Excimer and exciplex formation in van der Waals dimers of toluene and benzene

Structural isomerization dynamics of photoexcited van der Waals dimers of t oluene and benzene have been investigated by means of fluorescence and mass spectroscopic methods. Two isomeric forms of the heterodimer complex forme d between toluene and benzene, assigned to parallel displaced and T-shaped configurations, are found to rearrange into exciplexes when excited in the S-1 state with a small amount of vibrational energy. The toluene homodimer also reveals similar excess energy dependence on excimer formation. The exc ited-state dynamics are contrasted with those of the T-shaped benzene homod imer where no vibrational energy is required for structural isomerization. The excess energy dependence observed for the toluene complexes is attribut ed to a steric requirement of sandwich-type excimer geometry. Dissociation behavior of these exciplex/excimer complexes has been measured by photodepl etion spectroscopy and compared with that of the benzene excimer. These exc ited-state complexes are found to strongly absorb in the wavelength range b etween 440 and 560 nm. The absorption band is representative of the excimer /exciplex and is assigned to a transition into a charge-transfer state.