関: excimeric

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2018/04/08 13:13:12」(JST)

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An excimer^[1] (originally short for excited dimer) is a short-lived dimeric or heterodimeric molecule formed from two species, at least one of which has completely filled valence shell by electrons (for example, noble gases). In this case, formation of molecules is possible only if such atom is in an electronic excited state. Heteronuclear molecules 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 nanoseconds. Binding of a larger number of excited atoms form Rydberg matter clusters, the lifetime of which can exceed many seconds.

Formation and decay

Molecular orbitals

Under the molecular orbital formalism, a typical ground-state molecule has electrons 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 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 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

Excimer lasers
Laser	Reagents	Emission peak
XeCl	Xe + Cl ₂	308 nm
KrF	Kr + NF ₃	248 nm
ArF	Ar + F ₂	193 nm

Heterodimeric diatomic complexes involving a noble gas and 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. Emission of excimer molecules is also used as a source of spontaneous ultraviolet light (excimer lamps).^[2]

The molecule pyrene is another canonical example of an excimer that has found applications in biophysics to evaluate the distance between biomolecules.^[3]

In organic chemistry many reactions occur through an exciplex for example those of simple arene compounds with alkenes:^[4] The reactions of benzene and their products depicted are a [2+2]cycloaddition to the ortho product (A),^[5] a [2+3]cycloaddition to the meta product (B)^[6] and the [2+4]cycloaddition to the para product (C)^[7] 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 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

An excimer can be generated by a dielectric barrier discharge.^{[citation needed]}

Fluorescence quenching

Exciplexes provide one of the 3 dynamic mechanisms by which fluorescence is quenched. A regular exciplex has some 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.^[8]

As an exception to the conventional 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 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.^[9]

References

^ Birks, JB "Excimers", Rep. Prog. Phys. 1975, 38, 903-974.
^ M.I. Lomaev, V.S. Skakun, E.A. Sosnin, V.F. Tarasenko, D.V. Shitts and M.V. Erofeev (2003). Excilamps: efficient sources of spontaneous UV and VUV radiation. Phys.-Usp. 46(2):193–209. Link
^ 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.
^ Photochemistry of Arenes—Reloaded Jochen Mattay Angew. Chem. Int. Ed. 2007, 46, 663 – 665 doi:10.1002/anie.200603337
^ 1-cyanobicyclo[4.2.0]octa-2,4-dienes and their synthesis United States Patent 2,805,242 Issue Date: September 3, 1957 Link
^ 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
^ 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
^ Liang, JingXin; Nguyen, Quynh L.; Matsika, Spiridoula (2013). "Exciplexes and conical intersections lead to fluorescence quenching in π-stacked dimers of 2-aminopurine with natural purine nucleobases". Photochemical & Photobiological Sciences. 12 (8): 1387. doi:10.1039/c3pp25449f. ISSN 1474-905X.
^ Wang, Yingsheng; Haze, Olesya; Dinnocenzo, Joseph P.; Farid, Samir; Farid, Ramy S.; Gould, Ian R. (2007). "Bonded Exciplexes. A New Concept in Photochemical Reactions". The Journal of Organic Chemistry. 72 (18): 6970–6981. doi:10.1021/jo071157d. ISSN 0022-3263.

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English Journal

Titanate nanotube coatings on biodegradable photopolymer scaffolds.

Beke S, Kőrösi L, Scarpellini A, Anjum F, Brandi F.SourceDepartment of Nanophysics, Istituto Italiano di Tecnologia, via Morego 30, 16163 Genova, Italy. Electronic address: szabolcs.beke@iit.it.
Materials science & engineering. C, Materials for biological applications.Mater Sci Eng C Mater Biol Appl.2013 May 1;33(4):2460-3. doi: 10.1016/j.msec.2013.01.066. Epub 2013 Feb 4.
Rigid, biodegradable photopolymer scaffolds were coated with titanate nanotubes (TNTs) by using a spin-coating method. TNTs were synthesized by a hydrothermal process at 150°C under 4.7bar ambient pressure. The biodegradable photopolymer scaffolds were produced by mask-assisted excimer laser photoc
PMID 23498284

Formation of silicon nanoparticles by a pressure induced nucleation mechanism.

Kang MK, Kim SJ, Kim HJ.SourceSamsung Electronics Co., San #24 Nongseo-Dong, Giheung-Gu, Yongin-City, Gyeonggi-Do 446-711, Korea.
Nanoscale.Nanoscale.2013 Apr 21;5(8):3266-71. doi: 10.1039/c3nr34178j. Epub 2013 Mar 6.
Formation of silicon nanoparticles (SiNPs) was achieved using excimer laser crystallization of an amorphous Si (a-Si) thin film using a SiO2 capping layer (C/L) with improved thin-film transistor (TFT) performance due to the enlarged grain size of polycrystalline Si (poly-Si). After laser irradiatio
PMID 23467641

Ratiometric fluorescence sensing and intracellular imaging of Al(3+) ions driven by an intramolecular excimer formation of a pyrimidine-pyrene scaffold.

Das S, Sahana A, Banerjee A, Lohar S, Safin DA, Babashkina MG, Bolte M, Garcia Y, Hauli I, Mukhopadhyay SK, Das D.SourceDepartment of Chemistry, The University of Burdwan, Burdwan, 713104, West Bengal, India. ddas100in@yahoo.com.
Dalton transactions (Cambridge, England : 2003).Dalton Trans.2013 Apr 14;42(14):4757-63. doi: 10.1039/c3dt32908a.
An Al selective ratiometric fluorescent probe () has been synthesized by condensation of 4,5-diaminopyrimidine with 1-pyrenecarboxaldehyde. The structure of has been confirmed by single crystal X-ray analysis. In the presence of Al, exhibits a considerable excimer emission at 445 nm along with the d
PMID 23426128

Japanese Journal

白金(Ⅱ)錯体の発光色チューニング

篠﨑一英
Jasco report 57(1), 8-14, 2015-05
NAID 40020496371

Nitrogen doping of 4H-SiC by KrF excimer laser irradiation in liquid nitrogen

Ikeda Akihiro,Marui Daichi,Ikenoue Hiroshi,Asano Tanemasa
Jpn. J. Appl. Phys. 54(4S), 04DP02, 2015-01-21
… We report nitrogen (N) doping of 4H-SiC by KrF excimer laser irradiation in liquid N<inf>2</inf>. …
NAID 150000110208

Initial Experience of Percutaneous Extraction of Infected Cardiac Implantable Electric Devices Using Excimer Laser

OKADA Ayako,IZAWA Atsushi,MOTOKI Hirohiko [他],TOMITA Takeshi,MIYASHITA Yusuke,KOYAMA Jun,AIZAWA Kazunori,SHODA Morio,IKEDA Uichi
信州医学雑誌 63(2), 103-108, 2015
… We describe our initial experience of lead extraction from infected devices using an excimer laser that was approved for this application in 2010.Methods and results : We retrospectively analyzed the characteristics, type of devices and leads, as well as indications and complications in 13 patients with CIED infection between 2011 and 2014 at our hospital.The clinical presentation comprised isolated pocket infection (n＝12), lead endocarditis (n＝3) and methicillin-resistant Staphylococcus aureus-positive blood cultures. …
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