Quantum yield
Quantum yield is defined as the ratio of the number of photons emitted to the number of photons absorbed by a material or system.
Quantum yield=number of photons emited/number of photons absorbed
It is also describe as the fraction of absorbed photons that result in the desired process, and it is typically expressed as a percentage or a decimal value between 0 and 1.
Advantages of quantum yield:
Quantum yield has gained great importance in many fields, Some advantages of using quantum yield is given below:
- Provides quantitative information:
- Indicates reaction mechanisms:
- Enables optimization:
- Useful in material design
- Facilitates comparison:
- Non-destructive:
Provides quantitative information:
Quantum yield is a quantitative evaluation of the effectiveness of a photochemical or photophysical process, which allows for correct comparison and assessment of different materials and systems.
Indicates reaction mechanisms:
The quantum yield can give information about the mechanism of a photochemical or photophysical process, such as
- the number of steps included in the process or
- the intermediate species molded during the process.
Enables optimization:
By calculating the quantum yield under different conditions, it is likely to optimize the conditions for a photochemical or photophysical process to increase the quantum yield, which can lead to more competent and cost-effective processes.
Useful in material design:
Quantum yield can be found its application as a design parameter in the development of new materials for various applications, such as
- solar cells
- sensors
- and imaging agents.
Facilitates comparison:
The quantum yield permits for direct evaluation of different materials or systems, even if they have
- different absorption spectra or
- fluorescence emission spectra.
Non-destructive:
The calculation of quantum yield is a non-destructive method that does not need the sample to be devastated orused up, which is important for samples that are
- Rare
- Expensive
- or difficult to prepare.
Experimental determination of quantum yield
Various industrial method used for the determination of quantum yield a few important ones are given below:
- Fluorescence quantum yield:
- Phosphorescence quantum yield:
- Time-resolved spectroscopy:
- Actinometry:
- Absorption spectroscopy:
Fluorescence quantum yield:
This method includes relating the fluorescence intensity of a sample to a standard with a known quantum yield.
In fluorescence quantum yield method the quantum yield of the sample can be evaluated using the following equation:
Quantum yield = (Φ_sample/Φ_standard) x (I_sample/I_standard) x (A_standard/A_sample)
Where;
Φ = quantum yield
I = fluorescence intensity
A= absorbance of the sample and standard, respectively.
Phosphorescence quantum yield:
This method is similar to the fluorescence quantum yield method but involves measuring the phosphorescence intensity of a sample. The quantum yield can be calculated using a similar equation, but with different factors that account for the longer lifetime of the phosphorescence.
Time-resolved spectroscopy:
This method involves measuring the time-resolved fluorescence or phosphorescence decay of a sample using a time-correlated single-photon counting technique. By analyzing the decay kinetics, the quantum yield can be determined.
Actinometry:
This method involves using a known photochemical reaction with a known quantum yield as a standard to determine the quantum yield of a sample. The photochemical reaction is initiated using a known light source, and the resulting product is analyzed to determine the quantum yield.
Absorption spectroscopy:
This method involves calculating the absorbance of a sample at two points
- the excitation wavelength and
- at a wavelength where there is no absorption by the sample.
The quantum yield can be measured by using the following equation:
Quantum yield = (slope of the plot of ln(I_0/I) vs. absorbance) x (n_0/n) x (F/F_0)
Where;
I_0 = intensities of the excitation light at the beginning
I= intensities of the excitation light at the end
n_0 = refractive indices of the solvent at the beginning
n= refractive indices of the solvent at the end
F_0= incident light fluxes at the beginning
F= incident light fluxes at the end
The slope of the plot is linked to the quantum yield.