Avatar and Photochemistry: Chemiluminescence

Photochemistry for an Oscar? In the movie Avatar, it plays a central role, although I must admit I didn’t see it listed in the credits… The movie is set on a planet called Pandora, and at night time, the forests of Pandora light up to give some really beautiful cinema, all in 3D! This article explains the glow in the trees, insects, inhabitants and just about everything else on Pandora at night time. Back on Earth, we’re familiar with this glow too!

When some chemicals react, they can give off or require huge amounts of energy in doing so, as existing bonds are ripped apart and new ones form. This energy is in the form of heat – reactions can give off heat (exothermic) or require heat to proceed (endothermic). More unusually, the can give off large amounts of energy in the form of light. This light is called chemiluminescence. In photochemistry, we are usually concerned with providing molecules with light to activate a reaction. With chemiluminescence, it’s the other way around – a chemical reaction results in the emission of light. The classic demonstration of chemiluminescence is with a compound called, appropriately enough, luminol. Here’s a short Youtube video on it (with a rather excited chemist).

So what is happening?

Let’s look closer at the luminol reaction. When hydrogen peroxide (e.g. from household bleach) is added to luminol, in the presence of base and a catalyst (such as iron(III) which gets involved in the oxidation), 3-aminophthalate is formed. But the energy involved in the oxidation of luminol by the peroxide results in the phthalate having an electronically excited state. The releases this excess energy by emission of light, giving the blue colour observed.

Luminol Reaction

Luminol reacts with hydrogen peroxide to produce an electronically excited 3-aminophthalate, which emits in the blue (450 nm)

Applications of chemiluminescence

Natural World

One of the most common observations of chemiluminescence, as any inhabitant of Pandora will know, is bio-chemiluminescence, or bioluminescence, which is where natural world has exploited the use of chemiluminescence. The most commonly known example of this is the firefly (Photinus), which uses a reaction similar to that of luminol, and an enzyme, luciferase, in place of the peroxide, along with magnesium ions to produce a glow (the colour depends on the type of fly).

Oxidation of Luciferin

Oxidation of Luciferin by luciferase in the presence of magnesium ions gives emission (e.g. in the yellow region)

As well as Pandora, back on Earth, Irish swimmers came across some beautiful examples of bioluminescence off the coast at Killiney when “spectacular green neon flashes” in the sea were observed by swimmers as they swam through water. This was determined to be the plankton Noctiluca scintillans, which is reported to be known as “Sea Ghost” or “Fire of Sea”.

Image of Noctiluca

Image of Noctiluca Scintillans (taken from Maria Antonia Sampayo, http://planktonnet.awi.de, Creative Commons Attribution 3.0 License)

Analytical Applications

Given that emission spectroscopy is such a versatile analytical tool, it is perhaps no surprise that chemiluminescence has several potential applications in the area of chemical analysis. The intensity of luminescence is proportional the the concentration of reactant. In principle, analysis does not need the same level of instrumentation as emission spectroscopy – which needs a light source to excite the sample and emission my be detectable by eye. Therefore it can be used in crude analytical tests. Luminol is used to test for blood at crimescenes – a luminol spray on any suspected blood traces results in the iron in the blood catalysing luminol chemiluminescence, and glows for up to a minute after being sprayed.

But there is more scope for its use. Two problems to its adoption as an analytical technique are that the quantum yield of emission can be low, which means that at low concentrations, the detection may be difficult. In addition to potentially poor sensitivity, long lived emission (such as those observed in glow sticks and luminol), which makes for great demonstrations, means that response time is unnecessarily long. Some work on both of these areas is advancing. Coupling the chemiluminescence interactions with metal nanoparticles harnesses the surface plasmon resonance effect, where the emission from the chemiluminescence couples or resonates with the electron density of the nanoparticles, which enhance the signal, with reports of a 4 – 10 fold increase. The efficient transfer of energy from the excited state of the reagent to the nanorparticles also significantly reduces their lifetime, hence the lingering glow. Readers interested in this work are referred to Aslan and Geddes, given below.

This being said, chemiluminescence is already in use to study a wide range of medicinal and environmental-related compounds in a technique that couples chemiluminescence with liquid chromatography (HPLC-CL). The reagents used in this technique include the now familiar luminol, which is used to investigate lipid hydroperoxides, neurotransmitters such as dopamine (which enhance the chemiluminescence), and environmentally relevant species such as organophosphorus reagents (e.g. diclorvos). The always familiar Ru(bpy)32+, which as well as everything else can exhibit chemiluminescence, undergoing reduction by analytes in high energy electron transfer reactions to produce the excited state. This system has been used to study nitrosamines, N-methylcarbamates pesticides in pear and apple samples and domoic acid which can be a factor responsible for shellfish poisoning. There are several set-ups possible for interfacing the chemiluminescence set-up with the HPLC; most simply by having an injection point after separation for the chemiluminescent reagent prior to the emission detector. A very detailed review of the use of chemiluminescence in medicinal, food and environmental analytes is that by Gámiz-Gracia et al.

