Overdamped wavepacket motion along a barrierless potential energy surface in excited state isomerization

The wavepacket motion, associated with the barrierless excited state isomerization of the cyanine molecule 1,1′-diethyl-4,4′-cyanine in alcohol solutions, has been investigated using the fluorescence up-conversion technique. A viscosity-dependent ultrafast spectral shift of the fluorescence band was observed in agreement with predictions of earlier transient absorption studies. The shift appears as a fast decay (≈ 1 ps for propanol and ≈ 2.5 ps for hexanol) in the blue part of the fluorescence band and as a fast risetime for longer wavelengths. The observed spectral shift is a direct manifestation of the bound-twisting process of the isomerization and indicates that vibrational relaxation, solvent dynamics or Franck-Condon non-active modes do not play an important role.