Advanced photons in science

David Bradley is currently working with Argonne National Laboratory on a series of articles for the annual report of the ANL's Advanced Photon Source. For December's Elemental Discoveries, I will be offering a preview of the advanced science featured in the report.

Advanced Photon SourceElemental Discoveries: X-rays reveal some of the inner secrets of the world around us as seen under the illumination of ANL's Advanced Photon Source:

Are films ferroelectric?
Gold nanocrystals
Photosynthetic system
Dissecting the atom
Catalytic clues
SAXS and the water channel
Digging in the dirt
Folding Protein Sensors
X-ray movies

X-rays shed light on machinery of photosynthesis

by David Bradley

Flat, square-shaped molecules known as porphyrins are at the heart of natural and artificial photosynthesis, the conversion of sunlight into chemical energy. They provide a molecular springboard that captures photons of sunlight and bounces out energetic electrons. Porphyrins also have potential as light- powered catalysts and as components of photonics devices, such as information storage materials, that use light as their currency rather than electrons.

To help illuminate the inner workings of porphyrins, an Argonne National Laboratory team has used beamline BESSRC/XOR 11-ID to determine precisely how different porphyrin molecules respond to being excited by light under different chemical conditions. Their findings could help scientists fine-tune the chemical structure of porphyrins by changing the attached side-groups and the metal ions at their centre to make them respond to different wavelengths of light. Such modified porphyrins may one day form the building blocks of novel catalysts, photonic devices, and efficient solar power units.

ANL chemist Lin Chen and her colleagues point out that previous researchers have used Raman and other forms of spectroscopy to study the excited states of porphyrins. While these techniques provide useful information they are indirect in some ways and cannot reveal crucial structural details. For instance, Raman spectroscopy tells scientists only indirectly about the bond distances between the central metal ion and the chemical groups around the porphyrins ring bond distance, a key property that could be tweaked for optimum effect in a modified porphyrin. Raman spectroscopy also fails to reveal the number of bonds to the metal, its coordination number, and how many electrons it has lost or gain in order to form those bonds, and its oxidation state.

New X-ray techniques, such as time-resolved pump-probe X-ray absorption near edge and fine structure (XANES and XAFS) on the other hand can reach into the core of such molecules and pull out such information when the porphyrin is in solution phase. By carrying out such experiments with the pulsed synchrotron source available on BESSRC/XOR 11-ID, the ANL team could obtain information even for porphyrin molecules that exist only fleetingly with lifetimes of about 100 picoseconds. On this timescale it is possible to freeze the action as a porphyrin absorbs a photon and is excited and so determine its structure precisely in this state.

Chen and colleagues have focused on a derivative of the basic porphyrin molecule that carries eight hydrocarbon groups on the outside of the ring and a copper ion at the centre - the copper(II) octaethylporphyrin, CuOEP, molecule. CuOEP is an archetypal synthetic porphyrin that is essentially free from the interference of other complex chemical groups that are attached to the porphyrin ring in its natural counterparts. The researchers can therefore use CuOEP as a skeletal porphyrins model.

The researchers carried out their studies with two different solvent systems, one comprising the aromatic hydrocarbon toluene and the second containing both toluene and the oxygen-containing polar solvent tetrahydrofuran (THF). They found that toluene alone did not affect the way the copper- porphyrin system responds to light. However, in the mixed solvent system, the researchers found that a THF molecule can become attached temporarily through the copper atom to the porphyrin ring. The result is that an excited porphyrin forms in which the bonds between the copper atom and the porphyrin ring are stretched. The researchers explain how the formation of such a "pyramidal" shaped short-lived excited chemical species provides new insights into earlier spectroscopic studies and so will help researchers design new versions of the basic structure that have particular absorption properties "This study reveals the structural origin of solvent dependent properties of the CuOEP, such as lifetimes of the excited state and photoluminescence quantum yields," Dr Chen explains, "Such information will help in selecting media to facilitate a certain function of the porphyrin."

Source: Lin X. Chen, George B. Shaw, Tao Liu, Guy Jennings, Klaus Attenkofer "Exciplex formation of copper(II) octaethylporphyrin revealed by pulsed X-rays," Chem. Phys. 299, 215-223 (2004).

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