Formation of nanostructured silicas through the fluoride catalysed self-polymerization of Q-type functional silica cages.

Octa(dimethylsiloxy)silica cages (Q8M8H) undergo rapid self-polymerization in the presence of a fluoride catalyst to form complex 3D porous structural network materials with specific surface areas up to 650 m2 g-1. This establishes a new method to form bio-derived high inorganic content soft silicas with potential applications in filtration, carbon capture, catalysis, or hydrogen source.

Beads on a Chain Fluorescent Oligomeric Materials: Interactions of Conjugated Organic Cross-Linkers with Silsesquioxane Cages.

Organic electronic materials have advantages over inorganics in terms of versatility, cost, and processability. Recent advancements in organic materials for light-emitting diodes (OLED), field effect transistors (OFET), and photovoltaics have engendered extensive innovation potential on this field. In this research, we focus on synthesizing SQ (silsesquioxane) based oligomers cross-linked by dibromo-aromatic linkers and explore how the cross-linker influences their photophysical properties. Bis-trialkoxy silyl (linker) model compounds were synthesized to compare noncage photophysical properties with the oligomers. Several techniques such as UV/vis, fluorescence, FTIR, and thermal gravimetric analysis (TGA) have been used to characterize the systems. Time-resolved fluorescence and femtosecond transient absorption spectroscopy were used to understand the excited state dynamics of these materials. Studies were carried out to understand the differences between monomers and oligomers and potential energy transfer and charge transfer between the cages and cross-linking chromophores. Transient absorption showed lower energy absorption from the excited states, suggesting short-range communication between moieties. Single photon counting studies have shown distinct lifetime differences between most linkers and cages display possible excitation energy transfer through these materials. Transient absorption anisotropy measurements have shown signatures for excitation energy transfer between linker chromophores for oligomeric compounds. The silsesquioxane (SQ) backbone of the oligomers gives substantial thermal stability as well as solution processability, giving better flexibility for achieving energy transfer between linking chromophores.

Conjugated Copolymers That Shouldn't Be.

Multiple studies have explored using cage silsesquioxanes (SQs) as backbone elements in hybrid polymers motivated by their well-defined structures and physical and mechanical properties. As part of this general exploration, we report unexpected photophysical properties of copolymers derived from divinyl double decker (DD) SQs, [vinyl(Me)Si(O0.5 )2 ][PhSiO1.5 ]8 [(O0.5 )2 Si(Me)vinyl] (vinylDDvinyl). These copolymers exhibit strong emission red-shifts relative to model compounds, implying unconventional conjugation, despite vinyl(Me)Si(O-)2 siloxane bridges. In an effort to identify minimum SQ structures that do/do not offer extended conjugation, we explored Heck catalyzed co-polymerization of vinyl-ladder(LL)-vinyl compounds, vinyl(Me/Ph)Si(O0.5 )2 [PhSiO1.5 ]4 (O0.5 )2 Si(Me/Ph)vinyl, with Br-Ar-Br. Most surprising, the resulting oligomers show 30-60 nm emission red-shifts beyond those seen with vinylDDvinyl analogs despite lacking a true cage. Further evidence for unconventional conjugation includes apparent integer charge transfer (ICT) between LL-co-thiophene, bithiophene, and thienothiophene with 10 mol % F4 TCNQ, suggesting potential as p-type doped organic/inorganic semiconductors.

R-Silsesquioxane-Based Network Polymers by Fluoride Catalyzed Synthesis: An Investigation of Cross-Linker Structure and Its Influence on Porosity.

Silsesquioxane-based networks are an important class of materials that have many applications where high thermal/oxidative stability and porosity are needed simultaneously. However, there is a great desire to be able to design these materials for specialized applications in environmental remediation and medicine. To do so requires a simple synthesis method to make materials with expanded functionalities. In this article, we explore the synthesis of R-silsesquioxane-based porous networks by fluoride catalysis containing methyl, phenyl and vinyl corners (R-Si(OEt)3) combined with four different bis-triethoxysilyl cross-linkers (ethyl, ethylene, acetylene and hexyl). Synthesized materials were then analyzed for their porosity, surface area, thermal stability and general structure. We found that when a specified cage corner (i.e., methyl) is compared across all cross-linkers in two different solvent systems (dichloromethane and acetonitrile), pore size distributions are consistent with cross-linker length, pore sizes tended to be larger and π-bond-containing cross-linkers reduced overall microporosity. Changing to larger cage corners for each of the cross-linkers tended to show decreases in overall surface area, except when both corners and cross-linkers contained π-bonds. These studies will enable further understanding of post-synthesis modifiable silsesquioxane networks.

Thermally Stable Fluorogenic Zn(II) Sensor Based on a Bis(benzimidazole)pyridine-Linked Phenyl-Silsesquioxane Polymer.

