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.
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.
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, kq. 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, KSV. KSV is the product of the natural radiative lifetime (the lifetime in the absence of quencher, τ0, and the quenching rate constant, kq. 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).
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, Ks, 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.
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.)
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.
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.
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.
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!
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.
Ruthenium polypyridyl complexes certainly rank amongst the most researched family of compounds in inorganic photochemistry. They are interesting complexes to study, having relatively long (100′s ns) emission lifetimes and a range of applications. It was the oil crisis of the 1970′s that sparked interest in these compounds, as potential hydrogen fuel generators by the photochemical splitting of water, and as seen in other posts, they are currently at the forefront in terms of efficiency in dye-sensitised solar cells. In addition, they have been used as DNA probes and oxygen sensors. The photochemistry of these complexes is discussed below. Readers are recommended to be familiar with the concepts in the “Light Absorption and Fate of the Excited State” article before studying this material.
Like so many aspects of modern photochemistry, Ireland has some key researchers in ruthenium photochemistry and the article below draws from a recent perspective by John Kelly (TCD) and Han Vos (DCU). The fundamentals are discussed here with applications discussed in a forthcoming article.
1. Introduction to Inorganic Photochemistry
We have looked elsewhere at Jablonski diagrams for organic molecules. Inorganic molecules, or more specifically d-block complexes, add an extra layer of molecular orbitals to this Jablonski diagram, between the ground state (HOMO) of the organic compound (which is now the ligand) and the excited state (LUMO). This opens up a range of new transitions, aside from the HOMO-LUMO transition observed in organic chromophores. This latter transition in inorganic photochemistry is called a ligand-field or ligand-ligand transition, as in the excited state the electron is located on the ligand. As well as this, because of the presence of the metal’s molecular orbitals, three other transitions are available – a d-d transition, where an electron is excited from a metal orbital to an unoccupied metal orbital (this is usually referred to as a metal centred (MC) transition as well as transitions between the metal and the ligand. These can be either an electron excited from the ligand to the metal, called Ligand to Metal Charge Transfer (LMCT) or from the metal to the ligand (MLCT). Because of the energy differences between the various types of transitions, ligand field transitions are usually in the near-UV region (analogous to where we would expect organic molecules to absorb light), charge transfer transitions are in the visible region. The resulting emission from charge-transfer states is often highly coloured.
In order to discuss these transitions in context, we will focus on the, that is, the, inorganic photochemistry complex: Ru(II)(bpy)32+.
2. Fundamentals of ruthenium polypyridyl photochemistry
2.1 Absorption and Emission
Because of the incorporation of metal orbitals, the Jablonski diagram needs to incorporate the notation discussed above. Ruthenium in oxidation state II is d6, and so as an octahedral complex its electrons are in the low-spin t2g6 configuration. Incident light at about 450 nm promotes one of these electrons to a ligand anti-bonding orbital, a metal to ligand charge transfer. (We’ll discuss this, but you might consider how this was established.) Therefore we modify the S0 – S1 notation used in the Jablonski diagrams of organic molecules to one which denotes the type of excited state in inorganic ones – in this case 1MLCT. Transfer to 3MLCT is efficient (heavy atom effect) and so ruthenium complex’s photochemistry generally happens from here. [Remember intersystem crossing is effectively an electron flip, from a situation where electrons are paired to one where they are unpaired.]
The absorption and emission data are shown. Ruthenium absorbs at 450 nm (2.8 eV) and emits strongly at ~620 nm (~2.0 eV) in water. This emission is caused by radiative process from the 3MLCT state to the ground state. Emission lifetimes are approximately 200 ns in water in aerated solution and 600 ns in deaerated water. The oxygen in water is a very efficient quencher, and quenches emission with a rate of ~ 109 M-1 s-1. It is possible to map out the various deactivation processes of the excited state to investigate its kinetics:
The quantum yield of emission is therefore affected by how efficient the rate of emission is compared to the rates of deactivation and quenching. This is quantified by the Stern-Volmer relationship (oxygen quenches according to the dynamic quenching model) as discussed in the Quenching section, according to the equation below:
The rate constants, in particular the rate constant for deactivation, are dependent on how close the ground and excited states are. The excited state of this complex is a charge-transfer state (charge has moved from one region of the molecule to another), and therefore is very sensitive to solvent polarity – it will be stabilised in more polar solvents. Therefore, changing solvent polarity will affect the energy of the emitting state. It is found that on changing the solvent from water to acetonitrile, the emission lifetime increases from 635 ns to 870 ns, and the quantum yield of emission increases by 50% from 0.o4 to 0.o6. The emission maximum increases in energy from 627 nm to 615 nm.
