Photodynamic Therapy: An overview

The use of light in medical treatment is nothing new. In the ancient world, roots of the plant Dorstenia were applied topically on to irritated skin which would clear following a few hours of sunshine. The active ingredient, psoralen, is now the basis of PUVA (psoralen + UVA) therapy used to treat the effects of psoriasis and other skin ailments. Psoralen acts by intercalating between base pairs of DNA and upon UV irradiation, the two double bonds form [2+2] cyclo-adducts with thymine, kinking and destroying the DNA of the replicating cells, which was causing the skin irritations in the first place.

In the early twentieth century, following significant progess in synthetic chemistry of coloured dyes throughout the nineteenth century, two German scientists completed work on the toxic effect of eoisin, a methylene based dye. The effect was only noted with the presence of light, and at a later date the presence of oxygen. Thus the modern day science of photodynamic therapy (PDT) was discovered, consisting of three components: photosensitising dye, light, and oxygen. Despite positive results from trials, the work went relatively unnoticed, and it wasn’t until the 1970s that it really picked up again.

Fundamentals

The basic principles of PDT are relatively easy to consider. A light absorbing dye is applied or injected into the patient and after a time appropriate for maximum uptake into tumour, the affected area is irradiated with light. The dye absorbs the incident light and an electronically excited state is formed. This subsequently generates a reactive oxygen species (ROS) which destroys the tumour. The concept is beautifully simple, and as the dye is non-toxic in the absence of light, does not carry the negative effect of traditional chemotherapies which are much less discriminate in their action. In PDT, only irradiated areas of body tissue will generate activity leading to cell destruction. As mentioned, there are three components to PDT: light, photosensitiser and oxygen. the latter two are considered in turn, below.

Oxygen

The net result of dye irradiation is the generation of reactive oxygen species, and it is generally considered that singlet oxygen 1O2 is the ROS responsible for cell destruction in PDT. 1O2 is formed when oxygen, which in the ground state exists in as a triplet (3O2 ) absorbs energy. According to the MO diagram of the ground state 3O2 shown, the singlet is formed with the pairing of electrons in the LUMO. This is energetically unstable relative to the ground state, as there is a vacant orbital of the same energy available to the paired electron. Hence, 1O2 is energetically higher (thus more reactive) and will return to its triplet ground state. The lifetime of the singlet state in a cell is of the order of 100s of ns, and it has been estimated that it can diffuse less than 50 nm in this time. Therefore, in cells of the order of microns, the action is limited to cellular dimensions.

1O2 is formed by energy transfer from the photoexcited dye. But an alternative is possible. If the photoexcited dye transfers an electron to molecular oxygen, a superoxide anion is formed. Therefore, a crucial aspect of drug development in PDT is the nature of the ROS formed. Electron transfer forming superoxide anion is called a Type I reaction. Energy transfer forming 1O2 is called a Type II reaction. We can distinguish between these by understanding the chemistry behind them. Importantly for us here. Type II is detected because as singlet oxygen returns to its triplet ground state, it emits a small amount of infrared phosphorescence, which can be detected (see figure – the emission maximum is approximately 1270 nm). Type I on the other hand, can be detected by monitoring the redox chemistry of Fe3+ and subsequent formation of hydroxyl radicals (the photo-Fenton mechanism).

Singlet oxygen emission

Photosensitiser

The photosensitiser has several functions. It must locate in the tumour. This involves considering both hydrophilic and lipophilic components in the molecular design that are not covered here. It should also absorb in the far visible region (600 – 800 and preferably 700 – 800 nm). Haemoglobin is a significant component of body tissue, and absorbs strongly in the mid visible region (580 nm). This is obvious when we shine a light through our hand – only red light passes through. Therefore the ideal will be one which absorbs light where the body does not, allowing them to be used deep within body tissue (e.g. liver, pancreas).

As it stands, clinically approved PDT drugs are not yet optimised for longer wavelength light absorption, and hence PDT treatment is currently limited to areas that are easy to expose to a light source: skin, lung, oesophagus, etc. This issue of “penetration depth” was the subject of a recent court case, whereby a doctor justifiably claimed that PDT treatment would not be suitable for treatments such as liver cancer, as the liver was just too big for light to pass through. (The doctor was subsequently acquitted of all charges). The bar chart (Ref 1) shows how photosensitiser absorption capacity affects penetration depth, and this is the focus of current research (below). Penetration doubles once light at longer wavelength than the absorption of haemoglobin is achieved (630 nm) and doubles again at 700 nm. An ideal photosensitiser will therefore absorb between 700 – 800 nm.

Other factors for an ideal photosensitiser include: low toxicity in the absence of light and little post-treatment side affects. One of the most significant side effects is post-treatment light sensitivity, whereby patients have to avoid light for fear that healthy body tissue which have residual amount of photosensitiser present will generate unwanted activity.

