Excimer/exciplex formation in van der Waals dimers of aromatic molecules

Van der Waals dimers between benzofuran, dibenzofuran, and fluorene were studied in a supersonic jet. These show red-shifted excimer/exciplex emission following excitation to their locally excited states. The dibenzofuran dimer forms an excimer from its zero-point level, while the dibenzofuran-benzofuran and fluorene-benzofuran dimers require a moderate amount of excess vibrational energy for excimer formation. Exciplex formation in dibenzofuran-fluorene was inferred from spectral broadening. Excimer/exciplex formation was attributed to strong mixing of a charge transfer state with the exciton state. The charge transfer state has a low enough energy at its equilibrium geometry to mix, but this requires a large change in geometry from the equilibrium geometry of the locally excited state. There is a barrier in the adiabatic potential surface between the minima of the locally excited state and the excimer/exciplex state. This barrier is larger for greater differences in equilibrium geometry.