Evidence of strong vibronic coupling in the first excited singlet state of diphenylhexatriene by picosecond CARS spectroscopy

Applying the polarization-sensitive variant of coherent anti-Stokes raman scattering (CARS) with picosecond time resolution solvent-dependent vibrational shifts and depolarization ratios of vibrational resonances of the first excited singlet state of diphenylhexatriene were investigated. The depolarization ratios of all observed Raman lines (ρR∼0.72) near the transient absorption of the excited state show the dominance of anisotropic scattering originating from vibrations of the same symmetry species. Vibrational frequencies were determined under polarization conditions avoiding spectral interference with the electronic background. Frequencies of the two broad CC stretching bands in the spectral range above 1600 cm−1 increase with decreasing polarizability of the solvent indicating strong vibronic coupling in the first excited singlet state.