Chemical shifts: What's new in chemistry research? This bi-monthly column offers readers a concentrated look at the latest developments in chemistry from Canadian researchers. (Chemical Shifts).
In Suning Wang's lab, stars do more than just twinkle, they luminesce! Suning Wang, MCIC at Queen's University in Kingston, Ontario has been studying the luminescent behaviour of coordination compounds, particularly star-shaped co-ordination complexes. She has prepared several compounds that luminesce in the key blue region, which is important for applications in LEDs and as chemosensors. In a recent publication, Wang and her colleague R. Stephen Brown, MCIC, along with Jun Pang, Eric J.-P. Marcotte and Corey Seward, MCIC, report a compound that is a potent chemosensor for benzene (Angew. Chemie., Int. Ed. EngL 40:4042, 2001).
The novel star-shaped complex was obtained by a rapid route as shown above. The exact crystal structure of the complex depends on the solvents used to crystallize it. Two different forms have been found by the Queen's group. If the solvent is primarily dichloromethane with a small amount of benzene, a sandwich-type structure is formed (F igure 1) where the benzene molecule stacks with the central aromatic ring of the star-shaped structure. If the solvent is a 1:1 (benzene : Zn) structure is formed, where the beneze molecules interact with the outer part of the star, closer to the Zn atoms (Figure 2).
To study the potential of 1 as a fluorescence sensor, the Queen's group attached a bead containing compound 1 to a fused silica optical fibre. Upon exposure to benzene, the integrated intensity of the emission band of 1 decreases. After the removal of benzene vapour, the fluorescence recovers to the initial intensity. Even more interestingly, compound 1 is highly specific for benzene, even relative to very samilar molecules (toluene, ethyl benzene, and xylene). Fluroescence quenching by sacking interactions is well-documented, and is most likely at play in this system as well. Although fibre-optic benzene sensors based on fluorescence are known, fluorescence sensors based on solid-state materials for the selective detection of benzene vapour are still rare.
Measures of Control
Hapalosiphon fontinalis is a busy little blue-green algae. It is known to produce a multitude of organic compounds, including the hapalindole alkaloids. These compounds have promising biological properties (including antibacterial and antimycotic activity) and complex structures, which makes them targets for total synthesis. Michael Kerr, MCIC and graduate student Aaron Kinsman, MCIC at the University of Western Ontario in London, ON have recently synthesized a member of this class of compounds, Hapalindole Q (1). Their synthesis is complete in eight short steps, furnishing the desired product in 12.4% overall yield. The route features a Diels Alder cycloaddition as the key step. The Diels Alder reaction, first described by Otto Diels and Kurt Alder, who later won the Nobel prize for their discovery, is one of the most powerful reactions in organic synthesis. It brings two flat molecules together to form a cyclic molecule, often with high levels of control of stereo and regio chemistry.
The key Diels Alder reaction in the Kerr synthesis (2 + 3 to 4) was not without challenges. The diene component (2) is unreactive and thus a Lewis acid catalyst and high pressures (13kbar) were examined to facilitate the reaction. However, the diene polymerized under these conditions complictating the purification of 4. Increasing the reactivity of the other component (dienophile 3) by introduction of a nitro group did the trick, and the Diels Alder cycloadduct was obtained in 60% yield with 72% diastereoselectivity.
The next key step in the synthesis was to open one of the rings formed in the Diels Alder and extend the resulting Structure by two carbons. Although a ring opening metathesis was attempted, the olefin in 4 was too unreactive, even with a state of the art metathesis catalyst. The next standard reaction for this type of cleavage reaction is an ozonolysis reaction, which was attempted, but again, the olefin proved unreactive. Finally, the olefin succumbed to dihydroxylation and oxidative opening of the resulting diol. After a few functional group manipulations, the synthesis of Hapalindole Q was complete.
For the complete synthesis of Hapalindole Q, see the orginal paper, published in Organic Letters, 3:3189, 2001.
Prospecting for Gold
What do you need to make self-assembled monolayers (SAMs)? A clean room? A vacuum line? Scrupulously clean and expensive gold substrates? A quartz crystal microbalance? In fact, Hogan Yu, MCIC at Simon Fraser University in Burnaby, BC has shown that all you need is a few recordable compact disks (CD-Rs), some alkanethiols, a splash of nitric acid, and a lot of ingenuity.
Yu has shown in a recent paper in Analytical Chemistry (73:4743, 2001), that removing the outer polymer coating from CD-Rs permits their use as substrates for the preparation of SAMs. Unlike traditional methods, only nitric acid is needed to peel off the protective polymer film, not the potentially explosive piranha reagent [H.sub.2][SO.sub.4]/[H.sub.2][O.sub.2]) commonly employed for cleaning gold substrates prior to self-assembly.
Once the metal layer of the CD-R, which is pure gold, is exposed, it can be coated with alkanethiols that self- assemble into a tightly packed monolayer with the thiol end bound to the gold surface. To examine the structure of the monolayer that is formed, Yu uses surface IR, wetting measurements and electrochemistry. When compared with SAMs prepared on commercially available gold surfaces designed for such applications, Yu finds no difference within the experimental uncertainty of the applied measurements. The SAMs are as closely packed and as ordered as those formed on evaporated gold substrates.
What's more, the gold film from GD-Rs actually has a slightly flatter morphology than the commercial gold substrates prepared by evaporation. Examining these surfaces by scanning tunneling microscopy (STM) provides an extreme close-up of the surface. Both substrates are polycrystalline and have a topology similar to rolling hills. The gold substrates prepared from CD-Rs have relatively uniform 'grains' on the surface, with an average size of 20-40 nm in diameter, and typical peak-to-valley heights of 3-4 nm (Figure A). The commercial gold substrates have a similar surface, but the crystallites are non-uniform, and the typical peak-to-valley height is higher than that of the gold substrates from CD-R's (Figure B). The large-scale STM images shows the 'track trails' on CD-R gold substrates that are typically 800 nm apart.
Cathleen Crudden, MCIC is a university research professor at The University of New Brunswick.
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|Comment:||Chemical shifts: What's new in chemistry research? This bi-monthly column offers readers a concentrated look at the latest developments in chemistry from Canadian researchers. (Chemical Shifts).|
|Publication:||Canadian Chemical News|
|Article Type:||Statistical Data Included|
|Date:||Feb 1, 2002|
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