Like spin-containing magnetic metals that have unpair electrons in their d- or f-orbitals, magnetism can also be achieved by the introduction of radicals for organic molecules1. However, radicals in organic molecules are typically featured with low stability under ambient temperature and pressure2, which creates difficulties for the investigation of organic magnets. To solve the problem, fluorene derivatives were investigated because of its large aromatic system which helps to stabilize any introduced radical. A one-electron transfer was performed on the molecule 2,6-di-tert-butyl-4-(9H-fluoren-9-yl)cyclohexa-2,5-dien-1-one using sodium solid and the radical on the reaction product is detected by electron paramagnetic resonance. The electron paramagnetic resonance experiment revealed that due to the large π-conjugated system of the fluorene and the side chain, the compound is able to hold the radical under ambient temperature and pressure. The prepared fluorene derivative represents not only a stable organic radical species as itself but also a valuable precursor for a wide range of polymerizations. Propagation along 2,7 position of the fluorene polymerized with itself or other precursors may give substance that holds both magnetism and polymer properties.
1 Kamachi, M. J. Macromol. Sci., Polym. Rev.2002,42(4), 541–561.
2Zhang, K.; Monteiro, M. J.; Jia, Z. Polym. Chem.20167(36), 5589–5614.
Graphitic carbon nitride (CNx) is a promising photocatalyst that boasts facile synthesis and non-toxicity. Its development presents a potential alternative to fossil fuels and current solar energy conversion approaches. Promising photocatalytic hydrogen efficiencies have been reported using sacrificial electron donors but these routes are inherently unsustainable. Overall water splitting to generate O2 and high energy H2 is sustainable but current efficiencies are not high enough for commercial viability. Chemical and morphological alterations for CNx are being investigated to improve its photocatalytic activity.
The primary aryl amines of the CNx heptazine rings provide a potential site for post-synthetic substitutions. Additionally, successful substitution in CNx’s bulk form would capitalize on its scalable and facile synthesis. Post-synthetic substitutions were targeted to enhance its semiconductor qualities by amplifying favourable electronic characteristics such as increased π-π stacking and charge transfer complex formation.1,2 In this project, we adapted traditional amide synthesis procedures using aromatic acyl chlorides to CNx.
Direct substitution of the heptazine aryl amines to halogens via a diazonium intermediate was also attempted using the Sandmeyer reaction. Products were purified via phase extraction and column separation, characterized using FT-IR, and their photocatalytic activities were assessed using the degradation of the azo dye methyl orange.3
Theoretical yields were not determined due to the lack of direct substitution and the inability to determine the ratio of acyl chloride to CNx within the products. Amidation reactions yielded small amounts of organic-soluble product containing a mixture of acyl chloride and CNx and large amounts of insoluble bulk containing the acyl chloride. Although amide formation in the products was inconclusive, FT-IR spectra revealed that the acyl chlorides and their corresponding carboxylic acids are present in the CNx after extensive purification. These products have altered physical and spectral characteristics indicating intercalation of the acyl chlorides and potentially, substitution. Recovered bulk CNx from the purified halogenation reactions showed no spectral change compared to the reactant bulk CNx.
In this presentation, the efficacy of the chosen synthetic methods will be discussed and the trends in photodegradation rates will be related to spectral and physical properties of the products.
1Kobayashi, Y. et al. Nat. Mater. 2016 16(1), 109-114.
2Parini, V. P. Russ. Chem. Rev. 1962 31(7), 408-417.
3Yan, S. et al. Langmuir. 2010 26(6), 3894–3901.
