Picosecond time-resolved CARS spectroscopy of a mixed excited singlet state of diphenylhexatriene

Time-resolved coherent antistokes Raman spectroscopy (CARS) spectra from an excited singlet state of diphenylhexatriene (DPH) are reported. As determined from the CARS measurements, the excited-state population increases on a sub-picosecond time scale after excitation in the 1Ag–1Bu absorption band. We observe two strongly upshifted and anomalously broadened resonances in the double-bond stretching region. According to our semi-empirical calculations of vibrational frequencies, they can be assigned to polyenic p-chain modes of the 1Bu and 2Ag states, respectively. Frequency shifts as well as broadenings are extremely solvent-dependent, indicating a strong influence on the excited-state conformations. Our time-resolved measurements support a mixed excited singlet state with drastically different geometries of the local minima.