Jablonski Diagram for generation of singlet oxygen (ref 1)

From a photochemical point of view, one of the most important dye characteristics is that it will form a high concentration of triplet excited state. Since the mechanism of action is generation of singlet oxygen, a singlet state may not be long-lived enough to allow time for reaction with oxygen (singlet-singlet deactivation via fluorscence or non-radiative means is an allowed process, and therefore very fast). If the triplet forms via intersystem crossing, its deactivation is forbidden, and hence slower, allowing the energy transfer to oxygen to be more competitive. This again provides potential for future research (see below).

Photofrin

Photofrin (R)

The first clinically approved PDT drug was photofrin (R). Photofrin is a porphyrin -based compound and you may wish to examine its structure to identify hydrophilic and lipophilic components alluded to above. It absorbs at 630 nm, which is within the PDT window and has a respectable quantum yield of inter-system crossing of about 25%. However, its absorption is not significant, with a low extinction coefficient. Its worth noting here also that PDT is not limited to cancer based therapy, it has also been used as alternative to antibiotics and for gum disease (There is a good overview article in Chemistry World, ref 2, and some nice pictures for dental treatment at the Periowave site, ref 3)

Current Developments

Absorption spectra of chlorins and bacteriochlorins (Ref 1)

Photofrin’s limitation is primarily its light absorption. To get to a point where PDT can become more versatile, the photosensitiser needs first to absorb in the 700 – 800 nm window (and then subsequently satisfy all other demands re singlet oxygen generation, accumulation in tumours….). Reduction of one (chlorins) and two (bacteriochlorins) of the four pyrroles in porphyrin based compounds have been found to shift the wavelength of absorption to longer wavelengths. In the example shown, the absorption shifts to about 700 nm for chlorin-based molecules and to about 800 nm for bacteriochlorin based molecules. The compound shown has a high extinction coefficient of absorption and good oxygen generation capabilities. More information on these compounds is available in Ref 1.

Intersystem crossing can be enhanced by the heavy atom effect, and this is the subject of another class of boron-based compounds. It was noted for certain sites of substitution of iodine, the singlet oxygen generation capacity increased, attributed to an increased intersystem crossing yield caused by the iodine heavy atoms. More information on these compounds is available in reference 4.

Utilising heavy atom effect to enhance ISC (Ref 4)

Summary

PDT provides huge potential in treatment of cancerous tumours and a range of other antibiotic treatments. It has been called a very selective surgeon’s knife thanks to its ability to isolate the affected area for treatment with little collateral damage. At the core of future developments of PDT is an understanding of the photochemistry at its heart, and now a century after the first PDT action was discovered, it looks like it has a positive future.

References

1. (Primary reference for this article) Chem Soc Rev, 2011, 40, 340: Sections A, B, C.1, C.4, D, F.2, F.3

2. Chemistry World, 2012, April, 52, see also Education in Chemistry, 2004, May, 71.

3. Periowave blog (accessed December 2012)

3. Chem Soc Rev, 2013, 42, 77: pages 77 – 81.

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Origins of Flash Photolysis: George Porter

Flash photolysis revolutionised the science of photochemistry by allowing for rapid (and the definition of that term was era-dependent) monitoring of photochemical intermediates. Since its development in the mid-20th century, flash photolysis has been at the centre of studies of photochemical/physical processes. Developed by Porter, working with Norrish at Cambridge, at the microsecond timescale, the instrumentation has now evolved to the femtosecond timescale – nine orders of magnitude faster over four decades. Porter said in 1975 that he anticipated femtosecond spectroscopy within five years, and it was primarily instrumental issues which delayed his vision. His foresight was eventually realised with the work of Egyptian photophysicist Ahmed Zewail, working at Caltech. This first in a sequence of articles covers the historical development of flash photolysis, we will look in future posts at the progress leading up to Zewail’s development of femtosecond spectroscopy as well as outlining how it is used experimentally to study photochemical intermediates.

1. Development of Early Instrumentation

When Ronald Norrish, Goerge Porter and Manfred Eigen were awarded the Nobel Prize in 1967, for studies of extremely fast chemical reactions, effected by disturbing the equilibrium by means of very short impulses of energy, it was an acknowledgement of their pioneering work in developing apparatus to study microsecond chemical reactions in the microsecond timescale. Eigen’s work inolved using sound waves (a form of pressure) to perturb (or distort) systems, subtly, and Porter, working as a student of Norrish’s at Cambridge, used UV flashes to perturb systems creating electronically excited states. Prior to this development, “fast” reaction kinetics were capable of being studied only on the sub-second time-scale using stopped-flow apparatus, which was developed in the 1920’s. The concept was simple in principle – distort the system at equilibrium using a high-energy flash of light and detect how fast the system restores to equilibrium. The difference here from previous approaches to kinetic analysis was that studies, for example in stopped flow, examined how fast systems approached equilibrium on mixing, and hence were limited by how fast mixing could be effected.