It has been demonstrated Mo-Co based materials have great potential in light-harvesting processes – especially for photogeneration of H2, where MoS2 is used as an alternative to Pt- or Pd-based catalysts.1 Nevertheless, Mo-based materials are practically unexplored in the field of photocatalytic organic transformations, with only a few examples reported for applications in fine chemistry.2 The Scaiano group have worked with Co and Mo elements as co-catalysts (Mo-Co decorated TiO2) for light-induced transfer hydrogenations; however, in those cases the catalyst only works under UV light.1 Here we show the Mo-Co materials can show intrinsic photocatalytic activity in the absence of titania. To improve their heterogeneous properties, the materials are supported on glass wool (Mo-Co@GW), a strategy recently developed by the Scaiano group. The use of heterogeneous materials in catalysis usually facilitates the way the catalysts can be separated and reused.3 Glass wool is an ideal catalyst support because it is inexpensive, readily available, fairly robust, and its surface can be modified easily to achieve affinity towards catalytic materials.4
In this work we explore the use of MoCo@GW for different electron transfer reactions and we compare the efficiencies with the widely used Pd@TiO2. Particularly, MoCo@GW is capable of catalyzing radical formations of cyclic ethers using UVA or visible light irradiation.5 A radical-trapping species, TEMPO, is used to monitor the radicals’ formation. The TEMPO adduct is observed for cyclic ethers such as THF, dioxane, and benzodioxole. This material can also catalyse photoredox reactions, including the oxidation of 1-phenylethanol to acetophenone, indoline to indole, and tetrahydroisoquinoline to dihydroisoquinoline and isoquinoline under UVA or blue light irradiation.
Overall, the material is capable of catalyzing reactions involving one, two and up to four electrons, demosntrating the materials’ ability for charge transfer. Additionally, the MoCo@GW can act under visible light irradiation, showing better efficiencies than Pd@TiO2 photocatalysts.
1B. Wang, et al. J. Catal., 2019, submitted.
2Z. Li, et al. J. Photochem. Photobiol., C: Photochem. Rev., 2018, 35, 39-55.
3J. C. Scaiano, A. E. Lanterna, Pure Appl. Chem., 2019, submitted.
4A. Elhage, et al. Chem. Sci., 2018, 9, 6844-6852.
5A. Hainer, et al. J. Am. Chem. Soc. 2019, 141, 4531−4535.
Halogen bonding (XB) has been identified as a potentially useful tool in molecular electronics. Dependent on the movement of electrons, molecular electronics will result in the formation of radicals as electrons are transferred in a system. This fact indicates radical-involved XB systems to be of particular interest. Previously in the Kennepohl lab, the stable radical TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) has been observed to form weak XBs with cationic halogenated molecules. Expanding on this, the nature of the XB interactions between TEMPO and a series of halogenated pyridinium salts (where X = Cl, Br, and I) is explored using spectroscopic methods to compare the XB strength for different XB donors (Figure 1), as well as to investigate thermodynamic properties of XB systems in solution.
1H NMR titrations are performed to observe the relationship between increasing concentrations of TEMPO and the changing spectra of each pyridinium salt (constant concentration) in acetonitrile-d3 solvent at room temperature (25 °C) and low temperatures (-35°C). The data indicate binding constants too small to determine, which suggest fairly weak XB interactions. This weak interaction was further confirmed when preparing crystals: proved too weak to result in crystals containing both XB donor and acceptor.
When compared, the chlorinated salts show a greater shift in NMR spectra than their counterparts. These salts (M2ClP, M4ClP) are further analyzed using Cl K-edge X-ray absorbance spectroscopy (XAS). Interestingly, for the largest ratio of M2ClP:TEMPO the data show an increase in absorption edge energy. Understanding this phenomenon would require more experimentation.
Overall, the interactions between TEMPO and our chosen XB donors proved to be relatively weak, which impeded our investigation into the nature of radical-involved XB systems. This suggests the need for better XB pairs in future trials. However, investing further time into investigating to interpret the XAS data may be worthwhile.
Phenol de-aromatization constitutes a powerful synthetic approach to create highly functionalized 6-membered rings.1 The reaction requires hypervalent iodine reagents in polar protic solvent—hexafluoroisopropanol is typically used—and is theorized to proceed through an unexpected cationic-phenol (phenoxenium) intermediate.2 The mechanism of this reaction remains enigmatic due to the lack of experimental data in the literature.
