Quenching Mechanisms

Excited states can be deactivated in several ways – they can emit, giving off light energy, deactivate – resulting in a “vibrationally hot” ground state (i.e. energy loss as heat) or be quenched by another molecule. In this section, we will consider the process of quenching, and outline some ideas that use the process of quenching in applications. In addition, we will examine how the process of quenching can be studied to give us information on the nature of the excited state-quencher interaction. It is assumed the reader is familiar with the information presented in the Light Absorption and Fate of Excited State post.

Overview

Quenching of the excited state is a significant process because it is usually a very efficient process. The excited state of many organic compounds, for example, are efficiently quenched by the presence of oxygen, at rate constants several orders of magnitude faster than emission processes from the triplet state. (Emission from the triplet is spin forbidden, and hence has rate constants in the range 10 to 103 dm3 mol-1 s-1, whereas oxygen quenching may take place at rate constants of the order 109 dm3 mol-1 s-1. Therefore, to study the emission from triplets, we need to deaerate the sample (and have it at low temperature – see the experimental section). Quenching processes can occur by two processes – electron transfer or energy transfer. In both cases, the excited state energy of the luminophore (the luminescent species) is deactivated due to the presence of the quencher. There are two scenarios by which quenching is generally modelled, and these are discussed below.

Dynamic Quenching of an Excited State

If a solution with emitting species is studied, and for every 100 photons absorbed by the solution, 30 are re-emitted, the quantum yield of emission is said to be 0.3. What happens to the other 70? They are translated into radiationless transitions, such as deactivation. As mentioned in the Ruthenium polypyridyl photochemistry post, we can quantify the quantum yield of emission (or any process) as being the rate constant of that process (in this case emission) divided by the sum of all rate constants deactivating the excited state. If we divide the emission quantum yield in the absence of quencher by that in the presence of quencher, we can generate an expression known as the Stern-Volmer equation, as shown below.

SV_quenching

Derivation of the Stern-Volmer Equation based on considering rate constants of deactivation in the absence and presence of quencher

The Stern-Volmer equation models what is called dynamic quenching, quenching which occurs by the quencher diffusing through solution and interacting with luminophore, resulting in a deactivation of the excited state. The emission intensity is reduced, because as well as other deactivation pathways before the presence of quencher, the presence of quencher now adds another deactivation pathway in competition with luminescence. This quenching process is controlled by how fast the quencher can diffuse through solution and “collide” with luminophore, and as diffusion is usually a very fast process in solutions, it can be very efficient.

The Stern-Volmer equation is the equation of a straight line, and hence it allows for  very easy experimental determination of the quenching rate constant, kq. If the emission intensity (or lifetime) in the absence of quencher and then in the presence of incremental amounts of quencher is measured, and the resulting ratio of emission intensities (I(0)/I) is plotted as a function of quencher concentration, the resulting graph (called a Stern-Volmer plot) will have an intercept of 1 and a slope called the Stern-Volmer constant, KSV. KSV is the product of the natural radiative lifetime (the lifetime in the absence of quencher, τ0, and the quenching rate constant, kq. Knowing the slope and the natural radiative lifetime allows easy calculation of the quenching rate constant. An outline of a common experiment – quenching of a ruthenium polypyridyl complex emission with Fe3+ is shown below.

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The fact that quenching can be so efficient means that it can be a useful probe in studying systems with emission properties. For example, ruthenium polypyridyl complexes have been used successfully as oxygen sensors, whereby the complex has been incorporated into a silica matrix and the resulting stub located inside packaging. In the absence of oxygen, emission is observed when the stub is irradiated with light. However, if oxygen leaches into packaging, the emission observed will be substantially reduced, as it will be quenched by the oxygen. By calibrating the reduction in intensity using a Stern-Volmer plot, it is possible to estimate the concentration (partial pressure) of oxygen in the system. The concept has applicability in food packaging and for containers holding oxygen sensitive artefacts (e.g. paintings).

Static Quenching

Dynamic quenching results from collisions between excited state and quencher. However, if the quencher is somehow associated with the luminophore in solution prior to light absorption, the association may mean that the luminophore will not emit, due to induced changes in its properties because of presence of quencher. Therefore the reduction in emission intensity will be affected by the extent to which the quencher associates to the luminophore and the number of quenchers present. The reduction in emission intensity can be quantified as follows. If the luminophore, M, associates with quencher, Q according to an equilibrium constant of association, Ks, then this association constant can be quantified as the ratio of associated luminophore-quenchers luminophore-quenchers moieties ([M-Q]) to the product of unassociated luminophore and quencher; [M][Q]. Since the total concentation of luminophore, [M]0 is equal to the sum of associated and unassociated luminophore, substitution of this into the equilibrium expression, followed by rearrangement results in another equation of a straight line, very similar in form to the Stern-Volmer equation. However, while plotting I0/I (as emission intensity can be said to be proportional to concentration) against [Q] will result in a straight line for static quenching, analogous to dynamic quenching, interpretation of the slope is different. In this case, the slope quantifies the association constant between quencher and luminophore – and therefore is useful in providing information on how these two species interact in the ground state.

static-quenching

Derivation of an expression for static quenching

Dynamic or Static?

The question that immediately arises now is that if plots of emission intensity against quencher concentration both produce straight line graphs, how do we know which type of quenching is occurring? The answer lies in thinking again about the nature of each type of quenching. For dynamic quenching, all luminophores are affected by the quenching process as it is probable that they will all collide with a quencher during their excited state lifetime, so both emission intensity and lifetime reduced on increasing quencher concentration. For static quenching by association, only luminophore-quencher associations result in reduction in emission, unassociated luminophores are free to luminesce as if there was no quencher present. Increasing quencher concentration affects emission intensity, because there are more associations, but not emission lifetime, as the unassociated luminophores can emit in the absence of quencher. (Note that these two scenarios are the extremes, and there are cases where a mixture of both static and dynamic quenching may occur simultaneously.)

dynamic_versus_static_quenching

Schematic of dynamic versus static (association) quenching

Therefore the diagnostic test for assigning whether a quenching mechanism is dynamic or static is to compare how the emission intensity and emission lifetime changes as a function of increasing concentration. In the case of dynamic quenching, plots of relative emission intensities and emission lifetimes will be th same, changing on increasing quencher concentration. For static quenching, only a plot of relative emission intensity will change, the emission lifetime plot will have  slope close to zero.

diagnostic_plots

Model diagnostic plots to distinguish between dynamic and static quenching

Another model of static quenching is where the quencher is in a fixed position close to the luminophore (e.g. in a frozen matrix or a zeolite). This is modelled by the Perrin model of quenching, which will be discussed in the experimental techniques section when discussing phosphorescence.

References

MK Seery, N Fay, T McCormac, E Dempsey, RJ Forster, TE Keyes, Photophysics of Ruthenium Polypyridyl Complexes formed with lacunary polyoxotungstates with iron addenda, Phys. Chem. Chem. Phys., (2005), 19(7), 3426 – 3433. An example showing unusual static quenching between a quencher (large polyoxometallate clusters) and a luminophore (a ruthenium complex).

B Valeur, Molecular Fluorescence: Principles and Applications, Wiley: Weinheim, 2002. Discusses the principles of dynamic and static quenching well.

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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.