Mark MacLachlan

Professor

Research Classification

Nanomaterials
Supramolecules and Autoassembling
Inorganic and Organometallic Compounds
Biomaterials
Basic Materials

Research Interests

Supramolecular Chemistry
Nanomaterials
Biomaterials
Cellulose nanocrystals
Chitin
Materials Chemistry
Inorganic Chemistry

Relevant Degree Programs

 

Research Methodology

Electron Microscopy
Gas sorption
Synthesis
NMR spectroscopy

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Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - May 2019)
Supramolecular chemistry of palladium and platinum-containing macrocycles: from ring-expansion to host-guest complexes (2019)

No abstract available.

Chiral nematic mesoporous organosilica materials templated with cellulose nanocrystals (2018)

The synthesis and characterization of new mesoporous organosilica materials with twisted porous networks and defined nanostructures was explored. Spindle-shaped cellulose nanocrystals (CNCs) with nanoscale dimensions can be isolated through the acid-catalyzed hydrolysis of wood pulp. These CNCs behave as chiral nematic lyotropic liquid crystals and are advantageous for the soft-templating of mesoporous materials as they can produce helically arranged pores. As such, CNCs show promise as inexpensive and renewable precursors for the development of nanomaterials.In this thesis, CNCs were used to template chiral nematic mesoporous organosilica (CNMO) materials through the co-condensation of bridged alkoxysilanes, R(Si(OR′)₃)₂, with aqueous CNC suspensions. Removal of the CNCs produced free-standing organosilica films that have interconnected pore structures and long-range chiral order imparted by the CNC template. Organic functionality was introduced as an integral component of the organosilica structure by varying the alkoxysilane precursors. The syntheses of alkylene-bridged (C₁-C₆), aromatic-bridged, ethenylene-bridged, and sulfur-containing CNMO films are reported. It was determined that phase separation between the alkoxysilanes and CNC suspension could be minimized by using mixed solvent systems of water and DMF during self-assembly. The new CNMO films display tunable photonic properties resulting from the repeating helical structure as well as thermal stabilities and pore connectivity that depend on the organic linker used. The combination of chirality and mesoporosity in these organosilica films suggests applications in hard templating, chiral catalysis, and sensing. CNMO films were investigated as support materials by functionalizing them with spiropyran molecules for sensing or manganese salphen complexes for catalysis. Spiropyran-bound CNMO films were used for photopatterning and behaved as reversible divalent metals sensors. Heterogeneous manganese salphen/CNMO films displayed similar catalytic conversion to the homogeneous catalyst, however, a small enantiomeric excess (5%) was observed suggesting that the chiral environment of the films may affect catalysis. Finally, metal ferrites [MFe₂O₄ (M = Ni, Cu, Zn, Co)] with chiral nematic nanostructures were prepared through hard templating with chiral nematic mesoporous silica. These materials beautifully replicated the three-dimensional structure of the CNCs liquid crystalline phase and their crystallinity and pore sizes were controlled by altering the calcination temperatures.

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Liquid crystalline tactoids in microscopic ordered-disordered interfaces: emergence of self-assembly and topological defects (2018)

No abstract available.

Novel structures and unusual reactivity powered by tautomerism and electron delocalization in salicylimines (2017)

A family of tris(salicylaldimine) (TSAN) analogues was prepared by condensation of 2,4,6-triformylphloroglucinol (TFP) and various nitrogen-containing bases. Characterization of these compounds by ¹H NMR spectroscopy and single-crystal X-ray diffraction (SCXRD) revealed that some of them adopt the previously unknown enol-imine tautomeric form. Experimental data and ab initio modelling were used to establish which factors govern the keto-enamine/enol-imine tautomeric equilibrium, culminating in a simple structural model of TSAN behaviour. According to this model, π electron-withdrawing X groups in the NH₂X starting material stabilize the keto-enamine tautomeric form, whereas σ electron withdrawing X, e.g. electronegative heteroatoms, lead to the enol-imine form.The same tautomeric equilibrium has also been leveraged in a family of hydroxysalicylaldehyde Schiff bases to bring about facile exchange of specific aromatic CH hydrogen atoms when these compounds are dissolved in CD₃OD or D₂O under ambient conditions. The mechanism of this surprising isotopic exchange reaction has been investigated experimentally using ¹H NMR kinetic experiments on these Schiff bases and a number of control compounds as well as ab initio modelling. Both sources point to the involvement of the minor keto-enamine tautomeric form of the salicylimines, which facilitates electrophilic aromatic substitution of hydrogen with deuterium by stabilizing the sp³-hybridized Wheland intermediate formed in the course of the reaction.The impact of the keto-enamine tautomeric form on the electronic structure of TSANs has been studied in isolation by preparing a TSAN permanently “locked” in keto-enamine connectivity. This was achieved by replacing the labile proton present in regular TSANs with a non-labile methyl group. The resulting compound assumes the expected keto-enamine structure; however, a large degree of strain is introduced since coplanarity of the peripheral enamine arms with the central ring, required for electronic coupling between the two, results in steric repulsion between the methyl groups and the carbonyl oxygen atoms. The central ring of the product is consequently highly contorted.The unsatisfactory preparation of many of the salicylaldehyde precursors to the SANs above has been improved through a mild and efficient procedure using formamidine acetate and acetic anhydride. This method allows one to install up to three formyl group on various phenolic substrates without the harsh conditions encountered with common formylation techniques such as the Vilsmeier–Haack reaction.

