In particle physics, the quantum yield (denoted Φ) of a radiation-induced process is the number of times a specific event occurs per photon absorbed by the system.
Fluorescence quantum yield is measured on a scale from 0 to 1.0, but is often represented as a percentage.
Substances with the largest quantum yields, such as rhodamines, display the brightest emissions; however, compounds with quantum yields of 0.10 are still considered quite fluorescent.
Quantum yield is defined by the fraction of excited state fluorophores that decay through fluorescence:
where Non-radiative processes are excited state decay mechanisms other than photon emission, which include: Förster resonance energy transfer, internal conversion, external conversion, and intersystem crossing.
Thus, the fluorescence quantum yield is affected if the rate of any non-radiative pathway changes.
The quantum yield can be close to unity if the non-radiative decay rate is much smaller than the rate of radiative decay, that is kf > knr.
[2] The quinine salt quinine sulfate in a sulfuric acid solution was regarded as the most common fluorescence standard,[3] however, a recent study revealed that the fluorescence quantum yield of this solution is strongly affected by the temperature, and should no longer be used as the standard solution.
The quinine in 0.1M perchloric acid (Φ = 0.60) shows no temperature dependence up to 45 °C, therefore it can be considered as a reliable standard solution.
[4] Experimentally, relative fluorescence quantum yields can be determined by measuring fluorescence of a fluorophore of known quantum yield with the same experimental parameters (excitation wavelength, slit widths, photomultiplier voltage etc.)
where The subscript R denotes the respective values of the reference substance.
[5][6] The determination of fluorescence quantum yields in scattering media requires additional considerations and corrections.
[7] Förster resonance energy transfer efficiency (E) is the quantum yield of the energy-transfer transition, i.e. the probability of the energy-transfer event occurring per donor excitation event:
where A fluorophore's environment can impact quantum yield, usually resulting from changes in the rates of non-radiative decay.
[2] Many fluorophores used to label macromolecules are sensitive to solvent polarity.
The class of 8-anilinonaphthalene-1-sulfonic acid (ANS) probe molecules are essentially non-fluorescent when in aqueous solution, but become highly fluorescent in nonpolar solvents or when bound to proteins and membranes.
The quantum yield of ANS is ~0.002 in aqueous buffer, but near 0.4 when bound to serum albumin.
In a chemical photodegradation process, when a molecule dissociates after absorbing a light quantum, the quantum yield is the number of destroyed molecules divided by the number of photons absorbed by the system.
Since not all photons are absorbed productively, the typical quantum yield will be less than 1.
[10] Quantum yields of photochemical reactions can be highly dependent on the structure, proximity and concentration of the reactive chromophores, the type of solvent environment as well as the wavelength of the incident light.
Such effects can be studied with wavelength-tunable lasers and the resulting quantum yield data can help predict conversion and selectivity of photochemical reactions.
For example, a singlet to triplet transition quantum yield is the fraction of molecules that, after being photoexcited into a singlet state, cross over to the triplet state.