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.
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.
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.
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.
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.
- Molecule absorbs light and populates upper excited state S* with electrons
- Electrons in upper vibrational levels of S* undergo vibrational relaxation and the electrons move to the lowest vibrational level of S*.
- 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”.
- 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
- It may undergo fluorescence, giving off a photon of energy.
- It may undergo internal conversion as above.
- 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.
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.
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.