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The disparate symmetries in head-to-tail Schiff-base macrocycles (2017)

No abstract available.

Design and synthesis of new ceria-based materials for low-temperature methane oxidation (2016)

The main goal of this research is the development of new mesoporous materials for low-temperature methane oxidation that could be used in catalytic convertors of natural gas powered vehicles. Natural gas is a more environmentally friendly fuel than gasoline or diesel and is seen as a stepping-stone to renewable energy sources.The synthesis and characterization of novel ceria-based materials is reported. Modification of synthetic routes reported in the literature produced several new doped and undoped ceria samples, which were tested for catalytic activity for low-temperature methane oxidation. These tests showed that the presence of a second, redox active metal oxide results in materials with higher catalytic activity than those with only ceria. In addition, several new routes to ceria-based materials with various morphologies, including nanorods and hollow, mesoporous nanospheres, were developed. The nanospheres were successfully doped with lanthanum, giving rise to the first non-hydrothermal route to hollow, mesoporous nanospheres of both doped and undoped ceria.Further examination of materials containing two redox active metal oxides generated mesoporous cobalt oxide with doped and undoped ceria in the pores. Cobalt oxide was templated with KIT-6 silica to produce a material with highly ordered mesopores. The ceria/cobalt oxide materials have remarkably high activity for low-temperature methane oxidation given that they contain no noble metals. However, surprisingly, the mesoporous cobalt oxide on its own exhibited the highest catalytic activity with 50% complete methane conversion to carbon dioxide and water below 400 ˚C.Cerium-based precursors were also used as a starting material to synthesize Pd/ceria materials via a new method termed surface-assisted reduction. Surface-assisted reduction produces ceria with PdO highly dispersed on the surface. These materials showed exceptionally high activity for methane oxidation, with the best materials exhibiting 50% methane conversion below 300 ˚C. Exploration of the scope of surface-assisted reduction successfully produced ceria materials with gold or silver deposited on their surface.

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Synthesis of metal-organic frameworks from structure-directing iptycenyl ligands (2014)

No abstract available.

Chiral nematic mesoporous materials templated by nanocrystalline cellulose (2012)

The synthesis, characterization, and application of the first examples of chiralnematic mesoporous materials are reported. Nanocrystalline cellulose (NCC) was used asa liquid crystal template to generate NCC/silica composite films through evaporationinduced self-assembly. The NCC was removed from the composite films by calcinationto generate mesoporous silica films with high specific surface areas and chiral nematicstructures. The chiral nematic ordering in these films gives rise to photonic properties thatcan be tuned depending on the synthetic conditions. Thus mesoporous silica films withcolours spanning the visible spectrum were synthesized. The combination ofmesoporosity and chiral nematic ordering in these materials causes them to change colourin response to liquids and show strong circular dichroism signals that depend on therefractive index within the mesopores. Chiral nematic mesoporous silica can also be usedas a hard template to generate nanocrystalline films of anatase titanium dioxide. Thetitanium dioxide replicas are mesoporous and show chiral nematic ordering and photonicproperties that mimic the original silica films.By exploring different methods to remove NCC from the composite films, theprocedure used to synthesize chiral nematic mesoporous silica films was expanded toorganosilica. The resulting chiral nematic mesoporous organosilica films show similarproperties to the mesoporous silica films but have superior flexibility in some cases.Methods to control the pore size of the mesoporous silica and organosilica materials weredeveloped.Nanocrystalline cellulose/silica composite films were also used as a starting pointto synthesize chiral nematic mesoporous carbon films. This was achieved by pyrolyzingNCC/silica composite films followed by dissolving the silica in strong base. The silica used in this procedure was shown to be necessary for both the retention of chiral nematicordering and the introduction of mesoporosity into the carbonaceous material. Finally,chiral nematic mesoporous carbon (CNMC) was shown to be a promising material as asupercapacitor electrode material both on its own and as a composite with polyaniline.

