Saturday, June 18, 2011

Molecular visual illusion: Aromatic ring system reminiscent of M.C. Escher's Penrose stairs

Who hasn't seen M.C. Escher's famous picture of the stairs that appear to always go up even though they form a closed circle? This tricky visual illusion is also known as a Penrose stair, named after its discoverers, Lionel and Roger Penrose. Hiroyuki Isobe and a team at Tohoku and Tsukuba University (Japan) have now introduced a molecule in the journal Angewandte Chemie that looks like a Penrose stair.

In order to depict the composition and structure of molecules, these three-dimensional objects are represented as two-dimensional line drawings. An additional touch of perspective is often added in order to show the spatial orientation of the individual molecular components relative to each other.

Two-dimensional drawings can fool the eye so that the observer sees a three-dimensional object that is impossible in reality. We are fascinated and amused by looking at such visual illusions as M.C. Escher liked to draw them. One of his most famous works is a lithograph based on the Penrose stair; people in the image walk in a circle although they appear to be constantly descending the stair.

The scientists working with Isobe were reminded of this stair when they synthesized a molecule belonging to the cyclobis[4]helicene class and attempted to represent it as a line drawing. Helicenes consist of planar aromatic rings made of six . The rings are connected along one edge, forming an angle. For spatial reasons, the molecules are forced to twist into spirals. A [4]helicene is made of four connected rings. The Japanese researchers connected two such units through two single bonds.

Helices can twist around to the right or left. In these rings made of two helicenes, both helicene units twist in the same direction. When represented as a two-dimensional perspective line drawing, the double helicene gives the impression of a Penrose stair: both halves lead downstairs but after going around the circuit the eye is right back at the starting point.

How can this be? The molecule is clearly not an impossible object; it exits and its structure is not different than supposed. The solution to this conundrum is revealed when the molecule is represented from the side: the axial twist of the single bonds between the two building blocks makes the tilted orientation of the helicene ring systems possible.

Isobe hopes that this class of will open up new possibilities as a building block for liquid crystals.

More information: Hiroyuki Isobe, Illusory Molecular Equivalent of "Penrose Stairs" from an Aromatic Hydrocarbon, Angewandte Chemie International Edition, … ie.201102210

Provided by Wiley (news : web)

Efficiency record for flexible CdTe solar cell due to novel polyimide film


DuPont Kapton colorless polyimide film, a new material currently in development for use as a flexible superstrate for cadmium telluride (CdTe) thin film photovoltaic (PV) modules, has enabled a new world record for energy conversion efficiency. A team at Empa, the Swiss Federal Laboratories for Materials Science and Technology, has demonstrated a conversion efficiency of 13.8 percent using the new colorless film, leapfrogging their previous record of 12.6 percent and nearing that of glass.

Because Kapton film is over 100 times thinner and 200 times lighter than glass typically used for PV, there are inherent advantages in transitioning to flexible, film-based vs. rigid glass CdTe systems. High-speed and low-cost roll-to-roll deposition technologies can be applied for high-throughput manufacturing of flexible solar cells on polymer film as . The new polyimide film potentially enables significantly thinner and lighter-weight flexible modules that are easier to handle and less expensive to install, making them ideal for applications including building-integrated photovoltaics.

“Rather than transporting heavy, fragile glass modules on large trucks and lifting them by crane onto rooftop PV installations, one could imagine lightweight, flexible film-based modules that could simply be rolled up for transport, and easily carried up stairs,” said Robert G. Schmidt, new business development manager, Photovoltaics - DuPont Circuit & Packaging Materials. “With record-setting established through Empa, we’re confident this flexible, lightweight and durable material has the potential to revolutionize the industry by enabling flexible design and lowering balance of system costs.”

