Excimer

Under the molecular orbital formalism, a typical ground-state molecule has electrons in the lowest possible energy levels.

The term exciplex refers to the heterodimeric case; however, common usage expands excimer to cover this situation.

Sufficiently high energy (approximately 10 eV) is required to obtain a noble gas atom in the lowest excited electronic state, which provides the formation of an excimer molecule.

A regular exciplex has some charge-transfer (CT) character, and in the extreme case there are distinct radical ions with unpaired electrons.

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.