Abstract The excitation energy dependence of the intramolecular exciplex formation and the electronic relaxation has been studied in the collision-free vapor of 1-(9,10-dicyano-2-anthryl)-3-(pentamethylphenyl)propane. The fluorescence quantum yields and lifetimes of the 9,10-dicyanoanthryl moiety and the intramolecular exciplex in the collision-free vapor were observed to depend on the excitation energy within the S1 absorption band. From the quantum yields and the decay rate constants, the excitation energy dependence of the association and dissociation rate constants of the intramolecular exciplex formation were determined. The association rate constant of the exciplex formation increases more remarkably with increasing excitation energy than the nonradiative rate constants of 9,10-dicyanoanthryl moiety and the exciplex. The excitation energy dependence of the association rate constant was discussed in terms of the nonradiative transition from the initial optical state to the final exciplex and the level density of the final state, which were calculated in the simple model system by means of the semi-classical mechanics.