Beautiful Photochemistry

01Nov09

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/

 

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Quenching Mechanisms

30Oct09

Excited states can be deactivated in several ways – they can emit, giving off light energy, deactivate – resulting in a “vibrationally hot” ground state (i.e. energy loss as heat) or be quenched by another molecule. In this section, we will consider the process of quenching, and outline some ideas that use the process of quenching in applications. In addition, we will examine how the process of quenching can be studied to give us information on the nature of the excited state-quencher interaction. It is assumed the reader is familiar with the information presented in the Light Absorption and Fate of Excited State post.

Overview

Quenching of the excited state is a significant process because it is usually a very efficient process. The excited state of many organic compounds, for example, are efficiently quenched by the presence of oxygen, at rate constants several orders of magnitude faster than emission processes from the triplet state. (Emission from the triplet is spin forbidden, and hence has rate constants in the range 10 to 10³ dm³ mol^-1 s^-1, whereas oxygen quenching may take place at rate constants of the order 10⁹ dm³ mol^-1 s^-1. Therefore, to study the emission from triplets, we need to deaerate the sample (and have it at low temperature – see the experimental section). Quenching processes can occur by two processes – electron transfer or energy transfer. In both cases, the excited state energy of the luminophore (the luminescent species) is deactivated due to the presence of the quencher. There are two scenarios by which quenching is generally modelled, and these are discussed below.

Dynamic Quenching of an Excited State

If a solution with emitting species is studied, and for every 100 photons absorbed by the solution, 30 are re-emitted, the quantum yield of emission is said to be 0.3. What happens to the other 70? They are translated into radiationless transitions, such as deactivation. As mentioned in the Ruthenium polypyridyl photochemistry post, we can quantify the quantum yield of emission (or any process) as being the rate constant of that process (in this case emission) divided by the sum of all rate constants deactivating the excited state. If we divide the emission quantum yield in the absence of quencher by that in the presence of quencher, we can generate an expression known as the Stern-Volmer equation, as shown below.

Derivation of the Stern-Volmer Equation based on considering rate constants of deactivation in the absence and presence of quencher

The Stern-Volmer equation models what is called dynamic quenching, quenching which occurs by the quencher diffusing through solution and interacting with luminophore, resulting in a deactivation of the excited state. The emission intensity is reduced, because as well as other deactivation pathways before the presence of quencher, the presence of quencher now adds another deactivation pathway in competition with luminescence. This quenching process is controlled by how fast the quencher can diffuse through solution and “collide” with luminophore, and as diffusion is usually a very fast process in solutions, it can be very efficient.

The Stern-Volmer equation is the equation of a straight line, and hence it allows for  very easy experimental determination of the quenching rate constant, k_q. If the emission intensity (or lifetime) in the absence of quencher and then in the presence of incremental amounts of quencher is measured, and the resulting ratio of emission intensities (I(0)/I) is plotted as a function of quencher concentration, the resulting graph (called a Stern-Volmer plot) will have an intercept of 1 and a slope called the Stern-Volmer constant, K_SV. K_SV is the product of the natural radiative lifetime (the lifetime in the absence of quencher, τ₀, and the quenching rate constant, k_q. Knowing the slope and the natural radiative lifetime allows easy calculation of the quenching rate constant. An outline of a common experiment – quenching of a ruthenium polypyridyl complex emission with Fe³⁺ is shown below.

The fact that quenching can be so efficient means that it can be a useful probe in studying systems with emission properties. For example, ruthenium polypyridyl complexes have been used successfully as oxygen sensors, whereby the complex has been incorporated into a silica matrix and the resulting stub located inside packaging. In the absence of oxygen, emission is observed when the stub is irradiated with light. However, if oxygen leaches into packaging, the emission observed will be substantially reduced, as it will be quenched by the oxygen. By calibrating the reduction in intensity using a Stern-Volmer plot, it is possible to estimate the concentration (partial pressure) of oxygen in the system. The concept has applicability in food packaging and for containers holding oxygen sensitive artefacts (e.g. paintings).