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Assembly of Prussian blue analogue nanomaterials (2011)

Coordination polymers have many attractive properties but the development of applications has been hampered by the challenges associated with their processing and the preparation of nanosized analogues. In this thesis, the synthesis and characterization of new coordination polymer nanomaterials with previously inaccessible morphologies and compositions are reported. Prussian blue analogues (PBAs) were investigated as model compounds.Mesostructured PBAs were fabricated via a ligand-assisted liquid-crystal templating approach. Molecular surfactants having a charged iron cyanide complex as hydrophilic head group and metal-coordinated hydrophobic tails were synthesized. In formamide, the metal-containing template formed liquid-crystalline phases that were crosslinked into PBA mesostructures with the addition of transition metals. PBAs with well-ordered lamellar, hexagonal and cubic structures were obtained with a wide range of compositions. The materials made of iron(II) and iron(III) exhibited mixed-valency and ferromagnetic interactions in the PBA framework.A synthetic approach to attach a PBA precursor onto polymer-based structure-directing agents was developed. A preformed macromolecular backbone was functionalized with ionic pendent groups that can coordinate iron cyanide complexes. Metal-containing homopolymers and block copolymers were synthesized. In organic solvents, the ionic block copolymers behaved as a block ionomer and self-assembled into stable wormlike and toroidal reverse micelles whose cores were metallated with the iron cyanide complex or used as an ion confinement region for different cyanometallate compounds to be crosslinked into PBA-type frameworks. The soluble PBA nanomaterials are stable in solution, assemble into arrays on surfaces and were used as precursors for metal oxide nanostructures.Soluble hollow polymer capsules with PBA inner-shells were fabricated via emulsion-induced assembly of the iron cyanide block ionomer. The metal-containing amphiphilic macromolecules stabilized nanosized water droplets dispersed in organic solvent by assembling at the water-oil interface. The hydrophilic iron cyanide inner-shells were crosslinked into PBAs with zinc ions. The hollow capsules have selective permeability, are tunable in size, organize into hexagonal arrays in the solid state and were used as nanocontainers to encapsulate molecular compounds.A rigid structure-directing ligand was incorporated into the network of a PBA under solvothermal conditions to engineer the connectivity of a coordination polymer. A crystalline triptycene-scaffolded copper PBA was obtained.

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Construction of nanofibers from supramolecular self-assembly of Schiff-base macrocycles and metal salphen complexes (2010)

The work described in this thesis mainly covers the investigations of a series of conjugated Schiff-base macrocycles and metal salphen complexes. These compounds self-assemble into supramolecular structures through electrostatic or metal-ligand interactions, and their morphologies were studied by electron microscopy and atomic force microscopy. The Schiff-base macrocycles can bind alkali metal and ammonium cations into their crown-ether interior, leading to the formation of one-dimensional columns that can further organize into nanofibers with hierarchical organization. However, when macrocycles appended with long alkoxy chains were treated with the same conditions, lyotropic liquid crystallinity in organic solvents was observed under a polarized optical microscope. Among the metallosalphen complexes prepared, zinc(II)-containing salphen complexes were found to assemble into helical fibrous structures and exhibit gelation behavior in various solvents. Furthermore, modification of the peripheral substituents of the zinc(II) salphen complexes with carbohydrates further enhanced the helicity in the nanofibers. In addition, the surface texture and diameter of the nanofibers can be altered by the presence of ditopic 4,4′-bipyridine and the increase in hydrophobic effects during sample preparation.

