Ruthenium polypyridyl photochemistry

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.

Light absorption in d-block (octahedral) complexes resulting in from left: metal centred (MC), ligand to metal charge transfer (LMCT), metal to ligand charge transfer (MLCT) and ligand-ligand transition (L-L)

Light absorption in d-block (octahedral) complexes resulting in from left: metal centred transition (MC), ligand to metal charge transfer (LMCT), metal to ligand charge transfer (MLCT) and ligand-ligand transition (L-L)

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.]

Jablonski diagram for ruthenium polypyridyl complexes.

Jablonski diagram for ruthenium polypyridyl complexes. Solid lines and dashed lines are radiative and non-radiative processes respectively.

Absorption (top, source unknown) and emission (bottom, author's results) of Ru(bpy)3 complex in water

Absorption (top, source unknown) and emission (bottom, author's results) spectra of Ru(bpy)3 (2+) complex in water

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:

Deactivation processes of an excited state M* in the presence of a quencher (oxygen)

Deactivation processes of an excited state M* in the presence of a quencher (oxygen)

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:

Stern Volmer Equation for Quenching with oxygen as quencher

Stern Volmer Equation for Quenching with oxygen as quencher

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.

Schematic of Transient Absorption Spectroscopy Experiment: Laser excites sample and change in absorption is monitored by a xenon lamp. The simulated transient spectrum (top right) is the difference in absorption after laser flash, showing negative (dissapearance of ground state) and positive (formation of transients) absorbances. The absorption spectrum is shown on the bottom for comparison. Inset shows a kinetic trace of any of the transient peaks from which lifetime information can be gleaned.

Schematic of Transient Absorption Spectroscopy Experiment: Laser excites sample and change in absorption is monitored by a xenon lamp. The simulated transient spectrum (top right) is the difference in absorption after laser flash, showing negative (disapearance of ground state) and positive (formation of transients) absorbances. The absorption spectrum is shown on the bottom for comparison. Inset shows a kinetic trace of any of the transient peaks from which lifetime information can be gleaned.

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.

emission_decay

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.

3. Conclusion

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

Light Absorption and Fate of Excited State

Photochemistry is the study of what happens to molecules when they absorb light. Therefore it is important to consider the factors affecting whether and how efficiently molecules absorb. In addition, in the very short time-frame after a molecule has absorbed light, it can undergo a variety of processes. In applications, we may desire a particular process, so again an understanding of what pathways are available to excited states is important so that systems can be optimised as required (e.g. by changing solvent, modifying the molecule).

Students should note that this topic is traditionally approached from a quantum chemical background. All textbooks on photochemistry cover this well (for example see Turro or Gilbert and Baggott) so it is not necessary to relay it in too much detail here. Instead, a qualitative overview is presented for the purposes of providing a background to the material elsewhere on this site.

1. Light Absorption – Formation of the Excited State

Photochemistry is based on the reaction/reactivity of molecules in their excited state after they have absorbed light. By “light”, we mean that part of the electromagnetic spectrum that can promote electrons in the outer atomic orbitals to unoccupied orbitals – i.e. electrons near or at the highest occupied molecular orbital (HOMO) to orbitals near or including the lowest unoccupied molecular orbital (LUMO). To do this, the light must be of sufficient energy to promote electrons between electronic energy levels, and this is found to be light in the UV/visible region of the electromagnetic spectrum. For this reason, the region of the spectrum 200 nm < λ < 800 nm is sometimes referred to as the “photochemical window”. The range of wavelengths in the spectrum and the result of absorption by the atom/molecule is shown below.

Regions of the electromagnetic spectrum and their impact on atom structure

Regions of the electromagnetic spectrum and their impact on atom structure

Therefore, absorption of a photon of light of wavelength 200 – 800 nm may result in a HOMO-LUMO transition (dependent on other factors which we will discuss later). A very clear indication of this is observed in d-block complexes. For example, a ruthenium (II) complex has six d-electrons and has a low spin octahedral configuration t2g6. On absorption of visible light (λ ~450 nm), an electron is promoted to an eg orbital, giving the complex its red-orange colour. This transition is in the visible region. For d0 complexes such as TiO2, a d-d transition is not possible, and a transition from the oxide ligand to the metal centre – a ligand – to metal charge transfer (LMCT) transition occurs, but only if the molecule is irradiated by UV light (λ < 390 nm). Hence TiO2 is white, as it does not absorb any visible light.

Absorption of a photon of visible light causes a d-d transition in Ru(II) giving the molecule a visible colour

Absorption of a photon of visible light causes a d-d transition in Ru(II) giving the molecule a visible colour

As well as the type of transitions possible, a second factor to consider is the intensity of absorption as a function of wavelength. These absorptions, measured by UV/visible absorption spectroscopy for gases or solutions and diffuse reflectance spectroscopy (DRS) for solids will vary depending on the extinction coefficient, ε, of the molecule at that wavelength. The extinction coefficient is a measure of the probability of an electronic transition from ground to excited state, at a given wavelength. This probability is calculated via quantum chemical parameters that are beyond the scope of this course. However, in simple terms, the value of ε gives an indication of how “allowed” a transition is, where “allowed” is a meant strictly as a quantum chemical term. If ε is measured to be greater than 105 dm3 mol-1 cm-1, then the transition is “fully allowed” – all quantum chemical rules are passed. For transitions below ~100 [dm3 mol-1 cm-1 ,units implied from hereon], the transition is “forbidden”, indicating that all quantum rules are not passed, and the probability of transition is very low – i.e. the molecule does not absorb well at this wavelength.