The central focus of this study was to investigate the mechanisms proposed in the literature using density functional theory calculations as well as UV/Vis and Raman spectroscopy. Computational studies suggested the formation of an energetically favourable colourless iodine-phenol intermediate that dissociates into a coloured phenoxenium intermediate. We obtained good agreement between theoretical and experimental UV/Vis spectra on the formation of coloured intermediates consistent with phenoxenium intermediates for a range of phenolic compounds. Theoretical studies on various substituted phenols showed that butylated hydroxytoluene (BHT), a phenol with bulky R groups in the ortho position, produces a stable phenoxenium intermediate that can be freeze-quenched. A resonance Raman spectrum of this freeze-quenched red intermediate showed peak shifts that are indicative of a chemical transformation. We tentatively assign this Raman spectrum to a phenoxenium species and we are currently looking for phenoxenium reactive intermediates derived from other phenol derivatives. Our findings present promising strategies for elucidating the reaction mechanism, which will allow for advancements and practical implementation of this reaction in organic synthesis.
Reaction scheme for the de-aromatization of butylated hydroxytoluene (BHT) using hypervalent iodine in hexafluoroisopropanol (HFIP).
1 Roche, S. P. & Porco, J.A. J. Angew. Chem 2011 50(18), 4069.
2 Maertens G, Canesi S. J. Top Curr Chem 2016 373, 223-41.s
Glaser coupling involves the oxidative homocoupling of terminal alkynes to form symmetrical 1,3-diynes and is the most widely used procedure to form such molecules.1 These compounds have a wide variety of applications in the field of biology and material science including the synthesis of polymers and biologically active molecules.2 In the past, expensive palladium catalysts were the classical way to catalyze such reactions. Currently, the use of copper salts in the presence of a base and an oxidant serve as an alternative method to mitigate expenses, although not without its own drawbacks such as the use of higher temperatures and long reaction times.3 However, the use of heterogenous copper nanoparticle functionalized semiconductors as a photocatalyst in Glaser coupling remains largely unexplored.
For cuprous oxide nanoparticle synthesis, a simple and safe in-situ synthesis method was adopted from previously published results4 using the addition of economic and environmentally-friendly TiO2 as the supporting semiconductor. Furthermore, the photocatalytic capability of these Cu2O nanoparticles in the Glaser coupling of phenylacetylene was found to be highly efficient at 10mM concentrations in the presence of a strong base of equal concentration and methanol solvent. The reaction times ranged from 15 minutes to 2 hours at ambient temperatures – much lower than literature values.
By investigating the homocoupling efficiency at a variety of reaction conditions, it was determined that the reaction proceeds through a photooxidative mechanism reliant on the strength of the base used. In addition, the nanoparticles are fully recyclable at moderate light intensities and the reaction performance with TiO2 as a supporting semiconductor is on-par or better than other supports which were tested including KNbO3, KNb3O8, and optical-grade Nb2O5.
The promising nature of these results paves the way for a new environmentally friendly, economic, and highly-efficient standard by which Glaser coupling is carried out in the future.
1 Sindhu, K. et al. RSC Adv.. 2014, 4 (53), 27867–27887.
2 Zhang, S. et al. Adv. Synth. Catal. 2011, 353, 1463-1466.
3 van Gelderen, L. et al. Appl. Organometal. Chem. 2013, 27, 23-27.
4 Januário, E. R. et al. J. Braz. Chem. Soc.. 2018, 29 (7), 1527-1537.
Lanthanide (Ln)-doped upconverting nanoparticles (UCNPs) have fascinating optical characteristics which makes them attractive for potential biomedical applications such as drug delivery and biological imaging.1 In contrast to conventionally applied bioprobes that rely on the use of UV/visible light, the upconversion process that can take place in these materials enhances tissue penetration capability since NIR-light spectrally overlaps with the three biological windows.2 Based on preliminary results in the HemmerLab, NaGdF4-based UCNPs were synthesized using either Ln-chlorides or Ln-trifluoro acetates (Ln-TFAs) as precursors in a rapid microwave-assisted approach. Importantly, by choosing precursor chemistry in combination with reaction parameters (solvent, volume, time, temperature), two very different nanostructures became accessible.