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Studies of supramolecular chemistry through Schiff-base chemistry (2010)

This thesis describes the synthesis, characterization, and host-guest studies of a series of Schiff-base macrocycles. New [2+2] Schiff-base macrocycles were prepared by Schiff-base condensation. These macrocycles were shown to be wide-mouthed supramolecular hosts that can include organic cations such as pyridinium, paraquat and ammonium derivatives. A new kind of donor-acceptor-donor 3-in-1 complex was obtained in solution by combining macrocycle, cyclobis(paraquat-p-phenylene) and tetrathiafulvalene. Variations of these [2+2] Schiff-base macrocycles were prepared by modifying the substituents of the diformyl diol unit. In this way naphthalene-based macrocycles that undergo keto-enamine tautomerization were synthesized. These macrocycles can also combine with organic cations to form host-guest complex. The naphthalene-based [2+2] macrocycles can form lyotropic liquid crystals in chloroform and 1,2-dichloroethane. From the polarizing optical microscopy, it is proposed that the mesophases are lyotropic nematic liquid crystals based on a bilayer structure. A further study of these macrocycles shows that the host-guest complex can also form a lyotropic liquid crystalline phase.Covalently-linked macrocycles with isosceles triangle shapes were prepared by Schiff-base condensation. The molecular isosceles triangles proved to also be supramolecular hosts for pyridinium and ammonium cations, based on ¹H NMR, 2D-ROESY NMR, and mass spectrometry studies. In addition to the Schiff-base macrocycles, conjugated Schiff-base containing oligomers were synthesized via Gilch polymerization methods. The oligomers were characterized by gel permeation chromatography (GPC), thermogravimetric analysis (TGA) and UV-Vis spectroscopy. For further proof of the oligomeric structure, a model compound was prepared by the Wittig reaction.

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Supramolecular schiff base coordination chemistry : blueprints for self-assembling metallocavitands and nanotubes (2010)

Heptametallic zinc(II) and cadmium(II) clusters have been isolated after reactingthe metal-acetate salts with large diameter [3+3] Schiff base macrocycles. Two tetrazinccomplexes have been characterized and identified as intermediates in the formation of theheptazinc complexes. The heptametallic complexes are, in fact, templated by the Schiffbase macrocycles, a process that has been investigated with ¹H NMR spectroscopy andsingle-crystal X-ray diffraction. In the solid-state the heptametallic complexes have abowl-shaped geometry, reminiscent of organic cavitands, leading to them being calledmetallocavitands. Solid-state investigation of the heptazinc and heptacadmiummetallocavitands showed they organize into capsules with a cavity volume of 150 and215 ų, respectively. Solution dimerization was also observed in aromatic solvents andN,N-dimethylformamide (DMF). The thermodynamics of dimerization have beenquantified by van’t Hoff analyses of association constants measured with variabletemperature,variable-concentration ¹H NMR spectroscopy. Both metallocavitandsexhibit entropy-driven dimerization in all solvents in which dimerization occurs. Unusualfor dimerization of cavitands, this entropy-driven process can be attributed to theexpulsion of solvent from the monomeric cavity upon dimerization.Inside the cavity of heptacadmium metallocavitands is a μ₃-OH ligand where theproton is located at the base of the cavity and is capable of hydrogen bonding with guestmolecules. The μ₃-OH proton resonance is observable in low temperature 1H NMRspectra and exhibits two-bond J-coupling with three cadmium ions. Within capsules ofthe heptacadmium metallocavitands there are eight Lewis-acidic sites accessible to guestmolecules, six unsaturated cadmium(II) centers and two μ₃-OH ligands. Solid-stateanalysis shows that two DMF molecules are encapsulated in the heptacadmium capsulewhere they each simultaneously exhibit a host-guest hydrogen-bond and a dative metalligandinteraction.New methodology has been developed that facilitates synthesis of polydentate[2+2] Schiff base macrocycles with unsymmetrical salphen pockets. Also a [3+3]macrocycle with triptycenyl substituents has been synthesized to prohibit alkali-metalinduced solution aggregation.The one-pot twelve component head-to-tail self-assembly of Pt₄ rings directed bychelating imine-pyridyl donors has been demonstrated. These supramolecules exhibitextensive columnar organization in both solution and the solid-state, a phenomenon thatimparts liquid crystalline properties on the macrocycles.

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Conjugated schiff base-type metal-containing polymers (2008)

The work in this thesis describes the synthesis and characterization of a series ofconjugated polymers containing Schiff base transition metal complexes. High molecular weight poly(salphenyleneethynylene)s (PSPEs) were synthesized using the Sonogashira-Hagihara protocol and they were characterized using nuclear magnetic resonance spectroscopy and gel permeation chromatography. Their optical properties were investigated by UV-vis andfluorescence spectroscopies. PSPEs containing Zn saiphen moieties were found to exhibit strong aggregation that is facilitated by the presence of Zn to O interactions, and it was discovered thatthe polymers interact with various Lewis bases to undergo aggregation and deaggregation. New ladder-type conjugated polymers, as well as a series of model compounds that are representativeof the repeating units of the polymers, were synthesized using Schiff base condensation methods. The electronic and magnetic properties of these ladder-polymers were studied using cyclic voltammetry, electron paramagnetic resonance spectroscopy, and magnetic susceptibility measurements.

