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.
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