Synthesis and Structure-Photophysical Property Relationships of T8, T10, T12 and Oligomeric Organic Functionalized Silsesquioxanes.

Silsesquioxanes with conjugated organic tethers (chromophores) offer high orders of functionality (> 8 tethers), unusual enhanced absorption, emission and charge separation over free chromophores, excited state electron delocalization, and high thermal stability. This dissertation presents the synthesis and characterization of organic functionalized T10 and T12 [RSiO1.5]10,12 molecules, with emphasis on their synthesis by fluoride catalyzed rearrangement from [RSiO1.5]n and an understanding of their unique photophysical properties targeting components in optoelectronic devices. Initial discussion focuses on the synthesis of silsesquioxanes from silica via conversion of rice hull ash (RHA) silica to spirosiloxanes [i.e. Si(2-methyl-2,4-pentane-diolato)2] by reaction with 2-Me-2,4-pentanediol and catalytic NaOH. The resulting spirosiloxane reacts with selected arylLi reagents to form mono-aryl-spirosiloxane, suggesting a pentacoordinate silicon based mechanism. These aryl-spirosiloxanes are then converted through fluoride catalysis to novel aryl-silsesquioxanes [RSiO1.5]8,10,12. Thereafter we detail the development of [RSiO1.5]10,12 materials by fluoride catalyzed rearrangement and its mechanisms. F--catalyzed rearrangement of polymeric and octameric SQs is indispensable to the synthesis of [RSiO1.5]10/12, and mixed [R1R2SiO1.5]10,12 molecules in up to 95% yield. [PhSiO1.5]10 is synthesized in the highest reported yield to date (~50%), and is used as a model system for mechanism studies. The likely mechanistic paths taken to form T10 and T12 SQs are analyzed by experiment with MALDI/NMR to identify intermediates and computational modeling for the most likely pathways. The most favorable pathway to T10 from T8 involves coincidental participation of fluoride and water with a net enthalpy of ~-24 kcal/mol. We also explore in detail the photophysical properties of [StilbenevinylSiO1.5]8,10,12, which show similar absorption and emission in solution, but decreasing fluorescence quantum efficiencies with increasing cage size, suggesting more chromophore interactions and non-radiative decay. [StilbenevinylSiO1.5]10 shows the highest two-photon absorption cross-section of this series (5.7 GM/chromophore), offering the best polarization and charge transfer character. Fluorescence upconversion fluorescence lifetime studies on [StilbenevinylSiO1.5]8,10,12 find ultrafast charge transfer dynamics (<1 ps) indicative of chromophore-chromophore interactions in the excited state, unobserved for stilbenvinylSi(OEt)3, suggesting excited state charge delocalization.PHDChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111393/1/furgaljc_1.pd

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)) 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

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 Simet, 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.No detours: The base‐catalyzed depolymerization of SiO2 from different sources with diols led directly to distillable alkoxysilanes, including spirocyclic compounds, thus providing inexpensive routes to high‐purity silica and compounds with single Si−C bonds (see scheme): The alkoxysilanes could be converted either into Si(OEt)4 by treatment with EtOH and a catalytic amount of acid or into high‐purity precipitated (ppt) or fumed silica.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/137495/1/anie201506838-sup-0001-misc_information.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137495/2/anie201506838.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137495/3/anie201506838_am.pd

High Surface Area, Thermally Stable, Hydrophobic, Microporous, Rigid Gels Generated at Ambient from MeSi(OEt)3/(EtO)3SiCH2CH2Si(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 nBu4NF 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.Simple fluoride‐catalyzed polymerization of methyltriethoxysilane and bistriethyoxysilylethane leads to the formation of amorphous materials with little post‐synthesis processing. These materials have surface areas up to 1300 m2 g−1, densities as low as 0.06 g mL−1 and non‐polar solvent uptake of about 500 % by mass.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/141416/1/chem201704941.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/141416/2/chem201704941_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/141416/3/chem201704941-sup-0001-misc_information.pd

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. High definition : Two approaches for the formation of generation 5 PAMAM samples containing precise dye/dendrimer ratios are presented. The first approach, using direct separation based on dye hydrophobicity, generated a set of TAMRA‐containing dendrimers, and the second, using click chemistry, generated a set of fluorescein‐containing dendrimer (see figure).Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/106970/1/chem_201304854_sm_miscellaneous_information.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/106970/2/4638_ftp.pd

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. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 1373–1382The incorporation of a bis‐disulfide derivative of oltipraz (bisDS) in model thiol‐ene resin formulations affords an in situ generated pyrrolopyrazine‐thione. Subsequent exposure of these resins to atmospheric oxygen results in the generation of radicals via a redox cycle, in the presence of both thiol and molecular oxygen, permitting its utilization as an oxygen‐mediated initiating system for thiol‐ene polymerizations.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/136475/1/pola28507_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/136475/2/pola28507.pd

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

Beads on a Chain (BoC) Phenylsilsesquioxane (SQ) Polymers via F^– Catalyzed Rearrangements and ADMET or Reverse Heck Cross-coupling Reactions: Through Chain, Extended Conjugation in 3‑D with Potential for Dendronization

In this paper, we assess the utility of complementary routes to silsesquioxane based compounds using F^– catalyzed coupling to synthesize [vinylSiO_1.5]_<i>x</i>PhSiO_1.5]_{10‑<i>x</i>/12‑<i>x</i>} mixtures followed by copolymerization with divinylbenzene (via ADMET), or using reverse Heck coupling with 1,4-dibromobenzene and 4,4′-dibromo-stilbene to prepare lightly branched, nonlinear BOC systems. In another paper, we describe the use of Heck and Suzuki coupling to synthesize model conjugated <i>p</i>-R-stilbeneSQ BOCs starting from [<i>p</i>-IPh₈SiO_1.5]₈ and coupling with divinylbenzene (DVB) and 1,4-diethynylbenzene (DEB) finding extended 3-D conjugation in the DEB polymers. We find that the reverse Heck coupling (where the linker contains the bromo moieties) works best for these systems giving BoC oligomers with <i>M</i>_n of ∼6 kDa, in which extended excited state conjugation is observed for 1,4-dibromobenzene linked systems through ∼50+ nm red shifts in the emission spectra compared with DVB linked systems and model compounds. We compare and contrast the photophysical properties of the two sets of BOCs and the system where the conjugation length of the linker changes from divinylbenzene to divinylstilbene. We find that for a linker with a longer conjugation length, a red-shifted absorption and emission is observed; however, the difference in emission is much larger for the 1,4-dibromobenzene-linked system as compared to the model compounds, suggesting that a more rigid linker contributes to better orbital overlap with the cage and/or phenyl groups, increasing excited state conjugation interactions