Static Quenching

Dynamic quenching results from collisions between excited state and quencher. However, if the quencher is somehow associated with the luminophore in solution prior to light absorption, the association may mean that the luminophore will not emit, due to induced changes in its properties because of presence of quencher. Therefore the reduction in emission intensity will be affected by the extent to which the quencher associates to the luminophore and the number of quenchers present. The reduction in emission intensity can be quantified as follows. If the luminophore, M, associates with quencher, Q according to an equilibrium constant of association, K_s, then this association constant can be quantified as the ratio of associated luminophore-quenchers luminophore-quenchers moieties ([M-Q]) to the product of unassociated luminophore and quencher; [M][Q]. Since the total concentation of luminophore, [M]₀ is equal to the sum of associated and unassociated luminophore, substitution of this into the equilibrium expression, followed by rearrangement results in another equation of a straight line, very similar in form to the Stern-Volmer equation. However, while plotting I₀/I (as emission intensity can be said to be proportional to concentration) against [Q] will result in a straight line for static quenching, analogous to dynamic quenching, interpretation of the slope is different. In this case, the slope quantifies the association constant between quencher and luminophore – and therefore is useful in providing information on how these two species interact in the ground state.

Derivation of an expression for static quenching

Dynamic or Static?

The question that immediately arises now is that if plots of emission intensity against quencher concentration both produce straight line graphs, how do we know which type of quenching is occurring? The answer lies in thinking again about the nature of each type of quenching. For dynamic quenching, all luminophores are affected by the quenching process as it is probable that they will all collide with a quencher during their excited state lifetime, so both emission intensity and lifetime reduced on increasing quencher concentration. For static quenching by association, only luminophore-quencher associations result in reduction in emission, unassociated luminophores are free to luminesce as if there was no quencher present. Increasing quencher concentration affects emission intensity, because there are more associations, but not emission lifetime, as the unassociated luminophores can emit in the absence of quencher. (Note that these two scenarios are the extremes, and there are cases where a mixture of both static and dynamic quenching may occur simultaneously.)

Schematic of dynamic versus static (association) quenching

Therefore the diagnostic test for assigning whether a quenching mechanism is dynamic or static is to compare how the emission intensity and emission lifetime changes as a function of increasing concentration. In the case of dynamic quenching, plots of relative emission intensities and emission lifetimes will be th same, changing on increasing quencher concentration. For static quenching, only a plot of relative emission intensity will change, the emission lifetime plot will have  slope close to zero.

Model diagnostic plots to distinguish between dynamic and static quenching

Another model of static quenching is where the quencher is in a fixed position close to the luminophore (e.g. in a frozen matrix or a zeolite). This is modelled by the Perrin model of quenching, which will be discussed in the experimental techniques section when discussing phosphorescence.

References

MK Seery, N Fay, T McCormac, E Dempsey, RJ Forster, TE Keyes, Photophysics of Ruthenium Polypyridyl Complexes formed with lacunary polyoxotungstates with iron addenda, Phys. Chem. Chem. Phys., (2005), 19(7), 3426 – 3433. An example showing unusual static quenching between a quencher (large polyoxometallate clusters) and a luminophore (a ruthenium complex).

B Valeur, Molecular Fluorescence: Principles and Applications, Wiley: Weinheim, 2002. Discusses the principles of dynamic and static quenching well.

Filed under: Experimental, Light Absorption, Principles, Quenching, Ruthenium Photochemistry   |  Leave a Comment

Our Energy Future: Lecture by Prof Tom Meyer

21Oct09

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.

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.

Filed under: Applications, Dye-sensitized solar cells, Public Science, Ruthenium Photochemistry   |  Leave a Comment
Tags: Applications, DSSC, Dye-sensitized solar cells, Gratzel, inorganic photochemistry, Nobel Prize, ruthenium