Since the first report of the pyrene excimer by Forster,1 intermolecular excited-state interactions have become a key subject in photochemistry and photophysics. If the two chromophores are identical, the stabilizing interaction between a molecule in the ground state and a molecule in the excited state can be described by the configuration interaction model.2 An alternative way to estimate the stabilization energy takes into account properties such as electrostatic interactions, exciton resonance, intermolecular repulsion, charge-transfer interactions, dispersion forces, and overlap repulsions.3 If the two chromophores are different, an exciplex or an heteroexcimer can be formed.4 The stabilization of an excimer depends strongly on the overlap between the two chromophores.Calculations suggest a picture of the excited-state complex in which the two chromophores, if they are planar, are at an optimum distance of 3.5 Á in a planparallel orientation.

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