The in-between grey area, for ε values between ~102 and ~104, are where the transitions are “partially allowed”. The quantum mechanical rules are based primarily on two components – spin and symmetry. The spin component says that if a transition involves a change of spin (e.g. singlet to triplet) then the transition is forbidden. The symmetry component examines the symmetry of the ground and excited state, and depending on these symmetries the transition will be allowed or forbidden. But these symmetry calculations are based on a molecule idealised conditions, so the symmetry of the real molecule may be distorted by the presence of solvent or of a heavy atom on the molecule (the so-called “heavy atom effect” – we will return to later). Hence if a transition is spin-forbidden, symmetry allowed, then the probability is very low, and ε will be <100. But if it is spin-allowed, symmetry forbidden, then appreciable absorption may be observed (102 – 104) because of the symmetry distortions mentioned above.

The final factor to consider about light absorption, having discussed types of transition and intensity of absorption above is the shape of absorption spectra. Again, these relate to the discussions above on the value of ε at each wavelength, but for an individual electronic transition (e.g. HOMO – LUMO), transitions between vibrational levels of each orbital may be more intense than others. These transitions are governed by the Franck-Condon Principle, which states that:

the electronic transition in a molecule takes place so rapidly compared to nuclear motion, that immediately afterwards the nuclei have still very much the same nuclear geometry (position and velocity) as before the transition.

In simple terms, this means that electronic transitions take place so quickly the nuclear geometry differences between ground and excited states do not have time to adjust, or in even simpler terms, these transitions are vertical. Consider the potential energy diagram for a HOMO and LUMO shown below. Each electronic orbital has some of its vibrational levels shown. The probability of an electron being in one of these orbitals can be calculated, and are “mapped” using wavefunctions, as shown.

Looking at these qualitatively, we can say that the most probable transition between a vibrational level in the ground state (HOMO) and one in the excited state (LUMO) will be the one where the wavefunctions overlap the most in the vertical line above the ground state groud vibrational level. (in either a positive or negative direction). On the left hand side of the diagram, the greatest overlap is (hypothetically) the ground state vibrational level 1 and the excited state vibrational level 1, so we have a 0 – 1 transition (spectroscopists will get annoyed at this notation, but it is used here just to illustrate the principle). On the right hand side, the excited state geometry is different to the ground state (the potential energy diagram is shifted to the right a little), so in this case the hypothetical best overlap is between 0 in the ground state and 4 in the excited state. Tehrefore the shapes of the two absorption spectra in each of these scenarios is different. Of course, in practice we don’t see this fine structure, the absorption spectra are essentially a line drawn over the tops of the individual transition peaks, resulting in the broad, generally featureless absorption spectra we are used to. But if we were to do it in the gas phase (eg iodine vapour experiment) we would see this fine structure. If you’re wondering, the reason we don’t see fine structure in solution is because the molecules absorbing light are being battered around by solvent molecules, so the energy levels are constantly moving up and down a little, therefore blurring the transitions a little. Each electronic transition will have a suite of different vibrational transitions, so a molecule with, for example, three bands in the experimental absorption spectrum consists three of these processes happening. Because electronic transitions also vary in intensity, some of the bands may be more intense than others.

Ground and excited state potential energy curves with vibrational energy level wavefunctions shown. Left: The most probable transition is the 0 - 1 transition; Right: the different nuclear configuration of the excited state means that in this case, 0 - 4 transition is most intense. Curves over the PE diagrams show the resulting absorption spectra (Based on images presented in Gilbert and Baggott, which covers this area excellently)

Ground and excited state potential energy curves with vibrational energy level wavefunctions shown. Left: The most probable transition is the 0 - 1 transition; Right: the different nuclear configuration of the excited state means that in this case, 0 - 4 transition is most intense. Curves over the PE diagrams show the resulting absorption spectra (Based on images presented in Gilbert and Baggott, which covers this area excellently)

2. The Excited State

If a molecule absorbs light and forms an excited state, then it is in a very different state to one it was in the previous few sub-picoseconds. Excited states have been called “electronic isomers”, which rather underestimates their relevance. To emphasise the point, excited states are chemically different species to their corresponding ground states. This statement reflects the true beauty and power of photochemistry. For every photoactive molecule a second different molecule can be “created” by literally, the flick of a switch – this gives an inkling of the true potential of photochemistry as a discipline. Very often, these states are not accessible by thermal means because of the great differences in energy levels.

Excited states are energetically unstable and very short-lived. “Short” in this context means from sub-nano and nanosecond (if a process is allowed) to milliseconds and seconds, if a process is forbidden, such as phosphorescence. To put these numbers in context, the German photochemist and educator Michael Tausch has pointed out that the positive equivalent of a nanosecond (10-9 s), which is 10+9 s (or 1 gigasecond), is about the equivalent of a human lifetime.

