**The Photoluminescence Quantum Yield (PLQY) of a material or molecule is defined as the quantity of photons emitted as a fraction of the quantity of photons absorbed. This characteristic of a fluorescent or fluorophore molecule is critical for comprehending molecule behavior and interactions within a range of important materials.**

*Figure 1.** An integrating sphere fiber-coupled to a fluorometer for PLQY measurements.*

Similarly, a molecule’s Electroluminescence Quantum Yield (ELQY) is defined as the number of photons emitted by the electron current of a particular device. This measurement is essential in categorizing and monitoring display devices, lighting or other photovoltaic materials.

*Figure 2.** Electroluminescence can be measured using an integrating sphere (left) by fitting a powered device such as an LED into the sample tray. Center: The integrated intensity can be measured with input voltage or current. Right: Color can be plotted in CIE 1931 coordinates by measurement of the spectrum in the sphere.*

PLQY and ELQY are commonly used with:-

- Novel nanomaterials
- Nanoparticles
- Solar cells and photovoltaics
- Graphene or Single Walled Carbon nanotubes
- Quantum dots
- Coordination chemistry
- Lighting and display materials such as LEDs and OLEDs
- Electrovoltaics
- Films and coatings
- Paintings, colorimetry and coatings
- Cured and doped polymers, gels and hydrogels

Three methods are employed in order to measure PLQY. These are:

- The comparison method
- Fluorescence lifetime
- Direct method (integrating sphere)

Each of these methods is explored in further detail, below.

**How Do I Use the Comparative Method to Determine Quantum Yield?**

The comparative method makes use of a known reference standard – a sample with emission and absorbance properties that are close to the sample of interest and with a known PLQY value. Absorbance and fluorescence are measured for the reference standard, then the same measurements are undertaken for the sample being studied.

*Figure 3.** Left: The equation for calculating the fluorescence quantum yield of an unknown (Q*_{F}) by comparing it to the spectrum of a known standard. Right: A table of some known PLQY standards and their respective excitation wavelengths and quantum yields.

The equation below is used where Q_{F} is the quantum yield of an unknown fluorescent sample and Q_{R} is the quantum yield of the reference standard. Here, I_{F} and I_{R} are the integrated fluorescence intensities for the unknown and reference samples respectively, while A_{F} and A_{R} are the respective absorbance values of the unknown and reference samples.

The drawback to this method is that there exists a very limited amount of reference standards, so there are distinct limits to where this particular method can be applied.

**How Do I Use Fluorescence Lifetimes for Quantum Yield Determination?**

Fluorescence lifetimes can be used alongside different concentrations of a quencher in order to calculate a molecule’s quantum yield.

The equation below is used where t_{f} is the quantum yield, and k_{f}, k_{nr}, and k_{t} are the rate constants of fluorescence, non-radiative dissipation and energy transfer, respectively. τ_{f} is the fluorescence lifetime of the sample.

*Figure 4.** Fluorescence Quantum yield equation calculated by the rate constants of fluorescence (k*_{f}), non-radiative dissipation (k_{nr}) and energy transfer (k_{t}). Fluorescence lifetime calculated by one over the sum of the rate constants. And the quantum yield in relation to the Stern-Volmer quenching constant (K), the biomolecular quenching constant (k_{q}) and the lifetime ( t_{0}). (Lakowicz, 2006)

PLQY, in this instance, is determined by the rate constants of these non-radiative processes (for example Stern-Volmer quenching and FRET) as they compete with fluorescence.

The addition of a dilution series of quencher to a fluorescent solution allows for the calculation of PLQY by ascertaining the Stern-Volmer quenching constants (K) and the bimolecular quenching constant (k_{q}).

*Figure 5.** Left: Fluorescence excitation and emission spectra for different concentrations of sodium ascorbate quenching fluorescence from 9-aminoacridine. Center: Frequency-domain lifetime by modulation and phase measurements (center) for the same solutions. Right: Lifetime and intensity ratios (I/I*_{0} and t/t_{0}) versus concentrations. Linear fits to these plots yield quenching constants.

This method is certainly robust, but it does require a considerable amount of sample preparation, making it especially inconvenient for solid samples.

**How Do I Use an Integrating Sphere for Quantum Yield Measurements?**

The Integrating Sphere Method is a more direct method for measuring PLQY. With this technique, a sphere is coated with a reflective surface in order to capture all the light entering or leaving the sphere. This surface is generally made from barium sulfate-based materials or Spectralon^{®}.

*Figure 6.** Left: An integrating sphere where samples are placed on the inside of the sphere and then fluorescence is measured. Right: The reflectance spectrum of Spectralon*^{®} material that coats the inside of an integrating sphere. (LabSphere Spectralon(R) datasheet, 2017)

The fluorescence emission (E_{c}) and the scatter (L_{c}) of the sample are measured alongside the emission and scatter of a blank (L_{a} and E_{a}). These two spectral measurements (sample and blank) allow the PLQY to be directly calculated, as can be seen in the equation below.

*Figure 7.** Quantum yield (Ff) equation from measurement using an integration sphere.*

In this example, E_{b} is the integrated luminescence from the sample caused by indirect luminescence from the sphere, while A is the absorbance of the sample at the excitation wavelength.

The PLQY and any associated error analysis is then calculated by incorporating the two traces with appropriate spectral correction factors.

*Figure 8.** Calculation of PLQY from scatter (left) and fluorescence (right) of blank and sample.*

This information has been sourced, reviewed and adapted from materials provided by HORIBA Scientific.

For more information on this source, please visit HORIBA Scientific.