Excimer and Exciplex Formation in van der Waals Dimers of Toluene and Benzene

Structural isomerization dynamics of photoexcited van der Waals dimers of toluene and benzene have been investigated by means of fluorescence and mass spectroscopic methods. Two isomeric forms of the heterodimer complex formed 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 excited-state dynamics are contrasted with those of the T-shaped benzene homodimer where no vibrational energy is required for structural isomerization. The excess energy dependence observed for the toluene complexes is attributed to a steric requirement of sandwich-type excimer geometry. Dissociation behavior of these exciplex/excimer complexes has been measured by photodepletion spectroscopy and compared with that of the benzene excimer. These excited-state complexes are found to strongly absorb in the wavelength range between 440 and 560 nm. The absorption band is representative of the excimer/exciplex and is assigned to a transition into a charge-transfer state.