Results published in Science today (link opens in a new window)
report on the ubiquitous elementary reaction of electron transfer and how Ultra’s lasers have been used to direct the outcome of a type of light-induced processes – those operating via charge transport, which is fundamental to almost all light-induced phenomena around us, from solar energy conversion to DNA damage and the beginnings of cancer.
Julia Weinstein, University of Sheffield, UK, led the collaborative effort alongside the CLF’s Mike Towrie and Tony Parker, using the femtosecond pump-probe spectroscopy configurations available on the CLF’s Ultra system.
Schematic of the experiment. Credit: Science 2014
The work looks at what has often been deemed the “holy grail” of chemistry, whereby light is used to not only trigger a chemical reaction, for example photosynthesis and of late the photoprocesses behind tremendous progress in harnessing solar power, but how light can also be used to control what products are ultimately created.
These results offer a step towards “quantum control” of chemical reactivity using infrared light and may have fundamental implications to better understand, manipulate and improve both natural and artificial light driven chemistry.
Chemical reactions generally run from reactants to products and whilst usually a single product is made, the journey involves several steps and intermediates – in chemistry we think of these as road crossing points and junctions where the reaction path has to decide which is the most favoured route to follow.
These latest results published in the journal Science show how using low-energy infrared light can radically change which products are formed. When a laser pulse triggers a reaction, the natural product will be created but using a second, infrared, laser pulse the molecule is directed to change from its normal path to form a different product.
Julia Weinstein says “The key step has been to use the ultrafast tunable lasers that allow us to precisely select how much and when the extra energy should be applied. We have to do this within a million-millionth of a second of starting the reaction by another pulse of light. This extra energy must match a special molecular vibration – the one that determines which low-lying valley out of many available the high energy state will fall into.”
"This is early days but we have demonstrated a fundamental piece of science that indicates that we should be able to increase our ability to control photochemical reactions"
Prof. Mike Towrie, Ultra group leader, Central Laser Facility
"It’s great to see how science and technology come together in this work – we use highly engineered state-of-the-art lasers to study and influence fundamental chemical reactions which can be used to make electricity from solar radiation, or store energy in chemical bonds in 'solar fuels'"
Dr. Milan Delor, University of Sheffield
The study reports on the ubiquitous elementary reaction of electron transfer (ET) and how lasers direct the outcome of a particularly fascinating type of light-induced processes – those operating via charge transport, which is fundamental to almost all light-induced phenomena around us, from solar energy conversion to DNA damage and the beginnings of cancer. Molecules which make such light trickery possible are based on two types of chemical units joined together, one that provides an electron and the other which accepts an electron when illuminated with light. These electron donor-acceptor molecules are common in nature where they are used for photosynthesis.
What makes the model molecule (synthesised by Paul Scattergood) unique is that the two halves are joined by a conducting metal-organic “bridge” that provides a motorway for an electron to travel along. Triggered by the energy of an ultrafast visible laser pulse the electron begins its journey and first forms an electron/hole pair- a “charge transfer state”, and then – counterintuitively, and against Columbic recombination – a carefully designed energy gradient forces the charges to separate further from one another, forming a “charge-separated state” with independently reactive “negative” and “positive” charges.
However, if during this reaction, and within several picoseconds, a second pulse is applied that specifically knocks out of resonance a certain part of the molecule and a specific molecular vibration in the intermediate high-energy state, instead the charge separation is completely switched off and another high energy state is formed which is longer lived. The efficiency of this process can be as high as 100% per each photon of low-energy IR light absorbed , and such “pathway switch” has also been seen in calculations performed by Anthony Meijer.
"It is early days as this will take many years of research but the genie is out of the bottle "
Prof. Tony Parker, STFC Research Fellow, Central Laser Facility
The next step is to be able to block or unblock any chosen chemical pathway at will. This goal will require applying our new knowledge to synthesise new chemicals with properties that we can try and predict first, and which can be tested using the sort of experiments that we have shown to work.
M. Delor et al, Science, Vol. 346 no. 6216 pp. 1492-1495, 2014
http:/www.sciencemag.org/content/346/6216/1492.full (link opens in a new window)