Excited State Complex - Exciplex: Short lived complex formed between two or more different molecules when one of them is excited (usually by a photon).

The chemistry and physical properties of molecules change when the electron density changes upon excitation. This allows new types of interactions that did not occur in the ground state. The photophysics of exciplexes is often different from that of the uncomplexed fluorophore. However, an exciplex does not have to be fluorescent.

One specific type of exciplex is an excimer.