Increase in efficiency – toward achieving grid parity

Empa’s Laboratory for Thin Films and Photovoltaics is developing high-efficiency thin film solar cells with emphasis on novel concepts for enhancing their performance, simplifying the fabrication processes, and advancing device structures for next generation of more efficient and low-cost devices. They have been doing groundbreaking work in developing and optimizing a low temperature process (below 450 degrees Celsius) for high-efficiency CdTe solar cells on glass (reaching 15.6 percent efficiency) and (reaching 12.6 percent efficiency, the highest value before the recent improvement to 13.8 percent). Only a few weeks ago Tiwari's team also set a new world record in energy efficiency (of 18.7 percent) for another type of flexible based on copper indium gallium (di)selenide (also known as CIGS).

“Finding a film that could both be transparent and withstand high processing temperatures was a challenge initially, but the new Kapton colorless polyimide film had both the tolerance for high temperatures needed, and higher light transmittance due to its transparency that allowed it to exceed our previous world record in conversion efficiency of flexible CdTe solar cell,” said Ayodhya N. Tiwari, head of the laboratory. “As we continue to raise the standards for PV efficiency, materials make a distinct difference in the progress we make toward achieving grid parity. Of course, further development is needed for addressing cost and stability issues.” Tiwari plans to present a technical paper on the full findings at the 26th European Photovoltaic Solar Energy Conference and Exhibition in Hamburg, Germany, being held Sept. 5–9, 2011.

DuPont Kapton polyimide film has made innovative design solutions possible in a range of industries over the last 45 years including aerospace, automotive and industrial applications. With a unique combination of electrical, thermal, chemical and mechanical properties that withstand extreme temperature and other demanding environments, Kapton films have set the standards in high performance, long-term reliability and durability, and are ideally suited for applications in the PV industry. Three new Kapton PV9100 series films were introduced for the thin film PV market in 2010, including offerings for amorphous Silicon (a-Si) modules and Copper Indium Gallium Selenide (CIGS) photovoltaic applications.

Provided by Empa

New driving force for chemical reactions discovered

New research just published in the journal Science by a team of chemists at the University of Georgia and colleagues in Germany shows for the first time that a mechanism called tunneling control may drive chemical reactions in directions unexpected from traditional theories.

The finding has the potential to change how scientists understand and devise reactions in everything from materials science to biochemistry.

The discovery was a complete surprise and came following the first successful isolation of a long-elusive molecule called methylhydroxycarbene by the research team. While the team was pleased that it had "trapped" the prized compound in solid argon through an extremely low-temperature experiment, they were surprised when it vanished within a few hours. That prompted UGA professor Wesley Allen to conduct large scale, state-of-the-art computations to solve the mystery.

"What we found was that the change was being controlled by a process called quantum mechanical ," said Allen, "and we found that tunneling can supersede the traditional processes of kinetic and thermodynamic control. We weren't expecting this at all."

What had happened? Clearly, a chemical reaction had taken place, but only inert argon atoms surrounded the compound, and essentially no thermal energy was available to create new molecular arrangements. Moreover, said Allen, "the observed product of the reaction, , is the least likely outcome among conceivable possibilities."

Other authors of the paper include Professor Peter Schreiner and his group members Hans Peter Reisenauer, David Ley and Dennis Gerbig of the Justus-Liebig University in Giessen, Germany. Graduate student Chia-Hua Wu at UGA undertook the theoretical work with Allen.

isn't new. It was first recognized as a physical process decades ago in early studies of radioactivity. In , molecular motions can be understood in terms of particles roaming on a potential energy surface. Energy barriers, visualized as mountain passes on the surface, separate one chemical compound from another.

For a chemical reaction to occur, a molecular system must have enough energy to "get over the top of the hill," or it will come back down and fail to react. In quantum mechanics, particles can get to the other side of the barrier by tunneling through it, a process that seemingly requires imaginary velocities. In chemistry, tunneling is generally understood to provide secondary correction factors for the rates of chemical reactions but not to provide the predominant driving force.

(The strange world of quantum mechanics has been subject to considerable interest and controversy over the last century, and Austrian physicist Erwin Schrödinger's thought-experiment called "Schrödinger's Cat" illustrates how perplexing it is to apply the rules and laws of quantum mechanics to everyday life.)