A 2,6-bis(2-benzimidazolyl) pyridine-linked silsesquioxane-based semi-branched polymer was synthesized, and its photophysical and metal-sensing properties have been investigated. The polymer is thermally stable up to 285 °C and emits blue in both solid and solution state. The emission of the polymer is sensitive to pH and is gradually decreased and quenched upon protonation of the linkers. The initial emission color is recoverable upon deprotonation with triethylamine. The polymer also shows unique spectroscopic properties in both absorption and emission upon long-term UV irradiation, with red-shifted absorption and emission not present in a simple blended system of phenylsilsesquioxane and linker, suggesting that a long-lived energy transfer or charge separated state is present. In addition, the polymer acts as a fluorescence shift sensor for Zn(II) ions, with red shifts observed from 464 to 528 nm, and reversible binding by the introduction of a competitive ligand such as tetrahydrofuran. The ion sensing mechanism can differentiate Zn(II) from Cd(II) by fluorescence color shifts, which is unique because they are in the same group of the periodic table and possess similar chemical properties. Finally, the polymer system embedded in a paper strip acts as a fluorescent chemosensor for Zn(II) ions in solution, showing its potential as a solid phase ion extractor.

High Surface Area, Thermally Stable, Hydrophobic, Microporous, Rigid Gels Generated at Ambient from MeSi(OEt)3 /(EtO)3 SiCH2 CH2 Si(OEt)3 Mixtures by F- -Catalyzed Hydrolysis.

High surface area materials are of considerable interest for gas storage/capture, molecular sieving, catalyst supports, as well as for slow-release drug-delivery systems. We report here a very simple and fast route to very high surface area, mechanically robust, hydrophobic polymer gels prepared by fluoride-catalyzed hydrolysis of mixtures of MeSi(OEt)3 and bis-triethoxysilylethane (BTSE) at room temperature. These materials offer specific surface areas up to 1300 m2 g-1 , peak pore sizes of 0.8 nm and thermal stabilities above 200 °C. The gelation times and surface areas can be controlled by adjusting the solvent volume (dichloromethane), percent fluoride (as nBu4 NF or TBAF) and the BTSE contents. Polymers with other corners and linkers were also explored. These materials will further expand the materials databank for use in vacuum insulation panels and as thermally stable release and capture media.

Oxygen-mediated Polymerization Initiated by Oltipraz-derived Thiones.

A pyrrolopyrazine-thione derived from oltipraz, a compound that has been investigated as a chemopreventive agent, affords radicals in the presence of thiols and oxygen via a redox cycle, an attribute that suggests its suitability as an initiator for oxygen-mediated polymerization. Here, we explore the utilization of this pyrrolopyrazine-thione, generated in situ from a precursor, as an initiator for the radical-mediated thiol-ene polymerization. While the pyrrolopyrazine-thione was shown to be capable of generating radicals in the presence of atmospheric oxygen and thiol groups, the reaction extents achievable were lower than desired owing to the presence of unwanted side reactions that would quench radical production and, subsequently, suppress polymerization. Moreover, we found that complex interactions between the pyrrolopyrazine-thione, its precursor, oxygen, and thiol groups determine whether or not the quenching reaction dominates over those favorable to polymerization.

Avoiding Carbothermal Reduction: Distillation of Alkoxysilanes from Biogenic, Green, and Sustainable Sources.

The direct depolymerization of SiO2 to distillable alkoxysilanes has been explored repeatedly without success for 85 years as an alternative to carbothermal reduction (1900 °C) to Si(met) , followed by treatment with ROH. We report herein the base-catalyzed depolymerization of SiO2 with diols to form distillable spirocyclic alkoxysilanes and Si(OEt)4. Thus, 2-methyl-2,4-pentanediol, 2,2,4-trimethyl-1,3-pentanediol, or ethylene glycol (EGH2) react with silica sources, such as rice hull ash, in the presence of NaOH (10%) to form H2O and distillable spirocyclic alkoxysilanes [bis(2-methyl-2,4-pentanediolato) silicate, bis(2,2,4-trimethyl-1,3-pentanediolato) silicate or Si(eg)2 polymer with 5-98% conversion, as governed by surface area/crystallinity. Si(eg)2 or bis(2-methyl-2,4-pentanediolato) silicate reacted with EtOH and catalytic acid to give Si(OEt)4 in 60% yield, thus providing inexpensive routes to high-purity precipitated or fumed silica and compounds with single Si-C bonds.

D5h [PhSiO1.5]10 synthesis via F(-) catalyzed rearrangement of [PhSiO1.5]n. An experimental/computational analysis of likely reaction pathways.

