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Photoluminescence: Fluorescence and Phosphorescence


Photoluminescence is luminescence resulting from photoexcitation. In other words, photoluminescence is when a material emits light following the absorption of energy from incident light from another light source. The absorption of energy from a photon excites an electron in the material to a higher energy level, which then relaxes back to a lower energy level. This results in the emission of a lower energy photon. The study of this emitted light is known as photoluminescence spectroscopy.

Much like how photoluminescence is a form of luminescence, fluorescence and phosphorescence are two types of photoluminescence.

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Fluorescence vs. Phosphorescence


The most apparent difference between fluorescence and phosphorescence is that fluorescence generally only occurs while light is incident on the material, while phosphorescent materials can continue to glow for a period of time after the light source has been removed. On the atomic level, electron spin state is maintained throughout fluorescence, but changes direction when a phosphorescent material is excited, before reversing back again when the subsequent emission occurs.

Singlet and Triplet States

In the ground state, the total spin angular momentum of a molecule will be S=0. This is because all electronic orbitals are full (two electrons) and the electron spins in each orbital are anti-parallel (opposite direction). This is known as a singlet state.

When an electron from the HOMO is excited to the LUMO, however, there are then two unpaired electrons and the total spin angular momentum can be either S=0 or S=1. S=0 states are called singlets and S=1 states are called triplets. This is because the multiplicity of the spin states (the number of degenerate states) is given by 2S+1. When the excited electron retains its spin from the ground state (i.e. they are anti-parallel), S=0 and the multiplicity = 2(1/2 + -1/2) + 1 = 2(0) + 1 = 1. Hence, singlet. When the excited state electron changes spin state so it's the same as the ground state (parallel), S=1 and the multiplicity = 2(1/2 + 1/2) + 1 = 2(1) + 1 = 3. Hence, triplet.

Electron spin in singlet and triplet states
Electron spins in singlet and triplet states

According to Hund's rule of maximum multiplicity, the lowest energy state will be the one in which the multiplicity is largest. This means that electrons will fill orbitals singly and with parallel spins before pairing up. Triplets therefore tend to have a slightly lower energy than singlets.

As photons have spin-0, only transitions between states of the same spin, i.e. singlet-singlet, are allowed. The absorption edge and the emission peak of molecules, therefore, tend to correspond to the S0 → S1 and the S1 → S0 transitions, respectively, where S0 is the singlet ground state (HOMO) and S1 is the singlet excited state (LUMO). This type of emission is termed fluorescence and has a very short lifetime (~ns).

Although singlet-triplet transitions are forbidden by spin selection rules, they can still occur, but at a much slower rate compared to singlet-singlet transitions. This type of emission is termed phosphorescence and can have lifetimes of seconds or even as long as several hours.

Jablonski Diagrams


The most common way to illustrate electronic and vibrational states and the transitions between them is using Jablonski diagrams. The energy levels are arranged vertically according to energy levels and horizontally according to multiplicity. An example Jablonksi diagram is shown below.

Jablonski diagram
Jablonski diagram showing the singlet ground state (S0), the first two singlet excited states (S1 and S2), and the first triplet excited state (T1) (black lines). The vibrational energy levels (v0, v1, v2, and v3) are denoted by the grey lines.

Radiative transitions (such as absorption and fluorescence) are indicated by straight arrows, while non-radiative transitions (such as internal conversion and intersystem crossing) tend to be indicated by wavy arrows.

Stokes Shift in Fluorescence


When a photon of greater energy than the S0 → S1 transition is absorbed, an electron will be excited to a higher vibronic (electronic and vibrational) energy level, Sn, where n ≥ 1. The probability of a transition between vibronic states is given by the Franck-Condon principle, which relies on the relative overlap of the wavefunctions of the two states: the greater the overlap, the greater the probability of the transition. This applies to both absorption and emission processes.

Once excited, the electron will generally relax back down to the lowest vibrational level of the S1 state through non-radiative energy transfer. It will then relax to a vibrational state in the electronic ground state, S0, according to the Franck-Condon principle with the emission of a photon (fluorescence). Due to the vibrational energy levels involved, the energy of this photon will be less than the photon absorbed. This energy difference is known as Stokes shift.

Jablonski diagram for a stokes shift
Jablonski diagram illustrating the mechanism that results in a Stokes shift between the absorption (blue arrow) and emission (red arrow) peaks in organic molecules.
Stokes shift of BODIPY-Br
Example of Stokes shift. The normalised absorbance (blue) and fluorescence (red) spectra of a thin film of BODIPY-Br, a fluorescent small molecule, dispersed in polystyrene. It can be seen that there is a Stokes shift of 18 nm or 77 meV.

It should be noted that as the electrons tend to first relax non-radiatively to S1 and then radiatively from S1 to S0, the fluorescence spectrum is almost entirely independent of the excitation wavelength. This is known as Kasha's rule.

Fluorescence Quenching


Fluorescence quenching refers to a number of processes that can reduce the photoluminescence quantum yield (the amount of fluorescence) by allowing the electron to relax to a lower energy state non-radiatively (without releasing a photon).

