EXCIMER EXCIPLEX FORMATION IN VAN-DER-WAALS DIMERS OF AROMATIC-MOLECULES/

Van der Waals dimers between benzofuran, dibenzofuran, and fluorene we re studied in a supersonic jet. These show red-shifted excimer/exciple x emission following excitation to their locally excited states. The d ibenzofuran dimer forms an excimer from its zero-point level, while th e dibenzofuran-benzofuran and fluorene-benzofuran dimers require a mod erate amount of excess vibrational energy for excimer formation. Excip lex formation in dibenzofuran-fluorene was inferred from spectral broa dening. Excimer/exciplex formation was attributed to strong mixing of a charge transfer state with the exciton state. The charge transfer st ate has a low enough energy at its equilibrium geometry to mix, but th is requires a large change in geometry from the equilibrium geometry o f the locally excited state. There is a barrier in the adiabatic poten tial surface between the minima of the locally excited state and the e xcimer/exciplex state. This barrier is larger for greater differences in equilibrium geometry.