Lingli Ni

Ordered Hybrids from Template-Free Organosilane Self-Assembly

Despite considerable achievements over the last two decades, nonporous organic-inorganic hybrid materials are mostly amorphous, especially in the absence of solvothermal processes. The organosilane self-assembly approach is one of the few opportunities for creating a regular assembly of organic and inorganic moieties. Additionally, well-established organosilicon chemistry enables the introduction of numerous organic functionalities. The synthesis of periodically ordered hybrids relies on mono-, bis-, or multisilylated organosilane building blocks self-assembling into hybrid mesostructures or superstructures, subsequently cross-linked by siloxane Si-O-Si condensation. The general synthesis procedure is template-free and one-step. However, three concurrent processes underlie the generation of self-organized hybrid networks: thermodynamics of amphiphilic aggregation, dynamic self-assembly, and kinetically controlled sol-gel chemistry. Hence, the set of experimental conditions and the precursor structure are of paramount importance in achieving long-range order. Since the first developments in the mid-1990s, the subject has seen considerable progress leading to many innovative advanced nanomaterials providing promising applications in membranes, pollutant remediation, catalysis, conductive coatings, and optoelectronics. This work reviews, comprehensively, the primary evolution of this expanding field of research.

Self-Organized Poly(n-octadecylsilsesquioxane) Films via Sol–Gel Photopolymerization

We describe a novel solvent- and water-free sol-gel process for n-octadecyltriclorosilane (C(18)H(37)SiCl(3)) film catalyzed by photogenerated Brönsted acids. Driven by hydrophobic van der Waals interactions, a photoinduced self-assembly process occurs to afford a long-range ordered lamellar mesostructure, characterized by X-ray diffraction and transmission electron microscopy. Real-time Fourier transform IR spectroscopy was instrumental to probe the fast hydrolysis kinetics and assess the change of conformational behavior of the alkyl chains during UV irradiation. A unique combination of different solid-state NMR techniques ((29)Si, (13)C, (1)H) provided an insight into the supramolecular organization of this hybrid film.

Photoinduced synthesis and ordering of lamellar n-alkylsiloxane films†

Self-assembled organosilica films exhibiting various lamellar structures were prepared without solvent and watervia the fast photoacid-catalyzed sol–gel process of n-alkyltrimethoxysilanes (CnTMS, n = 8, 10, 12, 16 and 18). This facile photochemical route towards nanostructured hybrid films relies on superacid photocatalyst generated by the UV photolysis of a soluble lipophilic iodonium salt: (C12H25)2Φ2I+ SbF6−. A main emphasis has been on discussing the effect of the alkyl chain length on the level of ordering of the layered mesostructure (short- or long-range order), the alkyl chain packing arrangement (bilayered or interdigitated) and its conformational order (gauche and trans). X-Ray diffraction and electron microscopy (TEM and SEM) proved the high periodicity achieved with the long chain alkylsilanes (n = 16 and 18). Real-time Fourier transform infrared spectroscopy (RT-FTIR) was also instrumental in giving a unique insight into the sol–gel reaction kinetics and the progressive conformational ordering of the alkyl chains during the photoinduced self-assembly. Further, the combination of different solid-state NMR techniques (29Si and 13C) shed light on the siloxy layer microstructure and the conformational structure of the alkyl chain respectively.

Organic–inorganic tandem route to polymer nanocomposites: kinetic products versus thermodynamic products

We report a facile single-step photochemical methodology to afford alkylsiloxane-polymer hybrid films which relies on the tandem photoacid-catalyzed polymerization of n-alkyltrimethoxysilane precursors and a diglycidyl ether organic monomer. Photoacids liberated by the UV decomposition of iodonium salt triggers simultaneously the sol–gel process and epoxy cationic polymerization. Such conditions are intended to favour the formation of kinetic products trapped by cross-linking reactions instead of the thermodynamically most stable structures, whose preference would be for macrophase segregation. Organosilane precursor exhibiting different structures and chain lengths (n-butyl, n-octyl, n-dodecyl, n-hexadecyl, isobutyl and isooctyl) were systematically investigated to afford a range of transparent alkylsiloxane-polyether hybrids. The competitive organic–inorganic reaction kinetics were investigated using in situ real-time Fourier transform infrared spectroscopy. A main emphasis has been on discussing the effect of the alkyl substituent structure on the photoinduced polymerization kinetics and the silicate networks characterized by 29Si solid-state NMR. To avoid phase separation, the rate of formation of the two phases was tailored to favour concomitancy upon modulating several experimental parameters: film thickness, alkyl structure, photoacid generator concentration. Finally, the viscoelastic and surface properties were also assessed by dynamic mechanical analysis and water contact angle measurements, respectively.

Photopatterning of Multilayer n-Alkylsilane Films

Surface photopatterning of organosilane self-assembled monolayers (SAM) has received increasing attention since its introduction 20 years ago. Herein we report for the first time a cost-efficient soft photopatterning technique affording amplified 3D multilayer structures. The essential chemistry relies on a spatially controlled photoacid-catalyzed hydrolysis and polycondensation of n-alkyltrimethoxysilane precursors (n-C(12)H(25)Si(OCH(3))(3),). Amphiphilic siloxane species are photogenerated locally and are able to self-assemble spontaneously into a long-range-ordered lamellar mesostructure.

