Dielectric and acid-base bifunctional effects are elucidated in heterogeneous aminocatalysis using a synthetic strategy based on bulk silica imprinting. Acid-base cooperativity between silanols and amines yields a bifunctional catalyst for the Henry reaction that forms alpha,beta-unsaturated product via quasi-equilibrated iminium intermediate. Solid-state UV/vis spectroscopy of catalyst materials treated with salicylaldehyde demonstrates zwitterionic iminium ion to be the thermodynamically preferred product in the bifunctional catalyst. This product is observed to a much lesser extent relative to its neutral imine tautomer in primary amine catalysts having outer-sphere silanols partially replaced by aprotic functional groups. One of these primary amine catalysts, consisting of a polar outer-sphere environment derived from cyano-terminated capping groups, has activity comparable to that of the bifunctional catalyst in the Henry reaction, but instead forms the beta-nitro alcohol product in high selectivity (approximately 99%). This appears to be the first observation of selective alcohol formation in primary amine catalysis of the Henry reaction. A primary amine catalyst with a methyl-terminated outer-sphere also produces alcohol, albeit at a rate that is 50-fold slower than the cyano-terminated catalyst, demonstrating that outer-sphere dielectric constant affects catalyst activity. We further investigate the importance of organizational effects in enabling acid-base cooperativity within the context of bifunctional catalysis, and the unique role of the solid surface as a macroscopic ligand to impose this cooperativity. Our results unequivocally demonstrate that reaction mechanism and product selectivity in heterogeneous aminocatalysis are critically dependent on the outer-sphere environment.

The dynamic behavior of nanoscale mesoporous oxide materials exposed to aqueous solutions under biologically relevant conditions is shown to be highly dependent on composition, porosity, and calcination temperature. Dynamic processes were followed as a function of exposure on thin oxide films amenable to environmental ellipsometry porosimetry for the analysis of mechanical strength and pore size distributions as a function of exposure. Additionally, X-ray photoelectron spectroscopy was used for the elucidation of compositional changes as a function of exposure. Combined, this approach gives the first detailed, quantitative information of the degradation of nanoscale oxide materials under biologically relevant conditions. This approach also shows the utility of using film geometry as a convenient model system for the study of dynamic properties, as films are amenable to sensitive ellipsometric characterization. Pure silica films underwent a rapid degradation, occurring on the time scale of hours, while silica films mixed with 10% or less of zirconia or alumina were significantly more stable. These mixed metal oxide films showed structural changes on two time scales, undergoing a rapid partial degradation followed by a stabilization of the structure as the composition of the films evolved toward a depleted silica state. The time scales of these two processes were on the order of hours and days, respectively, and could be tuned by varying the composition and the calcination temperature of the films. These time scales are especially relevant to the culture and growth of mammalian cells and for drug release applications. Titania materials were shown to be stable under all conditions studied, making them suitable candidates for applications where the scaffold functions as a permanent support. These results yield unprecedented levels of detail on the kinetics of degradation and the dynamic structural and compositional changes occurring in these nanostructured materials.

In-situ thermal ellipsometric analysis is used to elucidate new and fine-scale details on the thermally driven densification, pyrolysis, crystallization, and sintering of dense and ordered mesoporous titania thin films prepared by evaporation-induced self-assembly. The role of the heating schedule, initial film thickness, nature of the substrate and templating agent, solution aging, and presence of water and other additives in the calcination environment is examined. Each of these parameters is shown to have unique and often substantial effects on the final film structure, while the technique itself provides detailed insight into the chemical origin and evolution of these effects. In-situ monitoring and control over the governing chemical processes, such as high-temperature adsorption phenomena that impact nanocrystal growth, is also demonstrated. The evolution of both the porosity and chemical processes occurring inside these materials are evaluated, including extraction of kinetic parameters for the pyrolysis of the template and crystallization of the matrix walls. The latter is shown to be strongly dependent on the presence of mesoscale ordering with ordered cubic films indicating a 1D diffusion-limited crystallization process and dense films following a 3D diffusion-limited process. Less well-ordered mesoporous films, despite similarities in pore volume and pore size distributions, are kinetically more reminiscent of dense films in terms of crystallization. In-situ thermal ellipsometry, by detailing the evolution of the thermally driven chemistry and ceramization that dictate the final film properties, provides immensely important insight into the synthesis and optimization of advanced functional materials based on titania and other metal oxide thin films.

