In the relative quantum yield method, the quantum yield of the sample interest is calculated by comparing its photoluminescence emission to that of a reference standard of known quantum yield. Quantum yields are therefore most commonly measured optically, using either the relative or absolute method. 14 These methods require specialised setups and are generally reserved for the determination of the quantum yield of important standards. Non-optical methods include the indirect measurement of the conversion of the excitation energy into heat and its dissipation to the solvent, 6, 12 and calorimetric methods, such as photoacoustic spectroscopy (PAS), 13 and thermal lensing. Quantum yield measurements can be split into non-optical and optical methods. High quantum yields are crucial for a wide range of applications including displays, lasers, bioimaging and solar cells, and accurate measurement of the quantum yield is therefore important. Quantum yield is one of the most important photophysical parameters when characterising luminescent molecules and materials. 11 By 1930 the term “quantum yield” as we know it today had been widely cited in a large number of textbooks and papers. 9 In 1924, Vavilov referred to the term “fluorescence yield”, based on Warburg’s earlier work, to calculate the fraction of absorbed rays of light to fluorescent rays of light, 10 whereas the term “molecules per quantum light absorbed” appears in a paper published in 1925 by Marshall to describe the photochemical reaction between hydrogen and chlorine. 8 His papers (in German) can be found on the German National Library website. He later named this procedure “quantum efficiency” and used the Greek letter phi ( Φ) to denote it.
Warburg published a series of papers over the period 1912-1921 in which he studied the conversion of ozone molecules into oxygen to acquire the ratio of the molecules produced to quanta absorbed. Einstein introduced the quantisation of light, i.e., a light beam consists of discrete quantum particles (quanta) carrying energy equal to, where is Planck’s constant and the light frequency. The roots of quantum yield can be traced back to the beginning of the 20 th century when Einstein’s revolutionary work on the photoelectric effect was published in 1905 (a copy of his paper translated in English can be found in Ref.
6 The quantum yield is therefore the probability that a system in the excited state deactivates through a radiative process to its ground state. The radiative transition rate ( ) denotes radiative (light emitting) processes such as fluorescence and phosphorescence whereas the sum of non-radiative rates ( ), includes processes such as internal conversion, intersystem crossing, and energy transfer. The quantum yield can therefore be rewritten in terms of these rates, The quantum yield of a system (such as a fluorescent molecule) is determined by the balance between the radiative and non-radiative transition rates within it (Figure 1). For example, if the system absorbs 100 photons and emits 30, then its quantum yield would be is 0.3 or 30%.įigure 1 Quantum yield and radiative and non-radiative decay processes. The quantum yield is reported as either a decimal fraction between 0 and 1 or as a percentage. However, the terms fluorescence, luminescence and photoluminescence quantum yield are also commonly used. This narrower definition is often simply called quantum yield with the emission of light being implicit given the context. However, it is most commonly written specifically for the emission of light (photoluminescence) by a system, 2-5 The IUPAC definition of quantum yield ( Φ) is the number of a certain event occurring per photon absorbed by the system, 1