Hydrophilic/Hydrophobic Film Patterning by Photodegradation of Self-Assembled Alkylsilane Multilayers and Its Applications

While the photopatterning of self-assembled monolayers (SAMs) has been extensively investigated, much less attention has been given to highly ordered multilayer systems. By being both thicker (0.5-2 μm) and more stable (cross-linked) than SAMs, patterned hybrid multilayers lend themselves more easily to the development of technology-relevant materials and characterization. This paper describes a facile two-step UV approach to patterning an alkylsilane multilayer by combining photoinduced self-assembly for multilayer synthesis and photodegradation through a mask for creating patterns within the film. In this second stage, a spatially resolved removal of the alkyl tail via a photooxidation mechanism took place, yielding regular and uniform silica microdomains. The result was a regular array of features (alkylsiloxane/silica) differing in chemical composition (hybrid/inorganic), ordering (crystal-like/disordered), and wettability (hydrophobic/hydrophilic). Such a photopatterned film was of utility for a range of applications in which water droplets, inorganic crystals, or aqueous polymer dispersions were selectively deposited in the hydrophilic silica microwells.

Photoinduced self-assembly of carboxylic acid-terminated lamellar silsesquioxane: highly functional films for attaching and patterning amino-based ligands

Recently, long n-alkyltrimethoxysilanes (H3C(CH2)nSi(OCH3)3) have proven to self-assemble into mesoscopically ordered passive lamellar films through an efficient solvent-free photoacid-catalysed sol–gel process. By using an analogue precursor architecture presenting a terminal ester group (H3COC(O)(CH2)10Si(OCH3)3), both functional and tuneable nanostructured organosilica films were synthesized, while keeping all the processing advantages of light-induced self-assembly. The subsequent attachment of a fluorescent amino-based ligand (Safranin O) was performed using a two-step procedure. The ester end groups were first hydrolysed in reactive carboxylic acid using standard methods, and activated with an amino ligand to form amide bonds. Hydrolysis and ligand coupling were assessed through infrared and solid-state 1H NMR spectroscopy. Direct patterning of the fluorescent ligand-functionalized silsesquioxane film was performed by exposure to deep UV light under a mask to cause the local degradation of the dye. The resultant photopatterned film was detected using fluorescence microscopy. This UV method could represent an effective and general approach for attaching and patterning amino-based ligands, with less restriction on substrate and surface preparation than self-assembled monolayers.

Non-symmetrical bis-silylated precursor can (also) self-direct the assembly of Silsesquioxane films

Symmetric α,ω-bis-silylated precursors are the standard building blocks of self-assembled bridged silsesquioxanes, a unique class of robust ordered nanomaterials prepared by sol-gel process without external surfactant. We report an unprecedented approach based on the utilization of a dissymmetric bis-silylated precursor, 1,2-bis(trimethoxysilyl)decane (1), in which the two alkoxy groups are carried by adjacent methylene groups. Extensive characterization—based on X-ray diffraction, real-time fourier transform infrared, electron and optical microscopy, 29Si solid-state nuclear magnetic resonance, thermogravimetry, and molecular modeling—shows surprisingly that such non-symmetrical precursor is highly conducive to achieve highly ordered lamellar mesostructure, able to sustain temperature up to 120 °C. To emphasize the effect of the alkoxy group functionality and position, comparison is made systematically with analogous silsesquioxanes derived from bis-(2) and mono-silylated (3) precursors. The sol-gel polymerization of 1 is unique by its ability to produce a homogeneous film possessing structural characteristic on multiple scales: uniform microcrystallites consisting of nanolamellar organosilica hybrid material. The most likely mesostructure consists of bilayers of slightly interpenetrated trans C8H9 chains, with a single central siloxy-hydrocarbon chain (Si2O(OH)4-C2H3)n. To permit a good lateral chain packing, the hydrocarbon chain of two adjacent silicon atoms point in opposite directions.Graphical abstract

Ordering minimalist bridged polysilsesquioxane films under visible LED light irradiation

ABSTRACT A visible light-mediated sol–gel process can promote the ordering of hexylene, octylene, and decylene-bridged polysilsesquioxane (SQ) with very simple organosilane building blocks having only a short alkylene bridging group: (CH3O)3SiCnH2nSi(OCH3)3 (n = 6, 8, 10). Photocondensation and self-assembly of films are promoted through exposure to a light-emitting diode (LED; 405 nm). The solvent-free synthesis involves very low light intensity (0.1 mW/cm2) and a minimum amount of photoacid generator (PAG; 0.5 wt%) acting as a photocatalyst. X-ray diffraction (XRD) and scanning electron microscopy (SEM) prove the high level of organization, while real-time Fourier transform infrared spectroscopy shows conformational ordering of alkylene chains throughout irradiation. Molecular modeling reveals a multilayer structure based on the stacking of a single polysiloxane chain, with organic groups attached to adjacent Si atoms showing up and down orientation.