Therefore the equipment and scientists which experimentally determine the processes which are discussed below should not be overlooked, and we will look at some of these in various articles (see Experimental). For now, it can be said that since the discovery of microsecond (x 10-6 sec) flash photolysis by Norrish and Porter in the 1950’s, each decade has seen another power of ten on the limit of time that can be studied culminating in Zewail’s development of femtosecond (x 10-15 sec) spectroscopy in the 1990’s. This is at the limit of atomic vibrations and indeed electron transfer, and so is probably a “true” limit, as beyond this the Heisenberg Uncertainty Principle becomes significant. Scientists at either end of the timescale, Norrish and Porter, and Zewail, won Nobel prizes for their efforts. these developments will be covered in more detail in a future article.

So what is the fate of the excited state? When a molecule absorbs light, it is a very fast process – on the order of picoseconds or lower. Depending on the wavelength of light used, and the Franck-Condon principle, above, the vibrational levels of some upper excited state will be populated with electron density. The various processes which occur can be represented on a Jablonski diagram, a sketch of the electronic energy levels in an atom together with their vibrational levels.

A Jablonksi diagram for an organic molecule. Radiative processes (those which are "vertical" in energy transfer) are shown in solid lines whereas non-radiative processes ("horizontal" energy transfer) are shown using dotted lines

A Jablonksi diagram for an organic molecule. Radiative processes (those which are "vertical" in energy transfer) are shown in solid lines whereas non-radiative processes ("horizontal" energy transfer) are shown using dotted lines. Indicative timescales are shown, although are molecule dependant.

In principle the Jablonski diagram is similar to the transitions in the potential energy curves, shown above, except the potential energy curves are usually not represented. A simple Jablonski diagram for an organic molecule is shown above. Note that a similar diagram for an inorganic compound will also include metal orbitals, so will be different in style. The processes which occur when a molecule absorbs light are below. We will discuss the kinetics of these processes in a separate post, looking at how they can be measured.

  1. Molecule absorbs light and populates upper excited state S* with electrons
  2. Electrons in upper vibrational levels of S* undergo vibrational relaxation and the electrons move to the lowest vibrational level of S*.
  3. The molecules very quickly dissipate this very high energy by internal conversion – the electron density moves to the lowest excited state, S1. Internal conversion occurs by the electron density transferring from the vibrational levels of the upper excited state to vibrational levels of a lower excited state which they are overlapping. Hence this is a “horizontal energy” transition, or a radiationless transition – it does not give off a photon of energy (light) as the electron density has not moved in one “big jump”.
  4. Vibrational relaxation again occurs, and the electron is now in the lowest vibrational state of S1. This is a statement of Kasha’s rule, which says that photochemical processes (fluorescence, quenching) happen from the lowest vibrational state of the lowest excited state (S1). The reason for this is that the processes described above leading to this situation all occur in a matter of picoseconds. The electron now has a choice of what to do next
  5. It may undergo fluorescence, giving off a photon of energy.
  6. It may undergo internal conversion as above.
  7. The electron may undergo intersystem crossing (ISC) to the triplet state. Once here, the molecule can undergo phosphorescence or deactivation. These processes are shown in the Jablonski diagram. Note the timescales involved in the various processes.

3. Conclusion

Light absorption can result in the formation of an (electronically) excited state, which has different chemical properties to the groud state. The intensity and shape of absorption spectra are a result of the nature of excitation between ground and excited states. Various processes result in the deactivation of the excited state.  The timescales of these indicate their efficiency, and we will look at these in more detail in future posts.

4. References

All general photochemistry texts discuss the principles of light absorption and deactivation of the excited state in good detail. some are given below, but any will give pretty much the same information.

Gilbert, A. and Baggott, J. E., Essentials of molecular photochemistry, Blackwell Scientific: London, 1991.

Turro, N. J., Ramamurthy, V. and Scaiano, J. C., Principles of molecular photochemistry: an introduction, University Science Books:Sausalito, 2009. Despite the title, a detailed text with lots on the various photophysical processes that occur on light absorption. These three authors are among the best known photochemists today. Turro’s classic, Modern Molecular Photochemistry, was for a long time the bible for photochemistry.

Luminescent PET Sensors

PET sensors are very simple in principle and can signal the presence or absence of a very small amount of analyte in solutions/biological samples by using a readily available instrument – the fluorimeter. This article looks at the design principles of PET sensors, as well as examining how they may be modified to enhance specificity/selectivity. [Aug 2009]

1. Introduction

Luminescent sensors provide for an easy, often visual, method for detection of a wide range of ions, physical properties such as pH and other components such as nanoparticles. Their wide variety of sensing capabilities are already used in commercial devices, and a greater understanding of their mechanism of operation is leading to newer exciting concepts, such as molecular logic gates for computing. This article aims to summarise the background theory to luminescent sensors – especially concentrating on photoinduced electron transfer (PET) which we can consider as the third type of quenching (Type I and Type 2 discussed elsewhere – links will be here) – and examining practical applications in the context of this theory. In summarising the work here, special attention is paid to the work of Prof A. P. de Silva who is based in Queen’s University, Belfast and who is one of the pioneers in the area. Interested students are referred to some of his papers in the references, where even in the formal constraints of academic journals, his passion for the subject is very evident.

