Fluorescence
Fluorescence is one of two kinds of photoluminescence, the emission of light by a substance that has absorbed light or other electromagnetic radiation. When exposed to ultraviolet radiation, many substances will glow with colored visible light. The color of the light emitted depends on the chemical composition of the substance. Fluorescent materials generally cease to glow nearly immediately when the radiation source stops. This distinguishes them from the other type of light emission, phosphorescence. Phosphorescent materials continue to emit light for some time after the radiation stops.
This difference in duration is a result of quantum spin effects.
Fluorescence occurs when a photon from incoming radiation is absorbed by a molecule, exciting it to a higher energy level, followed by the emission of light as the molecule returns to a lower energy state. The emitted light may have a longer wavelength and, therefore, a lower photon energy than the absorbed radiation. For example, the absorbed radiation could be in the ultraviolet region of the electromagnetic spectrum, while the emitted light is in the visible region. This gives the fluorescent substance a distinct color, best seen when exposed to UV light, making it appear to glow in the dark. However, any light with a shorter wavelength may cause a material to fluoresce at a longer wavelength. Fluorescent materials may also be excited by certain wavelengths of visible light, which can mask the glow, yet their colors may appear bright and intensified. Other fluorescent materials emit their light in the infrared or even the ultraviolet regions of the spectrum.
Fluorescence has many practical applications, including mineralogy, gemology, medicine, chemical sensors, fluorescent labelling, dyes, biological detectors, cosmic-ray detection, vacuum fluorescent displays, and cathode-ray tubes. Its most common everyday application is in fluorescent lamps and LED lamps, where fluorescent coatings convert UV or blue light into longer wavelengths, resulting in white light, which can appear indistinguishable from that of the traditional but energy-inefficient incandescent lamp.
Fluorescence also occurs frequently in nature, appearing in some minerals and many biological forms across all kingdoms of life. The latter is often referred to as biofluorescence, indicating that the fluorophore is part of or derived from a living organism. However, since fluorescence results from a specific chemical property that can often be synthesized artificially, it is generally sufficient to describe the substance itself as fluorescent.
History
Fluorescence was observed long before it was named and understood.An early observation of fluorescence was known to the Aztecs and described in 1560 by Bernardino de Sahagún and in 1565 by Nicolás Monardes in the infusion known as lignum nephriticum. It was derived from the wood of two tree species, Pterocarpus indicus and Eysenhardtia polystachya.
The chemical compound responsible for this fluorescence is matlaline, which is the oxidation product of one of the flavonoids found in this wood.
In 1819, E.D. Clarke
and in 1822 René Just Haüy
described some varieties of fluorites that had a different color depending on whether the light was reflected or transmitted. Haüy incorrectly viewed the effect as light scattering similar to opalescence. In 1833 Sir David Brewster described a similar effect in chlorophyll which he also considered a form of opalescence.
Sir John Herschel studied quinine in 1845 and came to a different incorrect conclusion.
In 1842, A.E. Becquerel observed that calcium sulfide emits light after being exposed to solar ultraviolet, making him the first to state that the emitted light is of longer wavelength than the incident light. While his observation of photoluminescence was similar to that described 10 years later by Stokes, who observed a fluorescence of a solution of quinine, the phenomenon that Becquerel described with calcium sulfide is now called phosphorescence.
In his 1852 paper on the "Refrangibility" of light, George Gabriel Stokes described the ability of fluorspar, uranium glass and many other substances to change invisible light beyond the violet end of the visible spectrum into visible light. He named this phenomenon fluorescence
Neither Becquerel nor Stokes understood one key aspect of photoluminescence: the critical difference from incandescence, the emission of light by heated material. To distinguish it from incandescence, in the late 1800s, Gustav Wiedemann proposed the term luminescence to designate any emission of light more intense than expected from the source's temperature.
Advances in spectroscopy and quantum electronics between the 1950s and 1970s provided a way to distinguish between the three different mechanisms that produce the light, as well as narrowing down the typical timescales those mechanisms take to decay after absorption. In modern science, this distinction became important because some items, such as lasers, required the fastest decay times, which typically occur in the nanosecond range. In physics, this first mechanism was termed "fluorescence" or "singlet emission", and is common in many laser mediums such as ruby. Other fluorescent materials were discovered to have much longer decay times, because some of the atoms would change their spin to a triplet state, thus would glow brightly with fluorescence under excitation but produce a dimmer afterglow for a short time after the excitation was removed, which became labeled "phosphorescence" or "triplet phosphorescence". The typical decay times ranged from a few microseconds to one second, which are still fast enough by human-eye standards to be colloquially referred to as fluorescent. Common examples include fluorescent lamps, organic dyes, and even fluorspar. Longer emitters, commonly referred to as glow-in-the-dark substances, ranged from one second to many hours, and this mechanism was called persistent phosphorescence or persistent luminescence, to distinguish it from the other two mechanisms.
Physical principles
Mechanism
Fluorescence occurs when an excited molecule, atom, or nanostructure, relaxes to a lower energy state through emission of a photon without a change in electron spin. When the initial and final states have different multiplicity, the phenomenon is termed phosphorescence.When a molecule in its ground state is photoexcited it may end up in any one of a number of excited states. These higher excited states are different vibrational levels, populated in proportion to their overlap with the ground state according to the Franck-Condon principle. These vibrational excited states typically decay rapidly by to S1, followed by radiative transition to the ground state or to vibrational states close to the ground state. This transition is called fluorescence. All of these states are singlet states.
A different pathway for deexcitation is intersystem crossing from the S1 to a triplet state T1. Decay from T1 to S0 is typically slower and less intense and is called phosphorescence.
Absorption of a photon of energy results in an excited state of the same multiplicity of the ground state, usually a singlet. In solution, states with n > 1 relax rapidly to the lowest vibrational level of the first excited state by transferring energy to the solvent molecules through non-radiative processes, including internal conversion followed by vibrational relaxation, in which the energy is dissipated as heat. Thus the fluorescence energy is typically less than the photoexcitation energy.
The excited state S1 can relax by other mechanisms that do not involve the emission of light. These processes, called non-radiative processes, compete with fluorescence emission and decrease its efficiency. Examples include internal conversion, intersystem crossing to the triplet state, and energy transfer to another molecule. An example of energy transfer is Förster resonance energy transfer. Relaxation from an excited state can also occur through collisional quenching, a process where a molecule collides with the fluorescent molecule during its excited state lifetime. Molecular oxygen is an extremely efficient quencher of fluorescence because of its unusual triplet ground state.
Quantum yield
The fluorescence quantum yield gives the efficiency of the fluorescence process. It is defined as the ratio of the number of photons emitted to the number of photons absorbed.The maximum possible fluorescence quantum yield is 1.0 ; each photon absorbed results in a photon emitted. Compounds with quantum yields of 0.10 are still considered quite fluorescent. Another way to define the quantum yield of fluorescence is by the rate of excited state decay:
where is the rate constant of spontaneous emission of radiation and
is the sum of all rates of excited state decay. Other rates of excited state decay are caused by mechanisms other than photon emission and are, therefore, often called "non-radiative rates", which can include:
- dynamic collisional quenching
- near-field dipole–dipole interaction
- internal conversion
- intersystem crossing
Fluorescence quantum yields are measured by comparison to a standard. The quinine salt quinine sulfate in a sulfuric acid solution was regarded as the most common fluorescence standard,
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 0.1 M perchloric acid shows no temperature dependence up to 45 °C, therefore it can be considered as a reliable standard solution.