A bulk heterojunction of ordered titania nanopillars and PbS colloidal quantum dots is developed. By using a pre-patterned template, an ordered titania nanopillar matrix with nearest neighbours 275 nm apart and height of 300 nm is fabricated and subsequently filled in with PbS colloidal quantum dots to form an ordered depleted bulk heterojunction exhibiting power conversion efficiency of 5.6%.

The synthesis of bulk, hydrophilic imprinted silica is reported via a novel method relying on the thermolysis of carbamates. This method is used to synthesize single-site materials consisting of either isolated primary amines or multiple organized primary amines within an imprinted site. Materials are characterized by multinuclear solid-state NMR spectroscopy, high-resolution thermogravimetric analysis, and potentiometric titration. Diffuse-reflectance UV/Vis and solid-state fluorescence spectroscopies of probe molecule-bound materials are used to demonstrate the site-isolated and polar nature of the imprinted sites resulting from this process. The polar nature is due to hydrophilic silanol groups ingenerate to the framework of the material. These permit the partitioning of polar reagents, such as 1,3,5trinitrobenzenesulfonic acid (TNBS), which are unable to access the imprinted sites when framework silanols are end-capped with hydrophobic trimethylsilyl functionality. The latter observation provides a mechanism to elucidate diminished reactivity of TNBS with lysine residues that are buried within the hydrophobic pocket of a protein.

Rational outer‐sphere design can be used to optimize heterogeneous catalysts. Thus, imprinting of bulk silica was used to prepare tethered active sites with hydrophilic (1) and hydrophobic environments (2). In the Knoevenagel condensation of isophthalaldehyde with malonitrile, for example, 1 gave rate enhancements of about 50 and 30 relative to 2 and a commercial catalyst consisting of a monolayer of 3‐aminopropyl groups on silica, respectively.

In this article we demonstrate the sub-nanometer patterning of mixed chemical functional groups consisting of thiol and primary amine functionality on the surface of silica and characterize the local organization of thiols and amines within these materials. Our approach has required the development of a synthetic method for the thermolytic imprinting of thiols based on a xanthate protection scheme, which enables bifunctional imprints containing carbamate and xanthate functionality to be condensed onto a silica surface and thermolytically deprotected in a single step. This imprinting process is demonstrated for the synthesis of bifunctional imprinted sites containing thiol−amine pairs and groups of two thiols and an amine per imprinted site, and is characterized using solid-state UV/Visible, 29Si CP/MAS NMR, and 13C CP/MAS NMR spectroscopies. Independent titration of thiols with Ellman's reagent and amines with perchloric acid demonstrates that the bulk surface coverage of thiol and amine functionality reflects the expected ratios based on imprint molecule stoichiometry, with yields of accessible bifunctional imprinted sites greater than 80% relative to the amount of imprint used. The high yields in the current methodology overcome previous limitations for the imprinting of mixed functional groups and are facilitated by the low entropic penalty for imprinting multiple groups provided by the unimolecular nature of the thermolysis process. Local chemical functional group organization is investigated by using o-phthalaldehyde as a specific binding probe for thiol−amine pairing on the length scale of ∼3 Å. The paired imprinted material shows 2.5-fold higher yield of paired sites relative to a mixed control material consisting of monofunctional thiol and amine sites at the same bulk surface coverage of 0.02 thiol and amine functional groups per nm2. This result demonstrates that silica surface imprinting can be used to control the local density of mixed chemical functional groups on sub-nanometer length scales in a fashion that is impossible to achieve with most other synthetic methods. Our ability to imprint mixed chemical functionality on the surface of silica and quantitatively characterize the resulting amount of thiol−amine pairing is further used to rigorously investigate imprinted site isolation in mixed control materials. Our results demonstrate that nonrandom surface distributions of immobilized imprint result in these materials, despite low surface coverages, and draw attention to the need for further investigating degree of site isolation when surface-imprinting silica.

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