Depending on the nature and concentration of the fluorophore (M) and the foreign molecules (Q), viscosity of the solvent, etc., the bimolecular interactions with the excited state fluorophore may involve encounter complex (M* . . . Q), as seen in dynamic quenching, excimer (MM)* or exciplex (MQ*) formation.

1M+hv⇌1M*
1M*+M⇌1(MM)(Excimer)
1M+Q⇌1(MQ)*(Exciplex)

When collisions between an excited fluorophore molecule and an unexcited identical fluorophore molecule lead to the formation of excited dimeric complexes, such excited dimeric complexes are called the excimers. The word "excimer" is . The word "excimer" is a combined contraction form derived from "excited dimer". Excimer formations have been observed with many aromatic hydrocarbons such as naphthalene, pyrene, etc. The "exciplex" is a combined contraction word derived from "excited complex". The exciplex is formed via collisions between an excited fluorophore molecule and an unexcited unlike molecule. One of the molecules (either fluorophore or the foreign molecule) acts as an electron donor and the other molecule acts as the electron acceptor. For example, exciplex formation has been observed between the electron donor molecule, N,N-diethylamine and excited anthracene molecule.

Excimer and exciplex have binding energies greater than the average kinetic energy and represent new chemical species with more or less well-defined geometrical structures. Excimers and exciplexes can lose their excitation energy through fluorescence or decaying processes. These processes result in quenching of the excited state and are therefore known as quenching processes. The formation of excimer and exciplex requires bimolecular interactions and therefore occurs at relatively high concentrations of the species that ensure large number of collisions among the molecules during the life-time of the excited fluorophore. Since the molecules have paired electrons, excimers only form when one of the dimer components is in the excited state when there is an rearrangement of electron distribution. When the excimer returns to the ground state, its components dissociate. The excimer or exciplex formation is therefore reversible. They break apart after emission and may form again on excitation. Excimers and exciplexes are accompanied with a broad structure-less emission band at higher wavelength than that of the monomeric fluorophore molecule. It has been observed that the exciplex emission band is red-shifted when the solvent polarity increases. Temperature and viscosity have also been observed to play important roles in intermolecular process like exciplex formation.