Here’s a short podcast outlining what we will cover in this section (with audio)
Vodpod videos no longer available.

2. Background

We’ve seen several times that molecules that have absorbed light may transfer their electron to another system – which is usually called a quencher, although is not constrained to this (see for example dye-sensitized solar cells). Because this electron is transferred after absorption of light by the molecule, the process is called photo-induced electron transfer (PET). It’s one of the great applications of photochemistry in general – excited states are better oxidisers and reducers than their ground state counterparts. Suppose we have donor (reducing agent, itself going to be oxidised) and acceptor (oxidising agent, itself going to be reduced) molecules as shown below, where the energy level of the donor HOMO is lower than the energy of the acceptor LUMO. As it stands, the donor cannot reduce the acceptor, as electron transfer is not energetically feasible. However, if we form a donor excited state, D*, by light excitation, the process becomes energetically feasible once the donor LUMO is higher than the acceptor LUMO. You should consider how excited states are also better oxidising agents.

Excited states are better reducing agents (and oxidising agents) than their corresponding ground states

Excited states are better reducing agents (and oxidising agents) than their corresponding ground states

In applying this general concept to this section, we will be looking at molecules that have both these components in one molecule. In this system, we will term them luminophore and receptor, which are analogous to the discussion of donor and acceptor respectively. The luminophore is the light absorber which results in an excited state and may luminesce, or accept an electron into the vacancy in the ground state, or donate an electron from the excited state. The receptor will accept/donate an electron from/to the luminophore. The two components are linked by a non-conjugated bridge (e.g. two sp3 carbon bridge). You may wish to consider why a non-conjugated bridge is required. The three components – luminophore-spacer-receptor – make up what we will term here a class III type of quenching; where the PET process occurs within one molecule.

Schematic of a Fluorophore-Spacer-Receptor model

Schematic of a Fluorophore-Spacer-Receptor model

The power of the model is that if the receptor’s energy level’s can be tweaked in the presence or absence of an analyte – for example an metal cation – and the energy levels between the receptor and luminophore are close, well the presence of the ion may turn on or shut off the electron transfer process. This concept is now a sensor, as luminescence is affected by the presence or absence of an analyte. If that luminescence can be measured (which it can), we have a very powerful, sensitive analytical tool to measure the extent of analyte present.

3. Details

3.1 Principles

The general principle discussed above is summarised in the diagram below. A luminophore gives off light in the absence of an analyte, but emission is quenched in the presence of an analyte. This is called an ON-OFF system – emission was turned off by the presence of an analyte. The systems become very powerful when the receptor – the docking area for the analyte – is selective to only a particular type of species (e.g. a halide) or even better a specific ion. An example of one of de Silva’s systems is shown below. The three components are obvious: the anthracene molecule is the luminophore (in this case a fluorophore as it emits from the excited singlet state); the bipyridyl component is the receptor and the two carbon-chain acts as a spacer. In the absence of an ion, this system fluoresces strongly. However, in the presence of either zinc ion of a proton the emission is shut off. So how can we explain what happened?

An example of an ON-OFF sensor: in the absence of protons or zinc ions, emission is observed (ON), whereas in the presence of either of these ions, emission is strongly quenched. (Based on de Silva, J. Am. Chem. Soc., 1999, 121, 1393)

An example of an ON-OFF sensor: in the absence of protons or zinc ions, emission is observed (ON), whereas in the presence of either of these ions, emission is strongly quenched. (Based on de Silva, J. Am. Chem. Soc., 1999, 121, 1393)

Examining the emission spectrum, we can see that the anthracene emission is very much reduced in the presence of one of the specific ions, but the shape or position of the bands do not change. Therefore its energy levels remain as they were prior to the presence of the receptor. It can be concluded that the receptor levels change, opening up a PET pathway that was not available prior to the presence of the analyte. The diagram below summarises the schematic, molecular and energetic processes.

Schematic (top); energy level (middle) and molecular (bottom) representations of an ON-OFF sensor. The dashed line in the energy level representation of the acceptor-analyte shows the energy level prior to binding of the analyte

Schematic (top); energy level (middle) and molecular (bottom) representations of an ON-OFF sensor. The dashed line in the energy level representation of the acceptor-analyte shows the energy level prior to binding of the analyte

3.2 Developing the idea further

In a paper discussing the developments of luminescent sensors so far this century, de Silva illustrates a scenario that is already a reality (Tetrahedron, 2005, 61, 8551 – 8588). An ambulance is called for a car accident trauma, and upon arrival paramedics take a blood sample. From this, they can alert the hospital en route of the required electrolyte levels required for a blood bag, having used a small portable instrument to determine concentrations of for example, sodium, potassium, calcium as well as oxygen levels and pH. The key here is that the the device incorporates a range of PET sensors, each of which have receptor components of the moleculethat selectively sense a particular of ion/molecule. Therefore, the question is how is this done?

