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 lower energy, than the absorbed radiation. The most striking example of fluorescence occurs when white light shines on a fluorescent material; in this case the material appears to glow with its own color.
Theoretically, any molecule or atom capable of absorbing electromagnetic radiation can also become a fluorescer. However, in practice, only a small subset of materials show appreciable levels of fluorescence. This is because for molecules to become excited and emit photons they must be able to rearrange themselves into an electronically excited state; many substances do not have such states available within their ground electronic configuration.
When an object absorbs incident photons it becomes raised to an excited state energy level above its ground state. While in the excited state, the molecule may either: return almost immediately to the ground state by emitting stimulating photons (fluorescence), fall back non-radiatively through intermediate energy levels (phosphorescence), or remain in the excited state until it collides with another molecule and returns to the ground state that way (resonance energy transfer). If electrons are not returned directly to the ground state after falling back from an excited singlet state then they may enter into one of several metastable triplet states before eventually decaying back down to the ground state and emitting photons (delayed fluorescence).
The time taken for an electron returning from an exited singletstateto decay back down through all three possible routes is calledthe radiative lifetimeand will determine how long ago something was last exposedto incident photons as well as which method(s) will be used for detectionin analytical chemistry applications involving fluorophores(substances which exhibit strong fluorescence). The radiative lifetimefor common organic fluorophores generally falls between2 ns – 100 μs although some rarer examples have been found withlifetimes up tp several milliseconds (molecular oxygen being oneexceptionally long-lived case). In contrast phosphorescent specieshave much longer lifetimes due largely to spin–orbit coupling effectswhich result in forbidden transitions and makeinternal conversionthe dominant deactivation pathway rather thanthe direct return path favored by rapid intersystem crossing eventsin singlet systems. Delayed fluorescence follows similar kinetics toboth phosphorescence and conventional fluorescence but possessesa different mechanism whereby electrons jump fromsinglet statesinto metastable triplet states(rather than directly returningto Ground as happens with regular fluroescence) before eventuallyundergoing intersystem crossing intosinglet formand decaying viaemissionof stimulating photons[citation needed]. Consequently delayedfluorophores tend to exhibit very long emission lifetimes oftenmeasured intosecondsand occasionally even minutes or hours dependingon molecular structure[citation needed].
It should be noted that there are also some ‘true’ phosphorescentmaterials which do not rely on metastable triplet intermediatestates but instead possess certain chemical groups whichdirectly facilitate spin–orbit couplingand thus enable them toovercome strict selection rules without resorting totriplet formation[citation needed]. These so-called ‘ligand centered’or ‘metal centered’ phosphors typically display quite shortemission lifetimes on account of their high quantum yield buthave nonetheless found significant use industrially where fastresponse times are required such as in displays technologies orenergy storage devices like batteries[citation needed]
Organic fluorophores tend to display absorption maxima within thenear ultraviolet or visible regionofthe electromagnetic spectrum(300nm – 700nm)[citation needed] while organic phosphors typically absorbwithin thenear infraredregion (>700nm)[citation needed]. This differenceis due largely tooverlapping molecular orbitals between atomsin conjugated systems which allow for easy promotionof electronsfrom occupied lowest lying orbitalsof relatively low energiesinto vacant highest lying unoccupiedorbitalsof relatively high energies(this process known assigma–pi*transition)[citation needed]. For this reason UV/visibleabsorbing fluorophoreshave much higher molar extinctioncoefficientsthan NIR absorbing phosphorescent materialsallowing them topossess strong signal even at very low concentrationswhile still remaining chemically stable over long periods oftimes [citation needed](an important consideration given thatUV/visible wavelengthsare readily scatteredby atmospheric gaseswhereas NIR wavescan penetrate much furtherbefore finally being scatteredback towards earth’s surface)[ citationneeded ].