Editing Excimer (section)

==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>