Porter has said that his work for the navy during World War II, as a radar scientist using pulses of electromagnetic radiation was the seed for his ideas at Cambridge when he went to work as Norrish’s graduate student after the war in 1945. Having been sent to get a replacement lamp for a torch for experiments he was conducting to study the CH2 radical (the torch was acting as a continuous light source), Porter saw flash-lamps being manufactured at the Siemen’s factory in Preston, UK and in 1947, introduced the idea of using flash lamps as a pulse of energy to “study transient phenomenon”. The second flash (the true genius of the development), after the burst of light creating the transient state, would essentially photograph the transient phenomenon – so the time scale of the flash was crucial. At the time, millisecond measurement was considered “far beyond direct physical measurement”. Flash photolysis would allow liftetimes 1000 times shorter to be measured by 1950.

The flash lamp used by Porter was a high-intensity pulsed lamp used by the Royal Navy at the time for night-time aerial photography. It was contained in a 1 m long quartz tube (2000 μF charged to 4 kV for those interested in electronics). It could be discharged in 2 ms. The probe flash – a less intense light source which would measure the changes in absorption after the initial flash was 50 microseconds, and both the initial flash (pump) and probe were timed using a timing wheel, which can be seen in the photograph of the original apparatus (to the right of the apparatus) below.

Left: Lord George Porter. Image generously donated by Lady Porter, (c) Lady Porter; used with permission. Right: The first flash photolysis apparatus. From Thrush, Photochem. Photobiol. Sci., 2003, 2, 453–454 - used by permission of the Royal Society of Chemistry - see link below)

Left: Lord George Porter at the time of the development of flash photolysis. Image generously donated by Lady Porter, (c) Lady Porter; used with permission. Right: The first flash photolysis apparatus. From Thrush, Photochem. Photobiol. Sci., 2003, 2, 453–454 - used by permission of the Royal Society of Chemistry - see link below)

2. Early Experiments

Interestingly, the first experiments the apparatus was used in have direct relevance to modern science – the study of hydroxyl radicals in hydrogen-oxygen-nitrogen dioxide systems and in the study of the ClO radical in chlorine-nitrogen-oxygen systems. These species and intermediates generated are at the heart of stratospheric chemistry research today. Early transient absorption spectroscopy experiments (see Windsor article in same issue, referenced below) were on triplet-triplet absorption in polyaromatic hydrocarbons (PAHs), again environmentally relevant species today. These studies looked at the kinetics of decay of the triplet state, in the microsecond timescale, and how they were affected by solvent viscosity, presence of oxygen, etc. These studies formed the backbone of a wide and ever-growing research into organic compounds (including my own on enone-alkene cycloadditions some 40 years later!).

Review

It’s difficult for us now to comprehend the true genius exhibited by Porter in his development of flash photolysis. I think it demonstrates magnificent scientific flair, taking together his previous experience, observance of available instruments parts and an obviously great understanding of chemistry, and combining to develop flash photolysis. The development of the technique revolutionised the fields of kinetics and photochemistry, with implications across a huge variety of fields, including, as we have seen above, stratospheric chemistry and organic photochemistry. Porter has sown the seeds for fast very fast and ultimately ultrafast reaction kinetics, which essentially required faster and faster laser pulses to achieve. By the late 1960’s, nanosecond spectroscopy was feasible.

In the next article on this topic, which will be linked here, we will look at Zewail’s work in developing femtosecond spectroscopy.

References

This article is sourced from various articles, as indicated below:

Van Houten, J. A Century of Chemical Dynamics Traced through the Nobel Prizes: 1967: Eigen, Norrish, and Porter, J. Chem. Educ., 2002, 79(5), 548 – 550. Overview of the development of Flash Photolysis in the context of Nobel Prizes in kinetics generally.

Thrush, B. A., The Genesis of Flash Photolysis, Photochem. Photobiol. Sci., 2003, 2, 453 – 454. Short article on the experimental details, as part of a special issue on George Porter’s work and influence in the area. The journal issue generally shows the scope of flash photolysis in photochemistry research today.

Farago, P., Interview with Sir George Porter, J. Chem. Educ., 1975, 52(11), 703 – 705. Great interview with Porter – what comes across so well is his keen interest in science and in promotion of good science communication. Well worth a  read.

Windsor, M. W., Flash photolysis and triplet states and free radicals in solution, Photochem. Photobiol. Sci., 2003, 2, 455 – 458. Wonderful personal account of Porter’s first student (as I can gather) and his work on the development of triplet-triplet absorption spectroscopy.

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.