Quenching mechanisms include:

  • Vibrational relaxation and internal conversion
  • Intersystem crossing
  • Förster resonance energy transfer
  • Dexter electron transfer
  • Radiative energy transfer

Thermally Activated Delayed Fluorescence (TADF)


Thermally Activated Delayed Fluorescence (TADF) is a process for generating fluorescence from electrons in the triplet state. Fluorescence from triplet states is forbidden, but electrons in TADF emitters can use surrounding thermal energy to first transition to the excited singlet state. The process through which this happens, reverse intersystem crossing, is slow, and hence the resulting fluorescence is 'delayed'.

TADF allows higher photoluminescence quantum yields to be achieved, and therefore has significant implications for OLEDs. 

Measuring Photoluminescence Quantum Efficiency


It is important to know the photoluminescence quantum efficiency (PLQE) or photoluminescence quantum yield (PLQY) of a sample in order to determine the efficiency with which it will be able to re-emit light and the ratio of its radiative to non-radiative rates.

Typically, photoluminescence quantum yield measurements use an integrating sphere fibre-coupled to an optical spectrometer. The sample is placed at the centre of the sphere at a slight angle, such that the light isn't directly reflected back out of the entrance port, and is irradiated by a monochromatic light source such as a laser. This light source is chosen so that the photon energy is higher than that of the sample emission and the sample has non-zero absorption at the incident wavelength. The inside of the sphere is coated with a diffuse, white reflective layer, which, through multiple reflections, distributes the light isotropically over the sphere's inner surface. Outgoing light, which consists of both source photons and sample emission, is then collected by the fibre.

Finally, the sample is removed and the measurement is repeated with a control. If the sample is in solution, this is likely a cuvette containing the solvent used, whereas if it is a thin film, it is likely a blank substrate. A typical PLQE setup is shown in the figure below.

Photoluminescence spectroscopy setup
Typical PLQE setup. Light from a monochromatic source - here, a laser - is focused onto a sample inside an integrating sphere. The laser light and the light emitted by the sample are reflected multiple times from the diffuse inner surface of the sphere, and collected by an optical fibre, which sends the light to a spectrometer.

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The two spectra can then be compared to calculate the amount of light absorbed and emitted by the sample. In the example PLQE data shown below, a portion of the excitation source is absorbed (red shaded region) when the sample is placed inside the sphere. Some of this light is re-emitted at longer wavelengths (blue shaded region). The PLQE, Φ, is then given by

PLQE equation

In practice, this is usually calculated by integrating the two shaded regions.

Example PLQE data
Example PLQE data. The red and blue solid lines show the spectrum from the sphere without and with the sample, respectively. The red and blue shaded regions show the light absorbed and emitted by the sample, respectively. The PLQE of the sample is given by the blue shaded area (which has been unrealistically scaled up for ease of viewing) divided by the red shaded area (x100).

If the sample's absorption and PL spectra overlap, it can reabsorb some of the photons it emits after they've been reflected from the sphere's inner surface. In this case, the PL emission in the integrating sphere measurements will have a different shape, with a reduction in emission at the high energy end - this is illustrated in the figure below. A self-absorption correction, which uses the actual PL spectrum, is therefore useful [1].

Reabsorption by sample in PLQE measurement
Example of reabsorption by sample in PLQE measurement. The green line represents the missing part of the PL spectrum and the green shaded area represents the reabsorbed photons.

The PLQE of a material depends on its electronic and vibrational energy levels. When a photon is absorbed, the excited electron can either decay radiatively or non-radiatively. If it decays radiatively, a photon will be released. If it decays non-radiatively, however, a photon will not be released. The PLQE is hence related to the radiative (fluorescence) rate, kr, and the sum (Σ) of all the non-radiative, knr, rates through

PLQE is related to the radiative (fluorescence) rate and sum of non-radiative rates

Φ is strongly affected by the molecules' environment - for example, solvent polarity. It can also be affected by aggregation. Aggregation can occur when molecules form complexes in the ground state and the electronic properties of the molecule are altered. This can result in broadened or shifted absorption and emission spectra and often results in an increase in non-fluorescent pathways and therefore a decreased PLQE. This is the case with aromatic molecules (those that contain planar, cyclic structures, such as benzene), which undergo π-π stacking, and when hydrophobic molecules are placed in polar solvents. Additionally, the close proximity of molecules in the solid state also increases the effects of aggregation if the length scales are short enough to allow energy transfer and intramolecular charge transfer to occur.

For this reason, thin films often have significantly reduced PLQEs compared to solutions. It is possible to reduce the effects of aggregation in films by dispersing the molecules in a transparent polymer matrix such as polystyrene or poly(methyl methacrylate) (PMMA). This is done simply by adding the polymer to the dye solution and processing as normal - for example, through spin coating. The polymer will help to space out the dye molecules, reducing intermolecular interactions and reducing aggregation. By increasing the concentration of polymer relative to the dye, aggregation can be further reduced; however, this is accompanied by a reduced absorption, which may not be desirable.

  

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References


  1. T.-S. Ahn, et al. Rev. Sci. Instrum. 78, 086105 (2007).

Contributing Authors


  • Kirsty McGhee
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