Finally, it is worth noting that a lot of gas-phase reactions result in chemiluminescence. This is the basis of gas analysers, for example the nitrogen monoxide analyser. Nitrogen monoxide reacts with ozone to produce an excited state nitrogen dioxide which emits in the far visible/infrared region. The extent of luminescence can be related tot he initial concentration of NO.

Your very own magical world

If you want to set up your own version of Pandora, which cost James Cameron $250M, you can do it for a few euros, by buying some glow sticks and dotting them around. Their glow lasts for a few hours, so all you need is a little imagination during this time…

References

Lights in Sea are Natural“, Irish Times, www.irishtimes.com, 18 October 2009

K. Aslan and C. D. Geddes, “Metal-enhanced chemiluminescence: advanced chemiluminescence concepts for 21st century“, Chem. Soc. Rev., 2009, 2556 – 2564.

Laura Gámiz-Gracia, Ana M. García-Campana, José F. Huertas-Pérez, Francisco J. Lara, “Chemiluminescence detection in liquid chromatography: Applications to clinical, pharmaceutical, environmental and food analysis—A review“, Anal. Chim. Acta, 2009, 640, 7 – 28.




Beautiful Photochemistry

I came across this nice blog recently and thought it was worth signposting here. It is called “Beautiful Photochemistry” and its author writes summaries  of recent articles from some leading chemistry journals which have a photochemical basis. There are some great synopses on a range of topics within photochemistry, including one I was very happy to see on enone-alkene cycloadditions.

Beautiful Photochemistry Blog: http://beautifulphotochemistry.wordpress.com/

 

Our Energy Future: Lecture by Prof Tom Meyer

Prof Tom Meyer, Energy Frontier Research Centre, University of North Carolina, was in Dublin to participate in a Dublin Region Higher Education Alliance Master Class on Solar Energy. Afterwards, he gave a public lecture on “Our Energy Future: Science, Technology and Policy Challenges for the 21st Century – A US Perspective“. The lecture was held at TCD, and was sponsored by the Royal Society of Chemistry Republic of Ireland Local Section. It considered the various current and future world energy demands, and the role renewable energies have to play in providing this energy. My summary is given below.

Prof Thomas J Meyer has been researching the photochemistry of ruthenium complexes since the late 1960’s. Much of what we know about electron transfer in ruthenium polypyridyl complexes today is due to work conducted by Meyer and others in this period. Meyer worked with Henry Taube, who won the Nobel Prize in 1983 “for his work on the mechanisms of electron transfer reactions, especially in metal complexes”, publishing a paper with him in Inorganic Chemistry (1968) on excited state oxidation potentials of ruthenium-amine complexes. This work was an important pre-cursor to a 1973 paper published by Taube, Meyer and co-workers on the reduction of oxygen by these complexes. In the mid-1970’s, at a time when the oil crisis of the time was reaching a peak, Meyer published a series of important papers in Journal of American Chemical Society on the nature and kinetics of quenching of ruthenium amine complexes (including ruthenium – tris-bipyridyl) which gave great kinetic and mechanistic insight into the electron transfer between the metal complexes and an array of quenchers. Meyer reiterated in an article written in 1975 the importance of understanding electron transfer in the study of energy conversion, especially so with metal complexes as these absorb strongly at wavelengths of solar interest.

Prof Meyer, speaking at TCD on "Our Energy Future"

A surge of interest in these systems was observed the oil crisis, which faded somewhat in the 80’s and it wasn’t until Gratzel’s work on dye-sensitised solar cells, reported in 1990, that generated efficiencies that would allow for devices to become realistic contributors to energy supply. Since that itme, work has been concentrating on enhancing light absorption capacity, currently champoined by a ruthrnium dy “N3” (see DSSC post), as well as considering and optimising electron transfer processes in the solar cell devices.

Meyer’s lecture in TCD considered the current and future status of energy demands. It was a message he has delivered to the american political system, across administrations, during his tenure at the Los Alamos National Laboratory. Meyer reported that in the US, energy costs make up 7 – 10% of the cost of living, and 7% of overall world trade. A large demand in energy increase has been observed since 1900’s and this surge is expected to continue until at least 2100. While current stable economies’ energy usage will level off, emerging and transitional ecomomies (China, India, etc) will place major demands on the world’s energy supply. In the six years since 1999, China and India increased their energy usage by 80% and 25%, respectively (Cicerone). (A presentation by Cicerone, Preseident of the National Academy of Sciences is reference below and places thes enegy demands in context). In summation, >100 TW of additional ‘clean’ energy will be required by 2100.

