text: Exciplexes provide one of the three 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.
  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.