Fluorescence

Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, than the absorbed radiation. A perceptible example of fluorescence occurs when the absorbed radiation is in the ultraviolet region of the electromagnetic spectrum (invisible to the human eye), while the emitted light is in the visible region; this gives the fluorescent substance a distinct color that can only be seen when the substance has been exposed to UV light. Fluorescent materials cease to glow nearly immediately when the radiation source stops, unlike phosphorescent materials, which continue to emit light for some time after.

Fluorescence has many practical applications, including mineralogy, gemology, medicine, chemical sensors ( fluorescence spectroscopy), fluorescent labelling, dyes, biological detectors, cosmic-ray detection, vacuum fluorescent displays, and cathode-ray tubes. Its most common everyday application is in ( gas-discharge) fluorescent lamps and LED lamps, in which fluorescent coatings convert UV or blue light into longer-wavelengths resulting in white light which can even appear indistinguishable from that of the traditional but energy- inefficient incandescent lamp.

Fluorescence also occurs frequently in nature in some minerals and in many biological forms across all kingdoms of life. The latter may be referred to as ''biofluorescence'', indicating that the fluorophore is part of or is extracted from a living organism (rather than an inorganic dye or stain). But since fluorescence is due to a specific chemical, which can also be synthesized artificially in most cases, it is sufficient to describe the substance itself as ''fluorescent''.

History

An early observation of fluorescence was described in 1560 by Bernardino de Sahagún and in 1565 by Nicolás Monardes in the infusion known as '' lignum nephriticum'' (Latin for "kidney wood"). 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 fluorescence in fluorites, Sir David Brewster described the phenomenon for chlorophyll in 1833
and Sir John Herschel did the same for quinine in 1845.

In his 1852 paper on the "Refrangibility" ( wavelength change) of light, George Gabriel Stokes described the ability of fluorspar and uranium glass to change invisible light beyond the violet end of the visible spectrum into blue light. He named this phenomenon ''fluorescence''
: "I am almost inclined to coin a word, and call the appearance ''fluorescence'', from fluor-spar .e., fluorite as the analogous term ''opalescence'' is derived from the name of a mineral."

The name was derived from the mineral fluorite (calcium difluoride), some examples of which contain traces of divalent europium, which serves as the fluorescent activator to emit blue light. In a key experiment he used a prism to isolate ultraviolet radiation from sunlight and observed blue light emitted by an ethanol solution of quinine exposed by it.

Physical principles

Mechanism

Fluorescence occurs when an excited molecule, atom, or nanostructure, relaxes to a lower energy state (usually the ground state) through emission of a photon without a change in electron spin. When the initial and final states have different multiplicity (spin), the phenomenon is termed phosphorescence.

The ground state of most molecules is a singlet state, denoted as S₀. A notable exception is molecular oxygen, which has a triplet ground state. Absorption of a photon of energy $h \nu_$ results in an excited state of the same multiplicity (spin) of the ground state, usually a singlet (S_n with n > 0). In solution, states with n > 1 relax rapidly to the lowest vibrational level of the first excited state (S₁) 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. Therefore, most commonly, fluorescence occurs from the first singlet excited state, S₁. Fluorescence is the emission of a photon accompanying the relaxation of the excited state to the ground state. Fluorescence photons are lower in energy ($h \nu_$) compared to the energy of the photons used to generate the excited state ($h \nu_$)

* Excitation: $\mathrm_0 + h \nu_ \to \mathrm_1$
* Fluorescence (emission): $\mathrm_1 \to \mathrm_0 + h \nu_$
In each case the photon energy $E$ is proportional to its frequency $\nu$ according to $E=h\nu$, where $h$ is Planck's constant.

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 (the quencher) collides with the fluorescent molecule during its excited state lifetime. Molecular oxygen (O₂) is an extremely efficient quencher of fluorescence just 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.

: $\Phi = \frac$

The maximum possible fluorescence quantum yield is 1.0 (100%); 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:

: $\Phi = \frac$

where $_$ is the rate constant of spontaneous emission of radiation and

:$\sum__$

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