Fluorescence Quenching and Non-Radiative Relaxation
Photoluminescence occurs when electrons radiatively relax from their photoexcited states. Emissions resulting from singlet-singlet transitions are known as fluorescence, however, there are a number of ways in which electrons in these excited states can relax non-radiatively. These are known collectively as fluorescence quenching.
Fluorescence quenching effects the photoluminescence quantum efficiency (PLQE), or photoluminescence quantum yield (PLQY), of a material. This is a measure of how many photons that the material emits compared to how many it absorbs, i.e. the efficiency at which absorbed light is re-emitted. If a material can only undergo radiative relaxation, it will have a photoluminescence quantum efficiency of 100%. However, if non-radiative relaxation (fluorescence quenching) is also possible, the PLQE will be less than 100%.
There are a number of different mechanisms through which quenching can occur. The main different types of fluorescence quenching are vibrational relaxation, internal conversion, intersystem crossing (ISC), Förster resonance energy transfer (FRET), Dexter electron transfer, and radiative energy transfer.
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Vibrational Relaxation and Internal Conversion
When, through the absorption of a photon, an electron is promoted to a higher vibrational state (v1, v2, v3, …), often the first relaxation process that occurs is vibrational relaxation (10-14 - 10-10 s). In this case, the electron will relax to the v0 vibrational energy level of the same electronic state, and the excess vibrational energy will be lost to other vibrational modes as kinetic energy. This occurs with either the same molecule or different molecule.
If there is sufficient overlap between vibrational modes of different electronic levels, it is possible for the electron to make this transition in a process known as internal conversion. Internal conversion is, however, very unlikely to facilitate a full relaxation to the ground state due to the large energy gap between the S1 and S0 levels.
Intersystem crossing (ISC) is a radiationless process through which an electron moves between two levels of the same energy but at a different multiplicity. For example, this can occur between an excited singlet state (S=0, multiplicity = 1) and an excited triplet state (S=1, multiplicity = 3). In this case, the electron will essentially flip its spin. This is more likely to happen if the vibrational levels of the two states overlap, meaning no or very little energy will be lost or gained in the transition.
In the case of ISC from an excited singlet to an excited triplet state, the electron can then relax radiatively to the singlet ground state through phosphorescence. Molecular oxygen, due to its unusual triplet ground state, is capable of enhancing the rate of ISC, resulting in fluorescence quenching.
In the Jablonski diagram below, a photon is absorbed, exciting a ground state electron to an excited vibrational state of the second singlet excited state, S2 (blue straight arrow). It then relaxes through internal conversion to an excited vibrational state of S1 (gold wavy arrow) and further relaxes to the ground vibrational state of S1 through vibrational relaxation (left-hand green wavy arrow). The electron then undergoes intersystem crossing (purple wavy arrow) to an excited vibrational state of the first excited triplet state (T1) and undergoes vibrational relaxation once more (right-hand green wavy arrow) to the T1 vibrational ground state. After a time (can be seconds or even hours), the electron relaxes back down to an excited vibrational state of the singlet ground state, S0, via phosphorescence (straight red arrow).
Förster Resonance Energy Transfer
Förster resonance energy transfer (FRET, also known as fluorescence resonance energy transfer) is an energy transfer process that occurs between two light-sensitive molecules: a donor and an acceptor. If the donor molecule is excited, it may transfer energy non-radiatively to the acceptor molecule as it relaxes back down to the ground state. The excited acceptor can then relax radiatively back to its own excited state, thus emitting a photon. It is very important to note that the donor does not emit a photon during this process and that the energy transfer is a non-radiative process.
In the Jablonski diagram below, the donor absorbs a photon (solid blue arrow), exciting an electron to an excited vibrational state of S1. Then, it will undergo vibrational relaxation to its vibrational ground state (green wavy arrow) and relax non-radiatively (red dashed arrow). This process will cause it to transfer energy to the acceptor (black dashed arrow), which excites a ground state electron to an excited vibrational state of the S1 level (blue dashed arrow). It then relaxes to the vibrational ground state of S1 (green wavy arrow) and relaxes radiatively to S0, emitting a photon (red solid arrow).
The efficiency of the FRET mechanism (quantum yield) depends on:
- The distance between the molecules (the efficiency is inversely proportional to the sixth power of their separation).
