Ultrafast energy relaxation dynamics of C₁₂₀, a [2+2]-bridged C₆₀ dimer

The energy relaxation dynamics of the photoexcited [2+2]-bridged C₆₀ dimer in o-dichlorobenzene has been investigated by the time-resolved fluorescence and transient absorption techniques. The absorption and fluorescence spectra of the C₆₀ dimer show that the monomeric moieties of C₆₀ are only weakly interacting with each other, indicating that the C₆₀ dimer can be considered as a nearly decoupled bichromophoric molecular system in the ground state. The lowest emitting state of the C₆₀ dimer is formed with ∼60-ps time constant, while its fluorescence decays nearly as fast (∼1.22 ns) as in the C₆₀ monomer (∼1.17 ns). The 60-ps decay component was suggested to be due to a transition from an optically nonemissive state to an emissive state in the C₆₀ dimer. It was also found that the overall photophysical properties of the C₆₀ dimer have much more to do with the disruption of the π-bond system than any bichromophoric interaction between the individual C₆₀ units.