Dynamics of Excimer Formation and Relaxation in the T-Shaped Benzene Dimer

The excited-state dynamics of the T-shaped van der Waals (vdW) dimer of benzene has been studied by probing fluorescence after S 1 excitation. Excimer fluorescence is observed when the dimer is excited into the S 1 origin and the ν 6 vibronic level. The results demonstrate that the excimer formation proceeds by tunneling through an energy barrier between vdW and excimer potential wells. Use of a pump−probe fluorescence depletion technique showed that the excimer absorbs strongly at 500 nm, consistent with the result in solution. Evidence is observed for an equilibrium between the vdW dimer and excimer states in the ν 6 excitation. These results are explained by invoking an excimer of a parallel stacked geometry, having a charge-transfer character. The excimer formation dynamics of the benzene dimer is discussed based on comparison with those of other aromatic dimers and cluster systems.