Selectivity has been achieved by a range of interesting approaches, and we have to be thankful to organic chemists for their synthesis of the diverse range of molecules available. Crown ethers are useful receptors, and are known to bind well to sodium ions. The image below is from de Silva’s homepage. It shows the use of a crown ether in a PET sensor that selectively senses for sodium. The principle is the same as above (except this is now an OFF-ON sensor – you should sketch out the energy levels to see how emission is turned on in the presence of sodium ions)

Fluorescent sensor for sodium, with emission spectra showing an incease in emission on additional increments of sodium concentration (From AP de Silva, used with permission)

Fluorescent sensor for sodium, with emission spectra showing an incease in emission on additional increments of sodium ion concentration (From AP de Silva website - link in text, used with permission)

Now that the general principle is understood, it is easy to get selective across a wide range of ions. By modifying the size of the crown ether, the sensor can lose its selectivity for sodium ions and move on to potassium ions (see image below). Various minor modifications can make it calcium selective. Very quicky, the range of ion selectivities can be acquired.

An example of a sensor that is potassium ion selective (because of the larger crown ether diameter compared to the sodium ion equivalent, above), from de Silva et al, Dalton Transations - used by permission of the Royal Society of Chemistry - see link below)

An example of a sensor that is potassium ion selective (because of the larger crown ether diameter compared to the sodium ion equivalent, above), from de Silva et al, 2003, Dalton Transations - used by permission of the Royal Society of Chemistry - see link below)

The principle can also be applied as a pH monitor, by developing a proton sensitive detector. In this case, amines linked to an anthracene molecule via a spacer make for a very simple but effective pH sensor. An example is shown below – a clever attribute of this design is that the use of two amines mean that much greater sensitivity across the range of pH is “built in” to the molecules design. The emission at various pH values is shown alongside the molecule. You should aim to sketch out the appropriate changes to the molecule on increasing acidity resulting in this change in emission. As well as simple pH monitors, these types of materials have been used in examining the effectiveness of cancer treatment. Cancer cells respond to treatment by developing acidic environments, and by examining the emission of a pH sensor on treating with radiation. The images, available in fig. 3 in the original paper show increased pH, easily detected, indicating that treatment is having an effect.

An amine-anthracene pH sensor, with changes in emission in at different pH's shown. (Source unknown)

An amine-anthracene pH sensor, with changes in emission in at different pH's shown. (Source unknown)

As well as cations, a lot of work has been invested into developing anion sensors. Gunnlaugsson, based in Trinity College Dublin is a leading researcher in this area and a suummary of many of his results can be found in the reference at the end. A video demonstrating the effect will be posted here September 09.

4. Conclusion

The principle of photo-induced electron transfer can be utilised in the development of molecular sensors. The mechanism of PET was examined above, as well as looking how shutting off or turning on this process provides information on the nature of ions interacting with the sensor. Sensors can be designed so that they  selectively sense an ion in the presence of others. Current research in this area is looking at using these principles in molecular computing, and this work will be surveyed in a future article.

[Aug 2009 – updates/amendments will be logged in comments.]

5. References

Newer optical-based molecular devices from older coordination chemistry, de Silva, A. P., McCaughan, B., McKinney, B. O. F. and Querol, M, Dalton Trans., 2003, 1902 – 1913. Really excellent overview of the area with lots of examples.

Luminescent sensors and switches in the early 21st century, Callan, J. F., de Silva, A. P. and Magri, D. C., Tetrahedron, 61, 8551 – 8588. Overview of  developments in 21st Century across range of analyte types (cations, anions, molecules)

Anion recognition and sensing in organic and aqueous media using luminescent and colorimetric sensors, Gunnlaugsson, T., Glynn, M., Tocci (nee Hussey), G. M., Kruger, P. E. and Pfeffer, F. M., Coord. Chem. Rev. , 2006, 250, 3094–3117. Good overview of anion sensors.

Dye-Sensitized Solar Cells (DSSC)

Increasingly, solar energy has a vital role to play in providing energy to cater for an ever increasing demand. This article looks at what dye-sensitised solar cells are and their current technological status, as well as what needs to be done to make them big hitters in the energy game. It summarises some recent reviews on the topic, interested students are pointed to the source material and other references at the end of the article. [Aug 2009]

1. Introduction

A FEW YEARS AGO when talking about dye-sensitised solar cells in lectures, a student asked me what the problem with them was. Gratzel had published his seminal paper in 1991, and now, over 15 years later, from an outside observer’s perspective, there wasn’t much progress in terms of developing a commercial applicable devide. It’s a good question, and one worth asking periodically of any innovation. We hear that drugs often take 10 – 15 years from bench to patient, so one might reasonably ask with DSSC – “What’s the delay?“.

Here’s a short podcast (with audio) introducing this section outlining what this article will cover:
Vodpod videos no longer available.

2. Context

Now that we are near the end of the first decade of the 20th century, future energy demands make for sobering reading. The world currently uses about 13 terawatts (TW) of energy, and it is predicted by 2050, an additional 10 TW will be required. Not only that, the additional energy required will be have to be carbon neutral. Not only that, we are heading for peak oil. Considering all of this, what is the role of solar energy?

