Excimer

An excimer is an excited state dimer, meaning that it is stable only in the electronic excited state. It dissociates to two independent species in the electronic ground state.

The energy of two identical molecules in the S₀ state, each denoted by A, as a function of their separation, typically shows three features:

• A flat region when the separation is very large. This flat region conveniently serves to define the zero of energy.
• A shallow minimum at a separation denoted as "sigma". This distance approximates "molecular contact". The absolute value of the energy in this minimum is often smaller than thermal energy, kT.
• Rapid onset of a severe repulsion as the separation falls below sigma.

If A and A* are capable of forming an excimer, we obtain a minimum in energy as the distance between A and A* decreases (see, for example, G Fig. 7.2).

• This minimum is at a closer approach for A + A* than is the minimum for A + A. In fact, the minimum energy for A + A* is at a geometry that would be repulsive if both molecules were in their electronic ground states. Therefore removal of the electronic excitation causes dissociation of the complex; the dimer is stable only in the presence of the excitation.

• The depth of the minimum in energy for A + A* can be larger than thermal energy. Therefore the excited state complex can indeed be stable ... as long as the excitation is present.

Emission from the excimer occurs at constant coordinates for the nuclear framework (Franck-Condon principle). This change in energy goes from the minimum in the curve for the excited state to a part of the strongly repulsive region for the ground state. Therefore the energy change is smaller than would have been the case for emission from the isolated chromophore, and the emission from the excimer is red-shifted. Furthermore, the emission is structureless, without vibrational fine structure, because the process leads to a strongly repulsive (and dissociative) geometry in the ground state.

Most of the excimers studied in polymer science are formed from pairs of planar aromatic systems. The excimer places the two planar molecules close together in a sandwich geometry. The two planes may be separated by only about 0.35 nm, which would be a repulsive configuration in the electronic ground state. The best characterized excimer is probably the one that involves two molecules of naphthalene, G Fig. 7.6 Other common excimer forming chromophores are benzene, carbazole, and pyrene. A separate page lists several examples of applications of excimers in polymeric systems.

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