed from commercially available acrylo POSS and polystyrene (PS). Functional thiols with different sizes, such as 2-mercaptoethanol, 1H,1H,2H,2H-perfluoro-1-decanethiol, 1-thio-β-D-glucose tetraacetate (sugar-SH), and 2-naphthalenethiol, were attached onto the head of the ACPOSS-PS conjugate by thiol-Michael and thiol–ene reactions. It was found that while both the methods offer a straightforward and highly efficient approach to prepare uniform and precise giant surfactants with small thiol ligands, only the former proceeds without apparent side reactions when large and bulky thiols, such as sugar-SH and 2-naphthalenethiol, are used. The former method also eliminates the need for UV irradiation or heat initiation. Therefore, the mild condition, high efficiency, and broad functional group tolerance of thiol-Michael chemistry should further expand the scope of POSS-based giant surfactants with unparalleled possibilities for head surface chemistry manipulation, which provides numerous opportunities for nanofabrication by the direct self-assembly of giant surfactants.

Precision synthesis of macrocyclic giant surfactants tethered with two different polyhedral oligomeric silsesquioxanes at distinct ring locations via four consecutive “click” reactions

The combined utilization of chemoselective “click” chemistry allows for the preparation of well-defined macromolecules with complex compositions and architectures. In this article, we employed the sequential “click” strategy to further expand the scope of synthetically available giant molecules by precisely constructing new giant surfactants based on polyhedral oligomeric silsesquioxane (POSS) tethered cyclic polymers. The general synthetic approach involves sequentially performed strain-promoted azide–alkyne cycloaddition (SPAAC) as a method for bimolecular homobifunctional ring closure, copper-catalyzed azide–alkyne cycloaddition (CuAAC) for POSS-polymer conjugation, and thiol–Michael/thiol–ene reactions for POSS surface functionalization. Specifically, a cyclic polymer tethered with two POSS cages of distinct surface chemistry at different locations of the chain has been prepared. This work promises to afford numerous cyclic polymers-based giant surfactants with diverse structural variations for further investigation on unexpected physical properties.

Hierarchical Self-Organization of ABn Dendron-like Molecules into a Supramolecular Lattice Sequence

To understand the hierarchical self-organization behaviors of soft materials as well as their dependence on molecular geometry, a series of ABn dendron-like molecules based on polyhedral oligomeric silsesquioxane (POSS) nanoparticles were designed and synthesized. The apex of these molecules is a hydrophilic POSS cage with 14 hydroxyl groups (denoted DPOSS). At its periphery, there are different numbers (n = 1–8) of hydrophobic POSS cages with seven isobutyl groups (denoted BPOSS), connected to the apical DPOSS via flexible dendron type linker(s). By varying the BPOSS number from one to seven, a supramolecular lattice formation sequence ranging from lamella (DPOSS-BPOSS), double gyroid (space group of Ia3̅d, DPOSS-BPOSS2), hexagonal cylinder (plane group of P6mm, DPOSS-BPOSS3), Frank–Kasper A15 (space group of Pm3̅n, DPOSS-BPOSS4, DPOSS-BPOSS5, and DPOSS-BPOSS6), to Frank–Kasper sigma (space group of P42/mnm, DPOSS-BPOSS7) phases can be observed. The nanostructure formations in this series of ABn dendron-like molecules are mainly directed by the molecular geometric shapes. Furthermore, within each spherical motif, the spherical core consists hydrophilic DPOSS cages with flexible linkages, while the hydrophobic BPOSS cages form the relative rigid shell, and contact with neighbors to provide decreased interfaces among the spherical motifs for constructing final polyhedral motifs in these Frank–Kasper lattices. This study provides the design principle of molecules with specific geometric shapes and functional groups to achieve anticipated structures and macroscopic properties.