- The degree of overlap of the donor’s emission spectrum and the acceptor’s absorption spectrum.
- The relative orientation of the donor emission and acceptor absorption dipole moments.
Because of the strong distance-dependence, the FRET efficiency can be used to determine the separation of the donor and acceptor in the range of 1-10 nm. It is sometimes called long-range energy transfer as it occurs over longer distances than Dexter energy transfer. Often, FRET is used for investigating biological structures and their dynamics.
Dexter Electron Transfer
Dexter electron transfer (or Dexter energy transfer or exchange energy transfer) is a mechanism by which electrons are exchanged between a donor and an acceptor when they are in physical contact. This exchange can occur either between two different molecules or two parts of the same molecule. As the process relies on the overlap of the electron wavefunctions (orbitals), Dexter electron transfer occurs over much smaller distances than FRET, which relies on dipole-dipole interactions. Hence, Dexter electron transfer will decay exponentially with separation and has a range up to only about 10 Å (0.1 nm). Because of this, it is sometimes referred to as short-range energy transfer.
During Dexter electron transfer, an electron in an excited state of the donor will be transferred to an excited state of the acceptor. Simultaneously, an electron in the ground state of the acceptor will be transferred to the ground state of the donor. In this way, the total number of electrons in each molecule remains constant. This exchange can occur either between singlets or triplets, as long as the multiplicity is kept constant. As with FRET, the donor emission and acceptor absorption spectra must overlap.
The left-hand diagram below shows a singlet-singlet exchange, and the right-hand panel shows the equivalent triplet-triplet exchange. The black arrows represent the initial state of the electrons (and the final state for those that don’t partake in the transfer process), the red dashed arrows represent their movement between donor and acceptor, and the grey arrows represent their final state.
Triplet-triplet annihilation (TTA)
A special case of Dexter electron transfer is known as triplet-triplet annihilation (TTA). In this process, if both the donor and acceptor are in an excited triplet state, they can undergo electron exchange to produce two singlet states. Again, the donor transfers its excited electron to an excited state of the acceptor, and the acceptor transfers its ground state electron to the donor’s ground state. However, in this case, the electron that started in the LUMO of the acceptor will relax to the HOMO to leave the acceptor in a higher energy excited singlet state.
In the left-hand diagram below, the black arrows represent the initial state of the electrons (and the final state for those that don’t partake in the transfer process). The red dashed arrows represent their movement between donor and acceptor, and the grey arrows represent their final state. The right-hand diagram represents the final state of the system.
One important application of TTA is TTA photon up conversion. This method allows two low energy photons to be “converted” into a single high energy photon. This is important in increasing the efficiency of solar cells, by allowing the absorption of photons with energy lower than the bandgap.
Radiative Energy Transfer
Radiative energy transfer occurs when an excited donor molecule relaxes to the ground state and releases a photon which is subsequently reabsorbed by an acceptor molecule. Therefore, for this mechanism to occur, the emission spectrum of the donor and the absorption spectrum of the acceptor must overlap. Radiative energy transfer typically occurs over a distance of tens of nm. The Jablonski diagram for radiative transfer is similar to that of FRET, except the dashed lines representing non-radiative transitions become solid lines representing radiative transitions.
In the Jablonski diagram below, the donor absorbs a photon (left-hand blue arrow), exciting an electron to an excited vibrational state of S1. It then undergoes vibrational relaxation to the vibrational ground state (left-hand green wavy arrow) and relaxes radiatively (left-hand red arrow), releasing a photon. The photon is absorbed by the acceptor, exciting a ground state electron to an excited vibrational state of the S1 level (right-hand blue arrow). It then relaxes to the vibrational ground state of S1 (green wavy arrow) and relaxes radiatively to S0, emitting a photon (right-hand red arrow).
Other Quenching Mechanisms
Other quenching mechanisms include:
- Static (or contact) quenching, in which two ground state molecules couple to form a non-fluorescent (dark) complex with a different absorption spectrum to either of the constituent molecules.
- Collisional quenching (or external conversion), in which a “quencher” molecule collides with an excited molecule, causing it to relax non-radiatively to the ground state. For example, this could occur in solution due to collisions between the organic and solvent molecules.
- Quenching through impurities and defects, in which electrons can get trapped in disorder states within the bandgap.
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