Kamat (2007) has summarised the role of solar energy very clearly with data that… ahem… blows other alternative/renewable energy sources out of the… ahem… water. Of the 10 TW additional energy required, building 1 GW nuclear power station every day for the next 50 years would meet the demand. (Nuclear isn’t strictly speaking renewable.) Hydroelectric could provide about 0.5 TW, tides and oceans could chip in 2 TW and wind power could blow 2 – 4 TW our way. Solar energy striking the earth amounts to 120,000 TW, yet only 0.01 – 0.04% of current energy usage is derived from solar sources. (Approximately 13% of current energy needs being supplied by renewable sources.) In theory, one hour’s solar irradiation is enough to supply a year’s global energy demands.

How would increasing the role of solar energy manifest itself on the ground? Solar flux at ground level is approximately 340 W/m2 in the world’s sunniest areas. Assuming 10% efficiency, each metre-squared of solar cell could generate 34 W. Plugging in the numbers, you’d be looking at a mere 4 x 10^11 m2, or about 618 km square to address the increase in world energy needs by 2050. While large, it’s not unrealistic. (Dublin county has an area of approximately 115 km square). The map below shows how, based on average sunlight irradiance measured over 1993-1994, how placement of solar energy “stations” at various locations around the planet would provide a substantial amount of solar-derived energy.

Total Primary Energy Supply: Required Land AreaRequired land area to supply an average of 18 TW (by Matthias Loster, 2006, reproduced with permission, full details on source website)

3. Overview

Dye-sensitised solar cells are as a concept, ingeniously simple. The idea was first conveived in the late 1970’s but since a Swiss photochemistr, Michael Gratzel, published a Nature paper in 1991 reporting 7% efficiency, the interest in the systemhas grown enormously. The outline of a DSSC is shown below and discussed in detail in section 4. Light harvesters gather in energy from a solar/light source and pass on the energy to an electrical circuit which does work (how do you think this compares with nature’s way of generating energy from sunlight?!).

Schematic of a Dye-Sensitized Solar Cell (DSSC) showing energy levels and electronic transitions

Schematic of a Dye-Sensitized Solar Cell (DSSC) showing energy levels and electronic transitions

Light harvesting dyes absorb solar radiation incident on them. This results in excitation of these molecules, who pass the energy obtained by means of transferring electrons onto a nanocrystalline TiO2 substrate onto which they are adsorbed. The electrons, now in the conduction band of titanium dioxide, conduct around a circuit and do work. At some counter electrode, a redox couple is utilised (usually iodide-triiodide) to regenerate the dye so the process can occur all over again. Assuming the dye is efficient at harvesting light, the transfer of electrons to titanium dioxide is efficient, the conduction of electrons in the circuit displays good potential and the dye can regenerate multiple-million times before being degraded, the concept works well with reasonable efficiency (~7 – 11%).

Slideshow: Electronic Pathways in Dye-sensitised Solar Cells:

The chemistry involved is sandwiched between two sheets of conducting glass, coated with a conductive layer (e.g. ITO) which is transparent. One plate of glass (working electrode) is coated with titanium dioxide nanoparticles that have the dye adsorbed onto the surface and the other (counter electrode) is coated with a catalyst (platinum or carbon). The plates contain electrolyte solution between them with the redox couple to regenerate the dye.

gratzel cell schematic showing forward and reverse processes

Forward and reverse electronic pathways in a dye-sensitized solar cell

Here’s a video of a DSSC in action:

4. Developing the idea further

The efficiency of dyes can be measured by consdering how many absorbed photons result in electron injection and how many of these injected electrons are collected to be used in the electrical circuit. This is expressed according to Equation (1), where IPCE(λ) is the incident photon to current efficiency – a measure of how many photons translate into electrical current.

IPCE(λ) = LHE(λ) × Φ(inj) × η(c)

The light harvesting efficiency (LHE) is the fraction of photons absorbed by the dye at a particular wavelength. The electron injection efficiency (Φ(inj)) is a measure of how many absorbed photons result in an injected electron into the semi-conductor and the charge collection efficiency is a measure of how many of these injected electrons are collected for electrical use. The equation essentially maps out each of the processes in the cell and considers their efficiency. All of the processes in the DSSC are kinetic – their efficiency is determined by how fast they occur relative to competing processes. We’ll consider below the various components of the dye and identify where any efficiencies could be improved upon in future developments. This section is based mainly on Hupp (2008 and subsequent more recent articles).

4.1 Dye Characteristics

The light harvesting dye is clearly a crucial component of the cell design and needs to fulfil several criteria; adsorption onto metal surface, overlap effectively with solar spectrum, inject electrons efficiently into metal oxide and be stable for many million cycles.