Through the use of Ln-chlorides with water, acetic acid and ethanol as the solvent system, relatively large Ln3+-doped NaGdF4 UCNPs with sizes in the 50-200 nm realm were synthesized. Doping with Tm3+(0.5%) and Yb3+(25%) or Er3+(2%) and Yb3+(20%) endows the NPs with visible luminescence under 980 nm irradiation. This optical behavior makes the NPs possible candidates for NIR-triggered drug release. Interestingly, the NPs obtained through this strategy appear porous in TEM and SEM. Such surface structure may be ideal for loading with photosensitizers or drugs and may overcome the need to grow a porous SiO2 shell,3 thus, opening the door to more efficient drug delivery and photodynamic therapies.
In contrast, the microwave-assisted decomposition of TFA-precursors in organic high-boiling point solvents resulted in UCNPs of less than 10 nm in size. To overcome surface defect induced luminescence quenching – critical at such a small size scale –, undoped NaGdF4shells were grown onto the doped NaGdF4: Yb3+(20%), Er3+(2%) core NPs. Spectroscopic studies revealed significantly brighter emission from core/shell (CS) structures with a 4 nm thick shell when compared to core-only UCNP.
Overall, through the microwave-assisted approach, optical probes for biomedical applications can be synthesized efficiently, while size and nanostructure can be controlled by using specific precursors, solvents, and reaction conditions.
1 J. Xu et al.J. Mater. Chem. B 2014, 2, 1791–1801.
2 E. Hemmer et al.J. Mater. Chem. B 2017, 5, 4365-4392.
3 S. He et al. Chem. Commun. 2015, 51, 431-434.
4 Y. Wang et al.CrystEngComm. 2011, 13, 1772-1774.
Lipid-derived electrophiles (LDEs) are by-products of the chain-mediated reaction between polyunsaturated fatty acids and molecular oxygen, termed autoxidation. Autoxidation proceeds through a radical mechanism to form lipid hydroperoxides, which can further decompose to LDEs. These resultant LDEs include a-b-unsaturated ketones and aldehydes.
To counteract this mode of membrane autodegradation, eukaryotic cells have evolved intricate antioxidant systems to ‘mop up’ LDEs before they can initiate further oxidation events. This detoxification process usually proceeds through enzyme-mediated reactions with antioxidants (ex. glutathione) and subsequent cellular excretion of LDE-antioxidant adducts1.
Previous work in our group has yielded a novel LDE-mimicking fluorogenic probe (AcroB), bearing an acrolein warhead that is sensitive to Michael addition. AcroB allows for real-time super-resolution mapping of cellular probe reactivity, as well monitoring of both cellular redox environments and the metabolic and excretion pathways for endogenously produced LDEs2. However, there is still much to be understood about the intracellular targets of AcroB, the mechanisms of its trafficking, and the pathways of its export.
In this talk, I will present ongoing efforts to unravel the mechanisms that lead to distinct LDE-analogue fluorescent intracellular morphologies by way of imaging, high-throughput assay, and cellular fractionation studies. Analogues of AcroB with greater lipophilicity were also synthesized and characterized. This investigation of the intracellular behavior of AcroB has yielded insights into endogenous LDE reactivity, thus cementing AcroB’s use as a tool to study intracellular LDEs.
1. Schopfer, Francisco J., et al. “Formation and Signalling Actions of Electrophilic Lipids.” Chemical Reviews, vol. 111, no. 10, 2011
2. Lincoln, Richard, et al. “Mitochondria Alkylation and Cellular Trafficking Mapped with a Lipophilic BODIPY–Acrolein Fluorogenic Probe.” Journal of the American Chemical Society, vol. 139, no. 45, 2017