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Master's Student Supervision (2010 - 2018)
Investigations of chitin- and cellulose-based materials (2018)

Chitin plastic was fabricated from Dungeness crab shells. Chitin was purified from the shells, deacetylated, and then formed a gel in weak acid. The gel was poured into molds and then dried to leave behind a plastic film, which was strengthened by treatment with sodium hydroxide. Scanning electron microscopy (SEM) images of the dried film indicated that it adopts a layered structure. One sample batch was deacetylated for eight hours, and the other for twelve hours. The batch deacetylated for eight hours had superior characteristics, with a tensile strength of 93 ± 35 N/mm² and maximum strain of (7.5 ± 3) % while dry; after soaking in water for an hour, the tensile strength decreased to 7 ± 7 N/mm² and the strain at fracture increased to (31 ± 9) %. Contact angle experiments showed that the material is hydrophobic. Infrared (IR) spectroscopy confirmed a high degree of deacetylation in the final material. Results from IR spectroscopy and X-ray diffraction suggest a low degree of crystallinity in the material. This strong and waterproof chitin bioplastic is promising for use as packaging and for medical technology. Additionally, a new type of polypyrrole-cellulose composite was developed. Polypyrrole was synthesized, mixed with an aqueous suspension of cellulose nanocrystals, and dried to a homogeneous black iridescent film. The film showed iridescence that is attributed to the chiral nematic structure characteristic of cellulose nanocrystal films. Surprisingly, the film was not found to have any conductive or capacitative electrical properties. When doped, these new composite materials could have interesting electrical properties owing to the chiral structure of the films.

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Photonic materials based on cellulose nanocrystals (2014)

The self-assembly of cellulose nanocrystals (CNCs) into a chiral nematic structure exhibiting photonic properties has garnered much interest in recent years. The development of free-standing chiral nematic films composed of mesoporous silica and organosilica using CNCs as a template has led to a number of studies on producing photonic films composed of inorganic compounds. These films can be tuned to reflect light within the visible spectrum, yielding an assortment of films that exhibit structural colour that is retained once formed, and can no longer be modified. The incorporation of a photonic structure into a flexible material, such as a hydrogel, would allow for colour changes to transpire after the film is formed. Here, the integration of a chiral nematic photonic structure into hydrogel films prepared from different monomers is reported. The swelling of the photonic hydrogels was explored through the use of UV-visible spectroscopy, and the strength of the gels was investigated. In addition to the formation of tunable photonic structures based on CNCs, also reported is the formation of hybrid photonic structures produced by combining two classes of photonic crystals. These films build onto the chiral nematic mesoporous silica films by introducing a secondary photonic structure, based on the close packed arrangement of nanospheres. These novel hybrid photonic structures were synthesized, and characterized using electron microscopy. The successful formation of composite photonic materials, such as CNC-hydrogels, and hybrid photonic films displays the potential for CNC to be used as a template to build photonic structures in a wide array of systems.

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Studies of Nitration of Cellulose - Application in New Membrane Materials (2014)

No abstract available.

Switching the reflection in chiral nematic mesoporous silica and organosilica films (2013)

Recently, a new class of free-standing chiral nematic mesoporous silica and organosilica films was developed using nanocrystalline cellulose as a template. Due to their unique structures these films are iridescent, and by varying synthetic conditions it has been shown that this iridescence can be tuned to selectively reflect incident light of different wavelengths across the entire visible spectrum. However, upon formation of the mesostructure, these optical properties are locked in and can no longer be altered. Herein I describe alternate techniques to modify the optical properties of these silica and organosilica films by infiltration with guest molecules after the films have been prepared.Liquid crystal mesogen 5CB and 8CB were loaded into the pores of unfunctionalized and octyl and phenyl functionalized silica films. Thermal cycling of these new composite materials elucidated a sharp and reversible optical change in the 8CB loaded octyl functionalized silica films. Additional studies including variable temperature POM and UV-Vis spectroscopy were conducted on this system prepared with organosilica films. This approach was then expanded on by doping an azobenzene derivative in 1, 5, and 10 % by weight into the 8CB liquid crystal before loading the mixtures into the films. A reversible optical change was then brought about in this system by irradiation at different wavelengths. Further development of these methods could lead to their implementation in the still emerging fields of sensing and display technologies involving colour information.

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Publications

 
 

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