We describe here the synthesis and analysis of the reaction pathways leading to formation of the rare D5h decaphenylsilsesquioxane (SQ) [PhSiO1.5]10via F(-) catalyzed rearrangement of [PhSiO1.5]nn = 8, 12, and oligomers initially synthesized from PhSi(OEt)3. Isolated yields of ∼50% [PhSiO1.5]10 are obtained via rearrangement of all starting materials. The recovered starting materials can be re-equilibrated using catalytic F(-) to generate similar yields in second batches. These yields arise because [PhSiO1.5]10 exhibits higher solubility and better energy stabilization (10 kcal mol(-1) theory) in CH2Cl2 compared to [PhSiO1.5]8 or [PhSiO1.5]12. Reaction intermediates were identified using time dependent (19)F NMR and MALDI-ToF mass spectrometry eventually equilibrating to form the 8 : 10 : 12 cages in a 1 : 3 : 1.3 equilibrium in CH2Cl2. Experimental results coupled with modeling using the Gamess computational package provide multiple reasonable pathways for SQ rearrangements to [RSiO1.5]10, starting from [RSiO1.5]8. Heats of reaction for interconversion of the model intermediates [HSiO1.5]x determined computationally, were used to select the most reasonable reaction pathways. The findings support a mechanism involving activation and cleavage of a T8 cage corner by F(-) attachment, followed by the corners stepwise removal as [i.e. RSi(OH)3], followed thereafter by reinsertion forming [RSiO1.5]9-OH followed by, insertion of another corner to form [RSiO1.5]10-(OH)2 and finally condensation to give [RSiO1.5]10. The most enthalpically favorable path (-24 kcal mol(-1)) involves a hybrid mechanism.

Expanding the Alternating Propagation-Chain Transfer-Based Polymerization Toolkit: The Iodo-Ene Reaction.

Analogous to the thiol-ene and phosphane-ene polymerizations, radical-mediated iodo-ene reactions are described here that proceed via alternating propagation and chain transfer (i.e., APT) reactions between perfluoroiodide- and vinyl-bearing monomers. The thermal polymerization of a diiodo/tetraene formulation yielded a cross-linked, homogeneous polymer that was approximately seven times as radiopaque as aluminum owing to its high iodine content. Visible-light photopolymerizations of model iodo-ene monomers were monitored using mid-IR spectroscopy, revealing that the perfluoroiodide functional group consumption exceeded that of the vinyl, a discrepancy that decreased with increasing irradiation intensities and hence polymerization rates. The functional group conversions in resin formulations with a large initial perfluoroiodide excess exacerbated secondary side reactions that led to off-stoichiometric functional group consumption; nevertheless, photopolymerization of resin formulations with excess vinyl stoichiometry proceeded according to the ideal APT mechanism.

Isolation and characterization of precise dye/dendrimer ratios.

Fluorescent dyes are commonly conjugated to nanomaterials for imaging applications using stochastic synthesis conditions that result in a Poisson distribution of dye/particle ratios and therefore a broad range of photophysical and biodistribution properties. We report the isolation and characterization of generation 5 poly(amidoamine) (G5 PAMAM) dendrimer samples containing 1, 2, 3, and 4 fluorescein (FC) or 6-carboxytetramethylrhodamine succinimidyl ester (TAMRA) dyes per polymer particle. For the fluorescein case, this was achieved by stochastically functionalizing dendrimer with a cyclooctyne "click" ligand, separation into sample containing precisely defined "click" ligand/particle ratios using reverse-phase high performance liquid chromatography (RP-HPLC), followed by reaction with excess azide-functionalized fluorescein dye. For the TAMRA samples, stochastically functionalized dendrimer was directly separated into precise dye/particle ratios using RP-HPLC. These materials were characterized using (1)H and (19)F NMR spectroscopy, RP-HPLC, UV/Vis and fluorescence spectroscopy, lifetime measurements, and MALDI.

Analyzing structure-photophysical property relationships for isolated T8, T10, and T12 stilbenevinylsilsesquioxanes.

Silsesquioxanes (SQs) are of considerable interest for hybrid electronic and photonic materials. However, to date, their photophysical properties have not been studied extensively, thus their potential remains conjecture. Here we describe the first known efforts to map structure-photophysical properties as a function of cage symmetry and size by comparing identically functionalized systems. Our focus here is on the solution photophysical properties of the title stilbenevinyl-SQs, which were characterized using single photon absorption, two-photon absorption, fluorescence emission, and fluorescence lifetime kinetics. We offer here the first detailed photophysical study of the larger pure T10 and T12 silsesquioxanes and show photophysical properties that differ as a function of size, especially in their fluorescence behavior, indicating that cage size and/or symmetry can strongly affect photophysical properties. We also find that they offer excitation-dependent emission (evidence of rare "red-edge" effects). The T10 stilbenevinyl-SQ offers up to a 10-fold increase in two-photon absorption cross section per chromophore over a free chromophore, signifying increased electronic coupling. The SQ cage compounds show "rise times" of 700-1000 fs and low anisotropy (~0.1) in fluorescence lifetime kinetic studies. These results indicate excited state energy transfer, unobserved for the free chromophores and unexpected for systems with "inert" silica cores and for 3-D hybrid molecular species. These findings provide the first detailed photophysical study of chromophore-functionalized T10 and T12 silsesquioxanes and show that SQs may be considered a separate class of compounds/materials with anticipated novel properties of value in developing new components for electronic and photonic applications.