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:

1. Fluorescence quantum yield:
2. Phosphorescence quantum yield:

• Time-resolved spectroscopy:

1. Actinometry:
2. 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.