Quantum yield

Quantum Yield

Quantum yield is a fundamental concept in the field of photochemistry and photophysics. It is a measure of the efficiency with which a molecule or a system of molecules converts absorbed photons into a desired photochemical or photophysical process. In other words, it quantifies the probability of a photon causing a specific reaction or event.

Definition

The quantum yield (Φ) is defined as the ratio of the number of desired events (N) to the number of absorbed photons (N_abs):

Φ = N / N_abs

The desired events can vary depending on the system under study. For example, in fluorescence, the desired event is the emission of a photon of lower energy (longer wavelength) than the absorbed photon. In photochemical reactions, the desired event could be the formation of a product or the initiation of a chemical reaction.

Factors Affecting Quantum Yield

Several factors can influence the quantum yield of a photochemical or photophysical process:

1. Absorption Efficiency: The ability of a molecule to absorb photons at a specific wavelength affects the number of absorbed photons and, consequently, the quantum yield. Molecules with high absorption cross-sections are more likely to absorb photons and exhibit higher quantum yields.

2. Competing Pathways: If there are multiple possible pathways for the absorbed energy to dissipate, the quantum yield may be reduced. For example, in fluorescence, non-radiative processes such as internal conversion or intersystem crossing can compete with the desired emission process, leading to a lower quantum yield.

3. Environmental Factors: The surrounding environment can also impact the quantum yield. Factors such as temperature, solvent polarity, and presence of quenchers or sensitizers can influence the efficiency of the desired process.

4. Molecular Structure: The molecular structure of the chromophore or fluorophore can significantly affect the quantum yield. Factors such as conjugation, electronic transitions, and molecular symmetry play a crucial role in determining the efficiency of the desired process.

Applications

Quantum yield is a crucial parameter in various scientific fields, including:

1. Fluorescence Spectroscopy: Quantum yield is used to quantify the efficiency of fluorescence emission, which is widely employed in biological imaging, materials science, and chemical analysis.

2. Photovoltaics: In solar cells, the quantum yield is used to assess the efficiency of converting absorbed photons into electrical energy.

3. Photochemistry: Quantum yield is essential in studying photochemical reactions, where it provides insights into the efficiency of light-induced chemical transformations.

4. Photobiology: Quantum yield is used to understand the efficiency of photosynthesis, photodegradation, and other light-dependent biological processes.