An excimer [Birks, JB "Excimers", "Rep. Prog. Phys." 1975, "38", 903-974.] (originally short for excited dimer) is a short-lived dimeric or heterodimeric
moleculeformed from two species, at least one of which is in an electronic excited state. Excimers are often diatomic and are formed between 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 nanoseconds. Binding of a larger number of excited atoms form Rydberg matterclusters the lifetime of which can exceed many seconds.
Formation and decay
molecular orbitalformalism, a typical ground-state molecule has electrons in the lowest possible energy levels. According to Hund's rule, at most two electrons can occupy a given orbital, and if an orbital contains two electrons they must be in opposite 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/LUMOgap. If the molecule absorbs light whose energy is larger than 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
bimolecularinteraction, 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.
The term "excimer" (excited state complex) 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
Heterodimeric diatomic complexes involving a
noble gasand a halide, such as xenon chloride, are common in the construction of excimer lasers, 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 interactions in the ground state. The molecule pyreneis another canonical example of an excimer that has found applications in biophysics to evaluate the distance between biomolecules[Conibear PB, Bagshaw CR, Fajer PG, Kovacs M, Malnasi-Csizmadia A. (2003). Myosin cleft movement and its coupling to actomyosin dissociation. "Nat Struct Biol" 10(10):831-5.] .
organic chemistrymany reactions occur through an exciplex for example those of simple arene compounds with alkenes: ["Photochemistry of Arenes—Reloaded" Jochen Mattay Angew. Chem. Int. Ed.2007, 46, 663 – 665 DOI|10.1002/anie.200603337] The reactions of benzeneand their products depicted are a [2+2] cycloaddition to the ortho product (A) ["1-cyanobicyclo [4.2.0] octa-2,4-dienes and their synthesis" United States Patent 2,805,242 Issue Date: September 3, 1957 [http://patimg1.uspto.gov/.piw?Docid=02805242&homeurl=http%3A%2F%2Fpatft.uspto.gov%2Fnetacgi%2Fnph-Parser%3FSect1%3DPTO2%2526Sect2%3DHITOFF%2526p%3D1%2526u%3D%25252Fnetahtml%25252FPTO%25252Fsearch-bool.html%2526r%3D1%2526f%3DG%2526l%3D50%2526co1%3DAND%2526d%3DPALL%2526s1%3D2805242.PN.%2526OS%3DPN%2F2805242%2526RS%3DPN%2F2805242&PageNum=&Rtype=&SectionNum=&idkey=NONE&Input=View+first+page Link] ] ., a [2+3] cycloaddition to the meta product (B) ["A Photochemical 1,3 Cycloaddition of Olefins to Benzene" K. E. Wilzbach and Louis Kaplan J. Am. Chem. Soc.; 1966; 88(9) pp 2066 - 2067; DOI|10.1021/ja00961a052] and the [2+4] cycloaddition to the para product (C) ["Photoaddition of benzene to olefins. II. Stereospecific 1,2 and 1,4 cycloadditions" Kenneth E. Wilzbach and Louis Kaplan J. Am. Chem. Soc.; 1971; 93(8) pp 2073 - 2074; DOI|10.1021/ja00737a052] with simple alkenes such as the isomers of 2-butene. In these reactions it is the arene that is excited.
As a general rule the
regioselectivityis 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.
Krypton fluoride laser
Wikimedia Foundation. 2010.
Look at other dictionaries:
Excimer — Die Bezeichnung Excimer, leitet sich her aus der Kurzform von „excited dimer“ (= angeregtes Dimer). Es handelt sich dabei um ein kurzlebiges Teilchen, das aus zwei oder mehreren zusammenhängenden Atomen besteht. Die Besonderheit gegenüber einem… … Deutsch Wikipedia
Excimer — eksimeras statusas T sritis radioelektronika atitikmenys: angl. excimer vok. Excimer, m rus. эксимер, m pranc. excimer, m … Radioelektronikos terminų žodynas
excimer — eksimeras statusas T sritis radioelektronika atitikmenys: angl. excimer vok. Excimer, m rus. эксимер, m pranc. excimer, m … Radioelektronikos terminų žodynas
Excimer — Ex|ci|mer [Kurzw. aus engl. excited dimer = angeregtes Dimer]; das; s, e, u. Ex|ci|me|re, das; en, en: eine durch Assoziation entstandene ↑ Molekülverbindung (AA)* aus einem angeregten Molekül (A*) u. einem nichtangeregten Molekül (A) derselben… … Universal-Lexikon
excimer — ex·ci·mer ek si (.)mər n 1) an aggregate of two atoms or molecules that exists in an excited state 2) EXCIMER LASER … Medical dictionary
excimer — /ek seuh meuhr/, n. Chem. a molecular complex of two, usually identical, molecules that is stable only when one of them is in an excited state. [1960 65; EXCI(TED) + (DI)MER] * * * … Universalium
excimer — noun Any diatomic molecule, at least one of whose atoms is in an excited state … Wiktionary
excimer — [ ɛksɪmə] noun Chemistry an unstable excited molecule which is formed by the combination of two smaller molecules and rapidly dissociates with emission of radiation. Origin 1960s: blend of excited and dimer … English new terms dictionary
excimer — ex·ci·mer … English syllables
excimer — ex•ci•mer [[t]ˈɛk sə mər[/t]] n. chem. a molecular complex of two, usu. identical, molecules that is stable only when one of them is in an excited state • Etymology: 1960–65; exci (ted) + (di) mer … From formal English to slang