The US currently uses 26% of the world’s oil supply, greater than the next five net using countries combined. 26% of the world’s oil is in the middle-east. Globally, the cost of oil is increasingly expensive to extract, as reserves become more and more difficult to source. Therefore additional energies from alternate sources is required to factor the loss in and increasing expensive of oil production; as well as the surge in energy demand from emerging economies. In addition, this energy supply must be in the context of envrironmental considerations, primarily global warming.

Meyer outlined several strategies to large scale energy production. Principal among these were nuclear, solar, and clean hydrocarbons. These and others are considered below.

Coal currently supplies 27% of the world’s energy demands, including half of US energy needs. It is also responsible for 35% of US carbon dioxide emissions. In principle, it could provide increased energy requirements until 2050, if 1% of GDP was used in dealing with carbon dioxide sequestration. The story of coal usage inclues the story of FutureGen – an initiative announced by the Bush administration in 2003. This was aimed at using coal as a clean fuel, with achieved targets of 275 MW of energy production with 90% carbon dioxide sequestration. However, the project was cancelled by the Bush administration in Jan 2008, due to massive cost overruns ($900M). The Obama adminsitration has restarted this work (June 2009), recognising that clean coal will be a crucial element to supplying energy demands in the forseeable future. Oil shale and tar sands are estimated to contain 2 trillion barrels of oil. However, it expensive (requireing a lot ofwater) and enviornmentally damaging to extract oil from these reserves.

Hyrdogen fuel is obtained from a variety of sources – primarily methane, but also from coal extraction and water electrolysis. In the latter case, electrolysis of water to produce hydrogen (and oxygen) is utililised by photochemical processes. Meyer identified the Idaho National Laboratory hydrogen programme as one which was making good progress in the production of hydrogen as a mass fuel. The advantages of hydrogen were good efficiency, and water and heat as emission products. However, the current costs (for transportation) are ca. $3500/kW, with a target of $35/kW. Another significant problem with the use of hydrogen was storage and transportation, which were expensive because of the nature of the fuel.

Nuclear energy provids ~20% of US energy, and increased usage would result in a significant decrease in greenhouse gases. There are 44 nuclear reactors currently being built internationally, and therefore these will be significnat contributors in to the future. The issues, well know, of nuclear power are what to do with waster, control (political issue), reprocessing and general safety issues.

Renewable energies provide an alternative approach to the solution. It is estimated that wind could provide 20% of US energy requirements. However, solar energy is a real viable option, given that 26,000TW per year of sunclight isiincident on the Earth’s surface (net amount after absorption etc). the technology is on the cusp of mass implementation, with some lingering problems regarding efficiencies. (In the US, there are also problems regardingthe arrangement of the national grid (see Grid 2030 project). Current estimates are that solar generation of 3 TW, assuming 10% efficiency solar cells, would cost approximately $60 Trillion (covering an area of 57k sq – miles). Current and future work will be focussed on reducing this cost.

Meyer reiterated the point in his talk, and again in questions, that there must be a political will to drive this work forward. Solar energy could have emerged as a major player much earlier, if work started after the oil crisis had continued apace. 6% of US energy is currently sourced from renewable sources; with 85% from coal, oil and gas. The hope is that by 2059, these numbers can be reversed!

References

C. R. Bock, T. J. Meyer, D. G. Whitten, Photochemistry of transition metal complexes. Mechanism and efficiency of energy conversion by electron-transfer quenching, J. Amer. Chem. Soc., 1975, 97, 2909 – 2911.

R. J. Cicerone, National Academy of Sciences, Address to the 145th Annual Meeting, available at: http://www.nationalacademies.org/includes/NASmembers2008.PDF [Oct 2009]

Las Alamos National Lab: National Security Science: http://www.lanl.gov/ [Oct 2009]

T. J. Meyer and H. Taube, Electron transfer reactions of ruthenium ammines, Inorg. Chem., 1968, 7, 2369 – 2371.

J. R. Pladziew, T. J. Meyer, J. A. Broomhea, and H. Taube, Reduction of oxygen by hexamammineruthenium(II) and by tris (ethylenediamine) ruthenium (II), Inorg. Chem., 1973, 12, 639 – 643.

H. Taube, Nobel Prize Lecture Nobel Prize 1983, http://nobelprize.org/nobel_prizes/chemistry/laureates/1983/taube-lecture.html [Oct 09]

R. C. Young, T. J. Meyer and D. G. Whitten, Kinetic relaxation measurement of rapid electron-transfer reactions by flash photlysis – conversion of light energy into chemical energy using Ru(bpy)3(3+)-Ru(bpy)3(2+*) couple, J. Amer. Chem. Soc., 1975, 97, 4781 – 4782.