View source for Excimer

{{Short description|Excited dimeric molecule containing a noble gas}}

[[Image:Excimer energy diagram.svg|right|Excimer energy diagram]]
An  '''excimer''' (originally short for '''excited dimer''') is a short-lived polyatomic [[molecule]] formed from two species that do not form a stable molecule in the ground state. In this case, formation of molecules is possible only if such atom is in an [[electron]]ic [[excited state]].<ref>{{cite journal |doi=10.1088/0034-4885/38/8/001|bibcode=1975RPPh...38..903B|title=Excimers|year=1975|last1=Birks|first1=J. B.|journal=Reports on Progress in Physics|volume=38|issue=8|pages=903–974}}</ref> [[Heteronuclear molecule]]s and molecules that have more than two species are also called '''exciplex''' molecules (originally short for '''excited complex'''). Excimers are often diatomic and are composed of two atoms or molecules that would not bond if both were in the [[ground state]]. The lifetime of an excimer is very short, on the order of [[nanosecond]]s.

==Formation and decay==
[[Image:Molecule HOMO-LUMO diagram.svg|thumb|300px|Molecular orbitals]]
Under the [[molecular orbital]] formalism, a typical ground-state molecule has [[electron]]s in the lowest possible energy levels. According to the [[Pauli principle]], at most two electrons can occupy a given orbital, and if an orbital contains two electrons they must be in opposite [[Spin (physics)|spin states]]. The highest occupied molecular orbital is called the HOMO and the lowest unoccupied molecular orbital is called the LUMO; the energy gap between these two states is known as the [[HOMO/LUMO|HOMO–LUMO]] gap. If the molecule absorbs light whose energy is equal to this gap, an electron in the HOMO may be excited to the LUMO. This is called the molecule's [[excited state]].

Excimers are only formed when one of the dimer components is in the excited state. When the excimer returns to the ground state, its components dissociate and often repel each other. The wavelength of an excimer's emission is longer (smaller energy) than that of the excited [[monomer]]'s emission. An excimer can thus be measured by fluorescent emissions.

Because excimer formation is dependent on a [[bimolecular]] interaction, it is promoted by high monomer density. Low-density conditions produce excited monomers that decay to the ground state before they interact with an unexcited monomer to form an excimer.

==Usage note==
The term ''excimer'' (excited state dimer) is, strictly speaking, limited to cases in which a true dimer is formed; that is, both components of the dimer are the same molecule or atom. The term '''exciplex''' refers to the heterodimeric case; however, common usage expands ''excimer'' to cover this situation.

==Examples and use==
{| class="wikitable" style="float:right"
|+ Excimer lasers
|-
! Laser !! Reagents !! Emission peak
|-
| [[Xenon chloride laser|XeCl]] || {{chem2|Xe + Cl2}} || 308&nbsp;nm
|-
| [[Krypton fluoride laser|KrF]] || {{chem2|Kr + NF3}} || 248&nbsp;nm
|-
| [[Argon fluoride laser|ArF]] || {{chem2|Ar + F2}} || 193&nbsp;nm
|}
Heterodimeric diatomic complexes involving a [[noble gas]] and a [[halide]], such as [[xenon monochloride|xenon chloride]], are common in the construction of [[excimer laser]]s, which are excimers' most common application. These lasers take advantage of the fact that excimer components have attractive interactions in the [[excited state]] and [[Repulsive state|repulsive interactions]] in the [[ground state]]. Emission of excimer molecules is also used as a source of spontaneous ultraviolet light ([[excimer lamp]]s).<ref>{{cite journal |doi=10.1070/PU2003v046n02ABEH001308|title=Excilamps: Efficient sources of spontaneous UV and VUV radiation|year=2003|last1=Lomaev|first1=Mikhail I.|last2=Skakun|first2=V. S.|last3=Sosnin|first3=E. A.|last4=Tarasenko|first4=Viktor F.|last5=Shitts|first5=D. V.|last6=Erofeev|first6=M. V.|journal=Physics-Uspekhi|volume=46|issue=2|pages=193–209}}</ref>

The molecule [[pyrene]] is another canonical example of an excimer that has found applications in [[biophysics]] to evaluate the distance between [[biomolecules]].<ref>{{cite journal |doi=10.1038/nsb986|pmid=14502269|title=Myosin cleft movement and its coupling to actomyosin dissociation|year=2003|last1=Conibear|first1=Paul B.|last2=Bagshaw|first2=Clive R.|last3=Fajer|first3=Piotr G.|last4=Kovács|first4=Mihály|last5=Málnási-Csizmadia|first5=András|journal=Nature Structural & Molecular Biology|volume=10|issue=10|pages=831–835|hdl=2381/134|url=https://figshare.com/articles/journal_contribution/10078475 |hdl-access=free}}</ref>