Adsorbtion of the dye onto the metal oxide surface is facilitated by incorporating a substituent that will adsorb readily. The most efficient studied are ruthenium dyes with carboxyl-substituted ligands – these carboxyl substituents adsorb onto the dyes surface.

run3-tio2

Solar Spectrum and Ru-N3 dye (left-most red spectrum) Additional spectra are predicted overlap from changing dye energygap (Hupp el at, 2008 Reproduced by permission of the Royal Society of Chemistry - original article linked below)

Top - Ruthenium based "N3" dye adsorbed onto a titanium dioxide surface; Bottom: Solar Spectrum and Ru-N3 dye (left-most red spectrum) Additional spectra are predicted overlap from changing dye energygap (Hupp el at, 2008 Reproduced by permission of the Royal Society of Chemistry - original article linked below)

The spectral overlap with the solar spectrum should be maximised so that as much of the sun’s energy as possible is utilised in exciting the dye, and promoting a high density of electrons into the excited state. In practice, dyes absorb in the visible and near infrared region (about 400 – 700 nm), capturing about half the available power and a third of the available photons from solar source. The ruthenium complexes which are currently “best in show” do have a limitation in that their exctinction coefficients are comparatively low (1 – 2 x 10e4 M-1 cm-1), requiring several hundred monolayers which in turn requires the metal oxide support to have a very high surface area. To achieve efficiencies of >15%, DSSCs will need to absorb bout 80% of light between 350 – 900 nm. Therefore using materials that have a higher absorption capacity may be a useful future strategy. Research here has included using osmium in place of ruthenium, which extended the absorption further into the red and enhanced the response of the cell to light relative to the ruthenium analogue. The 1MLCT to 3MLCT transition in osmium is much more intense than in ruthenium. Organic dyes have also been used successfully as attested by the very many articles and school projects on using fruit berries as the dye in these cells. A range of dyes are shown below, and while they vary in chemical structure, you should note a common factor between them and between these and the ruthenium dye shown above. Organic dyes have much larger extinction coefficients (5 – 20 x 10^5 M-1 cm-1), albeit across a narrow range than the ruthenium counterparts. (It seems that dyes will either absorb moderately well across a broad range or very well across a narrow range!)

dssc_dyes

Organic dyes utilized in (a) 9% efficient indoline, (b) 6.5% efficient coumarin, (c) 5.2% efficient hemicyanine, (d) 4.5% efficient squarine, (e) 7.1% efficient porphyrin, and (f) 3.5% efficient phthalocyanine-based DSSCs. (From Hupp et al, Reproduced by permission of the Royal Society of Chemistry - Original Article linked below)

In order for electronic transfer to be energetically favourable, the excited state energy of the dye should be higher in energy than the conduction band of the semiconductor. As well as this, the kinetics of electron injection into titanium dioxide should be faster than recombination of the dye (by luminescence or non-radiateive decay). This isn’t a problem with the N3 dye, above which injects on a femtosecond timescale and decays at a much more leisurely sub-picosecond timescale. (You might consider in your studies how this data would be determined experimentally). Electrons are injected from both the 1MLCT (on a sub-ps timescale) and the 3MLCT, which is formed within 100 fs by intersystem crossing (due to the heavy atom effect of ruthenium). Recent research (Durrant, J. Am. Chem. Soc., 2009, 131, 4808) has questioned whether similar difference in rates are observed in real DSSC (as opposed to model systems), based on results showing that in these real systems, the electron injection slowed down to ps timescale, allowing recombination to be competitive. Nevertheless, it is considered that this process is efficient, although caution is required in ensuring that efficiency isn’t reduced by other design factors on the cell.

Timescales involved for electron injection

Timescales involved for electron injection from Ru-dye to TiO2

Considering the discussion above regarding extending the dye’s absorption further into the red, this could be achieved by lowering the LUMO of the dye, although researchers have been reluctant to do this as since the LUMO-CB transition is so fast, the energy levels must be very well matched. (Goldilocks Principle: Not too high, not too low, just right).

Very recent research at Stanford University has coupled luminescent chromophores which absorb high energy photons and pass their energy on to the sensitising dye (Nature Photonics, 2009, 3, 406 – 411) – I’ll put more on this in a future article.

4.2 Metal Oxide Support

Following sucessful injection into metal oxide, the next phase is for the elctron to percolate through the oxide layer onto the working electrode. TiO2 is the most common substrate used. As a chemical, it is relatively inert, cheap and can be synthesised via the sol-gel process (offering flexibility and scalability). Unlike silicon solar cels, very high purity is not required. It exists in various forms, mainly anatase, rutile and brookite. Anatase has a high band gap (3.2 eV compared to rutile’s 3.0 eV) which gives several advantages. It absorbs very little of the solar spectrum, meaning it is transparent to incoming light source (so that the dyes rather than the metal oxide is activated by light). In addition, the larger band gap than rutile means that recombination is slower (ref the energy gap law) – some reports have determined a 30% lower efficiency with rutile.As with the dye, there are several factors to consider to maximise efficiency.

Gratzel’s innovation in his 1991 paper was among other things to use nanocrystalline TiO2. The nanomaterial’s much greater surface area meant that many more molecules of dye could adsorb onto the surface and pushed efficiency to the best reported at the time: 7%. Nanocrystalline surfaces in these cells are reported to have a surface: geometrical area of ca. 1200. Best performers actually have two distinct layers of metal oxide: a 12 micron thick transparent layer of 10 – 20 nm sized particles covered with a 4 micron thick layer of much larger (400 nm) particles. The larger particles scatter photons back into the film.

