Strong Grip: Unexpected interaction between organic semiconductors

 Jülich physicists have discovered an unexpectedly strong bond between organic layers. Such structures are still puzzling scientists throughout the world. These structures form the basis for novel electronic components made from organic semiconductors that are now increasingly used in smart phones and television sets.

The results have been published in the journal Physical Review Letters.

Organic semiconductors are cheap to produce, can be flexibly shaped and are relatively insensitive to external influences. In principle, they could in future even be simply printed on plastic foils. They are already widely used as organic light-emitting diodes (OLEDs), particularly in smart phones, because they consume so little power. Nevertheless, the electronic properties of these complex materials still remain largely unknown. Researchers are particularly interested in the interfaces because component performance decisively depends on how well contacts can be created with other organic and metallic conductors. The stronger the bond, the better electrons can pass from one material to the other -- and the more power or light can be produced by solar cells or light-emitting diodes.

However, organic molecules do not usually form such strong bonds. "Scientists have assumed that organic materials only interact among themselves via weak van der Waals forces. Only in contact with certain metals do they display stronger bonding known as chemisorption," says Dr. Christian Kumpf from Forschungszentrum Jülich. "For the first time, we have been able to demonstrate such chemisorption between two organic layers, which we applied to a silver crystal by chemical vapour deposition." Such sandwich-like structures are also found in OLEDs and usually consist of several organic layers between two metallic conductors.

For the analysis, Kumpf and his colleagues made use of PTCDA, an organic semiconductor material, and copper phthalocyanine, which is frequently used as a dye. They then investigated the layers, which are only one molecule thick, using various highly specialized measuring techniques. By means of ultraviolet photoelectron spectroscopy (UPS), the researchers were able to show that a change is transferred between the organic semiconductors. They also used scanning tunnelling microscopy (STM) and low-energy electron diffraction (LEED) to demonstrate that the arrangement of the molecules is transferred to the next layer in the order of the strong bonding, almost like a photocopy.

It has been known for some time that certain metals can establish such strong interactions with an organic semiconductor. In his earlier work, Kumpf himself contributed to research in this field, even before moving to Prof. Stefan Tautz's group in Jülich in 2008. "What is new is that the charge transfer takes place between these organic materials, that was rather unexpected. These findings will undoubtedly be exploited in the development of new organic semiconductors," says Tautz, director at the Jülich Peter Grünberg Institute (PGI-3: Functional Nanostructures at Surfaces). There is, however, still a long way to go since industrial manufacturing processes and laboratory requirements are quite different; the latter being more concerned with reproducibility and precision.

Story Source:

The above story is reprinted from materials provided by Forschungszentrum Juelich, via AlphaGalileo.

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Journal Reference:

Benjamin Stadtmüller, Tomoki Sueyoshi, Georgy Kichin, Ingo Kröger, Sergey Soubatch, Ruslan Temirov, F. Tautz, Christian Kumpf. Commensurate Registry and Chemisorption at a Hetero-organic Interface. Physical Review Letters, 2012; 108 (10) DOI: 10.1103/PhysRevLett.108.106103

Shedding light on the interaction between DNA and UVA radiation

Ultraviolet A (UVA) radiation is now known to cause skin cancers. The first information on the way in which UVA radiation acts directly on DNA has been revealed by a CNRS team from the Laboratoire Francis Perrin in collaboration with a CEA-Inac laboratory in Grenoble. The interaction between UVA and DNA results from the collective behavior of the bases of the DNA double helix, which causes chemical lesions that can induce carcinogenic mutations. This work is published on-line on 18 March 2011 in the Journal of the American Chemical Society.

Ultraviolet A (UVA) radiation represents over 95% of the solar UV radiation that reaches the Earth's surface. This UVA radiation is now known to cause skin cancers due to carcinogenic mutations brought about by chemical alterations of the four bases of DNA (adenine, cytosine, guanine and thymine). The most important chemical modification is thymine dimerization: two thymines next to each other in the DNA combine to form a new entity, known as “cyclobutane dimer”.

A CNRS team from the Laboratoire Francis Perrin (CNRS/CEA), in collaboration with researchers from the CEA Laboratoire Lésions des Acides Nucléiques, has examined the very first steps of the formation of such chemical lesions. They are publishing the first study describing the physical and chemical effects, prior to any biological effects, of UVA radiation on model DNA. The team of physical chemists examined the behavior of a synthetic DNA double helix (formed solely of adenine-thymine pairs) with regard to UVA photons. They then compared its behavior with that of two complementary single strands (containing only thymines or only adenines).

They found that DNA's capacity to absorb UVA photons results from the collective behavior of its bases. Studied individually, DNA bases (including thymine) are “transparent” to UVA. However, in this study, the scientists have shown that the absorption of UVA radiation substantially increases following the pairing of two single strands to form a double helix. In addition, the probability that a UVA photon absorbed leads to the formation of cyclobutanes is at least ten times higher in the case of a double strand than it is in the case of a single strand. These differences could be explained by changes induced by the UVA photons to the electronic structure of the bases. Following the absorption of a photon, the new electronic configuration adopted by the DNA, known as excited state, persists longer for a double strand than for complementary single strands. The thymines then have more time to undergo permanent alterations.

These experimental studies now need to be extended to more complex DNA sequences, similar to natural DNA. The stakes in terms of public health are high, especially since the quantity of UVA that reaches us is very high compared to UVB radiation (which represents less than 5% of the ultraviolet radiation that reaches the Earth's surface) and also because UVA radiation is still widely used in tanning centers.

More information: Base Pairing Enhances Fluorescence and Favors Cyclobutane Dimer Formation Induced upon Absorption of UVA Radiation by DNA, Akos Banyasz, et al. – Journal of the American Chemical Society, 18 March 2011.

Provided by CNRS (news : web)