Strong S₁-S₂ Vibronic Coupling and Enhanced Third Order Hyperpolarizability in the First Excited Singlet State of Diphenylhexatriene Studied by Time-Resolved CARS

A strong S₁-S₂ vibronic coupling effect is observed in the time-resolved coherent anti-Stokes Raman spectroscopy (CARS) spectra originating from the first excited singlet state of diphenylhexatriene. As determined from picosecond CARS measurements, the excited state spectrum appears on a subpicosecond time scale. An extremely high excited state hyperpolarizability |γ|_{exicted state} ≃ 3 × 10^-31 is derived from a CARS line shape analysis and is attributed to the increased delocalization after excitation in accordance with semiempirical calculations of bond lengths. We observe two strongly frequency-broadened vibrations being upshifted with respect to the C=C double bond stretching region of the ground state and assign them to the totally symmetric C=C stretching motion of the chain. Both frequencies depend on the solvent polarizability, giving evidence of strong S₁-S₂ vibronic coupling in the lowest excited singlet states. We discuss a model of S₁-S₂ vibronic coupling via an asymmetric low frequency mode. According to this model a double-well potential for the respective vibrational coordinate is generated in the first excited singlet state, resulting in two frequencies originating from the same type of vibration.