"We knew that the rate of a reaction can be significantly affected by quantum mechanical tunneling," said Allen. "It becomes especially important at low temperatures and for reactions involving light atoms. What we discovered here is that tunneling can dominate a reaction mechanism sufficiently to redirect the outcome away from traditional kinetic control. Tunneling can cause a reaction that does not have the lowest activation barriers to occur exclusively."

Allen suggests a vivid analogy between the behavior of methylhydroxycarbene and Schrödinger's iconic cat.

"The cat cannot jump out of its box of deadly confinement because the walls are too high, so it conjures a Houdini-like escape by bursting through the thinnest wall," he said.

The fact that new ideas about tunneling came from the isolation of methylhydroxycarbene was the kind of serendipity that runs through the history of science. Schreiner and his team had snagged the elusive compound, and that was reason enough to celebrate, Allen said. But the surprising observation that it vanished within a few hours raised new questions that led to even more interesting scientific discoveries.

"The initiative to doggedly follow up on a 'lucky observation' was the key to success," said Allen. "Thus, a combination of persistent experimentation and exacting theoretical analysis on methylhydroxycarbene and its reactivity led to the concept I dubbed tunneling control, which may be characterized as `a type of nonclassical kinetic control wherein the decisive factor is not the lowest activation barrier'."

While the process was unearthed for the specific case of methylhydroxycarbene at extremely low temperatures, Allen said that tunneling control "can be a general phenomenon, especially if hydrogen transfer is involved, and such processes need not be restricted to cryogenic temperatures."

Provided by University of Georgia (news : web)

UF researcher reduces allergens in peanuts using pulsed light

A University of Florida researcher has developed a new technique to make peanuts safer for people with peanut allergies.

Wade Yang, an assistant professor in UF’s food science and human nutrition department, used pulsed ultraviolet light, or PUV, to reduce the allergenic potential of peanuts by up to 90 percent. The study was published this week by the journal Food and Bioprocess Technology.

By releasing pulsed, or concentrated, bursts of light containing multiple wavelengths, PUV changes allergens so that human antibodies can’t recognize them and cause the release of histamines, which are responsible for allergy symptoms such as itching, rashes and wheezing.

“We believe the allergen can be controlled at the processing stage, before the product even goes to the shelf,” Yang said.

More than 3 million Americans are allergic to peanuts and tree nuts, and reactions can range from skin rashes to death. Peanuts have been found to cause the majority of deaths in the U.S. from anaphylaxis, or severe allergic reaction. Allergic reactions can occur from eating peanuts or from even the slightest exposure in some individuals. Currently, the best way for those with the allergy to be safe is to completely avoid peanuts.

Using PUV, Yang, a member of UF’s Institute of Food and Agricultural Sciences, reduced the allergenic potential of three of the most allergenic proteins in peanuts. The reduction of one of the proteins — Ara h2, the most potent of the three — marked the first time this reduction has ever been achieved with PUV.

Yang confirmed the allergy reduction using a biochemical test and by exposing the proteins to serum samples from patients with to see if an allergic reaction occurred.

Allergens were reduced in peanut extracts and peanut butter. Preliminary, unpublished results also demonstrate that PUV can significantly reduce the allergenic potential of whole peanuts.

Dr. Shih-Wen Huang, a pediatric allergist in UF’s College of Medicine, said epidemiological data show an increase in food allergies over the last 20 years.

Scientists don’t know why, he said, but there could be multiple factors involved, including living in a cleaner environment that shifts our immune response away from protecting against germs to reacting to innocent food substances. He also noted that increased peanut consumption is part of an overall trend toward healthier eating.

Huang said epinephrine is often recommended for treating severe allergic reactions, and for milder reactions, antihistamines.

And while epinephrine and antihistamines alleviate allergenic symptoms, Yang said he would like to prevent the allergy at the processing stage with PUV, before it reaches humans.

Yang’s future research involves developing a one-step roasting and allergen reduction process by PUV to produce hypoallergenic whole peanuts.

Provided by University of Florida (news : web)