Anexcimer (originally short forexcited dimer) is a short-lived polyatomicmolecule 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 anelectronicexcited state.[1]Heteronuclear molecules and molecules that have more than two species are also calledexciplex molecules (originally short forexcited complex). Excimers are often diatomic and are composed of two atoms or molecules that would not bond if both were in theground state. The lifetime of an excimer is very short, on the order ofnanoseconds.
Under themolecular orbital formalism, a typical ground-state molecule haselectrons in the lowest possible energy levels. According to thePauli principle, at most two electrons can occupy a given orbital, and if an orbital contains two electrons they must be in oppositespin 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 theHOMO–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'sexcited 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 excitedmonomer's emission. An excimer can thus be measured by fluorescent emissions.
Because excimer formation is dependent on abimolecular 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.
The termexcimer (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 termexciplex refers to the heterodimeric case; however, common usage expandsexcimer to cover this situation.
| 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 anoble gas and ahalide, such asxenon chloride, are common in the construction ofexcimer lasers, which are excimers' most common application. These lasers take advantage of the fact that excimer components have attractive interactions in theexcited state andrepulsive interactions in theground state. Emission of excimer molecules is also used as a source of spontaneous ultraviolet light (excimer lamps).[2]
The moleculepyrene is another canonical example of an excimer that has found applications inbiophysics to evaluate the distance betweenbiomolecules.[3]
Inorganic chemistry, many reactions occur through an exciplex, for example, those of simplearene compounds with alkenes.[4] The reactions ofbenzene and their products depicted are a [2+2]cycloaddition to theortho product (A),[5] a [2+3]cycloaddition to themeta product (B)[6] and the [2+4]cycloaddition to thepara product (C)[7] with simple alkenes such as the isomers of2-butene. In these reactions, it is the arene that is excited.
As a general rule, theregioselectivity is in favor of the orthoadduct at the expense of the meta adduct when the amount of charge transfer taking place in the exciplex increases.
For a noble gas dimer or noble gas halide it takes a noble gas atom in anexcitedelectronic state to form an excimer molecule. Sufficiently high energy (approximately 10eV) 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 anelectric discharge. That is why such excimer molecules are generated in aplasma (seeexcimer molecule formation) or through high energy electron beams.
Exciplexes provide one of the three dynamic mechanisms by whichfluorescence isquenched. A regular exciplex has somecharge-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 aconical 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 conventionalradical ion pair model, this mode of covalent bond formation is of interest to photochemistry research, as well as the many biological fields usingfluorescence spectroscopy techniques. Evidence for the bonded exciplex intermediate has been given in studies of steric andCoulombic effects on the quenching rate constants and from extensivedensity functional theory computations that show a curve crossing between the ground state and the low-energy bonded exciplex state.[9]