In [[organic chemistry]], many reactions occur through an exciplex, for example, those of simple [[arene compound]]s with alkenes.<ref>{{cite journal |doi=10.1002/anie.200603337|pmid=17143914|title=Photochemistry of Arenes—Reloaded|year=2007|last1=Mattay|first1=Jochen|journal=Angewandte Chemie International Edition|volume=46|issue=5|pages=663–665}}</ref> The reactions of [[benzene]] and their products depicted are a [2+2]cycloaddition to the [[Arene substitution patterns#Ortho.2C meta.2C and para substitution|ortho product]] (A),<ref>{{cite patent|country=US|status=patent|number=2805242|title=1-cyanobicyclo [4.2.0] octa-2, 4-dienes and their synthesis|gdate=1957-09-03|invent1=Ayer|inventor1-first=Donald|invent2=Buchi|inventor2-first=George}}</ref> a [2+3]cycloaddition to the [[Arene substitution patterns#Ortho.2C meta.2C and para substitution|meta product]] (B)<ref>{{cite journal |doi=10.1021/ja00961a052|title=A Photochemical 1,3 Cycloaddition of Olefins to Benzene1|year=1966|last1=Wilzbach|first1=K. E.|last2=Kaplan|first2=Louis|journal=Journal of the American Chemical Society|volume=88|issue=9|pages=2066–2067|bibcode=1966JAChS..88.2066W }}</ref> and the [2+4]cycloaddition to the [[Arene substitution patterns#Ortho.2C meta.2C and para substitution|para product]] (C)<ref>{{cite journal|doi=10.1021/ja00737a052|title=Photoaddition of benzene to olefins. II. Stereospecific 1,2 and 1,4 cycloadditions|year=1971|last1=Wilzbach|first1=Kenneth E.|last2=Kaplan|first2=Louis|journal=Journal of the American Chemical Society|volume=93|issue=8|pages=2073–2074|bibcode=1971JAChS..93.2073W }}</ref> with simple alkenes such as the isomers of [[2-butene]]. In these reactions, it is the arene that is excited.

[[Image:Arenephotocycloadditions.svg|400px|center|Arene photocycloadditions]]

As a general rule, the [[regioselectivity]] is in favor of the ortho [[adduct]] at the expense of the meta adduct when the amount of charge transfer taking place in the exciplex increases.

== Generation techniques ==
For a noble gas dimer or noble gas halide it takes a noble gas atom in an [[Excited state|excited]] [[electronic state]] to form an excimer molecule. Sufficiently high energy (approximately 10 [[Electronvolt|eV]]) is required to obtain a noble gas atom in the lowest excited electronic state, which provides the formation of an excimer molecule. The most convenient way to excite gases is by an [[electric discharge]]. That is why such excimer molecules are generated in a [[Plasma (physics)|plasma]] (see [[Excimer lamp#Excimer molecule formation|excimer molecule formation]]) or through high energy electron beams.

==Fluorescence quenching==
Exciplexes provide one of the three dynamic mechanisms by which [[fluorescence]] is [[Quenching (fluorescence)#Exciplex|quenched]]. A regular exciplex has some [[charge transfer complex|charge-transfer]] (CT) character, and in the extreme case there are distinct radical ions with unpaired electrons. If the unpaired electrons can spin-pair to form a covalent bond, then the covalent bonding interaction can lower the energy of the charge transfer state. Strong CT stabilisation has been shown to lead to a [[conical intersection]] of this exciplex state with the ground state in a balance of steric effects, electrostatic interactions, stacking interactions, and relative conformations that can determine the formation and accessibility of bonded exciplexes.<ref name="LiangNguyen2013">{{cite journal|last1=Liang|first1=JingXin|last2=Nguyen|first2=Quynh L.|last3=Matsika|first3=Spiridoula|title=Exciplexes and conical intersections lead to fluorescence quenching in π-stacked dimers of 2-aminopurine with natural purine nucleobases|journal=Photochemical & Photobiological Sciences|volume=12|issue=8|year=2013|pages=1387–1400|issn=1474-905X|doi=10.1039/c3pp25449f|pmid=23625036|pmc=5006741|bibcode=2013PhPhS..12.1387L }}</ref>

As an exception to the conventional [[radical ion|radical ion pair model]], this mode of covalent bond formation is of interest to photochemistry research, as well as the many biological fields using [[fluorescence spectroscopy]] techniques. Evidence for the bonded exciplex intermediate has been given in studies of steric and [[Coulomb's law|Coulombic effects]] on the quenching rate constants and from extensive [[density functional theory]] computations that show a curve crossing between the ground state and the low-energy bonded exciplex state.<ref name="WangHaze2007">{{cite journal|last1=Wang|first1=Yingsheng|last2=Haze|first2=Olesya|last3=Dinnocenzo|first3=Joseph P.|last4=Farid|first4=Samir|last5=Farid|first5=Ramy S.|last6=Gould|first6=Ian R.|title=Bonded Exciplexes. A New Concept in Photochemical Reactions|journal=The Journal of Organic Chemistry|volume=72|issue=18|year=2007|pages=6970–6981|issn=0022-3263|doi=10.1021/jo071157d|pmid=17676917}}</ref>

== See also ==
* {{annotated link|Excimer lamp}}
* {{annotated link|Excimer laser}}
* {{annotated link|Förster resonance energy transfer}}
* {{annotated link|Krypton fluoride laser}}
* {{annotated link|Noble gas compound}}

==References==
{{Reflist}}

{{Excimer lasers}}

[[Category:Photochemistry]]