One significant limitation of this model is the very slow time taken for electrons to percolate through the material and onto the transparent conducting electrode; which is of the order of 100’s of microseconds, in comparison with conduction band-dye recombination which is of the order of a couple of microseconds. (It is however faster than the other decay process: conduction band-redox couple, which is in the millisecond range). Because of this, researchers are looking at improving the dynamics of electron percolation through the film. One strategy is to use nanotubes/rods rather than particles. These have the disadvantage of having lower surface area, but an advantage of very much improved electron transport because they provide an physical electron pathway from the particle to the electrode. Pagliaro (2009) gives a nice summary of this and this and other work on ZnO nanorods will be summarised in a future article.

4.3 Electrolyte and Regeneration

The electrolye contins the redox couple which regenerates the oxidised dye whcih were formed byinjection of electron from the dye to the titanium dioxide layer, leaving D+. The redox couple has a difficult job – it must be very efficient at reducing the dye cation back to the original state for another cycle, but not intercept or capture one of the electrons being injected in the first place! The latter process, which would result in inefficiency involves transfer of an electron from the conduction band of titanium dioxide to the redox couple (to the triiodide ion I3-). Of the range of redox couples studied, iodide-triiodide has proved to be in the most efficient  cells, mainly because the transfer of electron from titania to trioidide is, as mentioned above, very slow (in the millisecond time range). One of the problems in trying to look for improved systems is that the redox chemistry of this redox couple is not well understood, so it is difficult to plan effective alternative dyes. One interesting approach is to use solid state redox couples, which allow for higher concentrations of the redox couple and extending the applicability of the device (because of removal of the liquid electrolyte).

5. Summary and Review

Dye-sensitized solar cells offer enormous potential as an alternative renewable energy provider. The principle of operation is for a light harverster to absorb light efficiently and pass on the energy to a metal oxide surface, which links in to a circuit generating current. The dye is regenerated at the counter-electrode via a redox couple. In reviewing dye sensitised solar cells, you should consider the two main themes covered in this article:

  1. Describe the principle of operation outlining the pathway the electron follow from light absorption to dye regeneration. Using the diagram below, represent these processes on an energy level diagram and for each of the stages outline the counter process available resulting in efficiency. By using time constants/scales, indicate whether these processes are likely to significantly impact on the efficiency of the cell.
  2. For each of the processes, outline improvement strategies that are being considered in current research in the area, indicating the rationale for such approaches.
Kinetic Processes in DSSC - you should consider the relative times for the various processes involved

Kinetic Processes in DSSC - you should consider the relative times for the forward(green) and reverse processes (purple arrows) involved

Addendum:

This diagram below, from Hupp et al., Chem. Eur. J., illustrates effectively the rates of competing processes:

Normalised rates of competitive processes in each stage of the DSSC. Note the "double hump" for charge injection - you should relate this to the schematic for charge injection above. (J. T. Hupp et al, New architechtures for dye-sensitized solar cells, Chem. Eur. J., 2008, 14, 4459. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.)

Normalised rates of competitive processes in each stage of the DSSC. Note the "double hump" for charge injection - you should relate this to the schematic for charge injection above. (J. T. Hupp et al, New architechtures for dye-sensitized solar cells, Chem. Eur. J., 2008, 14, 4458. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.)

Further Reading/References

  1. Meeting the clean energy demand: nanostructure architechtures for solar energy conversion, P. V. Kamat, J. Phys. Chem. C, 2007, 111, 2834 – 2860. Excellent article on the wide variety of roles nanotechnology may play in providing future energy needs. This paper was used to provide much of the context. Prof. Kamat’s website is also worth looking at. Google “Light Energy Conversion” and this is the web-page that comes up, with good reason.
  2. Advancing beyond current generation dye-sensitized solar cells, Hamann, T.W., Jensen, R. A., Martinson, A. B. F., Van Ryswyk, H and Hupp, J. T., Energy Environ. Sci., 2008, 1, 66 – 78. An excellent paper on the properties of DSSC and how they may be technically advanced in future developments. Section 4 is substantially based on this article. Link
  3. Dye-sensitized solar cells: a safe bet for the future, Gonçalves, L. M., de Zea Bermudez, V., Aguilar Ribeiro, H. and Magalhães Mendes, A, Energy Environ. Sci., 2008, 1, 655 – 667. A concise overview on the device aspect of DSSC, considering current and future implementation requirements.
  4. Nanochemistry aspects of titania in dye-sensitized solar cells, Pagliaro, M., Palmisano, G., Ciriminna, R. and Loddo, V., Energy Environ. Sci., 2009, 2, 838 – 844. Good overview of the photoanode requirements and research developments.
Now more than ever, solar energy has a vital role to play in providing energy to cater for an ever increasing demand. This post looks at what dye-sensitised solar cells are and their current technological status, as well as what needs to be done to make them big hitters in the energy game. It summarises some recent reviews on the topic, interested students are pointed to the source material and other references at the end of the article.

Published August 2009. Subsequent additions/amendments/corrections will be logged in comments.