These results can be explained as follows: on decreasing polarity of the solvent, the emitting state is destabilised by about 12 nm. This increase in energy difference between ground and excited state means that there is poorer overlap of the vibrational levels of the ground and excited state, so the deactivation process is not as efficient. Therefore the deactivation rate constant term is lower in the expression for the emission quantum yield in the presence of quencher, above, indicating a larger emission quantum yield. All of this is based on the assumption that the radiative rate constant remains unchanged, which is found to be true in practice. This observation is generally summarised as the Energy Gap Law – the larger the gap between ground and excited state, the less efficient deactivation processes are.
2.2 Nature of the Excited State
Absorption and emission spectra give initial information on the excited state, and are the photochemist’s initial tools to probe the excited state chemistry of molecules. To delve further, flash photolysis/transient spectroscopy give more detailed information. Flash photolysis, as mentioned elsewhere on this site, allows us to study the excited state by obtaining its lifetime and absorption spectrum. An experimental set-up is outlined below (more details onthe general details of flash photolysis in the Experimental article on Flash Photolysis). Excitation using, for example a Nd:YAG laser at 355 nm, generates the excited state which quickly equilibrates to the 3MLCT state. At this stage, a Xe or Hg/Xe obtains an absorption spectrum of the excited state. This was traditionally acquired point by point (i.e. measuring the change in absorption at 400, then 410, then 420 nm, etc) but iCCD (intensified charge coupled device) detectors are now the norm – these acquire information across a broad spectral range (~600 nm) at once. As well as providing structural information on the nature of the excited state by generating its absorption spectrum, flash photolysis also allows for the lifetime of this state to be measured, by acquiring a spectrum at intervals after the laser flash, therefore monitoring the decay of the excited state.
The transient spectrum is shown with the accompanying ground state absorption spectrum. In the transient spectrum, it can be seen that some peaks have negative changes in absorbance whereas others have positive changes. The negative changes in absorbance (“bleaching”) occur where the molecule shows absorbance bands in the ground state. Hence, with a transient spectrum, the lash flash results in the formation of the excited state, and the xenon lamp records the loss of ground state chromophores – any absorbance that was present because of these chromophores is now registered as negative changes in absorbance in the transient spectrum. On formation of excited/transient state, new chromophores are present, which are monitored by the xenon lamp, and hence appear as positive changes in absorption (remember ground and excited states are chemically different species). To generate a true transient spectrum, the differences in absorption is subtracted from the absorption spectrum, although this is rarely necessary. The decay curve, in the inset is the rate of decay of one of the peaks – e.g. the transient peak at 390 nm. Fitting this curve to an exponential function allows for the rate constant (and hence lifetime) of the transient state to be easily determined. For example, if the decay was found to be mono-exponential, the curve of intensity (I) versus time (t) would be fitted to the expressionand allow for calculation of k.
The above experiment discusses results from a nanosecond experiment, but if we were to push faster, into the picosecond and femtosecond domain, the processes of intersystem crossing and relaxation in the triplet state would be observed. These kind of experiments are how information such as charge injection rates in dye-sensitized solar cells can be determined.
The extent of positive absorbances in transient spectroscopy provide information on the nature of the transient species or excited state. Like conventional UV/vis spectroscopy, broad featureless bands very often don’t provide much direct information. However, considering the various types of transitions available, why is the excited state assigned as a MLCT state? This state, as indicated above, results in an extra electron residing on the bipyridyl (bpy) ligand, after an electron was transferred from the metal to it. Therefore, the transient spectrum should show characteristics of this bpy radical (called “bpy dot minus”). How can this be done? Well with the assistance of our electrochemical friends, we can electrochemically generate the bpy radical, and obtain its UV/vis spectrum (this technique is called spectroelectrochemistry). If it has characteristics similar to those in the transient spectrum (which in this case it does, the band at 368 nm), we can conclude that they must be attributed to the same chromophore.
In this first of two articles, we have looked at basic photophysical properties of a ruthenium complex and examined how absorption, emission and transient spectroscopic studies provide information on their excited state. In the second article, we will look at how these properties are used in a variety of applications.
4. References and Further Reading
Photochemistry of polypyridine and porphyrin complexes, K. Kalyanasundaram, Academic, London: 2002. Very comprehensive book on the area with excellent introduction covering theory in much more detail than above.
Vos, J. G. and Kelly, J. M., Ruthenium polypyridyl chemistry: from basic research to applications and back again, Dalton. Trans., 2006, 4869 – 4883. Good ooverview of the synthesis of these complexes and their variety of applications, especially looking at the role of Irish researchers in the area